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Theme 1:   The Organism at Work       

  1. Regulation of Internal Environment
  2. Nervous Co-ordination
  3. Sense Organs

Theme 2       The Organism And Its Environment   

  1. Ecology of Population
  2. Balance in Nature

Theme 3       Continuity of Life  

  1. Reproductive System and Reproduction in Humans
  2. Development of New Seeds
  3. Fruits
  4. Reproductive Behaviours
  5. Biology of Heredity (Genetics)
  6. Variation and Evolution
  7. Evolution



Theme 1:   The Organism at Work

  1. Regulation of Internal Environment


Homeostasis refers to the process by which organisms maintain a relatively stable internal environment. The internal environment comprises body fluids such as blood, lymph, and tissue fluid. In order for body cells to function optimally and for healthy growth, living organisms must be able to adapt to changes in the physical and chemical conditions of their body fluids. These conditions include temperature, pH, osmotic pressure, concentrations of dissolved substances, and mineral ions.



Homeostatic processes involve control mechanisms that detect and respond to changes in the organism’s internal environment. These mechanisms typically include:

  1. Sensory detectors that recognize changes in specific conditions and stimulate the relevant body parts.
  2. Effector organs or glands that react and restore the normal state.



In unicellular organisms, osmoregulation (homeostasis) is ensured by the use of a contractile vacuole. In multicellular organisms, various organs contribute to homeostasis, including the kidney, liver, skin, ductless glands (hormones), and the brain, which has overall control of the homeostatic process in the body.



The mammalian kidney is a reddish-brown, bean-shaped organ located in the posterior end of the abdomen. The right kidney is slightly lower than the left. On cutting a kidney longitudinally, two distinct regions are observed: an outer cortex and an inner medulla. Narrow tubules called urinary tubules (nephrons) pass through these two regions. The tubules open at the tips of triangular-shaped masses of tissues called pyramids, which in turn open into a funnel-shaped cavity called the pelvis. The kidney has many tiny capillaries, which are branches of the renal artery and renal vein. The pelvis continues as the ureter, a long narrow tube connecting the kidney to the urinary bladder.



The kidney serves as the chief osmoregulator and excretory organ in mammals, performing the following functions:

  1. Removal of toxic wastes and harmful substances.
  2. Production of heat during cold conditions.
  3. Excretion of nitrogenous wastes like urea.
  4. Regulation of water levels in the body.
  5. Assistance in regulating the body’s pH.
  6. Maintenance of salt or ion balance in the body.

The first three functions are related to excretion, while the last three are osmoregulatory functions of the kidney.



The kidney acts as an osmoregulator by maintaining the water, salt, and pH balance of the blood, primarily in the distal tubules and collecting ducts of the urinary tubules.


When the body is dehydrated, the osmotic pressure of the blood increases. Osmoreceptors in the hypothalamus detect these changes and stimulate the pituitary gland to secrete more antidiuretic hormone (ADH). ADH makes the walls of the urinary tubules more permeable, leading to increased water reabsorption into the blood. This results in less water being lost from the body, and concentrated urine is produced.

When the body is hydrated, the osmotic pressure of the blood is lowered, resulting in the secretion of less ADH. Consequently, the walls of the kidney become less permeable, and more water is lost from the body as dilute urine.


The concentration of sodium ions in the blood is regulated by excreting the excess or reabsorbing more, which is controlled by the hormone aldosterone.

The blood pH of 7.4 is maintained by excreting hydrogen ions when the pH becomes acidic and excreting hydrogen carbonate ions when it becomes alkaline.




  1. Nephritis: Inflammation of the blood vessels (glomeruli) in Bowman’s capsule of the nephron caused by bacteria (streptococci). This inflammation leads to increased permeability of the blood vessels, resulting in the leakage of useful materials from the blood into the glomerular filtrate. Inflamed blood vessels can also become blocked, leading to kidney failure.
  2. Diuresis: A condition where large quantities of dilute urine are produced because the cells of the kidney tubules are not reabsorbing water from the glomerular filtrate. Diuresis is common in patients suffering from diabetes insipidus.
  3. Kidney stones: Stony masses of minerals and organic matter formed in the urinary tubules. Low water intake combined with high salt intake predisposes individuals to this disease, causing the crystallization of mineral salts and obstructing the free flow of urine.
  4. Dropsy (edema): A disease condition in which the cells of Bowman’s capsule are unable to absorb water from the blood in the urinary tubules. This causes water retention in the blood or tissues, resulting in swelling of certain body parts.



  1. Presence of proteins and blood cells in urine (nephritis).
  2. Swollen face and ankles, constant weakness, and sluggishness (edema).
  3. Excessive urination leading to weight loss (diuresis).
  4. Abdominal pain, high blood pressure, and bloody urine (kidney stones).
  5. General body pains and fever (any renal disease).
  6. High blood pressure, dizziness, and fatigue.



  1. Use of drugs such as antibiotics (nephritis) and diuretics (edema).
  2. Kidney transplant (diuresis, nephritis).
  3. Dialysis: Utilizing a dialysis machine (artificial kidney) to filter waste out of the patient’s blood (nephritis).
  4. Reduction in water intake (edema).
  5. Increasing water intake and avoiding excessive consumption of calcium-rich foods (kidney stones).
  6. Surgical operation called nephrectomy (kidney stones). 


  1. Nervous Co-ordination

What are Hormones?

  1. Hormones are chemical substances produced or secreted by endocrine glands, which are ductless glands in the body.
  2. They are released into the bloodstream and travel to target organs to exert their effects.
  3. Hormones can speed up or slow down biological reactions in the body.
  4. They act as chemical messengers, produced in one part of the body and affecting specific target organs.
  5. Hormones play a role in homeostasis, growth, and development.
  6. After their actions, hormones are inactivated in the liver and excreted in urine.



There are five groups of animal hormones secreted by different glands.


The pituitary gland is located below the hypothalamus and consists of anterior and posterior parts. The anterior pituitary gland releases growth hormones and tropic hormones that regulate other endocrine glands. It is often referred to as the “master gland.” The hormones secreted by the anterior pituitary include:

  1. Gonadotropins (follicle-stimulating hormone and luteinizing hormone) which regulate reproductive functions.
  2. Adrenocorticotropic hormone, which stimulates the adrenal cortex to secrete corticosteroids.
  3. Thyroid-stimulating hormone, which regulates the thyroid gland.
  4. Prolactin, which stimulates milk production in mammary glands.
  5. Somatotropin (growth hormone), which promotes bone growth and metabolic rate.

The posterior pituitary gland secretes:

  1. Anti-diuretic hormone (ADH), which regulates water balance in the body.
  2. Oxytocin, which stimulates uterine contractions during childbirth and milk ejection.



The thyroid gland, located in the neck region close to the larynx, produces three main hormones: thyroxine, triiodothyronine, and calcitonin.

The primary hormone is thyroxine.


  1. Regulates the growth and development of all body cells.
  2. Increases the rate of glucose oxidation in body cells and heat production.
  3. Under secretion of thyroxine causes cretinism in children and sluggishness/goiter in adults.
  4. Over secretion of thyroxine causes hyperactivity and restlessness.



The parathyroid glands, which are small glands attached to the thyroid gland, secrete a hormone called parathyroid hormone (PTH). PTH plays a crucial role in regulating blood calcium levels. It achieves this by:


  1. Releasing calcium from the bones
  2. Increasing calcium absorption in the gut
  3. Reducing calcium excretion by the kidneys


The hormone calcitonin, produced by the thyroid gland, has the opposite effect and lowers blood calcium levels. Over secretion of parathyroid hormone leads to fragile bones prone to fractures, while under secretion causes muscle spasms.


The pancreas serves both as a digestive gland and an endocrine gland. It contains specialized cells called Islets of Langerhans, which produce two important hormones: insulin and glucagon.

Insulin lowers blood glucose levels by stimulating liver cells to convert excess glucose into glycogen for storage in the liver and muscles. Deficiency of insulin leads to diabetes mellitus, characterized by glucose excretion in urine, reduced appetite, and increased thirst. Excessive secretion of insulin causes a drop in blood sugar levels and constant hunger.

Glucagon, on the other hand, raises blood glucose levels by stimulating the liver to convert stored glycogen into glucose.



The adrenal glands, located above the kidneys, produce two distinct groups of hormones. The adrenal cortex produces corticosteroids, while the adrenal medulla produces adrenaline and noradrenaline, known as emergency hormones.

Corticosteroids include glucocorticoids, such as cortisol, which raise blood glucose levels during times of stress, and mineralocorticoids, such as aldosterone, which regulate sodium and potassium ion levels in body fluids.

Adrenaline, as an emergency hormone, prepares the body for immediate action in response to fear, danger, or anger. It achieves this by:

  1. Increasing muscular tone
  2. Accelerating heart rate and respiration
  3. Dilating pupils
  4. Enhancing the conversion of glycogen to glucose by the liver

Under-secretion of adrenaline results in a slow response to emergencies, low blood pressure, and heart rate, while over-secretion leads to excessive anxiety and excitement.



Reproductive hormones are produced by specific cells in the reproductive organs. In males, the testes produce testosterone, the male sex hormone, while in females, the ovaries produce estrogen and progesterone, the female sex hormones.

Testosterone stimulates:

  1. Growth and maturation of the penis, testes, and accessory sex structures
  2. Development of male sex characteristics such as muscle growth, pubic, armpit, chest, and facial hair, and deepening of the voice

Estrogen performs the following functions in females:

  1. Development of secondary sexual characteristics, including breast enlargement, pubic and armpit hair growth, widening of the hips, and fat distribution.
  2. Regulation of the reproductive or menstrual cycle


  1. Inhibits egg production (ovulation) during pregnancy
  2. Prepares and maintains the lining of the uterus
  3. Aids in implantation and development of the embryo in the uterus

Excessive secretion of reproductive hormones leads to excessive development of sexual organs and abnormal sexual urges, while under secretion results in poor development of secondary sexual characteristics, sexual organs, and decreased sexual drive.


Plant coordination relies on chemicals known as plant hormones. These hormones share similarities with animal hormones in that they:

  1. Are required in small amounts to exert their effects
  2. Are produced in one part of the plant and transported to other parts to elicit their effects

However, plant hormones differ in that they are not produced exclusively in specialized glands but have more generalized effects based on concentration and organ type. Plant hormones regulate growth, fruit formation, root development, leaf fall, and other activities. Combinations of plant hormones can result in unique responses beyond what each hormone can accomplish alone. Although slower than animal hormones, plant hormone responses mainly occur through growth processes.



The major groups of plant hormones include auxins, gibberellins, cytokinins, abscisic acid, and ethylene.


The primary naturally occurring auxin is indoleacetic acid (IAA), produced at the shoot apex. It influences cell division, elongation, and differentiation. Auxin:

  1. Influences stem growth towards light (positive phototropism) and root growth away from light (negative phototropism)
  2. Stimulates the development of lateral and adventitious roots for increased water and mineral absorption
  3. Inhibits the growth of lateral buds, causing apical dominance
  4. Promotes fruit development
  5. Breaks seed dormancy, promoting early germination
  6. Delays leaf fall



Gibberellins are found in root and stem apices. They promote growth by stimulating both cell elongation and division. They are also effective in promoting growth in dwarf plant varieties. Other effects of gibberellins include inducing seed germination and increasing fruit size.


Cytokinins are produced in roots and promote growth, similar to auxins and gibberellins. They stimulate cell division, ensuring normal stem and root growth. Unlike auxins, cytokinins stimulate lateral buds to grow into branches and delay aging in plants.


Produced in mature green leaves, fruits, and root caps, abscisic acid acts as a growth inhibitor that generally opposes the effects of auxins and gibberellins. It:

  1. Suppresses bud growth
  2. Induces dormancy in seeds and buds
  3. Promotes leaf aging
  4. Regulates the opening and closing of stomata, the tiny openings on leaves

The effects of abscisic acid help plants withstand adverse environmental conditions.


Ethylene, a simple hydrocarbon, is produced in leaves, stems, and young fruits. It inhibits lateral bud development and accelerates fruit ripening.



Both natural and synthetic plant hormones find applications in horticulture and agriculture. Some common uses include:

  1. Artificial vegetative propagation: Auxins are used in rooting powders to promote root formation in cuttings. Synthetic auxins aid in grafting by inducing wound tissue formation.
  2. Weed control: Synthetic auxins are used as selective herbicides to control weeds (e.g., 2,4-D).
  3. Harvesting: Auxins are applied to extend the shelf life of fruits on the plants.
  4. Parthenocarpy: Auxins and gibberellins induce fruit development without fertilization.
  5. Preservation: Cytokinins are used to prevent yellowing in stored vegetables, while growth inhibitors (e.g., abscisic acid) prevent sprouting in onions and potatoes. 






Living organisms have the ability to respond to changes in their environment, known as stimuli. These stimuli can be of various types, including mechanical, electromagnetic, chemical, or thermal. While most cells in an organism’s body are capable of sensing stimuli, certain cells specialize in detecting specific types of stimuli. These specialized cells are called sensory receptors or sense cells, and they are abundant in the human body, constantly monitoring the internal environment.

Mechanoreceptors are sensory receptors that respond to mechanical changes, while thermoreceptors, chemoreceptors, and photoreceptors are sensitive to heat, chemical substances, and light, respectively.

Sensory receptors convert the detected stimuli into electrical impulses, which are then transmitted to the brain. The brain interprets these impulses, translating them into visual images, sounds, smells, or taste sensations. Structures that contain sensory receptors are referred to as sense organs.

A sense organ is defined as a group of specialized cells or tissues that can receive, perceive, or detect stimuli and transmit the information to the central nervous system. In mammals, there are five types of sense organs:

  1. The skin, which detects touch, pain, pressure, heat, and cold.
  2. The eye, which detects light (sense of sight).
  3. The ear, which detects sound (sense of hearing and balance).
  4. The nose, which detects smell.
  5. The tongue, which detects taste.



The mammalian skin contains numerous sensory receptors that detect various stimuli such as touch, pressure, pain, cold, and heat. Unlike other sense organs, which specialize in detecting a single type of stimulus, the skin’s sensory receptors are capable of responding to multiple stimuli.

Sensory receptors are not evenly distributed throughout the skin. Each type of receptor is concentrated in specific regions of the body. Receptors sensitive to pressure, known as Pacinian corpuscles, are found deep within the skin and require stronger stimulation. Receptors sensitive to touch, called Meissner’s corpuscles, are predominantly distributed close to the surface of the skin, especially in hairless regions like the tongue, fingers, lips, and forehead. These receptors respond to gentle stimulation. Between the pressure and touch receptors, there are receptors that detect cold, heat, and pain.



The eye is the organ responsible for vision. It has a spherical shape and is protected by ocular or optical structures such as eye sockets, eyelids, eyelashes, tear or lacrimal glands, and conjunctiva.

  1. The eye sockets house the eyes.
  2. The eyelids (upper and lower) protect the eyes from foreign particles or mechanical injury.
  3. The tear or lacrimal glands, located at the meeting point of the eyelids, secrete a salty fluid called tears, which washes away dust and bacteria using a chemical substance called lysozyme.
  4. The eyelashes are rows of hairs on the eyelids that protect the eyeball from dust, excessive light, and shield the eye against sweat and water.
  5. The conjunctiva is a thin, transparent membrane that lines the inside of the eyelids and covers and protects the cornea. In case of infection, the conjunctiva can become inflamed, leading to conjunctivitis.



The wall of the eyeball consists of three layers, from the outermost to the innermost: sclera, choroid, and retina.

  1. THE SCLERA: This outermost white layer provides the eye with its shape and protects the inner parts. The sclera bulges out in front of the eye, forming the transparent cornea. The cornea allows light to enter the eye, brings the light to focus on the retina, and provides external protection.

2. THE CHOROID LAYER: This highly vascularized and pigmented layer (black) supplies nutrients and oxygen to the eye’s cells. The black pigment helps absorb light rays and prevents light reflection. The choroid layer includes the ciliary muscles, iris, pupils, suspensory ligaments, and the lens.

  1. IRIS: A band of muscle fibers that contracts and relaxes, altering the size of the pupils and controlling the amount of light entering the eye.
  2. PUPIL: The opening between the upper and lower iris that controls the amount of light entering the eye. Bright light causes the pupil to constrict, while dim light causes it to dilate.
  3. CILIARY MUSCLE: Consisting of circular and radial muscles, it contracts and relaxes to adjust the focal length of the lens, enabling the eye to focus on near and distant objects.
  4. SUSPENSORY LIGAMENTS: Hold the lens in place.
  5. LENS: A transparent, biconvex, elastic structure held in position by suspensory ligaments. It refracts light rays entering the eye and fine-tunes the focus of the object’s image on the retina.


3. THE RETINA: This part of the eye is sensitive to light. It is also vascularized, pigmented, and elastic. Light rays converge on the retina, forming images that are real, inverted, and smaller than the actual objects. The retina contains two types of sensory cells called cones and rods.

  1. CONES: Cells in the retina that are sensitive to colored vision and high light intensities. They contain a photochemical substance called iodopsin, which is not easily bleached by intense light.
  2. RODS: More numerous than cones, rods are sensitive to colorless vision and low light intensities. They contain a purple pigment-protein complex called rhodopsin, which is easily bleached by light.
  3. YELLOW SPOT (Fovea centralis): The most sensitive part of the retina, responsible for sending the most detailed visual information to the brain. It is the point where the image is focused.
  4. BLIND SPOT: The area on the retina where there are no cones or rods. The optic nerve exits the eye to the brain from this blind spot.
  5. OPTIC NERVES: These nerves transmit sensory impulses to and from the brain.
  6. AQUEOUS HUMOUR: A transparent, watery liquid that fills the space between the cornea and the lens. It contains protein, sugar, salt, and water solutions. The aqueous humor refracts light rays onto the retina and helps maintain the eye’s spherical shape.
  7. VITREOUS HUMOUR: A wider, transparent, jelly-like liquid that fills the space between the lens and the retina. It also consists of protein, sugar, salt, and water. The vitreous humor serves the same functions as the aqueous humor.



The eye performs two major functions: image formation and accommodation.

  1. Image Formation:

Light rays from an object enter the eye through the cornea, pass through the aqueous humor, lens, and vitreous humor, and finally reach the retina. These transparent structures refract (bend) the light rays, allowing them to converge on the retina. As a result, an image of the object is formed on the retina, appearing real, inverted, and smaller than the actual object. The light stimulus reflected from the object is received by the cones or rods, depending on the light intensity. These sensory cells convert the stimulus into electrical impulses, which are then transmitted through the optic nerve to the optic lobe of the brain. The

brain accurately interprets the image. To create a sharp image on the retina, all the refracted light rays must meet at a specific point on the retina known as the yellow spot.


  1. Accommodation:

This refers to the eye’s ability to focus on objects at different distances, enabling clear vision. Accommodation involves adjusting the focal length of the lens to focus near and distant objects onto the retina.

  1. To see near objects:
  2. The ciliary muscles contract, causing the suspensory ligaments to relax their tension on the lens.
  3. The lens becomes more convex, reducing its focal length and allowing the image to focus on the retina.
  4. To see distant objects:
  5. The ciliary muscles relax, causing the suspensory ligaments to contract and pull on the lens.
  6. The lens becomes flattened (elongated), increasing its focal length and allowing the image to focus on the retina.



Eye defects occur when an image cannot be properly formed on the retina. Some common eye defects include:

Short-sightedness (myopia):

  1. This defect causes a person to see nearby objects clearly, but distant objects appear blurred. It occurs when the eyeball is longer than normal from back to front, causing light rays from distant objects to focus in front of the retina.
  2. CORRECTION: Using spectacles or glasses with concave or diverging lenses that diverge the light rays from distant objects before they enter the eye. This allows the eye to bring the rays to a focus directly on the retina.


Long-sightedness (hypermetropia):

  1. This defect causes a person to see distant objects clearly, but nearby objects appear blurred. It occurs when the eyeball is shorter than normal, causing light rays from nearby objects to focus behind the retina.
  2. CORRECTION: Using spectacles or glasses with convex or converging lenses that converge the light rays from nearby objects before they enter the eye. This helps the eye bring the rays to a focus directly on the retina.



  1. This eye defect occurs as the lens and ciliary muscle lose their elasticity with age. Light rays from nearby objects are not bent inward sufficiently, causing them to focus behind the retina.
  2. CORRECTION: Using converging lenses.



  1. This defect is caused by an uneven cornea surface and can be corrected by using lenses that compensate for the uneven surface.



  1. Cataracts primarily affect older people, causing the eye lens to become cloudy. They can be corrected with a plastic lens or spectacles with suitable lenses.


Night blindness:

  1. Night blindness is caused by a deficiency of vitamin A.



  1. Conjunctivitis is the inflammation of the conjunctiva, usually caused by bacteria or irritants in the wind.




Mammals possess a bilateral auditory system consisting of two ears located on each side of the head. The majority of the auditory system is protected within the skull. Its primary functions are hearing and maintaining balance.



The mammalian ear can be divided into three main parts: the outer ear, the middle ear, and the inner ear.

  1. Outer Ear:

The outer ear comprises the following structures, starting from the outside of the organism:

  1. Pinna: This flexible structure is composed of soft cartilage covered with skin. The pinna collects sounds, detects their direction, and directs them into the external auditory meatus (ear canal).
  2. External Auditory Meatus: This canal is lined with fine hairs and glands that produce wax, which helps prevent the entry of germs, insects, and dust particles. It allows sound waves to pass from the pinna to the eardrum.
  3. Tympanic Membrane (Eardrum): This thin membrane vibrates when sound waves reach it. It separates the outer ear from the middle ear and transmits sound waves from the outer ear to the middle ear.


  1. Middle Ear:

The middle ear is a small air-filled chamber within the skull. It consists of three tiny bones called ear ossicles (malleus, incus, and stapes) and the Eustachian tube.

  1. Ear Ossicles: These bones, namely the hammer (malleus), anvil (incus), and stirrup (stapes), connect the outer ear to the inner ear. They transmit vibrations from the eardrum to the oval window, resulting in increased pressure within the window.
  2. Eustachian Tube: This narrow tube connects the middle ear to the pharynx. It typically opens during yawning, allowing air to enter or exit the middle ear and equalizing the air pressure on both sides of the eardrum.


  1. Inner Ear:

The inner ear consists of a complex bony structure known as the bony labyrinth, filled with perilymph fluid. Within the bony labyrinth, there are membranous sacs and tubes called the membranous labyrinth, which are filled with endolymph fluid. The cochlea and the semicircular canals are the two auditory structures connected to the utriculus and sacculus, respectively.

  1. Cochlea: Shaped like a snail’s shell, the cochlea is primarily responsible for hearing. It contains sensory hair cells (mechanoreceptors) that synapse with sensory neurons, forming the cochlear nerves. Together, they constitute the organ of Corti.
  2. Semicircular Canals: These three canals lie at right angles to each other and have swollen ends called ampullae. The ampullae contain sensory hair cells and otoliths (ear stones). The semicircular canals play a crucial role in balance and maintaining the body’s posture.



The ear performs two major functions: hearing and balancing.

Mechanism of Hearing:

The pinna collects and focuses sound waves, which then pass through the external auditory meatus. The sound waves cause the eardrum to vibrate, and these vibrations are transmitted to the ear ossicles, which amplify them. The oval window further magnifies the vibrations, sending them into the inner ear (cochlea) where the perilymph and endolymph fluids vibrate. These vibrations stimulate the organ of Corti within the cochlea, which converts sound vibrations into electrical impulses. The impulses then travel through the auditory nerves to the brain for interpretation.


Mechanism of Balancing:

Movement of the head in any direction causes the fluid within the semicircular canals and the otoliths in the ampullae to move. This movement generates impulses that travel through the vestibular branch of the auditory nerves to the brain for interpretation. The brain then relays impulses to the body’s muscles to maintain balance and body position.



Deafness is the primary disorder associated with the ear. It can be temporary or permanent and may result from various causes, including damage to the eardrum, Eustachian tube, or sensory cells in the cochlea. Factors such as wax blockage, ear infections, or exposure to loud noises can also lead to deafness.



To maintain ear health, it is important to follow these guidelines:

  1. Use cotton wool regularly.
  2. Avoid using sharp objects to clean the ear.
  3. Minimize exposure to loud noises.
  4. Protect the ear from strong blows that could damage the eardrum.
  5. Seek medical attention when experiencing any ear-related problems.



The nose serves as the olfactory organ in humans. The epithelial lining of the nasal cavity contains sensory nerve endings. Although humans have a relatively poor sense of smell compared to some animals like dogs, we can quickly detect smells, albeit for a limited duration.



The nose functions optimally when moist. Smell receptors within the nasal cavity are stimulated by chemical substances. Volatile particles in the air dissolve in the mucus layer covering the receptor cells in the nostrils. Stimulation of these receptors generates nerve impulses that travel through the olfactory nerve to the olfactory lobe of the brain. The brain then interprets the type of smell detected.



Taste buds, which are sensory cells, are responsible for detecting taste. These taste buds are located on small protrusions on the tongue’s exposed surface. Four sensory nerves connect the taste buds to the brain, enabling the interpretation of taste sensations. The tongue is sensitive to four primary tastes:

  1. Sweet: Detected by chemoreceptors at the tip of the tongue.
  2. Salty: Detected by chemoreceptors at the side (frontal area) of the tongue.
  3. Sour: Detected by chemoreceptors at the side (towards the back) of the tongue.
  4. Bitter: Detected by chemoreceptors at the back of the tongue.



When substances are placed in the mouth, chemicals dissolve in the saliva on the tongue’s surface. This dissolution stimulates the sensory nerve endings within the taste buds, which then transmit impulses to the brain for interpretation as sweet, bitter, sour, or salty tastes.

NOTE: Smell and taste are closely related sensations, and we often experience them together. When we eat or drink, taste receptors are stimulated, and simultaneously, flavor-producing chemicals dissolve in the moist air within the mouth and flow into the nasal cavity, stimulating smell receptors. The sensation of smell is often more pronounced than that of taste.



1. Advantages of Having Two Ears:

Having two ears provides several benefits, including improved sound localization and a more accurate perception of sound intensity. It also aids in better speech comprehension and enhances the ability to differentiate between different sound frequencies.


2.  Types of Deafness:

There are two main types of deafness:

  1. Conductive Deafness: This type occurs when there is an issue with sound conduction in the outer or middle ear, such as damage to the eardrum or ear ossicles.
  2. Sensorineural Deafness: This type is caused by damage to the sensory cells in the inner ear (cochlea) or the auditory nerve pathways.


3. Mechanism of Smelling:

Smelling involves the stimulation of smell receptors in the nasal cavity by volatile chemical substances. These receptors generate nerve impulses that are transmitted to the olfactory lobe of the brain for interpretation.


4. Functions of the Organ of Smell:

The organ of smell serves the following functions:

  1. Detection and interpretation of various odors.
  2. Triggering emotional and memory responses associated with certain smells.
  3. Influencing taste perception through the interaction between smell and taste sensations.
  4. Acting as a warning system by detecting potentially harmful substances or gases in the environment.



Theme 2: The Organism and Its Environment.

Ecology of Population.

  1. Ecology Population:

An ecological population refers to a group of individuals of the same species that live in the same area and interact with each other. This concept is fundamental to the field of ecology, which is the study of how organisms interact with each other and their environment. A population is defined by its specific geographic location, its members’ ability to interbreed, and its potential to increase in number through reproduction.

  1. Succession:

Succession is a process of ecological change in a specific area over time. It describes how the composition of species and the structure of an ecosystem evolve and change as an area transitions from a barren or disturbed state to a more mature and stable state. Succession can be classified into two main types: primary succession, which occurs in areas without any previous biological community, and secondary succession, which happens in areas that have been previously colonized by life but have been disturbed or disrupted.

  1. Structural Change in Species Composition, Variety, and Increase in Number:

This phrase describes the changes that occur in an ecosystem as it progresses through succession. Initially, in a barren or disturbed area, there may be only a few species present. As succession occurs, new species arrive, biodiversity increases, and the overall structure of the ecosystem becomes more complex. This is often accompanied by an increase in the population numbers of various species as the ecosystem becomes more stable and provides more resources.


  1. Primary Succession in Aquatic Habitat:

Primary succession in an aquatic habitat involves the colonization and establishment of life in a water body that previously lacked any significant biological community. This can happen, for example, in newly formed ponds or lakes created by geological processes such as glacial retreat. Over time, aquatic plants, algae, and animals gradually colonize and establish themselves, leading to a more complex and stable aquatic ecosystem.

This type of succession initiates from barren land, exposed rock surfaces, or unoccupied bodies of water.

The process of primary succession on land can be observed in scenarios like a construction site where a pile of subsoil, rocks, or concrete blocks remains after building. In aquatic environments, primary succession can be witnessed in newly formed artificial ponds.


The pioneers of any succession, referred to as primary colonizers, are typically autotrophic plants. These plants have uncomplicated life requirements and can endure harsh conditions. As the primary succession progresses into its second year, the growth of algae, lichens, and mosses may become more pronounced. Over time, these organisms contribute to soil formation through growth, decay, and decomposition.


Moving into the third year, small herbaceous plants might establish themselves. These plants play a role in transforming the habitat by outcompeting smaller plants, leading to their decline. The dropping of leaves from these plants further contributes to soil enrichment, making it more conducive for other organisms. As years pass, an increasing variety of species join the habitat, while some species dwindle. This succession of species continues until a climax community is achieved. This climax community heralds the presence of larger life forms like shrubs and trees.


  1. Meaning and Examples of Secondary Succession:

Secondary succession refers to the ecological process that occurs in an area that has previously supported life but has been disturbed or disrupted by events such as fires, clear-cutting, or abandoned agricultural land. The existing soil and some plant and animal species still remain, allowing for a faster and somewhat different recovery process compared to primary succession. An example would be a forest regrowing after a wildfire.

Secondary succession takes place in areas where soil and vegetation haven’t been completely eradicated. This process unfolds at a faster pace compared to primary succession, owing to the pre-existing presence of soil. Additionally, it transpires when a farmer abandons an old field. The commencement of secondary succession stems from a pre-established community that has experienced disturbances due to human activities and other influencing factors. Incidents such as fire, drought, and floods can trigger secondary succession.



1 Starts on a bare surface Starts on already colonized surface
2  It is slower or takes longer time to reach a climax community It is faster or takes a shorter time to reach a climax community
3 Starts with lower organisms Starts with fairly complex organisms



  1. Pioneer organisms are primarily plants, acting as producers.
  2. The population of organisms generally increases each year until a climax state is achieved.
  3. The diversity of species in the ecosystem grows progressively year by year.
  4. Succession follows a systematic and advancing pattern, commencing with microscopic green plants and culminating in mature trees.
  5. Each generation of species contributes to habitat transformation, enhancing soil quality through decay.
  6. Organisms engage in competition, vying for essential resources such as water, CO2, O2, light, and space. Superior competitors outcompete others.
  7. Changes in species composition occur as the fittest organisms thrive and less adapted species diminish.



  1. Structural modifications and community activities bring about alterations in the physical environment.
  2. The initial simple organisms are progressively replaced by more intricate ones in an evolutionary trajectory.
  3. Equilibrium is achieved as a variety of organisms colonize abandoned farmland.
  4. The ultimate result of succession is the establishment of a climax or stable community.


  1. Relationship Between Competition and Succession:

Competition plays a significant role in the process of succession. As species colonize an area, they often compete for resources like sunlight, water, and nutrients. Over time, certain species that are better adapted to the available resources become dominant, leading to changes in the composition of the ecosystem. Succession involves a gradual shift from early colonizers, which are usually r-strategists and opportunistic species, to later successional species that are often K-strategists and more specialized.


  1. Factors That Cause Overcrowding:

Overcrowding in a population can result from factors such as limited resources (food, water, shelter), high birth rates, low death rates, and reduced emigration. When a population exceeds the carrying capacity of its environment, it can lead to competition for resources and stress on the ecosystem.


  1. Ways of Avoiding Overcrowding:

Populations can avoid overcrowding by various means, including controlling birth rates through reproductive strategies, dispersing to new areas through emigration, and adapting behaviours to manage resource utilization. Predation, disease, and other ecological factors also play a role in naturally regulating populations.


  1. Effect of Food Shortage: Competition, Reproduction, Emigration Rat

Food shortage can have several effects on a population. Competition for limited food resources can increase as organisms strive to secure enough sustenance to survive and reproduce. This competition can influence reproductive rates, with some individuals potentially reproducing less if resources are scarce. Additionally, in response to food scarcity, the emigration rate might increase as individuals seek out areas with better resource availability. Overall, food availability can significantly impact population dynamics and ecosystem interactions.


  1. Balance in Nature


In the realm of nature, a state of dynamic equilibrium prevails. When environmental conditions influenced by population dynamics, encompassing both abiotic and biotic elements, are propitious, growth is encouraged. Conversely, when these conditions turn unfavorable, growth is hindered. A constraining element that curbs population expansion is termed a “limiting factor.” The amalgamation of all these constraining factors constitutes what is termed as “environmental resistance.”


The interplay between abiotic and biotic factors culminates in a state where the population size of organisms strives for a dynamic equilibrium, colloquially referred to as the “balance in nature.” As the population swells, so does the force of environmental resistance. A pertinent example is the depletion of food resources within a population, instigating a competitive struggle that ultimately leads to the demise of the weaker organisms. This cyclical process serves to maintain the population at a relatively steady level.



The term “population” alludes to the collective count of organisms belonging to the same species, coexisting within a designated area at a specific juncture. Within an ecosystem, the conglomeration comprises diverse species’ populations, forging a multifaceted biotic community. When these assorted populations within an established ecosystem display sustained equilibrium, the community attains a state of balance.

The multiplex factors that govern the populace of organisms in a particular habitat collectively constitute “environmental resistance.” These factors are broadly classified into two categories:


Abiotic Factors:

These encompass components such as heat, water, space, light, and nutrients.

  1. Heat: Organisms exhibit a propensity to evade excessively hot environments due to the potential to induce stress or fatality.
  2. Water: Survival rates within any population hinge on water availability. Adequate water supply bolsters population growth, while scarcity precipitates diminution.
  3. Space: Sufficient space is pivotal for normal growth and development. Spatial constraints lead to overpopulation and resultant inter-organism competition.
  4. Light: In the context of plant communities, light assumes paramount significance. Producers’ capacity to synthesize sustenance required by all habitat organisms hinges on light availability. Reduced light intensity culminates in feeble plant growth, impinging upon the overall nourishment supply.
  5. Nutrients: Nutrients in the soil are essential for plant sustenance synthesis. The scarcity of these nutrients imparts deficiency indications in plants, manifesting as stunted growth. Ultimately, this detrimentally influences crop yields.



These factors comprise the actions and interactions of organisms that reverberate upon one another. They encompass a spectrum of dynamics, including food availability, competition, natality, mortality, dispersal, parasites, predators, and pathogens.

  1. Food: This pivotal facet represents the essential substrate requisite for organisms’ sustenance, growth, development, and procreation.
  2. Competition: Competition ensues among organisms due to scarcities in space, sustenance, and other resources within the environment. Such pressures can escalate into confrontations and even cannibalistic behaviours among certain animals.
  3. Natality: The pace of birth, or natality, bears a direct impact on population numbers. Elevated birth rates trigger population growth, which in turn can lead to overcrowding.
  4. Mortality: The rate of mortality, or death, wields a substantial influence. Low death rates contribute to population escalation, whereas high death rates diminish it. However, exceedingly low death rates can trigger shortages in food resources and resultant overcrowding.
  5. Dispersal: Among plants, the dissemination of seeds and fruits through avenues like water, wind, animals, and insects curbs the risk of overcrowding. This mechanism serves to avert excessive population densities.
  6. Parasites: Organisms that subsist on or within another organism (the host) fall within this category. The actions of parasites can induce damage or demise in the host, thereby diminishing the population.
  7. Predators: These organisms prey upon weaker species (the prey). Intensified predation can curtail the population of the prey species.
  8. Pathogens: Pathogenic entities, encompassing bacteria, viruses, fungi, and protozoa, incite diseases. Their influence impinges upon the well-being of other organisms. When pathogenic assaults are rampant, the population of host organisms can be substantially reduced.



Within the natural world, a biological equilibrium manifests through the dynamic of predator and prey. In the realm of human beings, a parallel equilibrium is established through the practices of family planning and birth control. Family planning constitutes a mechanism through which couples ascertain the desired number of offspring and the timing of their arrival.


Birth control, on the other hand, encompasses techniques utilized to prevent a woman from conceiving for the duration she desires.

In the absence of effective family planning, a nation’s population could escalate without restraint, potentially outstripping the capacity of accessible nourishment and resources. This scenario could culminate in famine and loss of life.

Family planning is dedicated to averting pregnancies rather than terminating existing life. Meticulously devised family planning strategies serve to thwart unplanned pregnancies. The arsenal of birth control and family planning methods encompasses:

  1. Condom Usage
  2. Sterilization
  3. Spermicides
  4. Intra-uterine Device (IUD)
  5. Contraceptive Pills or Injections
  6. Rhythm Method
  7. Withdrawal Method


Theme 3.  Continuity of Life.

Reproductive System and Reproduction in Humans.

Female Reproductive System

  1. Ovaries: Two oval, cream-coloured structures located in the lower abdomen below the kidneys. These organs produce ova (eggs).
  2. Oviducts: Tubes that convey the ova from the ovaries to the uterus. Fertilization typically occurs in the upper portion of the oviducts.
  3. Uterus: The uterus is a hollow, muscular organ situated in the lower abdomen. It serves as the developmental site for the embryo. The inner lining, known as the endometrium, provides nourishment to the embryo. Implantation of the embryo into the endometrium supports its growth. The robust uterine muscles aid in the process of parturition (childbirth).
  4. Cervix: The cervix is a structure characterized by a ring of muscles that separates the uterus from the vagina. It constitutes the gateway to the uterus.
  5. Vagina: The vagina is a tube that opens externally and acts as both the copulatory passage and the birth canal through the vulva.

The male reproductive system

  1. Testis:

Each testis consists of numerous coiled tubes known as seminiferous tubules. Enclosed within a scrotal sac suspended between the thighs, this positioning ensures that sperm are maintained at a lower temperature than that of the body.

  1. Seminiferous Tubules:

The lining of the seminiferous tubules comprises actively dividing cells that give rise to sperm. Interspersed among the seminiferous tubules are interstitial cells responsible for producing male hormones such as androgens (e.g., testosterone). These tubules combine to form the epididymis, a coiled tube that temporarily stores sperm.

  1. Vas Deferens (Sperm Duct):

The vas deferens is the conduit through which sperm travel from the testes to the urethra.

  1. Seminal Vesicle:

The seminal vesicle produces an alkaline secretion that nourishes the spermatozoa.

  1. Prostate Gland:

The prostate gland produces an alkaline secretion aimed at neutralizing vaginal fluids.

  1. Bulbourethral Gland (Cowper’s Gland)

The bulbourethral gland secretes an alkaline fluid. Together with spermatozoa, these fluids constitute semen.

  1. Urethra:

The urethra is a lengthy tube responsible for conducting semen during copulation. It also facilitates the removal of urine from the bladder.

  1. Penis:

The penis is an intromittent organ used for insertion into the vagina during copulation.


Fertilization in animals is a pivotal reproductive process. It follows copulation, during which the erect penis is inserted into the vagina, leading to the release of semen. Within the female’s genital tract, sperms traverse to the upper region of the oviduct.

The sperm’s head penetrates the egg after the acrosome releases lytic enzymes, dissolving the egg membrane. The tail of the sperm is left behind. Subsequently, the sperm nucleus merges with the ovum’s nucleus, resulting in the formation of a zygote.

A fertilization membrane envelops the zygote, effectively preventing other sperms from entering it.


Following fertilization, the zygote begins mitotic division while moving towards the uterus. It becomes embedded within the uterine wall, a process termed implantation. At this stage, the zygote takes on the form of a hollow cell cluster referred to as a blastocyst or embryo.

In the uterus, the embryo develops villi that extend into the uterus, serving as a source of nourishment. Over time, these villi and the endometrium transform into the placenta.

Embryonic Membranes

Membranes that envelop the embryo start to develop. The outermost membrane, the chorion, forms finger-like projections known as chorionic villi, which nourish the embryo. The amnion encloses the embryo, creating a fluid-filled cavity where the embryo resides. This fluid acts as a protective cushion, guarding the fetus against physical harm and temperature fluctuations. The placenta is formed by the chorionic villi, allantois, and endometrium working in concert. The embryo connects to the placenta through the umbilical cord, housing both umbilical vein and artery.

Placental Functions:

  1. Protection:

Maternal and fetal blood circulation remains separate, preventing harmful pathogens and toxins from reaching the fetus. The placenta, however, allows maternal antibodies to pass into the fetus, enhancing its immunity.

  1. Nutrition:

The placenta facilitates the transfer of nutrients from the maternal bloodstream to the fetus.

  1. Excretion:

Nitrogenous waste removal from the fetal blood to the maternal blood is facilitated by the placenta.

  1. Gaseous Exchange:

Oxygen from the maternal blood moves into the fetal blood through diffusion, while fetal carbon dioxide diffuses into the maternal blood.

  1. Hormone Production:

The placenta produces hormones like progesterone and estrogen.

Fertilization initiates a series of intricate processes that culminate in the formation of the zygote, its implantation, and the development of crucial structures like the placenta. The placenta serves as a multifunctional interface between the maternal and fetal systems, providing protection, nutrition, waste removal, gaseous exchange, and hormone production for the developing fetus.

It seems like you’ve provided two points, but they might be related to a broader context. From the phrasing, it appears that you might be discussing physiological processes that occur during pregnancy. Let’s break down each point:

Selective exchange between mother and child

During pregnancy, the mother’s body and the developing fetus establish a connection through the placenta. The placenta is a temporary organ that develops in the uterus and serves as a means of exchanging nutrients, oxygen, waste products, and other substances between the mother’s blood and the fetus’s blood.

This process involves a selective exchange of molecules. Nutrients, such as glucose, amino acids, vitamins, and minerals, are transported from the mother’s blood to the fetus’s blood to support its growth and development. Oxygen is also transferred from the mother’s blood to the fetus’s blood, ensuring that the fetus receives the oxygen it needs for respiration.

Conversely, waste products like carbon dioxide and metabolic waste are transported from the fetus’s blood to the mother’s blood, where the mother’s body can eliminate them. This exchange is crucial for maintaining a healthy environment for both the mother and the developing fetus.

Removal of excretory products from the fetus:

As the fetus develops, it produces waste products from its metabolic processes, just like any living organism. These waste products include carbon dioxide, which is a byproduct of cellular respiration, and other metabolic waste substances.

The placenta, as mentioned earlier, facilitates the removal of these waste products from the fetus’s blood into the mother’s blood. Once in the mother’s blood, these waste products are transported to the mother’s organs, such as the lungs for carbon dioxide removal and the kidneys for processing other waste materials. The mother’s body then eliminates these waste products through processes like breathing and urination.

These two points are related to the physiological processes that allow the mother and developing fetus to share necessary substances while ensuring the removal of waste products. This exchange is facilitated by the placenta, a unique structure that develops during pregnancy to connect the maternal and fetal circulatory systems.


Development of New Seeds

Development of Zygotes in Plants

In flowering plants, the process of pollination is followed by fertilization, leading to the development of the zygote. This development eventually transforms the flower into a fruit that encloses the seeds. The male and female sex cells responsible for zygote formation are the pollen grain and the ovule, respectively. The following steps outline the processes involved:

  1. Pollination: After pollination, the pollen grain lands on the stigma and absorbs a sugary liquid, causing it to swell and germinate.
  2. Pollen Tube Growth: The outer coat of the pollen grain splits, and a pollen tube grows out and down inside the style.
  3. Division of Nucleus: The nucleus of the pollen grain divides into two parts—a large tube nucleus and a smaller generative nucleus. The male nucleus acts as the male gamete.
  4. First Fertilization: One of the male nuclei fuses with the ovule after its release into the embryo sac, resulting in the formation of a zygote. This zygote develops into the embryo, marking the first fertilization.
  5. Second Fertilization: The second male nucleus fuses with the secondary nucleus, forming the endosperm nucleus. This process leads to the production of endosperm, which serves as the food storage for the embryo.


Formation of Seeds and Fruit (Development of Embryo)

The zygote undergoes division through mitosis, forming multiple cells that differentiate and organize into an embryo. The embryo consists of the following parts:

  1. Plumule: The embryonic shoot.
  2. Radicle: The embryonic root.
  3. Cotyledon: One or two seed leaves.
  4. Endosperm: This may or may not be present and serves as a food storehouse for the embryo.

As the embryo develops, the ovary and ovules undergo changes and develop into fruits and seeds, respectively.



A seed is the ripened, fertilized, and developed ovule. It comprises the following parts:

  1. Seed Coat: The outer protective membrane of the seed.
  2. Hilum: The point of attachment of the seed to the seed stock.
  3. Micropyle: The tiny hole through which air and water enter the embryo of the seed.
  4. Embryo: The innermost part of the seed.

Seeds with a single seed leaf are known as monocotyledons, such as maize, millet, rice, sorghum, and wheat. Seeds with two seed leaves are called dicotyledons, such as mango and beans.


Germination of Seeds

Germination is the gradual development of the seed embryo into a seedling or young plant. Embryos of developed seeds often undergo a period of rest called dormancy until favorable conditions for germination arise. There are two types of germination:

  1. Epigeal Germination: The seedling emerges from the soil with the cotyledon(s) above the soil surface. This type is observed in dicotyledonous plants.
  2. Hypogeal Germination: The seedling emerges from the soil while the cotyledon(s) remain below the soil surface. This type is observed in monocotyledonous plants.

Conditions Necessary for Seed Germination

For seeds to germinate successfully, the following conditions are required:

  1. Adequate supply of water or moisture, which softens the seed coat and activates the soil for further development.
  2. Oxygen for respiration and energy generation is necessary for seed growth.
  3. Optimum temperature suitable for seed germination.
  4. Enzymes to accelerate the breakdown of food and release energy.
  5. Food or energy stored in the cotyledon(s) for dicot seeds or endosperm for monocot plants.
  6. Viable seeds capable of germination.





A fruit is a matured fertilized ovary of a flower containing one or more seeds. Contrary to this, some plants do not undergo fertilization for the formation of their fruit. Such fruits are called parthenocarpic fruits e.g. banana and pineapple. Such fruits are seedless.

A typical fruit has the following parts

  1. The fruit wall called pericarp which is made up of three layers from the inside to the outside (epicarp, mesocarp and endocarp).
  2. The seed or seeds
  3. The fruit stalk-the point of attraction between the fruit to the plant.



Fruits are classified based on their origin or structure. Common ways of classifying fruits are

  1. True and false fruits
  2. Simple, aggregate and composite fruits
  3. Fleshy and dry fruits
  4. Dehiscent and indehiscent fruits

True and false fruit:

A true fruit develops from a fertilized ovary and contains a pericarp and seed(s) e. g. mango, and cowpea while a false fruit develops from the ovary and other floral parts e. g. apple, and cashew.

Simple, aggregate and composite fruits

A simple fruit develops from a single flower with a single ovary e. g. cowpea, maize. An aggregate fruit develops from a single flower with several ovaries (each ovary develops into a fruitlet to form a cluster). The fruits have a common fruit stalk e. g. custard apple and strawberry. A composite fruit develops from an inflorescence e. g. fig, breadfruit.

Fleshy and dry fruits

Fleshy fruit is a fruit that has the whole pericarp or at least one of the pericarp thick, soft and succulent is a fleshy fruit. There are six types of fleshy fruits:

  1. Drupe: – A true, simple fruit with a thin epicarp, fleshy or fibrous mesocarp and a hard and woody endocarp which encloses the seed(s) e. g. mango, coconut, oilpalm fruits.
  2. Berry: – A true, simple fruit with a thin epicarp and succulent, edible mesocarp and endocarp e. g. tomatoes, guava etc.
  3. Hesperidium: – A special type of berry in which the epicarp and mesocarp are fused together and the endocarp form distinct chambers filled with succulent hairs e. g. oranges, lemon, grapes etc.
  4. Pome: – A simple, false fruit in which the fleshy edible part is derived from the receptacle and the core enclosing the seeds from the ovary e. g. apple and pear
  5. Sorosis: – A composite, false fruit formed from a dense inflorescence e. g. breadfruit, pineapple
  6. Synconium: – A composite false fruit that develop from a cup-like inflorescence enclosing numerous tiny male and female flowers e. g. fig

Dry fruit is a type of fruit in which the pericarp becomes dry, hard, woody or fibrous when the fruit ripens. Dry fruits can be grouped into dehiscent or indehiscent fruits

Dehiscent fruits split open to release the seeds when ripe. Four main types are

  1. Legumes: – The pericarp split open longitudinally along both side to release the seeds e. g. cowpea. Flamboyant etc.
  2. Follicle: – The pericarp split open longitudinally on one side only to release the seeds e. g. silk cotton, kola
  3. Capsule: – The pericarp slit along many sides to release the seeds e. g. okro, cotton etc
  4. Schizocarp: – Breaks up into units enclosing one seed each e. g. desmodium, cassia etc.

Indehiscent fruits fall to the ground when ripe, eventually decayed to release the seeds. Five main types are

  1. Achene e. g. clematis
  2. Cypsela e. g. tridax, sunflower, marigold
  3. Caryopsis e. g. maize, rice
  4. Nut e. g. cashew nut
  5. Samara e. g. combretum



This is the transfer of the seed or fruit from the parent plant to other places where such seed may germinate. The essence of dispersal includes the following:

  1. To avoid undue competition for nutrients, light space and water
  2. To prevent overcrowding of plants
  3. To prevent spread of disease
  4. To encourage colonization of new area for such plants


Agents of dispersal

These are the means by which seeds and fruits are removed from parents to other places. These agents include:

  1. wind
  2. water
  3. animals and man
  4. explosive mechanism


Features that aid methods of dispersal

  1. WIND – (i) Fruits or seeds are light. (ii) Fruits or seeds may have floss, tuff or pappus e. g. tridax, cotton, combretum etc.
  2. WATER – (i) Light fruits or seeds that can float in water (ii) Waterproof epicarp (iii) Fibrous mesocarp that can trap air to keep it afloat e. g. coconut
  3. ANIMALS AND MAN: – (i) The fruits or seeds may have hooks or hairs to attach to the animal skin (ii) The fruits are edible and the seeds indigestible e. g. pepper, desmodium
  4. EXPLOSIVE MECHANISM: – (i) Presence of one or more lines of fission or weakness e. g. cowpea, flamboyant, okro etc.


Reproductive Behaviours

  1. Courtship Behavior in Animals and Territoriality:

Courtship behavior in animals refers to a set of behaviors and displays that individuals of a species engage in to attract potential mates and establish a reproductive partnership. This behavior often involves various signals, sounds, postures, and movements that communicate the fitness and availability of an individual for mating. Courtship behaviors help animals recognize compatible partners and reduce the chances of mating with individuals from other species or those not ready for reproduction.


Territoriality, on the other hand, refers to the behavior exhibited by animals to establish and defend a particular area or territory. Territorial animals mark their territory with various cues, such as scent markings or vocalizations, to signal ownership and deter other members of the same species from entering. In the context of courtship, territoriality can play a role by allowing animals to secure a suitable area for mating and raising offspring. Displaying ownership of a territory can also be an attractive trait for potential mates, as it demonstrates the ability to provide resources and protection.


  1. Display of Colours and Body Parts:

Many animals use vibrant colors and distinctive body parts to communicate with each other and their environment. These displays serve various purposes, including courtship, warning predators, and establishing social hierarchy. The colors and body parts may be used to attract mates, signal readiness to mate, or indicate dominance within a group.

For example, in birds, males often have bright and colorful plumage that they display during courtship rituals to attract females. Peacocks are a classic example of this; their elaborate tail feathers are used in courtship displays. Similarly, some fish species change colors or patterns during breeding seasons to signal their readiness to mate. Insects, like butterflies, also use vibrant wing patterns to signal their presence to potential mates and to ward off predators.

  1. Seasonal Migration and Mutual Flight (e.g., Termites):

Seasonal migration is a phenomenon where animals move from one location to another in response to changing environmental conditions, usually related to factors like food availability, climate, or reproduction. This behaviour helps animals find better resources, and suitable breeding grounds, or escape unfavourable conditions.


Mutual flight, as seen in termites, refers to a synchronized mass movement of winged termites during their reproductive phase. Winged termites, known as alates, emerge in large numbers from their colonies and engage in flight to find new mates and establish new colonies. This phenomenon often occurs during specific times of the year, and the termites are drawn to light sources as they fly. Once they find a suitable partner, they shed their wings and start the process of creating a new termite colony.

These behaviours are all fascinating examples of how animals adapt and communicate in order to ensure successful reproduction, secure resources, and thrive in their environments.


Biology of Heredity (Genetics)


  1. Genetics, the scientific study of heredity and variation in all living organisms, was termed by Dilliam Bateson in 1906.
  2. Heredity/Inheritance pertains to the passage and manifestation of traits from parents to offspring, accounting for the common resemblances between them.
  3. Variation encompasses the dissimilarities existing among parents, offspring, and siblings.
  4. Genes are hereditary units located on chromosomes, responsible for transmitting traits from parents to offspring. Coined “genes” by Johannsen in 1909.
  5. Chromosomes are thread-like structures within cell nuclei, housing genes.
  6. Characters denote inheritable attributes possessed by organisms, such as height, complexion, and colour.
  7. Gamete is a mature sex cell participating in sexual reproduction. Examples include pollen grains, ovules, sperms, and ova. Gametes are typically haploid.
  8. Zygote, a single cell, forms upon the fusion of male and female gametes. It is diploid.
  9. Allelomorphs (alleles) are gene pairs on specific chromosome positions, controlling contrasting traits.

10.A Locus denotes the gene’s location on a chromosome.

  1. Genotype encompasses an individual’s inherited genetic makeup from both parents, comprising dominant and recessive traits.
  2. Phenotype encompasses all observable traits of an organism, encompassing physical, physiological, and behavioural characteristics.
  3. A Dominant trait is expressed in offspring resulting from parents with different traits. It is governed by a dominant gene (e.g., Tt).
  4. A Recessive trait remains hidden when a dominant trait is present. It is regulated by a recessive gene.
  5. Homozygous refers to an individual with identical genes for a specific trait (e.g., TT or tt).
  6. Heterozygous refers to an individual with differing alleles at a gene locus (e.g., Tt).
  7. Filial generations encompass the offspring of parent organisms, denoted as F1, F2, F3, and so on, with each generation leading to the next.
  8. A Hybrid results from the crossbreeding of genetically distinct parents of the same species.
  9. Hybridization involves mating plants with differing traits, encompassing monohybridization (crossing two pure traits) and dihybridization (crossing two pairs of contrasting traits).
  10. Haploid organisms possess one set of chromosomes in their gametes (23 in humans), represented as “n.”
  11. Diploid organisms possess two sets of chromosomes in somatic cells (46 in humans), represented as “2n.”
  12. Mutation refers to genetic changes in an organism, leading to new inheritable characteristics.



Hereditary variation denotes differences among individuals transmitted from parents to offspring, except in identical twins. It arises due to:

  1. Genetic reshuffling during meiosis, involving independent assortment and segregation, resulting in novel combinations.
  2. Crossing over during meiosis, where chromatids exchange genetic material, causing variation.

Transmittable characters in animals encompass physical traits (e.g., body stature, skin color), genetic conditions (e.g., sickle cell anemia), and sensory abilities (e.g., taste).

Transmittable characters in plants include traits related to growth, fruit, leaf, color, disease resistance, and more.

Transmission of characters occurs through genes. Diploid organisms produce haploid gametes through meiosis. Upon fertilization, gametes fuse into diploid zygotes, each containing two gene copies.

Hence, characters determined by genes are transmitted from parents to offspring through gamete


In the study of genetics, Gregor Mendel (1866) worked with the garden pea

(Pisumsativum) because of three unique properties present in it. These are

  1. Peas are self-pollinating
  2. They have a very short lifespan
  3. They have several unique genetic characteristics e.g. round or wrinkled seeds, tallness or shortness, seeds /pods/ flowers colouration, pod texture etc.

The Mendel’s methods of studying genetics are two:

  1. Monohybrid inheritance
  2. Dihybrid inheritance


Monohybrid inheritance

Mendel crossed two different plants which differ in one pair of contrasting characters e.g. tall and short plants. The procedures he followed are as follows:

  1. He planted tall plants for several generations and got all tall plant offspring. Likewise, the short plants he planted yielded all short plant offspring.
  2. He then planted tall and short plants. When the flowers were produced, he cross pollinated the pollen grains (male gamete) of the tall plant with the stigma (female gamete) of the short plant.
  3. He then planted the seeds of the cross in the procedure (ii) above and obtained all tall plants. This he called the first filial generation(F1,)
  4. He then crossed the F1 plants, collected their seeds and sowed them. He got tall and short plant in ratio 3: 1. This he called second filial generation (F2)

This experiment resulted into Mendel’s first law of inheritance which is based on the principle of complete dominance.

Mendel’s first law of inheritance

Mendel’s first law of inheritance otherwise known as the law of segregation of genes states that paired alleles segregate from each other when the homologous chromosomes on which they reside separate during meiosis. Each gamete receives one of the two alleles. The actual segregation occurs in F2 generation.


Gregor Mendel crossed plants which differ in two pairs of contrasting characteristics e.g seed shape (round or wrinkled seeds) and seed colour (yellow or green seeds). He crossed plants having round and yellow seeds with these having wrinkled and green seeds. The F1 seeds were having round and yellow seeds. Self-pollinating F1 plants produced the F2 plants of four (4) types

  1. Round and yellow seeds : 9
  2. Wrinkled and yellow seeds : 3
  3. Round and green seeds : 3
  4. Wrinkled and green seeds : 1

This experiment resulted into Mendel’s second law of inheritance

Mendel’s second law of inheritance

Mendel’s second law of inheritance otherwise known as law of independent assortment of genes state that alleles of genes on different chromosomes assort independently during meiosis


A dominant phenotype has the genotypic patterns RR, Rr. The genotype is determined using test cross or back cross

Test cross is the crossing of an organisms with an homologous recessive organism

Back cross is the crossing of an organism with an homologous recessive organism from the original parental generation


This deal with the ability of two contrasting alleles to interact and produce a heterozygous phenotype that is different from the two homologous phenotypes: Examples of organisms exhibiting incomplete dominance include: Mirabilisjalapa,4 o’clock plant, Audlausian fowl. This principle opposes Mendel’s principle of complete dominance.


In co-dominance, both alleles in the heterozygous individuals are fully expressed. The effect of one is not modified by the presence of the other. Therefore, three distinct phenotypes are produced e.g Inheritance of human “ABO” blood group

Allele 1A implies the addition of antigen A to the cell surfaces of red blood cells resulting in a person with group A blood. Likewise Allele 1B implies the addition of antigen B to the cell surfaces of red blood cells resulting in a person with Group B.

In a heterozygous individual, (1A 1B) both antigens A and B are added to the cell surfaces of red blood cells. So the individual has blood group AB.

NOTE:  1A and 1B are co-dominants while 1Ois recessive.

Multiple alleles

Genes that have more than two alleles in the population are said to have multiple alleles e.g. the human ‘ABO’ blood grouping


Sex determination in human beings

Each body cell of human beings has 23 pairs (46) of chromosomes, 22 of which are autosomes and a pair is sex chromosome. In male the two sex chromosomes in each body cell are X and Y chromosomes, therefore, each male gamete carries either X or Y chromosome. In the female, all egg cells of the body contain two X chromosomes. Therefore all egg cells contain one X chromosomes each. At fertilization, the combinations of an egg with a sperm carrying either X or Y chromosome occurs by chance. The formation of a male or female offspring has equal chances as shown below parents:



Chromatin granules (thread – like structures) found in the nucleus of eucaryotic cells are the precursors or raw materials of chromosomes.

Chromosomes occur in pairs known as homologous chromosomes. Each chromosome is made up of two threads called chromatids joined at a point called centromere. Each human somatic cell has 46 chromosomes. These are present in 23 pairs of homologous chromosomes. The number of chromosomes in each somatic cell of an organism is called diploid number (2n).


Each chromosome is made up of protein units in a strand of deoxyribonucleic acid, DNA (in double helix). Along its length are genes arranged which are actually DNA segments. The DNA is a very large molecule made up of repeating units called nucleotides. Each nucleotide is made up of deoxyribose (a sugar molecule), phosphate and an organic nitrogenous base which may be adenine, guanine, thymine or cytosine. Guanine always pairs with cytosine and adenine with thymine. The two helical chains are referred to as complementary strands of DNA since one is the exact opposite of the other.


Sex chromosomes and autosomes

There are forty-four autosomes which are similar in shape and size in both male and female. The last pair is called sex chromosome which are of genotype XX in female and XY in male. Exception to this is in birds, moths and butterfly where the female has genotype XY and the male XX. Also, in certain grasshoppers, the Y chromosome is absent so that the male has the genotype XO.

Just before cell division, the protein bundles come together and the DNA strands coil tightly around them, causing the chains to shorten and become visible under the light microscope. This process is called condensation


Each DNA molecule is made up of thousands of genes. The DNA molecules coil around the 23 pairs of chromosomes. In human body cells are about 50,000 genes. Each DNA molecule can make an exact copy of itself in a process called replication. This forms the basis for the transmission of hereditary materials from parents to the offspring.



Genes are the expression of hereditary characters in organisms and are located on the chromosomes of a body cells. Therefore chromosomes are responsible for the transmission of characters from parents to offspring. Chromosomes are arranged in pairs known as homologous chromosomes (exactly alike in shape and size and carry genes responsible for the transmission of the same characteristics). The genes relating to the same character e. g. tallness and shortness occupy identical loci on the homologous pair. The genes on homologous pair of chromosomes determine whether the individual will be homozygous or heterozygous for certain characters.



  1. The chromosomes pass the genes into the gamete during meiosis.
  2. Homologous chromosome separate into two daughter cells during the first stage of meiosis.
  3. The two chromatids of each chromosome separate during the second stage of meiosis. Each gamete therefore has one set of chromosomes hence one copy of genes.
  4. During fertilization, the gametes fuse together to form a zygote. The zygote receives two genes for the same character (one from one chromosome in the egg and the other from one chromosome in the sperm).
  5. When the two genes are the same, the offspring is a homozygous but when they are different, the offspring is a heterozygous (hybrid).



Sex–linked traits are characteristics whose genes are carried on the X chromosome of the sex chromosomes instead of autosomes. Such genes are inherited along with such X chromosomes. They are all controlled by a recessive gene. Examples of Sex-linked traits are: colour blindness, haemophilia, baldness, sickle cell anaemia and albinism.

  1. Colour blindness: A colour blind person cannot distinguish near colours. It is an abnormality of the gene that controls the production of cone cells (light receptors) in the retina of the eye.
  2. Haemophilia: This is a disorder in which bleeding takes an abnormally long time to stop or fails to stop because blood clotting will not occur. In haemophiliac (the victim) small injuries can result to bleeding to death e.g. Queen Victoria’s lineage (gene for haemophilia arose as a mutation in Queen Victoria or one of her parents) in British Royal Family.
  3. Baldness: The recessive gene controlling this trait causes the hair on the upper part of the head to pull out prematurely. It is more common in male human beings.
  4. Albinism: This is the condition in which the skin of an animal is non – pigmented because of lack of the pigment called melanin.
  5. Sickle cell anaemia: The recessive gene controlling this abnormality causes some of the red blood cells to be sickle shaped. The haemoglobin of the affected red blood cells is abnormally shaped thereby making it inefficient in transporting oxygen. In a condition of low oxygen concentration, the haemoglobin breaks down causing the cells to be sickle shaped. This then leads to the blockage of the cavities of the small blood vessels in the body thus hindering free flow of blood. The body part affected receives lower blood, oxygen and nutrients. Therefore, the victim goes into crisis at such periods characterized by pains in the bones and joints.



Probability is usually expressed in units ranging from 0 – 1. Mendel’s works were based on probability.


Probability = No of times an event occurs

Total no of trials

The two guiding principles of probability in genetics are:

  1. The result of one trial of a chanced event does not affect the result of latter trials of the same event.
  2. The chance that two independent events will occur together simultaneously is the product of their chances of occurring separately.



In agriculture

Genetics is relevant and has led to the following:

  1. Cross fertilization &self-fertilization procedures
  2. Development of early maturing varieties of organisms.
  3. Development of disease–resistant varieties of organisms.
  4. Production of crops and animals that can adapt to climatic conditions.
  5. Improvement of quality and quantity of product


In medicine

Genetics helps in the following:

  1. Determination of paternity of a child.
  2. Blood transfusion
  3. Diagnosis of diseases
  4. Sex determination
  5. Marriage counseling to avoid cases of genetic disorder.
  6. Knowing and choosing the sex of a baby.
  7. Development of test tubes babies.


NOTE:  All the applications listed above sum up the relevance of biology to life in what is now termed biotechnology. In biotechnology the DNA is now being manipulated to the benefits of humanity i.e. genetic engineering


 Variation and Evolution


Population is a group of organisms of the same specie living in a specified area within a given period of time. Variation refers to the differences which exist between individuals of the same species

Types of variations

  1. Morphological variation
  2. Physiological variation.


Morphological variation is the noticeable physical appearance of individuals of the same species. This physical appearances change gradually within a population. The feature observed shows a gradual transition between two extreme forms (continuous variation) e.g. size (height or weight), colour and finger prints.

Physiological variation is the difference in the ways individuals of the same species behave or react to conditions in their environment. It is not visibly apparent like morphological variation. It relates to the functioning of the body. In physiological variation, organisms can be grouped into two or more classes within a population without any graduation or intermediate between or among them (discontinuous variation). Examples of such variation are behaviour which can be temperamental, accommodating, excited or calm, blood groups, ability to roll the tongue, ability to taste phenylthiocarbamide etc.



There are two causes. They include:

  1. Genetic differences
  2. Environmental influence.

Genetic differences

A sudden change in a gene called mutation can be inherited when sex linked. This then brings about variation e.g. a gene responsible for green fruits in plants may be altered to produce a yellow fruit in the same plant

Environmental differences

Environmentinclude housing, food, healthcare, educational facilities, parental care etc. e.g. an intelligent person exposed to an unfavourable environment becomes dull.



  1. Crime detection: Use of finger prints which can be arch, loop, whorl or compound.
  2. Determination of paternity using blood group
  3. Development of hybrids of desired traits in agriculture
  4. Classification of human race based on skin colour, shape of nose, texture of the hair into Caucasoid (European), Negroid (Black African) Mongoloid (Chinese and Japanese), Australoid (Australian)
  5. Blood transfusion: The blood group of the donor must be compatible with that of the recipient. If not, the donor’s RBC will clump in the recipient’s blood vessels causing serious harm to the recipient. Each blood group is characterized by specific proteins in the blood which are antigens in the RBC and antibodies in the blood plasma. The table below shows antigen-antibody reactions between donor and recipient bloods.






(Antigen a)


(Antigen b)


(Antigen a + b)



 A (Antibody b) + +
B (Antibody a + +
AB (None) + + + +
O (Antibody a + b) +

+    means positive reaction (no clumping)

Note: O is a universal donor while AB is a universal recipient




Competition is the process by which living organisms in the habitat struggle with one another for limited essential needs in the environment. Such scarce resources in plants include; light, space, nutrient and water while animals complete for food, space or mate.

Competition finally results in survival of the fittest and elimination of the unfit.

Types of competition

Intraspecies competition: competition between organisms of the same species. e.g. many maize seedlings grown in a small area.

Interspecies competition: that between different species of organisms. e.g. many maize and pepper seedlings growing in a small area.


Relationship between competition and succession

Succession is the change in a population caused by the replacement of the old members with new ones as a result of competition. The newly formed habitat is gradually colonized by different species of plants in a stepwise manner until a relatively stable community is established and later the habitat will be inhabited by animals. As soon as a habitat is established, competition sets in. The early inhabitant modify the habitat by their activities while the later arrivals compete and outgrow the previous inhabitants which gradually loss out.



Adaptation is the possession of special features which improve the chances of an organism to survive in its environment. All organisms have adaptive structures which could be structural or morphological and behavioural in nature. These enable them to live successfully in their habitat.

There are three modes of adaptation:

Structural Adaptation

Adaptive colouration

Behavioural Adaptation



This is a special modification of structures which help organisms to survive better in their environment. Examples include;

Structural adaptation to obtain food e.g. a toad has a long tongue to catch its prey; birds have sharp, strong and curved claws for catching their prey; Insects have modified mouth parts for feeding; Insectivorous plants (e.gutriculariaspi.e. bladderwort, Droseriasp i.e. sundew, etc) have special structural adaptive features.

Structural adaptation for escape and defence. Escape adaptation can be grouped into camouflage (concealing ccolouration), individual and group responses e.g. caterpillars taking the colour of leaves. Defence adaptation may be inform of physical structure e.g. spines and shell, scales etc, chemical defence e. g. snakes attack their enemies by spitting venom, bees and scorpion have stings and mimicry (looking like an uninteresting objects) e. g. stone plant.

Structural adaptation to attract matese.g. Adult male agama lizard displays its bright colour to attract its mates, flowering plants attract insects for pollination, bright coloured feathers of male domestic fowls and peacock etc.

Structural adaptation to regulate body temperature e.g. mammals have fat layer, sweat gland, feathers and subcutaneous fat in birds in birds etc. All serve to regulate heat loss.

Structural adaptation for water conservation: e.g. some plants have small needle like leaves (conifers), thick bark (acacia), waxy cuticles etc to reduce the rate of transpiration. Likewise some animals possess scales, exoskeleton, feathers etc. to reduce water loss.



This is the possession by an organism of a colour which enables it to catch its prey, avoid its predators or enemies, secure mates and ensure their survival. Adaptive colouration may be grouped into

Concealing (cryptic) colouration to help organisms blend with their background and remain unnoticed by predators

Colour blendingwith the environment e. g. green cuticles of grasshopper, green snakes etc.

Counter shading by animals possessing a dark dorsal surface and light ventral surface as in tilapia fish to remain unnoticed by predator above and below.

Colour change(camouflague)to match the environment as in chameleon, grasshopper etc.

Disruptive colouration as patterns to break the body outline of animals against the dark and light shades of their backgroung as in giraffe, leopard, tiger, lady bird beetle etc.

Warning colouration to announce the presence of the organism(s) to potential predator to avoid them because they have some unpleasant features e. g. variegated grasshoppers, black and yellow bands of wasps.

Mating colouration as in male agama lizard, peacock.

Mimicry in harmless organisms resembling a distasteful or harmful one for the enemies to avoid such e.g. stick insects, swallow tail butterfly.

Bright colouration of insect pollinated flowers and pitchers of insectivorous plants.



Behaviour is basically adaptive, everything used by organisms to promote their survival. Examples include:

Behavioural adaptation in predators e.g. Lion with high speed chases its prey; spider spins its webs for its prey

Behavioural adaptation to protect prey from predators e.g bats hold tree branches with heads upside down (which is described as swaying in the air), Antelopes escape with speed, beetles secreate offensive odour, toad puffs itself up

Behaviuoral adaptation for avoiding harsh weather conditions e.g. aestivation i.e. passive period of existence. It is practiced by crocodiles; Hibernation i.e. sleep period to survive food scarcity or winter (low temperature) exhibited by insect-eating bats; migration of certain animals (e.g cattle egrets) to favourable habitats

Behavioural adaptation in plants: e.g some plants shed leaves in dry season (deciduous plants); some like yam tuber, potatoe die down and survive as underground stem; plant seeds can remain dormant, plant shoot moves towards light (Positive phototropism)

Gregarious behaviour (movement in groups) is expressed by elephants zebra, birds, fishes, social animals (bees, termites) etc.



Social animals are those in which individuals of the same species live together cooperatively in organized communities known as societies (colonies). Examples of social animals are: social insects (like termites, honey bees or wasps, ants etc), wolves, foxes, baboons etc.

Characteristics of Social Insects

They live together

They display division of labour

They show distinct castes

Members communicate with one another within the colony.



Habitats of termites: They are found living together in large communities in nest which may be tunnels in dead wood or ant hills (termitaria).

Note Termites are blind: they communicate through touch and smell.


Castes of termites:

They have three castes: The reproductive, soldiers, workers.

The reproductive are of three types: king, queen and winged reproductive. The king has no wing, is smaller than the queen and it fertilizes the queen. The queen has a small head, small thorase and large abdomen. It is the largest of all the castes. Only one queen at a time is found in a colony. The queen lays eggs. The winged reproductive are fertile and are potential kings and queens of new colonies.

The soldiers are sterile, wingless and blind. They have big heads with stony maxillae and mandibles. Soldiers are of two types: (i) The mandibulate soldiers with strong mandibles and (ii) Nasute soldiers with projective mouth paths. The soldiers defend the colony against enemies.

The workers are wingless, blind, and sterile. They form the majority in the colony and possess well developed mouth paths. They build and repair the termitaria; provide food for colonial members; look after the eggs laid by the queen and baby termites (nymphs).They feed the nymphs and the queen and cultivate fungus gardens.


Life history of termites

Termites exhibit incomplete metamorphosis i.e.

Egg                  nymph                         adults

The nymphs develop into soldiers and reproductive. And those which fail to develop become workers. When the winged reproductives are mature, they exhibit nuptial or wedding flight i.e. swarming out from the existing colony to build new ones.


Behavioural adaptation of termites for survival

They move in groups to ward off their enemies

They have a wide variety of diet; feeding on both living and dead plants.

They burrow into the soil or wood to build tunnels for protection against their enemies.

The habit of feeding on dead members helps to keep the colony clean.

Their ability of massive production of off springs promotes their survival.


Economic importance of termites

Termites while building their tunnels help in loosening and mixing the soil.

They decompose wooden materials as they feed.

They add humus to the soil through their decomposition activities.

They act as good source of protein and fat.

The ant hill clay can be used to build the surface of tennis court.



Habitat of Honey bees – These are social insects living in hives made up of chambers or cell.

Castes of honey bees

The bee colony has 3 castes, namely:

The drone (in hundreds per colony)

The queen (only in 1 per colony)

The workers (in thousand per colony)

The drone is the winged with shorter abdomen than the queen but bigger than the workers. The drone mates with the queen during the nuptial flight after which it dies.

The queen is a fertile female, winged and much bigger than the workers. It is fed with royal jelly and lays egg.

The worker is a sterile female, winged and is smaller than the queen or drone. It possesses eyes and a sting. Also with modified mouth paths for collecting nectar and building the hive. The workers legs are also modified for collecting pollen grains from flowers. The workers perform a special dance called rail wagging as it locates a food source. The workers secrete wax for building the hive, ventilate the hive and clean the cells, guard the hive, make honey from nectar and pollens and feed the larvae with royal jelly or honey.


Economic importance of honey bees

They help to pollinate flowers.

They produce honey which has high nutritive and medicinal value.






Organic evolution is the sum total of adaptive changes from pre–existing or old forms that has taken place over a long time resulting in diversity of forms, structures and functions among organism. The basis of evolution is that all organisms have pre–existing ancestors.


Evidences of evolution

Fossil record: A fossil is an impression of a plant or an animal that lived a very long time ago. The age of fossil is determined using radioisotope dating. Fossils are normally preserved in sedimentary rocks. Depending on the source, fossil records can be referred to as geological or paleontological or archaeological or historical record.

Geographical distribution: Based on the effect of climate on all living things, variations in their forms, structures and functions can occur. After several years of isolation, organisms of one climate tend to differ slightly from organisms of another climate.

Comparative anatomy: Evolution is obvious in anatomical comparison of vertebrates. The Pisces or fishes have simple heart with one auricle and one ventricle. The amphibians have two auricles and one ventricle. Reptiles have two auricles and a partially divided ventricle. Aves and mammals have two auricles and two ventricles.

Embryological evidence: The embryo of man in the womb at different stages of development resembles the embryo of fish, amphibians and reptiles.

Evidence of vestigial organ: Vestigial organs are minute and incomplete organs that have no special function. Evolutionarily, these organs are believed to be once functioning e.g. appendix and rudimentary tail in man.

Evidence from domesticated animals: These include cats, dogs, hensetc which live with humans for many years.



There are three prominent theories

Jean Lamarck’s theories

Jean Lamarck’s theories of use and disuse which states that Changes in the environment lead to changes in the species of organism.

The changes cause the organisms to form new structures or habits to adapt to environmental changes.

The organisms then develop specialized characters by use and disuse of organs.

Frequently used organs become well developed and the unused ones degenerate or become useless.

The well-developed or dominantly acquired characters are inheritable.

Lamarck’s theory is unacceptable to modern scientists who have proved that only characters represented on genes are inheritable not physical ones got through use and disuse.


Charles Darwin’s theory   

Charles Darwin’s theory of natural selection (survival of the fittest) which states that Species of organisms can produce large number of offspring to the environment with limited resources.  This then results in competition among the offspring.

The survivors must have inherited the useful traits which are passed on to the offspring at reproduction. Those that could not survive the competition die off. As the population gradually becomes better adapted to the environment, new species emerge. This theory is widely acceptable to many scientists to date.

NOTE: Both Lamarck and Darwin recognized the importance of the environment.


Modern theories of evolution:

Based on the combination of natural selection (Darwin’s) and the genetic origin of variation. This theory states that:

There exist variations in the species population. Some of the variations have special survival advantages Individuals with favourable variations are more adaptive to their environment than others.

The individuals have to struggle for existence in the environment. The fittest contribute more offspring to the next generation than the unfit ones.The main causes of variations are mutation and recombination of genes.


Roles of mutation in evolution

Mutation is a sudden change in DNA structure leading to a change in the phenotype of the species concerned. When mutation occurs in the gene of gametes, it leads to production of new species.

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