Question 1. Explain the following:
- (a) Blood of higher animals (15 Marks)
- (b) Macromolecules in biological systems (5 Marks)
Question 2. With the aid of well labeled diagram, explain the process of gaseous exchange in insects. (Marks)
Question 3.
- (a) Explain the process of diffusion across cell membrane (15 Marks)
- (b) Outline five (5) properties of enzymes (5 Marks)
Question 4. Draw and label the structure of a synapse and explain the process of synaptic transmission (20 Marks)
Question 5.
- (a) Explain in detail the Layers of Gastrointestinal Tract (GIT) (15 Marks)
- (b) Mention five (5) adaptations in toad (5 Marks)
Question (b) Assuming uniform colour (C) is dominant over spotted (c) in cattle. When a cross is made between a pure breed (homozygous) black bull (BBCC) with a white spotted cow (bbcc):
- (i) Use Punnet diagram to show the result of the F₁ generation
- (ii) What will be the phenotype of the calves in the F₁ generation?
- (iii) Show the types and ratio of offspring that are possible when two of the F₁ generations are mated.
- (iv) Name all the new combinations that may arise from the cross above (1b iii). (20 Marks)
Question (c) Fill in the blank spaces. Spellings of one word answer and scientific names must be correct to score. (10 Marks)
- (i) Reduction of loss of water in reptiles is reduced by the presence of a
- (ii) b neuron conducts nerve impulses towards the central nervous system
- (iii) Endocrine hormones reach their target organ through the c
- (iv) The d filters fluid from the blood in the kidney
- (v) Mosquito larva breathe air using the structure called e
- (vi) The selectively permeable part that provides shape and protection of the inner organelle of the cell is called f
- (vii) The g are photoreceptors in the eye that are sensitive to high light intensity
- (viii) Glandular cells are responsible for producing h
- (ix) The grinding/digestion of food occurs in i of insects
- (x) The DNA consists of two strands, arranged in a j
SOLUTIONS
QUESTION 1
(a) Blood of Higher Animals (15 Marks)
Blood is a specialized connective tissue that circulates through the body via the cardiovascular system. It consists of a liquid matrix called plasma and cellular components.
Composition of Blood:
1. Plasma (55% of blood volume)
- Straw-coloured liquid composed of about 90% water
- Contains dissolved substances: proteins (albumin, globulins, fibrinogen), glucose, amino acids, hormones, enzymes, urea, salts, and gases
- Functions: transport of nutrients, waste products, hormones; regulation of osmotic pressure and pH
2. Red Blood Cells / Erythrocytes
- Biconcave, disc-shaped, lack nucleus at maturity (in mammals)
- Contain haemoglobin — a conjugated protein that binds oxygen to form oxyhaemoglobin
- Produced in red bone marrow; lifespan ~120 days
- Function: transport of O₂ and CO₂
3. White Blood Cells / Leucocytes
- Nucleated, larger than RBCs, fewer in number
- Types: Neutrophils (phagocytosis), Lymphocytes (antibody production), Monocytes (phagocytosis), Eosinophils (allergic responses), Basophils (release histamine)
- Function: Immune defense
4. Platelets / Thrombocytes
- Tiny cell fragments, no nucleus
- Function: Blood clotting (haemostasis)
- Release thromboplastin which initiates the clotting cascade → fibrinogen → fibrin mesh
Functions of Blood:
- Transport of oxygen, carbon dioxide, nutrients, hormones, and waste
- Regulation of body temperature
- Defense against pathogens (immunity)
- Clotting to prevent excessive blood loss
- Maintenance of pH and osmotic balance
(b) Macromolecules in Biological Systems (5 Marks)
Macromolecules are large, complex molecules formed by polymerization of smaller subunits (monomers).
| Macromolecule | Monomer | Example | Function |
|---|---|---|---|
| Carbohydrates | Monosaccharides | Starch, Glycogen, Cellulose | Energy storage, structural support |
| Proteins | Amino acids | Enzymes, Haemoglobin | Catalysis, transport, structure |
| Lipids | Fatty acids + Glycerol | Fats, Phospholipids | Energy store, membrane structure |
| Nucleic Acids | Nucleotides | DNA, RNA | Genetic information, protein synthesis |
QUESTION 2 — Gaseous Exchange in Insects
Insects exchange gases through a system of trachea and tracheoles, not through lungs or blood.
Process:
- Air enters through spiracles — small pores along the thorax and abdomen, controlled by valves to reduce water loss.
- Spiracles lead into a network of tracheal tubes — rigid, air-filled tubes lined with chitin rings (taenidia) that prevent collapse.
- Trachea branch repeatedly into finer tubes called tracheoles (diameter < 1 µm), which penetrate directly into tissues and cells.
- At the tips of tracheoles, gases diffuse directly between the air and respiring cells — oxygen diffuses into cells, CO₂ diffuses out.
- Ventilation is aided by rhythmic body movements that compress and expand the tracheal system, creating a pumping action.
Diagram Description (well-labeled):
Spiracle → Trachea → Tracheoles → Body cells
↕ (valves) (ringed/chitinous) (thin-walled, direct contact)
Key labels: spiracle, trachea, taenidia (chitinous rings), tracheole, body/muscle cell, fluid-filled tip of tracheole, oxygen direction (in), CO₂ direction (out).
QUESTION 3
(a) Diffusion Across Cell Membrane (15 Marks)
Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, until equilibrium is reached.
Types of diffusion across the cell membrane:
1. Simple Diffusion
- Small, non-polar molecules (O₂, CO₂, ethanol) pass directly through the phospholipid bilayer
- No energy required (passive)
- Rate depends on: concentration gradient, temperature, size of molecule, surface area
2. Facilitated Diffusion
- Polar or charged molecules (glucose, amino acids, ions) cannot cross the lipid bilayer directly
- Requires carrier proteins or channel proteins (protein pores)
- Still passive — moves down concentration gradient, no ATP needed
- Carrier proteins undergo conformational change to transport molecules
3. Osmosis (special case)
- Movement of water molecules from high water potential to low water potential through a selectively permeable membrane
Factors affecting rate of diffusion (Fick's Law):
Rate of diffusion ∝ (Surface area × Concentration difference) / Thickness of membrane
- Larger surface area → faster diffusion
- Steeper gradient → faster diffusion
- Thinner membrane → faster diffusion
- Higher temperature → faster diffusion (more kinetic energy)
Structure of membrane relevant to diffusion:
- Phospholipid bilayer — hydrophobic core restricts polar molecules
- Integral proteins — form channels and carriers
- Cholesterol — regulates membrane fluidity
(b) Five Properties of Enzymes (5 Marks)
- Biological catalysts — Speed up metabolic reactions without being consumed or permanently changed
- Specificity — Each enzyme acts on a specific substrate (lock and key / induced fit model); determined by the shape of the active site
- Sensitive to temperature — Activity increases with temperature up to an optimum; beyond this, the enzyme denatures (active site shape changes irreversibly)
- Sensitive to pH — Each enzyme has an optimum pH; extreme pH causes denaturation
- Reusable — Enzymes are not used up in reactions and can catalyze multiple reactions repeatedly
QUESTION 4 — Structure of Synapse and Synaptic Transmission
Structure of a Synapse
Key labeled parts:
- Pre-synaptic neuron (knob/terminal bouton)
- Synaptic vesicles (containing neurotransmitter, e.g., acetylcholine)
- Pre-synaptic membrane
- Synaptic cleft (~20–40 nm gap)
- Post-synaptic membrane
- Receptor proteins (on post-synaptic membrane)
- Mitochondria (in pre-synaptic knob — provide ATP)
Process of Synaptic Transmission:
- A nerve impulse (action potential) travels along the pre-synaptic neuron to the terminal bouton.
- Depolarization causes voltage-gated calcium ion (Ca²⁺) channels to open; Ca²⁺ floods into the pre-synaptic knob.
- Ca²⁺ causes synaptic vesicles to fuse with the pre-synaptic membrane and release neurotransmitters (e.g., acetylcholine) into the synaptic cleft — this is exocytosis.
- Neurotransmitter molecules diffuse across the synaptic cleft.
- They bind to specific receptor proteins on the post-synaptic membrane, causing ion channels to open.
- Influx of Na⁺ ions causes depolarization of the post-synaptic membrane → generating a new action potential.
- The neurotransmitter is then broken down by enzymes (e.g., acetylcholinesterase breaks down acetylcholine into choline + acetate) or re-absorbed (reuptake) into the pre-synaptic neuron, terminating the signal.
- The products are recycled to re-synthesize neurotransmitter.
Synapses are unidirectional — transmission only goes from pre- to post-synaptic neuron because vesicles and neurotransmitters are only on the pre-synaptic side and receptors only on the post-synaptic side.
QUESTION 5
(a) Layers of the Gastrointestinal Tract (GIT) (15 Marks)
The wall of the GIT consists of four main concentric layers (from innermost to outermost):
1. Mucosa (Innermost layer)
- Lines the lumen of the GIT
- Composed of:
- Epithelium — secretes mucus and digestive enzymes; absorbs nutrients
- Lamina propria — loose connective tissue with blood vessels and lymphatics
- Muscularis mucosae — thin smooth muscle layer that creates folds
- Highly folded (villi and microvilli) to increase surface area for absorption
2. Submucosa
- Thick layer of dense connective tissue
- Contains blood vessels, lymph vessels, and nerves
- Contains Meissner's (submucosal) plexus — nerve network that controls secretions
3. Muscularis Externa (Muscular layer)
- Contains two layers of smooth muscle:
- Inner circular muscle — constricts the lumen
- Outer longitudinal muscle — shortens the gut
- Together they produce peristalsis — wave-like contractions that propel food
- Contains Auerbach's (myenteric) plexus between the two muscle layers — controls motility
4. Serosa / Adventitia (Outermost layer)
- Serosa: single layer of squamous epithelium (mesothelium) + connective tissue; covers organs within the peritoneal cavity; secretes serous fluid to reduce friction
- Adventitia: connective tissue without mesothelium; found where the GIT is attached to surrounding structures (e.g., oesophagus)
(b) Five Adaptations of a Toad (5 Marks)
- Moist, permeable skin — allows cutaneous respiration (gas exchange through skin), supplementing lung breathing; skin must remain moist to allow diffusion of gases
- Camouflage (cryptic coloration) — skin coloration blends with the environment (brown/grey/green), helping avoid predators
- Mucus glands in skin — keep skin moist for gas exchange and reduce desiccation
- Parotoid glands — secrete toxic/poisonous secretions on the skin surface as defense against predators
- Long, sticky, protrusible tongue — attached at the front of the mouth, can be rapidly flicked out to capture insects and other prey efficiently
- (Bonus) Tympanic membrane (eardrum) — visible on the surface behind the eye; detects sound vibrations, important for communication during mating
GENETICS QUESTION (b) — Cattle Coat Colour
Given:
- Uniform colour (C) = dominant
- Spotted (c) = recessive
- Black (B) = dominant
- White (b) = recessive
- Pure black bull = BBCC
- White spotted cow = bbcc
(i) F₁ Punnet Diagram
Cross: BBCC × bbcc
All gametes from bull: BC
All gametes from cow: bc
| bc | |
|---|---|
| BC | BbCc |
All F₁ offspring = BbCc
(ii) Phenotype of F₁ Calves
All calves are BbCc — they carry dominant alleles for both traits.
Phenotype: Black and uniform coloured (Black, non-spotted)
(iii) F₁ × F₁ Cross — BbCc × BbCc
Gametes from each BbCc parent: BC, Bc, bC, bc
16-square Punnett grid:
| BC | Bc | bC | bc | |
|---|---|---|---|---|
| BC | BBCC | BBCc | BbCC | BbCc |
| Bc | BBCc | BBcc | BbCc | Bbcc |
| bC | BbCC | BbCc | bbCC | bbCc |
| bc | BbCc | Bbcc | bbCc | bbcc |
Phenotypic classes:
| Phenotype | Genotype combinations | Ratio |
|---|---|---|
| Black uniform (B_C_) | BBCC, BBCc, BbCC, BbCc | 9 |
| Black spotted (B_cc) | BBcc, Bbcc | 3 |
| White uniform (bbC_) | bbCC, bbCc | 3 |
| White spotted (bbcc) | bbcc | 1 |
Ratio = 9 : 3 : 3 : 1
(iv) New Combinations Arising from the F₁ × F₁ Cross
The parental types were:
- BBCC = Black uniform
- bbcc = White spotted
The new (recombinant) combinations are:
- Black spotted (B_cc) — e.g., BBcc, Bbcc
- White uniform (bbC_) — e.g., bbCC, bbCc
These combinations did not exist in the parent generation.
QUESTION (c) — Fill in the Blanks
| Blank | Answer |
|---|---|
| (i) a | Scales (scales/scutes reduce water loss through the skin of reptiles) |
| (ii) b | Afferent/Sensory (sensory neuron conducts impulses toward the CNS) |
| (iii) c | Bloodstream (hormones travel through the blood to target organs) |
| (iv) d | Glomerulus (the glomerulus filters blood in the kidney) |
| (v) e | Siphon/Air tube (mosquito larvae use a siphon/air tube to breathe at water surface) |
| (vi) f | Tonoplast (the selectively permeable membrane surrounding the vacuole) |
| (vii) g | Cones (cone cells are photoreceptors sensitive to high light intensity and colour) |
| (viii) h | Secretions/Hormones (glandular cells produce and release secretions) |
| (ix) i | Gizzard (the gizzard grinds and digests food in insects) |
| (x) j | Double helix (DNA consists of two strands in a double helix arrangement) |