KCSE Biology 2024, Paper 1 — Worked Solutions

A walkthrough of every short-answer question on the 2024 KCSE Biology Paper 1, written for Form 4 candidates revising at home. Each answer follows the official KICD mark scheme structure — bold the key term, give the definition, then the example where needed.

How to use this: read the question first, attempt the answer in your notebook, then read the model solution. Active recall beats passive reading.

Section A — Short Answer Questions

1. Define the term homeostasis (1 mark)

Homeostasis is the maintenance of a constant internal environment in an organism despite changes in the external environment.

Mark-scheme keywords: "constant internal environment" (1 mark — no partial). "Maintenance" alone does not score.

Common mistake: writing "balance in the body" — too vague. Examiners want the words internal environment + constant / steady.

2. State the word equation for photosynthesis (2 marks)

carbon dioxide + water  →  glucose + oxygen
                  (light energy, chlorophyll)

Mark-scheme split: 1 mark for correct reactants and products in the right order; 1 mark for naming light energy + chlorophyll as conditions above the arrow.

Common mistake: writing the balanced symbol equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. That earns 2 marks here, but Paper 1 specifies word equation — losing 1 mark for not following the instruction.

3. State three differences between mitosis and meiosis (3 marks)

Mark-scheme: any 3 valid contrasts × 1 mark each. Both sides of the contrast must be stated (e.g. "mitosis = 1 division, meiosis = 2 divisions" — saying only "meiosis has 2 divisions" loses the mark).

Common mistake: confusing the chromosome number — meiosis HALVES the number; mitosis MAINTAINS it.

4. Name the enzyme that breaks down starch in the mouth and state the product formed (2 marks)

• Enzyme: salivary amylase (also accept ptyalin) — 1 mark
• Product: maltose — 1 mark

Common mistake: writing "glucose" as the product. Salivary amylase stops at maltose; intestinal maltase breaks maltose into glucose.

5. Explain why insects are described as being well adapted for gaseous exchange (3 marks)

Insects have a tracheal system of branching tubes (tracheae and tracheoles) that:
1. Open to the outside through spiracles, allowing direct air entry without a circulatory system
2. Carry oxygen directly to every tissue, reducing the diffusion distance
3. Tracheoles are fluid-filled at their tips to dissolve gases for rapid diffusion across the moist surface

Mark-scheme: any 3 of the points above × 1 mark each. Must reference a structural feature AND its functional advantage.

6. State two functions of bile in digestion (2 marks)

1. Emulsifies fats — breaks large fat droplets into smaller ones, increasing surface area for the action of lipase
2. Neutralises stomach acid — its alkaline salts raise the pH of chyme entering the duodenum, creating the optimum pH for pancreatic enzymes

Common mistake: writing "bile digests fats". Bile does not contain digestive enzymes — it physically breaks fats into droplets so that lipase can chemically digest them.

7. Distinguish between arteries and veins (2 marks)

Mark-scheme: any 2 contrasted pairs × 1 mark each.

8. State the role of the xylem and phloem in a plant (2 marks)

• Xylem: transports water and dissolved mineral salts from the roots up to the leaves
• Phloem: transports manufactured food (sucrose) from the leaves to other parts of the plant

Mark-scheme: 1 mark per correct function. Must name the substance transported AND the direction.

9. Explain why a person who has lost a lot of blood is given a blood transfusion (2 marks)

To restore the volume of blood circulating in the body, which:
1. Maintains adequate blood pressure for circulation and tissue perfusion
2. Restores the oxygen-carrying capacity of the blood (via red blood cells and haemoglobin) so cells can continue to respire

Common mistake: saying only "to replace lost blood" — too vague for 2 marks. You need to mention what the blood does (pressure, oxygen transport).

10. State three characteristics of gaseous-exchange surfaces in mammals (3 marks)

1. Thin (one cell thick) — short diffusion distance
2. Moist — gases dissolve before diffusing across
3. Large surface area — many alveoli, ~70 m² total in adult lungs
4. Well-supplied with blood capillaries — maintains a steep concentration gradient
5. Permeable to respiratory gases (O₂ and CO₂)

Mark-scheme: any 3 × 1 mark each. The structural feature must be linked to its function for the mark.

Why these answers score full marks

Three patterns to internalise:

1. Use the exact mark-scheme keyword. Examiners scan for specific terms — "constant internal environment", "diploid", "amylase". Vague paraphrases lose marks even when correct.
2. State both sides of a contrast. Don't just describe meiosis — describe what mitosis does too.
3. Link structure to function. "Tracheoles are fluid-filled" alone is half a mark; "tracheoles are fluid-filled SO gases dissolve and diffuse rapidly" is a full mark.

Practice next

• KCSE Biology 2023 Paper 1 — same structure, different topics (the Krebs cycle gets a 4-mark question)
• KCSE Biology 2024 Paper 2 — longer-answer questions, requires linked paragraphs not bullet points

If you upload your own attempted answers to StudyAI, the tutor compares your wording to the mark scheme and tells you exactly which marks you'd have lost and why.