KCSE Biology Paper 1 (231/1) Questions and Answers

Updated 1 November 2026. Looking for KCSE Biology Paper 1 questions and answers? Below are ten original practice questions in the 231/1 style, each with a worked answer that shows where the marks sit, plus a topic map and a timing plan. The 2026 paper is on Tuesday 10 November, 2.00 to 4.00 pm, according to the KNEC 2026 KCSE Examination Timetable. These are not KNEC papers and we do not sell “marking schemes”; they are teacher-checked revision questions.

How KCSE Biology Paper 1 (231/1) Is Set

Biology Paper 1 is a two-hour theory paper of short-answer questions, all compulsory, marked out of 80. Expect around 25 to 30 questions worth one to six marks each, spread across Form 1 to Form 4 work. There is no choice, so a weak topic cannot be avoided; it can only be revised. Paper 2 (231/2) comes on Thursday 12 November and the practical, 231/3, on Friday 13 November.

The paper rewards precise terms. In the 2025 KCSE, Biology was one of the subjects where male candidates posted a higher mean score than female candidates, as reported by Citizen Digital in its 2025 KCSE results performance report, which shows there is ground for every candidate to make up.

KNEC allows a non-programmable calculator in Biology, as listed in the instructions section of the timetable, but you will rarely need more than a ratio or magnification calculation. Pencil is for diagrams only; answers go in blue or black ink.

Topics That Carry the Most Marks

KCSE Biology Paper 1 Questions and Answers (Original Practice)

Time yourself: these ten questions total 28 marks, so aim to finish in about 40 minutes.

1. State two differences between a plant cell and an animal cell. (2 marks)

Worked answer: A plant cell has a cellulose cell wall, an animal cell has none; a plant cell has a large central sap vacuole, an animal cell has small temporary vacuoles. (Chloroplasts present versus absent is also accepted.) One mark per correct contrast; the difference must be stated for both cells.

2. A cell measures 2 cm in a drawing magnified ×400. Calculate its actual size in micrometres. (2 marks)

Worked answer: Actual size = drawing size ÷ magnification = 20 mm ÷ 400 = 0.05 mm = 50 µm. One mark for the correct formula with substitution, one for 50 µm with the unit.

3. Explain why red blood cells burst when placed in distilled water. (3 marks)

Worked answer: Distilled water is hypotonic to the cell sap of the red blood cell; water enters the cell by osmosis; the cell has no cell wall, so it swells and bursts (haemolysis).

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

Worked answer: Salivary amylase (ptyalin); maltose.

5. State three adaptations of the alveoli for gaseous exchange. (3 marks)

Worked answer: Numerous, giving a large surface area; walls one cell thick, giving a short diffusion distance; moist lining that dissolves gases; dense capillary network that maintains a concentration gradient. Any three.

6. Write the word equation for anaerobic respiration in yeast. (2 marks)

Worked answer: Glucose → ethanol + carbon dioxide + energy. Missing “energy” commonly costs a mark.

7. Give the role of the loop of Henle. (2 marks)

Worked answer: It reabsorbs water and sodium ions from the filtrate, creating a high salt concentration in the medulla so that concentrated urine can be formed.

8. In a capture-recapture study, 60 grasshoppers were marked and released. Later 50 were caught, of which 10 were marked. Estimate the population. (3 marks)

Worked answer: Population = (first capture × second capture) ÷ marked recaptured = (60 × 50) ÷ 10 = 300 grasshoppers. Formula, substitution, answer: one mark each.

9. State two differences between mitosis and meiosis. (2 marks)

Worked answer: Mitosis gives two daughter cells, meiosis gives four; mitosis keeps the diploid number, meiosis halves it to haploid; crossing over occurs in meiosis only. Any two contrasts.

10. A heterozygous tall plant (Tt) is crossed with a short plant (tt). Work out the ratio of offspring. (4 marks)

Worked answer: Parental genotypes Tt × tt; gametes T, t and t, t; offspring Tt, Tt, tt, tt; phenotypic ratio 1 tall : 1 short. Examiners award marks for gametes shown and a correct ratio, so set the cross out fully, ideally with a Punnett square.

Where Candidates Lose Marks in Biology Paper 1

• Writing “water moves by osmosis” without saying from where to where, or across what.
• Misspelling technical terms so they read as a different word, such as “ureter” for “urethra”.
• Listing three points when two are asked; a wrong extra point can cancel a correct one.
• Forgetting units in magnification and population questions.
• Giving the function of a structure when the question asks for its adaptation.

A Two-Hour Plan for 80 Marks

Read the whole paper in the first five minutes and tick the questions you can answer at once. Work at roughly one mark per 1.3 minutes, which leaves about 15 minutes at the end to revisit gaps and check that every answer is in the space provided. Never leave a blank: a correct key term often earns a mark even when the explanation is incomplete.

Keep Revising Biology Paper 1

Go deeper on each topic in our KCSE Biology Paper 1 revision guide, then test speed in the timed Biology Paper 1 challenge or the KCSE Biology quiz. For predicted focus areas see KCSE 2026 Biology Paper 1 most tested topics, and for the other papers visit KCSE Biology practice papers, the full KCSE quiz list and the KCSE hub.

Frequently Asked Questions

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