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Updated 1 November 2026. This page gives you KCSE Biology Paper 2 questions and answers in the 231/2 format: structured questions, a data-and-graph question and an essay, each with the points that earn marks. Paper 2 rewards precise vocabulary and full explanations more than any other science paper, so the worked answers below show exactly how much detail a mark needs. Use them as a timed drill before the paper on Thursday 12 November.
These are original CBC Guides questions written in the KCSE style, not a KNEC paper. The answers are worked answers checked by a teacher, not an official marking scheme.
How KCSE Biology Paper 2 (231/2) Is Set Out
Biology Paper 2 is out of 80 marks. Section A has five compulsory structured questions worth 40 marks. Section B has three questions: question 6 is compulsory and is usually a data question in which you plot or interpret a graph (20 marks), and you then choose either question 7 or question 8, both essays worth 20 marks. The KNEC 2026 KCSE Examination Timetable, Instructions and Guidelines places Biology 231/2 on Thursday 12 November 2026 from 8.00 to 10.00 am, so you have two hours. Budget roughly 50 minutes for Section A, 35 minutes for the graph question and 30 minutes for the essay, leaving a few minutes to check.
Paper 2 draws heavily on Form 3 and Form 4 topics: ecology, genetics, evolution, reproduction, growth and development, response, coordination, and support and movement. Form 1 and 2 topics such as transport, gaseous exchange, respiration and excretion also appear, especially in the essays.
Biology is one of the most widely taken sciences, so a good grade here strengthens the best-five subjects that count towards your mean. For scale, the 2025 KCSE had 993,226 candidates, as reported by the Kenya News Agency in CS Ogamba releases KCSE 2025 results, so small differences in marks change your position a lot.
Section A: Structured KCSE Biology Paper 2 Questions and Answers
1. Genetics (8 marks)
In garden peas, the allele for tall stems (T) is dominant over the allele for dwarf stems (t). A heterozygous tall plant is crossed with a dwarf plant. (a) Show the cross using a genetic diagram. (b) State the phenotypic ratio of the offspring. (c) What is a test cross and why is it useful?
Worked answer: (a) Parental genotypes Tt × tt; gametes T and t from the tall parent, t and t from the dwarf; offspring Tt, Tt, tt, tt (award marks for parents, gametes, fusion lines and offspring). (b) 1 tall : 1 dwarf. (c) A test cross is crossing an organism showing the dominant trait with a homozygous recessive one; it reveals whether the dominant-looking parent is homozygous (all offspring tall) or heterozygous (half dwarf). Saying “1:1” without naming the phenotypes loses the mark.
2. Ecology: estimating population (8 marks)
(a) Students threw ten 1 m2 quadrats at random in a 200 m2 plot and counted 45 blackjack plants in total. Estimate the population. (b) In a capture-recapture study, 60 grasshoppers were caught, marked and released. Later 50 were caught, of which 15 were marked. Estimate the population and state two assumptions.
Worked answer: (a) Mean density = 45 ÷ 10 = 4.5 plants per m2; population = 4.5 × 200 = 900 plants. (b) N = (60 × 50) ÷ 15 = 200 grasshoppers. Assumptions: marks do not fall off or harm the insects; marked insects mix randomly with the rest; no births, deaths or migration between samples.
3. Respiration (8 marks)
(a) Calculate the respiratory quotient (RQ) of a fat whose oxidation is: 2C51H98O6 + 145O2 → 102CO2 + 98H2O. (b) What does an RQ of about 1.0 tell you about the substrate? (c) Name the product of anaerobic respiration in muscle and explain oxygen debt.
Worked answer: (a) RQ = CO2 produced ÷ O2 used = 102 ÷ 145 = 0.70. (b) The substrate is carbohydrate, such as glucose. (c) Lactic acid. Oxygen debt is the extra oxygen needed after vigorous exercise to oxidise the lactic acid that built up when oxygen supply was too low.
Section B: The Data Question and the Essay
6. Data question (20 marks, shortened here)
The dry mass of germinating maize seedlings was recorded: day 0, 2.0 g; day 2, 1.8 g; day 4, 1.6 g; day 6, 1.7 g; day 8, 2.2 g; day 10, 2.9 g. (a) Plot a graph of dry mass against time. (b) Account for the change from day 0 to day 4. (c) Account for the change from day 6 to day 10.
Worked answer: (a) Marks go to a title, labelled axes with units, a suitable scale, correctly plotted points and a smooth curve or straight joins. (b) Dry mass falls because stored food in the endosperm is broken down and used in respiration to provide energy for growth, while the seedling has no green leaves to photosynthesise. (c) Dry mass rises because the first leaves have opened and photosynthesis now exceeds respiration, so the seedling makes more food than it uses. Write “photosynthesis exceeds respiration”, not just “it photosynthesises”.
7. Essay: urine formation (20 marks)
Describe how urine is formed in the human kidney.
Worked answer (marking points, maximum 20): blood enters the glomerulus through the afferent arteriole, which is wider than the efferent arteriole, creating high pressure; ultrafiltration forces water, glucose, amino acids, salts and urea into Bowman’s capsule; blood cells and plasma proteins stay behind because they are too large; the filtrate passes into the proximal convoluted tubule; all glucose and amino acids are reabsorbed by active transport; the tubule cells have microvilli and many mitochondria for this; water is reabsorbed by osmosis; in the loop of Henle sodium ions are pumped out to make the medulla concentrated, drawing water from the collecting duct; in the distal convoluted tubule salts are reabsorbed and pH is regulated; antidiuretic hormone (ADH) makes the distal tubule and collecting duct more permeable to water when the body needs to save water; the remaining fluid, urine, contains water, urea and excess salts and passes through the collecting duct to the pelvis and ureter to the bladder. Essays lose marks for listing without explaining; each step needs the “how”.
Common Mistakes in Biology Paper 2
- Writing genetic crosses without labelling gametes and the parents’ phenotypes.
- Drawing graphs with uneven scales or without units on the axes.
- Using everyday words (“food goes into blood”) instead of terms such as absorption, diffusion and active transport.
- Choosing the essay you like rather than the one you can write 20 points for. Plan each in one minute before deciding.
How This Page Beats a PDF Download
Most results for this search are old papers and schemes in PDF with no reasoning. Here each answer shows why a point scores. Revise topics in the KCSE Biology Paper 2 revision guide, then test speed in the KCSE Biology Paper 2 challenge. Quick drills are in the KCSE Biology quiz, and more practice is on the KCSE Biology practice papers page. Every subject is listed on the KCSE quiz index and the paper-by-paper challenges.
Frequently Asked Questions
How many marks is KCSE Biology Paper 2?
80 marks: 40 in Section A (five structured questions) and 40 in Section B (a compulsory data question plus one essay chosen from two).
How long is Biology Paper 2 (231/2)?
Two hours. In 2026 it runs on Thursday 12 November from 8.00 to 10.00 am.
Which topics are most common in Biology Paper 2?
Genetics, ecology, evolution, reproduction, growth, response and coordination, with transport, respiration and excretion common in essays.
Is this the KCSE Biology Paper 2 marking scheme?
No. These are original questions with worked answers. KNEC does not release marking schemes before the exam, so any “scheme” for the coming paper sold online is fake.
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