The 2023-2024 Ordinary Level national exam in Biology and Health Sciences I was sat on 30 July 2024, and it covered a wide spread of the S3 syllabus: blood and circulation, genetics, reproduction, mutations, ecology, and a full experimental biology question on osmosis. If you sat this paper or you're using it to prepare for your own exams, this guide walks through every question with a full explanation, not just the final answer.
For related preparation, you can also check our S3 Chemistry 2025 worked solutions and our S3 Mathematics I 2025 worked solutions, since these subjects are usually revised together for the O Level national exams.
DOWNLOAD THE QUESTION PAPER HERE
Section A: Attempt ALL Questions (55 marks)
Question 1: Blood Cells and Haemoglobin
Answer: c) Red blood cells.
Red blood cells (erythrocytes) contain haemoglobin, the iron-rich protein that binds oxygen for transport around the body. Lymphocytes and phagocytes are white blood cells involved in immunity, and platelets are responsible for clotting, not oxygen transport.
Question 2: The Cornified Layer of the Skin
Answer: The cornified layer's main role is protection, specifically preventing uncontrolled water loss by evaporation and forming a barrier against pathogens and physical damage.
This corresponds to option d) in the original list. Temperature regulation through sweating is carried out by sweat glands, not the cornified (outermost dead) layer, and sensation is detected by nerve endings deeper in the dermis, not the cornified layer.
Question 3: Acid Produced During Oxygen Deficiency in Muscles
Answer: a) Lactic acid.
When muscles don't receive enough oxygen during intense exercise, they switch to anaerobic respiration, which produces lactic acid as a byproduct instead of the carbon dioxide and water produced during aerobic respiration.
Question 4: Biologically Incorrect Statement
Answer: d) Amphibians have scales.
This statement is biologically incorrect. Amphibians, such as frogs, have moist, permeable skin without scales, which they use for gas exchange. Scales are a feature of reptiles and fish, not amphibians.
Question 5: Function of Joints
Answer: b) Allows bone growth.
Bone growth happens at the growth plates within the bone itself, not at the joints. Joints exist to allow articulation between bones, enable mobility, and support the body, which are options a, c, and d.
Question 6: Contraceptive Methods (Table Completion)
| Method | Type |
|---|---|
| Spermicide | Non-hormonal (barrier/chemical) |
| Condom | Barrier |
| Implant | Hormonal |
| IUD | Non-hormonal (though hormonal IUD versions also exist) |
| Pills | Hormonal |
Question 7: True or False on Mutations and Genetics
a) False. Mutations can absolutely be caused by copying errors during cell division; this is one of the most common natural causes of mutation.
b) True. Environmental factors such as radiation and certain chemicals can cause mutations.
c) True. Some mutations are severe enough to be lethal, particularly if they disrupt essential genes.
d) True. Albinism results from a mutation affecting melanin production.
e) False. Genotypic variation refers to differences in genetic makeup, which is not directly visible; only its resulting phenotypic expression can be observed.
f) True. Phenotypic variations, the observable physical traits, are visible.
Question 8: Fill in the Blanks (Meiosis and Fertilization)
"The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes compared to the parent cell. During fertilization, the fusion of gametes from two individuals restores the diploid condition. Gametes are haploid (containing only one set of chromosomes) while the zygote is diploid (containing two sets of chromosomes). Thus, sexually reproducing organisms alternate between haploid and diploid stages. In sexual reproduction, two individuals produce diploid offsprings that have characteristics from both parents. The gametes are produced from parent cells via the process called meiosis."
Question 9: Matching Blood Components to Functions
White blood cells → Defend the body against pathogens
Red blood cells → Transport of respiratory gases
Platelets → Clotting of blood
Question 10: Yes or No Statements on Respiration
a) No. ATP is not simply "released" in the mitochondrion; it is synthesized (produced) there during aerobic respiration, primarily through the process of oxidative phosphorylation.
b) Yes. Aerobic respiration releases significantly more energy (ATP) per glucose molecule than anaerobic respiration, since it fully breaks down glucose using oxygen.
Question 11: Gamete Production Sites
a) Sperm cell: produced in the testes.
b) Ovum: produced in the ovaries.
Question 12: Nutritional and Health Needs Across Ages
a) Who needs more milk, the boy or the grandfather?
The 3-year-old boy needs more milk.
b) Reasons:
Young children are in a critical stage of rapid growth and bone development, and milk provides the calcium and protein essential for building strong bones and tissues at this stage. The grandfather, being fully grown, no longer needs the same volume of milk for growth, though he still benefits from calcium for maintaining bone density.
c) Why does a 28-year-old pregnant mother need more iron than a man of the same age?
A pregnant woman needs more iron because her blood volume increases significantly to support the developing foetus, and iron is essential for producing the additional haemoglobin required to carry oxygen to both her own tissues and the foetus. She also needs to build iron reserves for the baby's early development after birth.
Question 13: Why Athletes Are Given Glucose, Not Sucrose
Glucose is a simple sugar that can be absorbed directly into the bloodstream without needing to be digested first, providing a fast, immediate source of energy during intense physical activity. Sucrose is a disaccharide that must first be broken down into glucose and fructose by digestive enzymes before it can be absorbed, which takes more time, making it less suitable for the rapid energy needs of athletes during exercise.
Question 14: Basic Measure to Protect Against Disease
Maintaining good personal hygiene, such as regular handwashing with soap, is one of the most basic and effective measures to protect against contracting many infectious diseases, since it removes pathogens before they can enter the body.
Question 15: Why Cereal Crops Should Grow Close Together
Cereals such as wheat, barley, and rice are wind-pollinated crops. Growing them close together increases the chances of pollen from one plant successfully reaching the flowers of nearby plants, improving pollination rates and therefore increasing grain yield.
Question 16: Dealing with Sexual Assault
A survivor of sexual assault should seek immediate safety and medical attention, since timely medical care can address injuries and provide preventive treatment for infections or pregnancy. It's also important to report the incident to trusted authorities, such as the police or a local health center, and to seek psychological support and counseling, since emotional recovery is just as important as physical care. Survivors should be treated with confidentiality, dignity, and without judgment throughout this process.
Question 17: Why Industrial Plants Must Treat Waste Water
Untreated industrial waste water often contains harmful chemicals, heavy metals, and pathogens that can severely pollute rivers and lakes if dumped directly. This contamination can kill aquatic organisms, disrupt entire ecosystems, and make water unsafe for human use, including drinking and irrigation. Treating waste water before disposal removes or neutralizes these harmful substances, protecting both the environment and public health.
Question 18: Fill in the Blanks (Immunity)
a) Antigens bind at the antigen-binding site forming antigen-antibody complexes.
b) Natural (innate) immunity is a naturally acquired immunity.
Section B: Attempt Any THREE Questions (30 marks)
Question 19: Cholera and Daily Health Practices
a) How can we help someone with cholera?
The most urgent step is to prevent and treat dehydration, since cholera causes severe diarrhoea and fluid loss. This is done by giving oral rehydration solution (ORS) to replace lost fluids and salts, and by seeking urgent medical attention, since severe cases can be life-threatening within hours if untreated.
b) Four daily practices to ensure good health:
- Washing hands regularly with soap, especially before eating and after using the toilet
- Drinking clean, safe water, boiled or treated if the source is uncertain
- Eating a balanced diet with adequate fruits, vegetables, and protein
- Getting regular physical exercise and adequate sleep
Question 20: Monoculture and Biological Diversity
Monoculture, growing a single crop species across a large area, significantly reduces biological diversity in an ecosystem. It eliminates the variety of habitats and food sources that different species depend on, since only one type of plant is available. This makes the ecosystem far more vulnerable to pests and diseases, which can spread rapidly and devastate the entire crop when there's no genetic diversity to provide natural resistance. Monoculture also depletes specific soil nutrients repeatedly rather than allowing natural nutrient cycling, and it reduces the population of pollinators and beneficial insects that thrive in more diverse plant environments. Over time, this can lead to soil degradation and a decline in the overall resilience of the ecosystem.
Question 21: Blood
a) What is blood plasma?
Blood plasma is the pale yellow, liquid component of blood in which blood cells (red blood cells, white blood cells, and platelets) are suspended.
b) Role of blood plasma:
Plasma transports blood cells, nutrients, hormones, waste products, and dissolved gases around the body, and it also plays a role in regulating blood pressure and body temperature.
c) The four ABO blood groups:
A, B, AB, and O
d) The factor that determines positive (+) or negative (−) blood type:
The Rhesus (Rh) factor
e) Why it's important to know a person's blood group:
Knowing a person's blood group is essential for safe blood transfusions, since transfusing incompatible blood types can trigger a dangerous immune reaction. It's also important during pregnancy, particularly regarding Rh factor compatibility between mother and baby.
f) Function of platelets in blood:
Platelets are responsible for blood clotting, helping to stop bleeding when a blood vessel is damaged.
Question 22: Blood Group Genetics (Punnett Square)
A father with blood group A marries a mother with blood group B, and one of their children has blood group O.
a) Genotypes of the parents:
Since blood group O is only possible with the genotype ii (the recessive allele from both parents), each parent must carry a hidden "i" allele. The father's genotype must be Iᴬi and the mother's genotype must be Iᴮi.
b) Punnett square and possible phenotypes:
| Iᴬ | i | |
|---|---|---|
| Iᴮ | IᴬIᴮ (Blood group AB) | Iᴮi (Blood group B) |
| i | Iᴬi (Blood group A) | ii (Blood group O) |
The possible phenotypes and their ratio among the children are: 1 AB : 1 A : 1 B : 1 O. This means each child has a 25% (1 in 4) chance of having any one of the four blood groups.
Question 23: Mitosis
a) What is mitosis?
Mitosis is a type of cell division in which a single parent cell divides to produce two genetically identical daughter cells, each with the same number of chromosomes as the parent cell.
c) "Mitosis is essential in all living organisms." Reasons to support this statement:
Mitosis is fundamental to life for several reasons. It enables growth, since multicellular organisms increase in size by producing new cells through mitosis rather than simply enlarging existing ones. It's essential for tissue repair and wound healing, replacing damaged or dead cells with genetically identical new ones. It allows asexual reproduction in many single-celled and some multicellular organisms, letting them reproduce without a mate. It maintains the correct chromosome number across generations of cells, ensuring genetic stability within an organism's body. Finally, it supports cell replacement in tissues that continuously wear out, such as skin cells and blood cells, which must be constantly renewed throughout an organism's life.
Section C: Compulsory Question (15 marks)
Question 24: Demonstrating Osmosis
a) Apparatus and chemicals for demonstrating osmosis:
- Visking (dialysis) tubing or a potato/plant tissue
- Concentrated sugar or salt solution
- Distilled water
- Beaker
- String or clips (to seal the tubing)
- Ruler or measuring scale (to measure changes)
b) Suggested control:
A control setup could use distilled water both inside and outside the tubing (or on both sides of the plant tissue), so there's no concentration gradient. This shows that any change observed in the experimental setup is due to osmosis and not some other factor.
c) Result(s) of the demonstration:
Water moves into the tubing or plant tissue containing the concentrated sugar or salt solution, causing it to swell and increase in mass or size. In the control setup, no significant change occurs.
d) Explanation of the result(s):
Osmosis is the movement of water molecules from an area of higher water concentration (the dilute solution outside) to an area of lower water concentration (the concentrated solution inside), through a partially permeable membrane. Water moves in this direction to try to equalize the concentration on both sides of the membrane.
e) How storing food in salty water prevents bacterial or fungal growth:
A high salt concentration outside microbial cells creates a hypertonic environment. By osmosis, water is drawn out of the bacterial or fungal cells and into the surrounding salty solution, causing the cells to dehydrate and shrink. This process, called plasmolysis, prevents the microorganisms from surviving and multiplying, which is why salting has been used to preserve food for centuries.
f) Another way of preventing food from spoilage:
Refrigeration or freezing, which slows down microbial growth and enzyme activity by lowering the temperature, is another common and effective method of food preservation.
How to Use This Guide Effectively
Don't just read through these answers passively. Cover the solutions, attempt each question on paper first, and only then compare your working against what's shown here. Pay particular attention to Section A, since with 55 of the 100 marks concentrated there, small recurring mistakes, like mixing up genotypic and phenotypic variation, or forgetting the direction water moves during osmosis, cost the most marks overall. Genetics questions like Question 22 are also worth extra practice, since Punnett square problems appear almost every year in some form.
If you're preparing for the next national exam and want structured, one-on-one support in Biology, Chemistry, Physics, or Mathematics, Mathrone Academy can match you with a qualified tutor who can work through past papers like this one with you in real time. You can also explore more worked past paper solutions and other national exam preparation guides on our news page.
Get tutoring support:
WhatsApp: +250 786 684 285
Email: [email protected]
⚠️ Important Disclaimer
The solutions on this page are prepared by the Mathrone Academy team for revision and learning purposes only. This is not an official NESA marking scheme or REB-approved answer guide. While every effort has been made to ensure accuracy, answers and explanations may differ from the official examiners' marking guide. Always refer to your school teacher or the official NESA publications for authoritative marking guidance. Mathrone Academy accepts no responsibility for any discrepancies between these solutions and official results.