S4 Biology Practice Paper: Enzymes And Cells, Cell Division, Respiration, Photosynthesis, Transport And Ecology (100 Marks)

S4 Biology Practice Paper: Enzymes and Cells, Cell Division, Respiration, Photosynthesis, Transport and Ecology (100 marks)

2026-11-13

Biology

S4 Biology paper, 24 parts on enzymes, cell division, respiration, photosynthesis, transport and ecology, with computed answers and marking points.

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Paper details　S4 Biology practice paper with 24 parts on enzyme activity, cell division, respiration, photosynthesis, plant and human transport and energy flow, with computed answers.

Section A: Enzymes and Cells (16 marks)

Show the formula and the unit in calculations.

#	Question	Answer and marking notes
1	Enzyme rate at five temperatures, 40 C equals 100 percent: 10 C 15, 30 C 85, 40 C 100, 50 C 70, 60 C 20. (a) State the optimum. (b) How many times faster is 30 C than 60 C? (c) Explain the fall after 60 C. (d) Why is 0 C inactive but not destructive?	Answer: (a) 40 C. (b) 85 divided by 20 equals 4.25 times. (c) Heat denatures the protein, so the active site changes shape and substrate cannot bind, and this is irreversible. (d) Cold only slows molecular motion, so activity returns on warming. (Marking: 1 mark each)
2	Pepsin works best near pH 2 in the stomach and trypsin near pH 8 in the small intestine. (a) How does pH affect enzyme activity? (b) Why do the two enzymes act in different organs? (c) Why is an enzyme specific? (d) Explain the rise in activity from 20 C to 40 C.	Answer: (a) Extreme acidity or alkalinity changes the charges and shape of the active site, denaturing the enzyme. (b) Each enzyme has its own optimum pH matching its organ. (c) The active site shape is complementary to one substrate only. (d) Warmer molecules move faster, so collisions between enzyme and substrate increase.
3	140 g of sucrose solution contains 7 g of sucrose. (a) Find the percentage concentration by mass. (b) Find the new concentration after adding 260 g of water. (c) A cell with 5 percent cell sap is placed in that new solution; state the net movement of water. (d) Why does a plant cell not burst in pure water?	Answer: (a) 7 divided by 140 times 100 equals 5 percent. (b) 7 divided by 400 times 100 equals 1.75 percent. (c) Water moves into the cell, because 1.75 percent is lower than 5 percent. (d) The rigid cell wall resists turgor pressure.
4	Compare animal and plant cells. (a) Name three structures found in plant cells but not in animal cells. (b) State one function of the large vacuole. (c) Name one structure missing in a mature human red blood cell and its consequence. (d) State one function of the cell membrane.	Answer: (a) Cell wall, chloroplast and large vacuole. (b) It stores water and solutes and maintains turgor to support the tissue. (c) No nucleus, so it cannot divide and has a limited life. (d) It controls what enters and leaves the cell.

Section B: Cell Division and Genetic Material (16 marks)

Distinguish stage names from changes in number.

#	Question	Answer and marking notes
5	A body cell has 46 chromosomes. (a) How many cells result after three mitotic divisions? (b) How many chromosomes does each daughter cell contain? (c) Starting with 5 such cells, how many cells exist after three divisions? (d) In which stage of the cell cycle does DNA replication occur?	Answer: (a) 2 to the power 3 equals 8 cells. (b) 46, as mitosis keeps the chromosome number constant. (c) 5 times 8 equals 40 cells. (d) Interphase, in the S phase.
6	Describe the four stages of mitosis. (a) What happens to chromosomes in prophase? (b) Where do they lie in metaphase? (c) Where do sister chromatids go in anaphase? (d) What does cytokinesis produce in telophase?	Answer: (a) Chromatin condenses into visible chromosomes, the nuclear envelope breaks down and spindle fibres form. (b) They line up on the equator of the cell. (c) Spindle fibres pull sister chromatids to opposite poles. (d) The cytoplasm divides into two daughter cells.
7	Gamete formation in humans involves meiosis. (a) A cell with 46 chromosomes divides by meiosis; how many chromosomes per daughter cell? (b) How many gametes does one such cell finally produce? (c) State one way meiosis creates genetic variation. (d) Contrast the number of daughter cells with mitosis.	Answer: (a) 23. (b) 4. (c) Independent assortment and crossing over of homologous chromosomes give new gene combinations. (d) Meiosis gives 4 daughter cells from one cell, mitosis gives 2.
8	DNA is the genetic material. (a) How is a gene related to DNA? (b) State the basic unit of DNA. (c) How can a gene mutation affect a protein? (d) Give one reason why a mutation may not change the phenotype.	Answer: (a) A gene is a section of DNA carrying genetic information. (b) A nucleotide, made of a base, deoxyribose and phosphate. (c) A changed base sequence changes codons and the amino acid chain, altering protein shape or function. (d) The code is degenerate, or the change lies outside coding regions.

Section C: Respiration and Metabolism (18 marks)

State the unit in every energy calculation.

#	Question	Answer and marking notes
9	1 g of glucose releases about 16 kJ when fully oxidised. (a) Find the energy released from 10 g of glucose. (b) Write the word equation for aerobic respiration. (c) Name the main organelle for aerobic respiration. (d) Why does muscle make lactic acid during hard exercise?	Answer: (a) 16 times 10 equals 160 kJ. (b) Glucose plus oxygen gives carbon dioxide plus water plus energy. (c) The mitochondrion. (d) Oxygen supply is short, so glucose is only partly broken down anaerobically, giving lactic acid.
10	Compare aerobic and anaerobic respiration. (a) How do the energy yields differ for the same mass of glucose? (b) What does yeast produce without oxygen? (c) What does human muscle produce without oxygen? (d) State one immediate benefit of anaerobic respiration to an athlete.	Answer: (a) For the same mass of glucose, aerobic respiration releases far more energy. (b) Ethanol and carbon dioxide. (c) Lactic acid. (d) Energy is still supplied quickly so the muscle keeps working for a short time.
11	A student uses 0.4 dm3 of oxygen per minute, and each dm3 releases about 20 kJ. (a) Find the oxygen used in 15 minutes. (b) Find the total energy released. (c) Find the average energy per minute. (d) In what form is much energy lost? (e) Name one cooling response.	Answer: (a) 0.4 times 15 equals 6 dm3. (b) 6 times 20 equals 120 kJ. (c) 120 divided by 15 equals 8 kJ per minute. (d) Mostly as heat. (e) More sweating and widening of skin blood vessels.
12	Case C: student A sits for 30 minutes at 20 C while student B runs for 30 minutes at 20 C. (a) Who has the higher heart rate? (b) Explain. (c) Is respiration here catabolic or anabolic? (d) What is the role of enzymes in metabolism? (e) Suggest one way to measure breathing rate.	Answer: (a) Student B. (b) Working muscle needs more oxygen and makes more carbon dioxide, so the heart pumps faster. (c) Catabolic. (d) Enzymes lower the activation energy and speed up reactions without being used up. (e) Count breaths per minute, or measure the rate of carbon dioxide release.

Section D: Photosynthesis (18 marks)

Explain trends and limiting factors.

#	Question	Answer and marking notes
13	Photosynthesis happens in chloroplasts. (a) Write the word equation. (b) Name the two raw materials. (c) Name one useful product and its use. (d) State the role of chlorophyll.	Answer: (a) Carbon dioxide plus water gives glucose plus oxygen, using light energy and chlorophyll. (b) Carbon dioxide and water. (c) Glucose, used in respiration for energy or stored as starch. (d) It absorbs light energy and converts it to chemical energy.
14	An aquatic plant makes 12, 24, 28 and 29 oxygen bubbles per minute at light intensities 20, 40, 60 and 80 units. (a) Find the change from 20 to 40 units. (b) Find the percentage rise from 40 to 60 units. (c) Why does the curve flatten after 60 units? (d) Suggest one change that raises the curve again.	Answer: (a) From 12 to 24, a rise of 12 bubbles, that is double. (b) 28 minus 24, divided by 24, times 100 equals 16.7 percent. (c) Light is no longer limiting; carbon dioxide or temperature limits the rate. (d) Raise the carbon dioxide concentration or move towards the optimum temperature.
15	A leaf of 100 cm2 makes 0.6 cm3 of oxygen each hour. (a) Find the oxygen made in 5 hours. (b) Find the hourly output of a 250 cm2 leaf. (c) Find the output of that leaf in 4 hours. (d) Why is there no photosynthesis at night? (e) Name one gas exchange that still occurs at night.	Answer: (a) 0.6 times 5 equals 3 cm3. (b) 250 divided by 100 times 0.6 equals 1.5 cm3. (c) 1.5 times 4 equals 6 cm3. (d) Without light the light reactions stop, so no glucose or oxygen is made. (e) Respiration continues: oxygen in, carbon dioxide out.
16	Compare photosynthesis and respiration. (a) How do their energy changes differ? (b) How do their timings differ? (c) Name one gas involved in both. (d) How is net oxygen release decided by day? (e) Describe one experiment showing that light is needed.	Answer: (a) Photosynthesis stores light energy as chemical energy; respiration releases chemical energy for the cell. (b) Photosynthesis needs light; respiration runs day and night. (c) Carbon dioxide, or oxygen. (d) Oxygen is released only when production exceeds use in respiration. (e) Cover half a leaf, expose it to light, then test for starch with iodine.

Section E: Transport in Plants and Humans (16 marks)

Cover both sides in comparison questions.

#	Question	Answer and marking notes
17	Plants transport materials in vascular bundles. (a) Name two substances carried in xylem. (b) Name one substance carried in phloem. (c) Where does the pull for transpiration come from? (d) Give two features that suit root hairs for absorbing water.	Answer: (a) Water and mineral salts. (b) Sugars such as sucrose. (c) Water lost through stomata lowers water potential in the leaf, pulling water upward. (d) They are numerous with a large surface area, thin walls and strong osmotic uptake.
18	One plant transpires 1.2 L of water per day on average. (a) Find the water lost by 10 plants in 7 days. (b) Find the water lost by one plant in 30 days. (c) State one benefit of transpiration. (d) Why does the rate fall when humidity is high?	Answer: (a) 1.2 times 10 times 7 equals 84 L. (b) 1.2 times 30 equals 36 L. (c) It draws water and minerals up to the leaves and helps cool the plant. (d) A small water potential difference between leaf and air slows evaporation.
19	At rest a person has a heart rate of 72 beats per minute and a stroke volume of 70 mL. (a) Find the output per minute in mL. (b) Convert it to litres. (c) Find the daily output in litres over 1440 minutes. (d) Give two reasons why output rises during exercise.	Answer: (a) 72 times 70 equals 5040 mL. (b) 5.04 L. (c) 5040 times 1440 equals 7,257,600 mL, that is 7257.6 L. (d) Heart rate rises and stroke volume increases.
20	Blood has several components. (a) Which component carries oxygen? (b) Which component causes clotting? (c) State one function of white blood cells. (d) How does vaccination protect a person?	Answer: (a) Red blood cells, through haemoglobin. (b) Platelets. (c) They engulf pathogens or make antibodies against antigens. (d) It makes antibodies and memory cells, so a later infection is fought quickly.

Section F: Ecology and Energy Flow (16 marks)

Every explanation needs a cause and an effect.

#	Question	Answer and marking notes
21	Food chain: grass, insects, frogs, snakes. Grass fixes 50,000 kJ and about 10 percent of energy passes to the next level. (a) Find the energy available to insects. (b) Find the energy available to snakes. (c) Give two reasons for heavy energy loss. (d) Why do food chains rarely have more than five levels?	Answer: (a) 50,000 times 10 percent equals 5,000 kJ. (b) 5,000 times 10 percent times 10 percent equals 50 kJ. (c) Much energy is lost as heat; uneaten remains and waste are broken down by decomposers. (d) Only a tenth passes on, too little to support more levels.
22	Carbon cycles through ecosystems. (a) Which process turns atmospheric carbon dioxide into organic carbon? (b) Name one process that returns organic carbon to the air. (c) What is the role of decomposers? (d) How does burning fossil fuels affect the carbon cycle?	Answer: (a) Photosynthesis. (b) Respiration, decomposition or combustion. (c) They break down dead matter and waste, releasing carbon dioxide. (d) It releases carbon stored for ages, raising atmospheric carbon dioxide and strengthening the greenhouse effect.
23	Producers in an ecosystem have a biomass of 2,000 kg and about 10 percent of energy passes on. (a) Find the primary consumer biomass. (b) Find the secondary consumer biomass. (c) Why is a biomass pyramid usually upright? (d) Give one case of an inverted biomass pyramid.	Answer: (a) 2,000 times 10 percent equals 200 kg. (b) 200 times 10 percent equals 20 kg. (c) Energy and biomass fall at each level, so producers hold the most. (d) In the sea, fast growing phytoplankton can have less biomass than the zooplankton feeding on them.
24	Human activity disturbs ecological balance. (a) How can water pollution kill fish? (b) How does eutrophication lower dissolved oxygen? (c) Suggest one measure to conserve biodiversity. (d) How does falling biodiversity affect ecosystem stability?	Answer: (a) Pollutants cut dissolved oxygen or poison the fish directly. (b) Nutrients trigger algal blooms; decomposers then use up oxygen breaking down the dead algae. (c) Set up reserves, limit hunting or restore habitats. (d) Fewer species simplify food webs, so the system resists change and recovers less well.

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