CSEC Biology — Paper 2 Solutions

Roti is made of wheat flour (starch), water and oil — so digestion produces sugars (from starch) and fatty acids/glycerol (from oil).

1. Glucose (or maltose) — from the digestion of starch by amylase + maltase.
2. Fatty acids and glycerol — from the digestion of the cooking oil by lipase.

1 mark each.

Chicken is mostly protein and some fat.

1. Amino acids — produced when protease enzymes (pepsin, trypsin, peptidases) break down proteins.
2. Fatty acids and glycerol — from the lipase digestion of fats in the chicken skin.

1 mark each.

1 mark per dietary change + 1 mark per matched consequence = 4 marks.

The conversion of excess blood glucose into glycogen is driven by the hormone insulin, secreted by the β-cells of the pancreas (1 mark).

In diabetics — especially Type 1 — the pancreas produces little or no insulin (or, in Type 2, the body's cells are insulin-resistant). Without effective insulin signalling, the liver and muscles do not take up glucose to store it as glycogen, so blood-glucose levels stay high while glycogen stores are NOT replenished (1 mark).

(i) Why was the mixture kept in a water bath? To keep the amylase enzyme at the optimum temperature (≈ 37 °C, body temperature) so it works at its maximum rate. (1 mark)

(ii) Why must the solutions be kept at the same temperature? So that temperature is a controlled variable — only the time of digestion is allowed to change. This makes the comparison between samples at different times fair/valid. (1 mark)

(iii) Why were starch and amylase NOT mixed at the start? So that both solutions could first reach the same target temperature (~37 °C) before the reaction began — otherwise the enzyme rate would change as the mixture warmed up. (1 mark)

(iv) Suggest a suitable aim. "To investigate how long it takes amylase to completely break down starch into sugar at body temperature." (1 mark)

(v) Time for digestion to complete: The colour stops changing after the 14-minute sample (light orange-brown) — every drop after that gives the same light-orange colour. So digestion was effectively complete by 14 minutes (some mark schemes accept 12 minutes — when the blue-black colour was effectively gone). (1 mark)

(vi) Explanation: Iodine turns blue-black in the presence of starch and stays orange-brown when no starch is present. As amylase progressively hydrolysed the starch into maltose/glucose, the amount of starch left fell. By 14 minutes virtually no starch remained, so iodine no longer turned blue-black — it kept its own native orange-brown colour. (2 marks: 1 for explaining the iodine indicator; 1 for linking the colour change to declining starch concentration.)

(vii) At 22 minutes, the cavity will still appear light orange-brown (the natural colour of iodine) — there is no starch left for it to react with. (1 mark)

(viii) THREE precautions:

1. Use the same volume of starch solution and amylase solution in every trial (controlled volume).
2. Maintain the same temperature in the water bath (avoid temperature drift) — use a thermometer.
3. Use a clean dropper / fresh cavity for each sample to avoid cross-contaminating one time point with the previous one.
4. Other valid: use the same concentration of iodine; use the same time intervals (every 2 min); avoid stirring inconsistently.

1 mark each (any 3) = 3 marks.

(ix) Reagent to identify what's left after 20 minutes: Benedict's solution — heating with the mixture would give a brick-red precipitate, confirming the presence of reducing sugars (maltose / glucose). (1 mark)

(x) Conclusion: "Amylase digests starch into sugar; given enough time at the optimum temperature, all the starch is broken down so that iodine no longer turns blue-black." (1 mark)

(xi) Would similar results be obtained if starch is replaced by roti? YES, similar results would be expected. (1 mark)

(xii) Reason: Roti is made from wheat flour, which is rich in starch. Amylase still hydrolyses the starch in the roti to maltose/glucose, so iodine would again gradually stop turning blue-black. (Slight differences may occur because the roti also contains oil/protein, so the rate may be slower while starch gets gelatinised by water.) (1 mark)

Photosynthesis is the process by which green plants (and some other autotrophs) use light energy — absorbed by chlorophyll — to convert carbon dioxide (from the air) and water (from the soil) into glucose and oxygen.

1 mark for "light energy converted into chemical energy"; 1 mark for "CO₂ + H₂O → glucose + O₂".

6 CO₂ + 6 H₂O →^{(light + chlorophyll)}→ C₆H₁₂O₆ + 6 O₂

1 mark for correct reactants/products; 1 mark for correct balancing (and including the light/chlorophyll above the arrow).

1. Water enters the root hair cells from the soil by osmosis — moving from a higher water concentration in the soil to a lower water concentration inside the root.
2. It moves across the root cortex (cell-to-cell or via the apoplast) into the xylem vessels, then is pulled up the stem in a continuous column by transpiration pull, finally diffusing out of the leaf veins into the mesophyll (photosynthetic) cells.

1 mark for osmosis at the root; 1 mark for xylem transport up the stem.

Intercellular air spaces (between the spongy mesophyll cells) provide a large internal surface area over which gases can diffuse to and from every photosynthetic cell. They act as a reservoir of CO₂ within the leaf. (1 mark)

The stomatal pore is the actual opening to the outside: it is where CO₂ enters the leaf from the atmosphere and where O₂ (and water vapour) leaves the leaf. Its opening and closing is controlled by the surrounding guard cells. (1 mark)

On a very hot day, the rate of transpiration rises sharply and the leaf can lose water faster than the roots can supply it. To prevent excessive water loss, the guard cells lose turgor and the stomata close. Once the stomata are closed, very little CO₂ can enter the leaf, so CO₂ becomes a limiting factor and the rate of photosynthesis falls (1 mark).

Additionally, very high temperatures can begin to denature the photosynthetic enzymes (e.g., Rubisco) above their optimum temperature, further reducing the rate (1 mark).

Chlorophyll absorbs light energy (mostly red and blue wavelengths, reflecting green) and converts it into chemical energy (ATP and NADPH) that drives the synthesis of glucose.

1. Source of food / energy. Photosynthesis produces glucose, the foundation of every food chain on Earth. Plants (producers) feed herbivores, which feed carnivores; without photosynthesis there would be no food. (2 marks)
2. Source of atmospheric oxygen and CO₂ sink. Plants release O₂ as a by-product, replenishing the oxygen that animals need for aerobic respiration, and at the same time they remove CO₂ from the atmosphere — buffering against climate change. (2 marks)
3. Other valid: photosynthesis stores energy in fossil fuels (formed from prehistoric plants) which we still use today.

The standard CSEC carbon cycle has these labelled arrows:

• Photosynthesis — atmospheric CO₂ → green plants (sugars).
• Respiration — animals/plants/microbes → CO₂ back into the atmosphere.
• Combustion — burning of fossil fuels and wood → CO₂ into the atmosphere.
• Decomposition / Decay — bacteria + fungi break down dead organisms → CO₂.
• Feeding — carbon transfers from plants to animals via food chains.

1 mark per correctly named process = 3 marks.

Carbon is the building block of all organic molecules — carbohydrates, lipids, proteins, nucleic acids — that make up the structure and metabolism of every living cell.

(i) Reuse — using an item again, in its original form (or for a new purpose), instead of throwing it away. E.g. refilling a glass bottle.

(ii) Reduce — minimising the amount of waste/raw material we generate or consume in the first place. E.g. using fewer single-use plastic bags.

(iii) Recycle — collecting used items and reprocessing them into new products. E.g. melting glass bottles to make new bottles.

1 mark each.

1. Conserves natural resources / reduces mining. Recovering aluminium, gold, silver and copper from old phones means less ore has to be dug up — which protects ecosystems and saves energy.
2. Reduces electronic waste / pollution. Heavy metals in discarded phones (lead, mercury, cadmium) can leach into soil and water; recycling keeps these toxins out of landfills.
3. Other accepted: recovers high-value metals at much lower cost than mining; reduces greenhouse-gas emissions associated with smelting.

1 mark each.

2 marks per pair of clearly contrasted differences (any two pairs) = 4 marks.

1. Mulch / compost — branches and leaves can be chipped and spread on gardens or farms to enrich soil with nutrients and reduce water loss.
2. Lumber / firewood — the trunk and large branches can be cut into planks for construction or used as firewood for cooking, providing low-cost building/heating material to the community.
3. Other accepted: furniture-making employs local craftspeople; sawdust used as animal bedding; wood ash used as a soil amendment.

1 mark each.

Transpiration is the loss of water vapour from the surface of a plant — primarily from the leaves through the stomata — by evaporation and diffusion.

1. Pulls water up from the roots ("transpiration pull") in a continuous column through the xylem, supplying every part of the plant with the water needed for photosynthesis and turgor.
2. Cools the leaves (evaporative cooling) on hot days, preventing heat damage to the photosynthetic enzymes and chloroplasts.
3. Other valid: moves dissolved mineral ions (nitrates, phosphates, etc.) up to the leaves; maintains turgor pressure for support of soft (non-woody) tissues.

1 mark each.

1. Humidity of the surrounding air.
2. Wind speed (air movement).
3. Light intensity (which controls how widely the stomata open).
4. Other accepted: water availability in the soil; leaf surface area; stomatal density / leaf adaptations.

1 mark each.

1. Humidity: When humidity is high, the water-vapour gradient between the inside of the leaf and the outside air is small — water vapour has nowhere to diffuse to — so transpiration slows down. When humidity is low, the gradient is steep and transpiration is rapid. (2 marks)
2. Wind speed: Moving air sweeps away the layer of saturated water vapour just outside the stomata, maintaining a steep diffusion gradient — so transpiration increases with wind speed (up to a point). In still air, water vapour accumulates and transpiration slows. (2 marks)
3. Light intensity: In bright light, the stomata open to allow CO₂ in for photosynthesis; the wide-open stomata also let more water vapour out, so transpiration increases. At low light or in darkness, stomata close and transpiration is greatly reduced. (2 marks)

More frequent and intense droughts (or hurricanes / sea-level rise / saltwater intrusion). For example: rising temperatures and prolonged dry spells reduce soil water; transpiration outpaces uptake; crops wilt, photosynthesis slows, yields fall. Caribbean farmers — already vulnerable on small island nations — face poorer harvests of staples like sugarcane, cocoa, citrus and bananas, threatening food security and livelihoods. (2 marks)

Other valid one-way explanations: stronger hurricanes flatten plantations and destroy infrastructure; rising sea levels salt-poison coastal soils; warmer oceans bleach coral reefs and reduce fisheries.

Any one of: switch to renewable energy (solar, wind, geothermal) and away from fossil fuels; protect and reforest existing forests to absorb CO₂; impose carbon taxes / emissions caps; promote public transport and EVs; reduce industrial emissions / improve energy efficiency.

(i) Gene — a section of DNA that codes for a specific protein and therefore determines a particular characteristic (e.g., flower colour, blood group).

(ii) Allele — one of the alternative forms of a gene found at the same locus on a chromosome (e.g., the gene for flower colour has a "purple" allele and a "white" allele).

(iii) Chromosome — a thread-like structure made of tightly coiled DNA wrapped around histone proteins, found in the nucleus, that carries many genes. Humans have 23 pairs (46 in total).

1 mark each.

The Figure shows the standard meiosis diagram with TWO homologous pairs of chromosomes in the parent cell. Typical labels are:

• A — Homologous pairing & crossing over (Prophase I). Each pair of homologous chromosomes lines up side-by-side; non-sister chromatids exchange segments at the chiasmata, producing genetic recombination.
• B — Independent assortment / first division (Anaphase I). Homologous chromosomes are pulled to opposite poles. Each pole receives a random mix of maternal and paternal chromosomes — independent assortment generates further variation.
• C — Second meiotic division (Anaphase II / Telophase II). Sister chromatids separate to form the final four haploid daughter cells, each genetically distinct from the parent and from each other.

1 mark each.

Errors in meiosis (e.g., non-disjunction — chromosomes failing to separate properly) would produce gametes with the wrong chromosome number. If such a gamete fuses at fertilisation, the resulting embryo could have missing or extra chromosomes, leading to genetic disorders such as Down syndrome (trisomy 21), miscarriage, or other developmental problems.

Genetic variation produces individuals with different traits, some of which give a survival advantage in a particular environment. Over generations, natural selection acts on this variation; populations diverge as different traits are favoured in different environments, ultimately leading to the formation of new species (speciation).

If a tall pure-bred (homozygous) plant is crossed with another plant and ALL offspring have the same phenotype, then the second parent must also have been homozygous. The simplest case (and the standard CSEC example): tall is dominant over short.

Define the alleles: let T = allele for tallness (dominant), t = allele for shortness (recessive). (1 mark)

Parents' phenotype: Tall × Short (1 mark)

Parents' genotype: TT × tt (1 mark)

Cross / Punnett square:

(2 marks)

Offspring genotype: All Tt (heterozygous) (1 mark)

Offspring phenotype: 100% Tall pea plants (1 mark) — confirming the observation that all offspring have the same phenotype.

Any one of: protection (mechanical barrier against pathogens, UV, mechanical injury); sensation (touch, pressure, pain, temperature receptors); excretion (sweat removes urea, salts and water); vitamin D synthesis; storage of fat (subcutaneous layer).

1. Sweat glands — produced sweat that travelled up the sweat ducts onto the skin's surface; as the sweat evaporated, it absorbed latent heat from the skin and cooled John's body, preventing overheating. (2 marks)
2. Arterioles supplying the skin (vasodilation) — the smooth muscle in the walls of the skin arterioles relaxed, widening the vessels. More warm blood flowed close to the skin's surface where heat could be lost by radiation and convection to the surroundings, lowering body temperature. (2 marks)
3. Other valid: hair erector muscles relaxed, lying hairs flat to reduce trapped insulating air; reduced metabolic rate.

Jan is Caucasian and her husband Anthony is African-American. Anthony's skin contains much more melanin — a brown/black pigment produced by melanocytes that absorbs UV radiation and protects the deeper skin layers from DNA damage. Jan has less melanin, so her skin is more easily damaged by UV → quicker sunburn and a higher long-term risk of skin cancer (1 mark).

Sunscreen breaks down on contact with sunlight, sweat and water — it loses effectiveness within a couple of hours. Reapplying frequently keeps the SPF level high enough to compensate for Jan's lower natural melanin protection (1 mark).

Homeostasis is the maintenance of a constant internal environment within an organism (e.g., body temperature, blood-glucose level, water/salt balance, blood pH) despite changes in the external environment.

1. On a hot day, the body loses water through sweat and the blood becomes more concentrated (lower water potential / higher salt concentration). (1 mark)
2. Osmoreceptors in the hypothalamus detect the fall in blood water content; the posterior pituitary gland is stimulated to release more antidiuretic hormone (ADH). (1 mark)
3. ADH travels in the blood to the kidneys, where it makes the walls of the distal convoluted tubule and collecting duct more permeable to water. (1 mark)
4. More water is therefore reabsorbed back into the blood, producing a smaller volume of more concentrated urine. The blood's water content rises back to normal — homeostasis is achieved. (1 mark)

1. Build-up of toxic urea/creatinine in the blood (uraemia) → fatigue, nausea, vomiting, confusion.
2. Fluid and electrolyte imbalance → swelling of ankles/face (oedema), high blood pressure, irregular heartbeat from K⁺ build-up.
3. Other accepted: anaemia (kidney makes EPO); weakened bones (kidneys activate vitamin D); increased risk of cardiovascular disease.

1 mark each.

Any one of: controlled his hypertension by taking blood-pressure medication consistently; maintained a low-salt, low-fat, balanced diet rich in vegetables; regular exercise + healthy weight; limited alcohol; avoided OTC NSAIDs which damage kidneys; routine medical check-ups for kidney function.