CSEC Biology — Paper 2 Solutions

Electron microscope (EM) — specifically a transmission electron microscope. The fine internal detail (organelles, membrane systems) shown in a "micrograph" of an animal/plant cell can only be resolved by an electron microscope.

Drawing should occupy at least half of the answer space and show clean, single-line outlines (no shading), with neat label-lines drawn with a ruler that do NOT cross. Required labels:

1. Cell wall — outer rigid layer (made of cellulose).
2. Cell membrane — partially permeable layer just inside the cell wall.
3. Cytoplasm — jelly-like medium where reactions occur.
4. Nucleus — large oval body containing the genetic material.
5. Large central vacuole — fluid-filled sac containing cell sap.
6. Chloroplasts — green organelles where photosynthesis occurs.
7. Mitochondrion — site of aerobic respiration.

Mark allocation (typical 9-mark drawing): 2 marks size and clarity (large, clean lines); 1 mark each for any 7 correct labels with clean ruled label-lines.

1. Both contain a nucleus with genetic material (DNA).
2. Both contain cytoplasm bounded by a cell membrane.
3. Other accepted: both contain mitochondria for aerobic respiration; both contain ribosomes; both have endoplasmic reticulum and Golgi apparatus.

1 mark each (any two).

Nucleus (or, more specifically, the chromosomes within the nucleus).

Both have a cell wall (although the chemistry differs — cellulose in plants, peptidoglycan/murein in bacteria). Both have cytoplasm enclosed by a cell membrane and contain ribosomes.

2 marks per structure-and-benefit pair = 6 marks for the three blanks.

2 marks per pair (1 for each cell's distinct feature) = 4 marks.

The bacterial cell has a cell wall (visible as a distinct outer rigid layer) and no membrane-bound nucleus — its genetic material lies free in the cytoplasm. An animal cell has neither of these features (no cell wall + has a defined nucleus).

The original cell shows two pairs of homologous chromosomes (4 chromosomes in total). After meiosis there are 4 daughter cells, each with TWO chromosomes (haploid). Cell P shows two distinct chromosomes; cell S shows another genetically different pair (suggesting crossing over occurred). Q and R should each show one chromosome from each homologous pair — different combinations from those already drawn in P and S.

Acceptable for Q and R: each shows two single chromosomes (one from each homologous pair), but in different combinations from P and S, reflecting the random assortment that occurs during meiosis.

Marks: 1 for correct chromosome number (2 per cell); 1 for showing single chromosomes (not paired); 2 for genetically different combinations from P and S, illustrating the variation produced by independent assortment + crossing over.

Haploid — each daughter cell has half the chromosome number of the original (parent) cell.

Any one of: testes (sperm production), ovaries (egg production). (Acceptable: gonads.)

Chromosome (acceptable: chromatin / DNA).

1. Random fusion of gametes at fertilisation. Each child results from a different sperm fertilising a different egg — so each daughter inherits a different combination of alleles from her parents. (2 marks)
2. Independent assortment + crossing over during meiosis. When gametes form in the parents' gonads, homologous chromosomes line up randomly and exchange segments at the chiasmata. Each gamete therefore carries a unique mix of maternal and paternal alleles. So each daughter gets a unique allele combination, leading to differences in observable traits like hair colour, eye colour, height, etc. (2 marks)

1. Halves the chromosome number to make haploid gametes — so that when two gametes fuse at fertilisation, the diploid number is restored. Without meiosis the chromosome number would double every generation.
2. Generates genetic variation through crossing over and independent assortment, which is the raw material for evolution by natural selection.

1 mark each.

Male gamete = pollen / pollen grain (sperm nucleus). Female gamete = ovum / egg cell (within the embryo sac of the ovule).

1 mark each.

1. Kingdom Plantae (plants).
2. Kingdom Fungi (mushrooms, moulds, yeasts).
3. Kingdom Protoctista / Protista (single-celled eukaryotes — Amoeba, Paramecium, algae).
4. Other accepted: Kingdom Monera / Bacteria (prokaryotes — bacteria, blue-green algae).

1 mark each.

Looking at the six fish (A–F):

1. Body shape (elongated/streamlined vs flattened/compressed).
2. Patterning / colouration (striped, spotted, plain).
3. Mouth/snout shape (long pointed snout vs short blunt mouth).
4. Other accepted: fin shape and number; presence/absence of scales; size; tail shape (forked / rounded); eye position.

1 mark each.

Example: using body patterning:

1 mark for naming the characteristic; 1 mark for correct grouping headings; 2 marks for correctly placing all six fish based on the chosen feature.

Conservation is the careful management and protection of natural resources — including living organisms and their habitats — so that they continue to exist and remain in good condition for future generations.

1. Establish marine protected areas (MPAs) / no-fishing zones on coral reefs. Within these zones, fishing of vulnerable species is banned, allowing fish populations to recover and providing safe breeding grounds. This directly reduces predation by humans / overfishing. (2 marks)
2. Pollution control / waste management. Enforce strict laws on dumping sewage, industrial waste, agricultural runoff and plastics into the sea. Treat sewage before discharge and educate the public about reducing single-use plastics that end up in the ocean. This minimises the chemicals and microplastics that damage reefs and poison fish. (2 marks)
3. Other accepted: public education campaigns; closed seasons during breeding; size limits on catches; captive breeding and re-stocking programmes; coral nursery / replanting projects.

Any one of: Amoeba, Paramecium, Euglena, yeast, Escherichia coli (bacterium), Chlorella.

1. Oxygen / carbon dioxide (respiratory gases).
2. Nutrients (glucose, amino acids, fatty acids, mineral ions, vitamins).
3. Hormones (chemical messengers like insulin, adrenaline).
4. Other accepted: water; urea / metabolic waste; antibodies; heat (for thermoregulation).

1 mark each.

1. Surface-area-to-volume (SA:V) ratio. A unicellular organism has a very high SA:V — every part of the cell is close to the surface, so simple diffusion delivers gases and nutrients fast enough for survival. A large multicellular organism has a much lower SA:V; diffusion alone is too slow to supply cells deep inside the body. (2 marks)
2. Distance and demand. Specialised tissues in multicellular organisms have high metabolic demand and are far from the body surface. A dedicated transport system (blood circulation in animals, xylem/phloem in plants) is needed to deliver oxygen, glucose and hormones rapidly to every cell, and to remove wastes. (2 marks)

Translocation — the transport of dissolved organic substances (mainly sucrose, but also amino acids and hormones) from where they are made (the leaves) to where they are stored or used (roots, fruits, growing shoots) through the phloem. (1 mark)

Transpiration — the loss of water vapour from the surface of a plant (mainly the leaves) by evaporation through the stomata, which pulls water and dissolved minerals up the xylem from the roots. (1 mark)

1. Hollow tube made of dead cells without end-walls. Once the cells die, their cytoplasm and end-walls dissolve, leaving a continuous unobstructed pipe from root to leaf. Water and minerals can flow upward in an unbroken column with very little resistance. (2 marks)
2. Walls strengthened with lignin (rings/spirals). Lignin makes the walls rigid and waterproof; this prevents the vessels from collapsing under the strong tension of transpiration pull, and stops water from leaking sideways before it reaches the leaves. (2 marks)

The phloem consists of sieve-tube elements joined end-to-end through perforated sieve plates. The cytoplasm flows from one element to the next through the holes, allowing dissolved sucrose and amino acids to be moved efficiently. Each sieve element is also accompanied by a companion cell packed with mitochondria, which provides the ATP needed for active loading/unloading of solutes.

Diffusion — the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient. (1 mark)

Breathing (ventilation) — the mechanical process by which air is moved into and out of the lungs (inhalation and exhalation) through movement of the diaphragm and rib muscles. (1 mark)

Respiration — the chemical process inside cells by which glucose is broken down (with or without oxygen) to release energy as ATP. (1 mark)

1. Diaphragm — sheet of muscle below the lungs.
2. External intercostal muscles — between adjacent ribs; contract to pull ribs up and out during inhalation.
3. Internal intercostal muscles — between adjacent ribs; contract during forced exhalation to pull ribs down and in.

1 mark each.

1. Air rich in O₂ enters the alveoli after passing down the trachea, bronchi and bronchioles. (1 mark)
2. Each alveolus is one cell thick and surrounded by a dense network of capillaries, also one cell thick. The total path between air and blood is therefore only two cells thick, allowing rapid diffusion. (1 mark)
3. O₂ diffuses from the alveolar air (high O₂) into the blood (low O₂), where it binds to haemoglobin in red blood cells. (1 mark)
4. CO₂ diffuses from the blood (high CO₂) into the alveoli (low CO₂) and is breathed out. (1 mark)
5. The alveoli's huge surface area (~70 m² total), thin walls, moist lining (which allows gases to dissolve before crossing the membrane) and continuous blood supply maintain steep concentration gradients for fast, efficient gas exchange. (1 mark)

1. During strenuous exercise, the muscles' demand for energy outstrips the supply of oxygen via the lungs and circulation. (1 mark)
2. The muscle cells switch (partially) to anaerobic respiration: glucose → lactic acid + 2 ATP. (1 mark)
3. This produces extra ATP for muscle contraction, but lactic acid accumulates and lowers muscle pH, causing the burning sensation/fatigue/cramping in the muscles. (1 mark)
4. After the activity ends, the body breathes deeply and rapidly to take in extra O₂ — the oxygen debt — and oxidises the lactic acid back to CO₂ and water in the liver and muscles. (1 mark)

Egestion — the elimination of undigested food (faeces) from the digestive tract. The substances expelled were never part of the body's metabolism — they simply passed through the gut. (1 mark)

Excretion — the removal of metabolic waste products (substances actually produced by the cells of the body — e.g., CO₂, urea, excess salts) from the body. (1 mark)

2 marks per organ-and-substance pair (1 organ + 1 substance) = 4 marks for any two.

1. Vomiting causes Joy to lose a large volume of fluid → her blood becomes more concentrated (low water content). (1 mark)
2. Osmoreceptors in the hypothalamus detect the drop in blood water and stimulate the posterior pituitary gland to release more antidiuretic hormone (ADH). (1 mark)
3. ADH travels in the blood to the kidneys, increasing the permeability of the distal convoluted tubule and collecting duct to water. (1 mark)
4. More water is reabsorbed back into the blood, producing a small volume of concentrated urine. The blood's water content rises back to normal — homeostatic balance is restored. (1 mark)

Drink plenty of water and oral rehydration salts (ORS) / electrolyte drinks to replace lost water, sodium and potassium. In severe cases, intravenous (IV) saline drip may be needed.

1. Waxy cuticle on the leaf surface. A thick, waxy, waterproof layer (cutin) covers the upper epidermis of the leaf and reduces the amount of water that can escape directly through the leaf surface. Drought-adapted plants (xerophytes) have especially thick cuticles. (2 marks)
2. Stomatal control / sunken stomata. The guard cells close the stomata when water is scarce (e.g., at midday or under drought stress), preventing water vapour from leaving. Many xerophytes also have stomata in pits or surrounded by hairs that trap a layer of humid air, slowing diffusion of water vapour out of the leaf. (2 marks)
3. Other accepted: reduced leaf surface area / leaves modified to spines (cacti); rolled leaves to trap moisture (marram grass); CAM photosynthesis — open stomata only at night when temperatures are lower; deep tap-roots to reach groundwater; storing water in succulent stems/leaves.