CSEC Biology — Paper 2 Solutions

The set-up uses the raw potato as a "membrane bag" between two solutions: pure (or near-pure) water in the petri dish on the outside, and a very concentrated salt solution forming inside the well as the salt crystals dissolve.

Water has a higher concentration of water molecules in the petri dish than inside the salt-filled well. By osmosis, water moves through the partially permeable membranes of the potato cells, down its water-concentration gradient, from the petri dish → through the potato → into the well.

What you should draw and label after 2 hours:

• The well now contains a noticeably higher level of salt solution (the salt crystals have dissolved and water has flowed in).
• The water level in the petri dish has dropped.
• The raw potato itself looks slightly turgid — its cells have absorbed water and become firmer.
• Label arrows showing the direction of water movement: petri dish → potato → well.

Marks (typical): 1 for raised solution level in well; 1 for lowered water level in dish; 1 for showing potato as a continuous membrane between the two; 1 for correctly labelled water-movement arrows.

"An investigation into osmosis using a raw potato as a partially permeable membrane."

(Any title that names the process — osmosis — and the model membrane — potato — is acceptable.)

A hypothesis is a testable prediction stating what is expected to happen and why.

"Water will move by osmosis from the petri dish (high water concentration) through the cell membranes of the raw potato into the salt-filled well (low water concentration), causing the level of liquid in the well to rise."

1 mark for predicting the direction of water movement; 1 mark for explaining it via osmosis / water-concentration gradient.

Boiling denatures the cell membrane proteins of the potato cells, so the membranes are no longer partially permeable — they leak freely in both directions.

1. Little or no change in liquid levels. Because the membranes have lost their selective permeability, water and salt can move freely both ways and the net osmotic flow into the well is greatly reduced — no clear rise in the well or fall in the dish.
2. The boiled potato becomes soft / flaccid rather than firm/turgid; salt may also leak out of the well into the surrounding water (the dish water becomes salty).

1 mark each.

1 mark for each correct process + 1 mark for each correct reason = 8 marks.

2 marks per structure-and-explanation pair.

2 marks per structure-and-explanation pair = 4 marks.

From the diagram (Figure 2), count the number of chromosomes visible at the prophase / metaphase stage where they are clearly distinguishable.

The parent cell contains 4 chromosomes (the typical example used in CSEC mitosis diagrams shows 4 chromosomes / 2 homologous pairs). Mark whatever number is shown by the prophase image.

The standard sequence of mitosis is: Interphase → Prophase → Metaphase → Anaphase → Telophase.

• Interphase — chromosomes not yet visible (DNA replicated)
• Prophase — chromosomes condense and become visible
• Metaphase — chromosomes line up at the equator (cell middle)
• Anaphase — sister chromatids separate to opposite poles
• Telophase — two new nuclei form; cell starts to divide (cytokinesis)

Match each labelled diagram (A–E) to one of these stages. Award 2 marks for the fully correct sequence; 1 mark if the order is mostly correct with one slip.

1. Growth — mitosis produces new identical body cells, increasing cell number so the organism grows in size.
2. Repair / replacement of damaged cells — mitosis replaces cells lost through wear and tear (e.g., skin cells, blood cells) with genetically identical copies.
3. Other valid: asexual reproduction in some plants, fungi and simple animals; healing of wounds.

1 mark each (any two).

1. Mutation — random changes to DNA produce new alleles.
2. Sexual reproduction (specifically: random fusion of gametes, independent assortment of chromosomes during meiosis, and crossing over between homologous chromosomes).

1 mark each.

Parents' phenotypes: Male × Female

Parents' genotypes: XY × XX

Gametes: (X, Y) × (X, X)

Offspring genotypes (Punnett square):

Offspring phenotypes: 2 XX (female) : 2 XY (male) → ratio 1 : 1.

Conclusion: Each conception has a 50% chance of producing a male and 50% chance of producing a female — therefore the statement is accurate.

Mark allocation: 1 phenotypes; 1 genotypes; 1 gametes; 2 cross/offspring genotypes; 1 conclusion = 6 marks.

Haemophilia is X-linked recessive. Use X^H for normal allele, X^h for the haemophilia allele.

• Father (haemophiliac) genotype: X^hY — he can only pass on X^h to a daughter.
• Mother (carrier) genotype: X^HX^h — she can pass on X^H or X^h.

So daughters receive X^h from dad PLUS either X^H or X^h from mum:

1. X^HX^h — carrier daughter (heterozygous, no symptoms).
2. X^hX^h — haemophiliac daughter (homozygous recessive).

1 mark each.

2 marks for each correct substance + storage-site pair = 4 marks (any two pairs).

1. Energy reserve — provides energy for periods when food intake is low or impossible (e.g., hibernation, dormancy, drought, winter).
2. Growth and development — stored food fuels the early growth of seedlings before they can photosynthesise, and supports growth at every life stage.
3. Reproduction — stored food in seeds/eggs nourishes the embryo until it can feed itself.
4. Other accepted: emergency reserve in extreme conditions; supports tissue repair and immune function.

1 mark each (any three).

During germination the seed is metabolically very active but cannot photosynthesise yet, so it must mobilise its stored reserves.

1. Stored proteins are broken down by protease enzymes into amino acids (1 mark).
2. The amino acids are transported to the embryo / growing regions (1 mark).
3. They are then used to build new proteins/enzymes for growth, including the new structural proteins of the radicle, plumule and developing leaves (1 mark).

So the protein in storage tissues decreases over time because it is being mobilised, broken down, and reassembled into the seedling's own tissues.

The starch level falls sharply during germination:

1. The enzyme amylase hydrolyses stored starch into the soluble sugar maltose, then maltase breaks maltose into glucose.
2. The glucose is used by the embryo for aerobic respiration to release ATP for growth (or for synthesising cellulose for new cell walls).

That is why the starch curve drops while the sugar curve rises during germination.

Glucose (or maltose). Glucose is the primary respiratory sugar produced from starch breakdown during germination.

Starch is insoluble in water and osmotically inactive. Because it does not dissolve, it does not affect the water potential of the storage cells — large quantities can be stored without causing water to rush in by osmosis (which would otherwise burst the cell). It also stores compactly: a single starch grain holds many glucose units in a small volume, and it is easily broken back down to glucose by amylase when needed.

1 mark for stating the property (insoluble / osmotically inactive); 1 mark for the consequence (storable in bulk without osmotic damage).

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + Energy (ATP)

1 mark for correct reactants and products; 1 mark for correct balancing of atoms.

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

During an all-out sprint, James's leg muscles demanded oxygen faster than his lungs and circulation could supply it.

1. His muscle cells switched to anaerobic respiration, partially breaking down glucose without oxygen and producing lactic acid.
2. Lactic acid built up in the muscles, lowering pH and causing the burning, cramping pain he felt (1 mark).
3. After the race he was in oxygen debt — he needed extra oxygen to oxidise the accumulated lactic acid back to CO₂ and H₂O in the liver and muscles.
4. To repay this oxygen debt his breathing rate and depth increased dramatically — that is why he was gasping, even after he stopped running (1 mark).

Marking guide: 1 mark for switch to anaerobic respiration; 1 mark for naming lactic acid; 1 mark for linking lactic acid to pain; 1 mark for explaining gasping as repayment of oxygen debt.

1. Lung damage / Chronic bronchitis & emphysema. Tar in cigarette smoke paralyses and destroys the cilia that line the airways, so mucus and pathogens accumulate, causing a "smoker's cough", repeated chest infections and chronic bronchitis. Nicotine and other chemicals also break down the walls of alveoli (emphysema), reducing the surface area for gas exchange and leading to breathlessness.
2. Cardiovascular disease. Nicotine raises blood pressure and heart rate; carbon monoxide reduces the blood's oxygen-carrying capacity by binding to haemoglobin to form carboxyhaemoglobin. Together these increase the risk of coronary heart disease, atherosclerosis, heart attack and stroke.
3. Other accepted: increased risk of lung cancer (and other cancers — mouth, throat, oesophagus, bladder); reduced fertility; weakened immune system.

2 marks per effect: 1 for naming, 1 for explanation. Any two = 4 marks.

Any one of:

• Increase taxes on tobacco products to make cigarettes more expensive and reduce consumption.
• Ban smoking in public spaces (workplaces, restaurants, parks).
• Mandate graphic health warnings and plain packaging on cigarette packs.
• Ban tobacco advertising and sponsorship.
• Public-education campaigns + free smoking-cessation programmes.
• Raise the legal age for purchasing tobacco.

(i) Stimulus: a change in the internal or external environment that is detected by a receptor and produces a reaction in an organism.

(ii) Response: the reaction or change in behaviour or activity of an organism brought about by a stimulus.

1 mark each.

The standard reflex arc — receptor → sensory neurone → relay (interneuron, in spinal cord) → motor neurone → effector — is labelled across the diagram. Common assignments:

• A — Receptor (stretch receptor in patellar tendon) — detects the stimulus (sharp tap).
• B — Sensory neurone — carries the impulse from the receptor to the spinal cord.
• C — Spinal cord / relay neurone — synapses with the motor neurone (no brain involvement → reflex).
• D — Motor neurone (or effector / quadriceps muscle) — carries the impulse to the muscle, which contracts to kick the leg.

1 mark per correct label = 4 marks.

Any one of:

• Damage to the reflex arc — e.g. injury to the sensory or motor neurone, or to the spinal cord at the level that controls the knee.
• Damaged / detached patellar tendon so the stretch receptor was not activated.
• Tense / contracted quadriceps muscle (he was not relaxed), so the stretch receptor was not stretched by the tap.
• A nervous-system disease (e.g., peripheral neuropathy) interfering with impulse conduction.

Millipedes show taxes — directional movements towards or away from a stimulus. Pick any two of the stimuli you would expect in a basement and link them to the taxis:

3 marks per stimulus: 1 for naming the stimulus, 1 for naming the type of taxis (positive/negative), 1 for explaining the survival benefit. Any two stimuli = 6 marks.

1. Daphnia stay deeper / hidden. Because Daphnia normally come up to the surface in the dark to feed on algae, artificial lighting suppresses this migration — they do not feed properly, populations fall, and predator fish that depend on them lose a food source.
2. Algal blooms. With Daphnia no longer grazing the surface algae at night, algae multiply unchecked. This can lead to algal blooms that block sunlight, deplete dissolved oxygen at night, and kill fish and other aquatic life — disrupting the entire food web.

1 mark each.

1. Condom (male or female) — barrier method.
2. Combined oral contraceptive pill — hormonal.
3. Intrauterine device (IUD/coil) — long-acting.
4. Tubal ligation (female sterilisation) or vasectomy (male sterilisation) — surgical.
5. Other accepted: diaphragm, cervical cap, contraceptive injection (Depo-Provera), implant, abstinence, rhythm/calendar method, withdrawal.

1 mark each.

1. Condom — a thin latex or polyurethane sheath that physically blocks sperm from entering the vagina (or, for the female condom, from reaching the cervix), preventing fertilisation. Bonus: also reduces the spread of sexually transmitted infections, including HIV.
2. Oral contraceptive pill — contains synthetic oestrogen and/or progesterone; these hormones suppress ovulation (so no egg is released), thicken cervical mucus (so sperm cannot pass), and thin the uterus lining (so any fertilised egg cannot implant).

1 mark per outline. Other valid methods: tubal ligation cuts the Fallopian tubes so eggs and sperm cannot meet; the IUD prevents implantation/fertilisation; abstinence prevents sperm from entering the vagina.

Total people living with HIV = 330 000. People accessing treatment = 220 000. People NOT accessing treatment = 330 000 − 220 000 = 110 000.

Percentage = (110 000 / 330 000) × 100 = 33.3% (1 dp) — about one-third of HIV-positive Caribbean people were not on treatment in 2020.

1 mark for setting up the correct fraction; 1 mark for the answer.

1. Stigma and discrimination — fear of being identified as HIV-positive by family, employers or community can stop people from going to clinics, especially in small Caribbean towns where confidentiality is hard to maintain. (2 marks: 1 for the reason, 1 for the explanation.)
2. Financial / access barriers — cost of clinic visits and follow-up tests; long travel distances to ARV-supply centres; no transport; no health insurance. (2 marks)
3. Other accepted: denial of diagnosis; lack of knowledge that effective treatment exists; side-effects of antiretrovirals; religious or cultural beliefs that favour alternative remedies; unstable supply of medication.

2 marks per reason × 2 = 4 marks.

1. HIV specifically attacks and destroys helper T-lymphocytes (CD4 cells), the white blood cells that coordinate the body's immune response (1 mark).
2. As CD4 numbers fall, the immune system is severely weakened (immunodeficiency) and can no longer mount an effective response against pathogens (1 mark).
3. Microbes that healthy people fight off easily — like Mycobacterium tuberculosis, Pneumocystis jirovecii, fungi, or other viruses — now cause severe, uncontrollable infections (e.g., TB, pneumonia). It is these opportunistic infections, not HIV itself, that are usually the immediate cause of death (1 mark).