2026 paper questions

Cortisol (cortisone in the book) is a glucocorticoid of the adrenal cortex: it helps to regulate the blood glucose level. Epinephrine comes from the adrenal medulla, the mineralocorticoid is aldosterone (salt and water balance), and androgen is only produced in small amounts by the cortex.

Air passes down the respiratory tract in this order: nose, pharynx, larynx, trachea, bronchi, bronchioles and finally alveoli. Of the three structures in option A, the trachea comes first, then the bronchi, then the alveoli, so A is in the correct order. Option B starts with the bronchi, which lie below the pharynx and trachea. Option C starts at the alveoli, which are the end of the tract. Option D is tempting because the pharynx does come before the trachea, but it is listed after it, and the bronchi are skipped so that the list jumps straight to the alveoli.

Shivering is rapid contraction and relaxation of muscles. Shivering muscles give out four or five times as much heat as resting muscles, so shivering increases heat production when the body is cold. A and D are the opposite of its effect, and B is wrong because the purpose is to raise or hold body temperature, not lower it.

The A band keeps the same length. It is the whole length of the thick (myosin) filaments, and in contraction the thin filaments only slide in between them, so the thick filaments do not change length. The tempting choices change: the I band gets shorter, the H zone disappears, and the Z lines (the ends of the sarcomere) move closer together. A Z line is only a thin disc that marks the boundary, so its length is not the point; the distance between Z lines is what shrinks.

The cerebrum is divided into two cerebral hemispheres, and the corpus callosum is the band of axons that joins them, so each hemisphere can pass information to the other. Option A describes the cerebellum (balance and coordination of voluntary movement), option B the thalamus (a relay centre), and option D the midbrain, so they are the tempting wrong answers.

Hydrochloric acid and intrinsic factor are both made by the parietal (oxyntic) cells of the gastric glands. Chief cells (also called zymogenic or zymogen cells, so options A and D are the same cell type) make only the inactive enzyme pepsinogen. Goblet cells are mucus-secreting cells; the mucus of the stomach comes from its surface mucous cells.

Monoclonal antibodies (mAbs) bind one specific antigen, so they can be aimed at one kind of cell. The book lists targeted therapies (cancer, COVID-19, cardiovascular and autoimmune diseases) and immunotherapy under the therapeutic use of mAbs, so targeted cancer therapy is correct.

C is the tempting option, but diagnosis of diseases is a diagnostic use (ELISA, Western blot, imaging with labelled mAbs), not a therapy. B and D are not uses of mAbs at all.

The bicuspid (mitral) valve lies between the left atrium and the left ventricle and shuts when the ventricle contracts, so blood cannot flow back into the atrium. The aortic valve is the tempting option, but it stops blood falling back from the aorta into the left ventricle. The tricuspid valve is on the right side, and the pulmonary valve guards the exit of the right ventricle.

Tubular secretion is the addition of substances such as ammonium and hydrogen ions from the blood and the cells of the tubule into the filtrate, which helps keep the pH of urine at 4.8 to 7.5. The movement is therefore from blood to filtrate. Option B is the opposite process, selective reabsorption (filtrate to blood). Options A and D name organs of the urinary tract that urine passes through, not a movement of substances between blood and tubule.

The baby receives ready-made antibodies from its mother, so the baby's own immune system has not made them: the immunity is passive. It comes through the placenta without any injection or medical procedure, so it is natural. This is natural passive immunity, and it lasts only a few weeks because the antibodies are broken down.

C is the tempting option, but natural active immunity needs the body itself to make antibodies after an infection. B and D are artificial, which needs a vaccine or an injection of antibodies.

A PCR run begins with denaturation. Heat at 94 to 95 °C breaks the hydrogen bonds and separates the double-stranded template into single strands. Without single strands the primers cannot anneal and Taq polymerase has nothing to copy, so every later step depends on it.

B and C (primer annealing and primer extension) are the second and third steps of each cycle. A is wrong because blotting is not a step of PCR.

Ocean acidification lowers the pH of seawater and reduces the carbonate ions that calcifying organisms need to make calcium carbonate. Shell-forming organisms (corals, oysters, clams, sea urchins and plankton such as pteropods) therefore find it hard to build and keep their shells and skeletons.

Fish are affected mainly in behaviour and through loss of food, and marine mammals and birds only suffer indirectly through the food web. They are not the group that is most affected.

Synthetic biology combines biology, engineering and computer science to design and construct new biological parts, devices and systems for useful purposes. Creating useful biological products and systems is its main goal.

Preventing mutations, eliminating all microorganisms (engineered microbes are in fact used to make drugs and fuels) and studying ancient DNA are not aims of synthetic biology.

The thin myofilament is made of actin (the core), tropomyosin (spiralling along the actin) and troponin (at regular intervals). Myosin is the protein of the thick myofilament, so it is the one that does not correspond to the thin filament. Tropomyosin and troponin are tempting because they are not actin, but both sit on the thin filament and control its contact with myosin.

A plasmid has an origin of replication (ori), so it can copy itself independently of the chromosome in the host cell. This gives many copies of the gene inserted into it, which is what a vector must do.

A is wrong because a plasmid readily carries foreign genes once they are joined to it. B is wrong because a plasmid does not destroy viral DNA, and D is wrong because plasmids are found mainly in bacteria (and some yeasts and plants), not only in viruses.

Range is the difference between the highest and the lowest value: Range $=$ highest $-$ lowest $=15-10=5$. The data in order are 10, 11, 12, 13, 15.

The tempting options are the end values themselves: 15 is only the highest value and 10 is only the lowest. The range is the gap between them, not either of them. 2.5 is half of the range and has no meaning here.

Homologous structures are similar because the organisms descended from a common ancestor. The human arm and the whale flipper are built on the same bone plan inherited from a common ancestor, even though they are used differently.

The other pairs have a similar function but a different origin: fins of fish and wings of birds, wings of birds and insects, and the eyes of a human and an octopus all do a similar job in unrelated structures. Similar function without a common ancestral structure is not homology.

Pyelonephritis is the type of urinary tract infection (UTI) in which the kidneys are infected. The book names the three types of UTI by the site of infection.

• It is caused by bacteria (for example E. coli, the commonest cause of UTI), which usually pass from the faecal matter around the anal opening to the opening of the urethra and travel upwards.
• It is more common in women, because the female urethra is short (1.5 inches compared with 8 inches in men).
• Because it is bacterial, it is cured by antibiotic therapy. Drinking plenty of water and personal hygiene help to prevent it.

The respiratory surface is the area where gaseous exchange with the environment actually takes place. An efficient respiratory surface has these properties (any three, 1 mark each):

A kidney transplant places a healthy kidney into a person with chronic kidney failure. The book gives these problems (any three, 1 mark each):

1. Rejection. The recipient's immune system recognises the new kidney as foreign and attacks it.
2. Toxic effects of cyclosporine. The drug that is given to prevent rejection is itself toxic to the patient.
3. Lifelong medication. Drugs such as cyclosporine must be taken to suppress the immune system. If the patient stops them at any time, rejection can occur even ten or fifteen years after the transplant. Rejection and the toxic effects of the drug are treated together by adding extra doses of steroids.

Respiratory pigments are coloured molecules that act as oxygen carriers by binding reversibly to oxygen. They contain a coloured non-protein part, for example haem in haemoglobin. The two pigments of man are haemoglobin and myoglobin. Their role (three points):

1. Haemoglobin transports oxygen. The iron of its four haem groups combines loosely with oxygen where the oxygen concentration is high (lungs) and releases it quickly where the concentration is low (tissues). Each molecule carries four oxygen molecules.
2. Myoglobin stores oxygen in muscles. It has one chain and one haem group and holds on to oxygen until the partial pressure falls below 20 mmHg, so it gives up oxygen only when the supply of oxyhaemoglobin has been used up.
3. Myoglobin passes oxygen on. It acts as an intermediate that transfers oxygen from haemoglobin to the aerobic metabolism of the muscle cells. It also makes meat look red.

Component (1 mark)

The bile salts (sodium glycocholate and sodium taurocholate). Bile contains no digestive enzymes, so it is the bile salts that help digestion.

How they help (2 marks)

• Emulsification: bile salts reduce the surface tension of large fat globules and break them into tiny droplets. The droplets have a much greater total surface area, so pancreatic lipase can act on the fat faster.
• Absorption: bile salts also help the fatty acids to be absorbed from the intestine.

In the sliding filament model the thin (actin) filaments slide inwards between the thick (myosin) filaments, and neither set of filaments changes length. The changes are:

Median (1 mark)

Mean (2 marks)

a. How ECG detects abnormalities (1 mark)

An electrocardiogram records the electrical changes (depolarization and repolarization) of the heart as the P wave, QRS complex and T wave. A normal ECG shows that the heart is working properly, so a trace whose waves, timing or rhythm differ from the normal values (for example PR interval 0.12 to 0.20 s, QRS 0.08 to 0.10 s) points to a fault in the heart.

b. Two conditions (2 marks)

• Arrhythmias (irregular heartbeat).
• Conduction defects (a fault in the pathway of the impulse).
• Damage to the heart muscle, impaired blood flow to the heart muscle, the size and position of the chambers, and the effect of cardiac medicines or pacemakers can also be shown.

Identification (1 mark). The structure is a synapse, the junction where the swollen end of one neuron (the synaptic knob) meets the next cell, here a banded (striated) cell, as at a neuromuscular junction. The two cells are not in contact: a gap, the synaptic cleft, separates them, and the impulse crosses it as a chemical, the neurotransmitter.

Labelling A to D (2 marks)

The graph is the action potential of a neuron: the potential rises from the resting level (−70 mV) past the threshold (−55 mV) to about +40 mV, then falls and dips below the resting level before returning to it.

Definition (1 mark)

Macrophages are large phagocytic white blood cells formed from monocytes. Monocytes leave the blood and settle in tissues and organs such as the lungs, liver, spleen, kidney and lymph nodes, where they become macrophages. They belong to the second line of defence (nonspecific).

Two functions (2 marks)

• Phagocytosis: they engulf and destroy microorganisms that enter the tissue.
• Antigen presentation: they display parts of the microbe on their surface, so they are antigen presenting cells (APCs) and inform other cells; they secrete proteins that raise the body temperature (interleukin-1) and stimulate the specific immune response.
• Wound healing and tissue repair: they remove dead cells and debris.

a. Release of neurotransmitter at the synaptic knob (2 marks)

b. Effect on the postsynaptic membrane (1 mark)

The neurotransmitter binds to receptor molecules on the postsynaptic membrane. This opens ion channels, so Na⁺ diffuses in and the membrane depolarises towards the threshold (an excitatory postsynaptic potential, EPSP). If threshold is reached, an action potential starts in the postsynaptic neuron. The neurotransmitter is then broken down by enzymes (acetylcholine by acetylcholinesterase) so the membrane can reset.

Both arthritis and osteoporosis affect the skeleton, but they affect different structures.

a. Double-walled sac (1 mark)

The pericardium is a closed sac around the heart made of two layers: the visceral layer (inner), which covers the heart directly (the epicardium), and the parietal layer (outer), which forms the sac around the heart and contains the fluid of the pericardial cavity between the layers.

b. Two functions (2 marks)

• It prevents overfilling: the pericardium is inelastic, so the heart cannot be overstretched with blood.
• It lubricates the heart with serous fluid, preventing friction with the surrounding organs.
• (Also: it holds the heart in position and keeps a hollow space in which the heart can expand.)

a. Antivirals (1.5 marks)

Antivirals are a class of drugs used to treat viral infections. They work by inhibiting the ability of viruses to multiply and spread within the body. They target a diverse group of viruses, such as herpes, hepatitis and influenza viruses.

b. Antiretrovirals (1.5 marks)

Antiretrovirals are a subset of antivirals used especially against retrovirus infections, mainly HIV. A retrovirus uses RNA as its genetic material and reverse transcriptase to replicate. Antiretrovirals inhibit the replication of retroviruses, acting on different stages of the HIV life cycle, which helps to control HIV infection and reduces the risk of progression to AIDS.

Why it produces HCl (1.5 marks)

• Gives the low pH (about 2) at which pepsin works best.
• Kills bacteria that enter with the food.
• Stops the digestion of carbohydrates by inactivating salivary amylase.
• Denatures proteins so that proteolytic enzymes can reach the peptide bonds inside them.
• Changes inactive pepsinogen into active pepsin.

Why it produces mucin (mucus) (1.5 marks)

The mucus of the stomach is viscous and alkaline. It forms a thick layer over the epithelial cells that lubricates the lining and protects it from the acidic chyme and from pepsin, so the acid and enzyme digest the food and not the stomach wall.

Both are sex hormones secreted by the gonads, and both are steroids. They differ as follows.

Most marine bony fishes (teleosts) are hypotonic to sea water, so they face three challenges:

• Loss of water. Water leaves the body by osmosis, especially across the gill epithelium.
• Replacing the water. To make up the loss they must drink large amounts of sea water, unlike freshwater fishes.
• Excess salts. Drinking sea water brings in too much salt. The fish removes $\mathrm{Na}{}^{+}$, $\mathrm{Cl}{}^{-}$ and some $\mathrm{K}{}^{+}$ across the gill epithelium, excretes the divalent ions $\mathrm{Mg}{}^{2+}$ and $\mathrm{Ca}{}^{2+}$ through the kidney, and some fishes also have rectal glands that remove salts into the gut.

Any three of the following harmful effects, one mark each.

T cells differentiate in the thymus. When they are activated they divide and give four types, each with its own role in the cell-mediated immune response. Any three earn the marks (1 mark each).

Kidney failure (1 mark)

Kidney failure (renal failure) is a decline in kidney function, particularly in the efficiency of the filtering process. Chronic renal failure is the irreversible deterioration of renal function; it is gradual and occurs over a period of years.

Four causes of chronic renal failure (2 marks)

1. Bacterial infection of the pelvis and the surrounding tissue of the kidney.
2. Nephritis, inflammation of the glomeruli.
3. Damage due to high blood pressure.
4. Diabetes mellitus.

a. Genetically modified organism (1 mark)

A genetically modified organism (GMO) is an organism whose genetic material (DNA) has been altered in a way that does not occur naturally by fertilisation or natural recombination. Genetic engineering is used to insert foreign genes, remove existing genes or modify specific genes in it.

b. Two applications of transgenic bacteria (2 marks)

• To produce human proteins, for example human insulin made in Escherichia coli (first done in 1978).
• To improve plant growth.
• To remove environmental pollutants.
• To extract metals from low grade ores.

Any two applications with a short explanation (1.5 marks each).

1. Synthetic microbes

Bacteria and yeast are engineered to make valuable chemicals, fuels and drugs. For example, bacteria can turn sugar into ethanol, a renewable alternative to fossil fuels, and the human insulin gene inserted into E. coli makes insulin that is harvested and purified for medical use. Engineered microbes can also clean up pollutants such as oil spills (bioremediation).

2. Gene editing

DNA is modified precisely to correct genetic disorders, improve crops and livestock, and study disease. For example, it offers treatment for sickle cell anaemia, and golden rice is engineered to make beta-carotene, which gives vitamin A, to fight vitamin A deficiency.

Other applications

• Bio-sensing devices: engineered cells give a detectable signal for a target substance, for example contaminants in water or glucose level in diabetic patients.
• Biological computers: cells and proteins perform computations and could detect disease inside the body.

Extinction is the complete disappearance of a species. Any two of the species below, with the name and a short discussion, earn the marks (1.5 each).

1. Bramble Cay melomys (Melomys rubicola)

A small rodent of the tiny island of Bramble Cay off the coast of Australia. It is considered the first mammal to go extinct because of climate change: rising sea levels and storm surges wiped out its habitat.

2. Golden toad (Incilius periglenes)

It lived in Costa Rica's Monteverde Cloud Forest and is believed to have gone extinct in the late 1980s. Climate change brought drought and shifts in temperature that made the forest less hospitable, and disease further reduced the population.

Other species from the book

• Pinta Island tortoise (Chelonoidis abingdonii), Galapagos Islands: the last individual, Lonesome George, died in 2012; climate change disrupted its food and water.
• Atelopus frogs (harlequin frogs): many species were lost as the chytrid fungus spread with rising temperature and changing humidity.

a. Sequence homology (1.5 marks)

Sequence homology is the presence of shared sequences between two or more DNA, RNA or protein molecules, which indicates a common ancestral origin. It helps to understand evolutionary relationships, functional conservation and genetic diversity.

Example: the sequence of the protein BauA in 25 samples is the same in most positions (conserved). Another example is the rRNA sequence, which shows homology in all organisms.

b. Structural homology (1.5 marks)

Structural homology is the similarity of the 3D structures of biological molecules such as proteins, RNA or DNA. It shows shared architectural features and suggests a common ancestral origin or functional relationship.

Example: the 3D shape of the myoglobin molecule is similar in human, horse, sperm whale, sea turtle and tuna.

Both are imaging tests that give detailed pictures of the inside of the body, but they work on different principles.

A homeostatic mechanism has three parts: a receptor, a control centre and an effector.

Receptor (1.5 marks)

Detects that something in the external or internal environment is changing (a stimulus) and sends the information to the control centre. Examples are thermoreceptors and mechanoreceptors; thermoreceptors in the skin detect a change in temperature.

Control centre (1.5 marks)

Receives and processes the information from the receptor, compares it with the set point and sends commands to the effectors. The brain (hypothalamus) is the control centre for temperature; the renin-angiotensin system is a control centre for blood pressure and fluid balance.

Clinical development starts with the investigational new drug application: the preclinical data are submitted to the regulatory authorities (such as the FDA) to obtain permission to begin clinical trials. The trials then run in four phases.

Structure of the nephron (4 marks)

The nephron is the functional unit of the kidney. Each nephron has a renal corpuscle and a renal tubule, and the tubule drains into a collecting duct that receives many nephrons.

1. Renal corpuscle: a knot of capillaries called the glomerulus, surrounded by the thin, double-walled Bowman's capsule at the closed end of the tubule. Blood enters by the afferent arteriole and leaves by the efferent arteriole, which is half its diameter, so the pressure in the glomerulus is high.
2. Proximal convoluted tubule: the highly coiled part that leads away from the capsule and dips towards the renal pelvis.
3. Loop of Henle: a sharp loop into the medulla, with a descending limb and an ascending limb.
4. Distal convoluted tubule: the ascending limb returns to the region of the renal corpuscle and becomes coiled again, then joins the collecting duct.
5. Collecting duct: receives many nephrons; many collecting ducts join to form larger ducts that empty into the renal pelvis.

Blood supply: the afferent arteriole supplies the glomerulus; the efferent arteriole forms the peritubular capillaries around the tubules, including the vasa recta that run with the loop of Henle; these drain into the renal vein.

Labelled diagram (3 marks)

The sliding filament theory states that during contraction the thin myofilaments slide past the thick ones so that they overlap to a greater degree. The filaments do not shorten; the sarcomere shortens because the thin filaments are pulled towards its centre.

The three states

• Relaxed: the thick and thin filaments overlap only at the ends of the A band, and the H zone is wide.
• Partially contracted: the thin filaments have slid inwards; the Z lines are closer and the H zone is narrower.
• Fully contracted: the thin filaments meet at the M line; the H zone has disappeared and the I bands are very short. The A band keeps its length.

How the filaments slide (cross-bridge cycle)

Muscle contraction is started by a nerve impulse. It passes along the sarcolemma and down the T-tubules to the sarcoplasmic reticulum, whose calcium gates open and release calcium into the cytosol. The sliding then takes place in the following steps:

These cross-bridges form and break many times during a contraction, acting like tiny ratchets that pull the thin filaments towards the centre. Because this happens in all the sarcomeres at once, the whole fibre shortens.

Result

The thyroid gland lies on either side of the trachea, below the larynx. It secretes three active hormones: tri-iodothyronine (T3), thyroxine (T4) and calcitonin (2 marks each).

1. Tri-iodothyronine (T3)

• Contains three atoms of iodine. About 10 per cent of the thyroid secretion; four times more potent than T4 but acts for a shorter time.
• Controlled by TSH from the anterior pituitary.
• Raises the basal metabolic rate, enhances glucose catabolism and cholesterol synthesis in the liver, and promotes growth of the skeleton and development of the nervous system and muscles.
• Excess causes hyperthyroidism (Graves' disease); lack causes hypothyroidism.

2. Thyroxine (T4, tetra-iodothyronine)

• Contains four atoms of iodine. About 90 per cent of the thyroid secretion; its action lasts four times longer than T3.
• Has the same effects as T3: raises the basal metabolic rate, helps the development of the nervous system in the foetus and infants, and keeps the gut moving normally.
• Under secretion: myxedema in adults, cretinism in infants, and goitre when iodine is lacking.

3. Calcitonin

• Made by the C-cells of the thyroid; has no iodine.
• Released when the blood calcium level is too high.
• Lowers blood calcium: increases deposition of calcium in the bone matrix, reduces absorption of calcium by the intestine and its reabsorption by the kidney tubules.
• If deficient, high blood calcium disturbs muscles and nerves and may cause kidney stones.

Path of blood (4 marks)

Pulmonary circulation (heart to lungs and back)

Systemic circulation (heart to body and back)

Labelled diagram (3 marks)

Swallowing

Swallowing moves the bolus of food from the mouth to the stomach. It is a set of reflexes in three phases.

Phase 1: oral phase (voluntary)

Phase 2: pharyngeal phase (reflex)

Phase 3: esophageal phase (involuntary)

Peristalsis

Peristalsis is a wave-like, involuntary muscular contraction that pushes food forward (anterograde) along the digestive tract. It is controlled mainly by the myenteric plexus, which lies between the longitudinal and circular muscle layers.

Peristalsis moves food down the esophagus, mixes it with juices and moves chyme along the small intestine, and drives waste towards the rectum in the large intestine.

Definition

Polymerase chain reaction (PCR) is a technique that amplifies a single gene or piece of DNA into thousands to millions of copies by in vitro replication. It was invented by Kary Mullis in 1983 and is carried out in a thermocycler (PCR machine), which controls the time and temperature of each step.

Components of the reaction mixture

• Template DNA: the DNA to be amplified.
• Primers (forward and reverse): short pieces that mark where copying begins.
• dNTPs: the free nucleotides, the building blocks of the new strands.
• ***Taq* polymerase:** a heat-stable DNA polymerase from Thermus aquaticus that synthesises the new strands.
• Buffer: the medium of the reaction.

Steps (5 marks)

Flow sheet (1 mark)

When the blood becomes warmer than normal, the hypothalamus (the thermostat of the body) starts two physiological responses that increase heat loss: vasodilation and sweating.

Vasodilation (4 marks with sweating)

The muscles in the walls of the arterioles near the skin relax, so these arterioles dilate and more blood flows through the capillaries of the skin. The blood loses heat to the environment by radiation, and the body cools.

Sweating

Sweat glands secrete a salty solution onto the skin. As the sweat evaporates it absorbs heat from the body, which cools the skin and the blood flowing through it. Sweating works best in dry air; in humid air the sweat cannot evaporate, so cooling is poor.

Homology is the similarity of structure, physiology or development of different species based on their descent from a common evolutionary ancestor. Structural homology is therefore a similar structure due to common ancestry.

Option D (similar function but different structure) describes analogous structures, which are the opposite case. Options A and B describe features that are different or unrelated, so they do not show homology.

An endocrine gland has no duct and releases hormones into the blood. The salivary gland is exocrine: it pours saliva into the mouth through a duct. The pancreas (islets of Langerhans), parathyroid and testis (testosterone) all secrete hormones into the blood; the pancreas is also exocrine, which makes it the tempting option.

The epiglottis is the flap of cartilage that closes the opening of the larynx (the glottis) during swallowing, so food goes down the oesophagus and not into the trachea. The larynx is the tempting wrong answer because the epiglottis belongs to it and it holds the vocal cords, but the larynx as a whole stays open for air and does not close. The pharynx is the common passage for food and air, and the bronchi are the branches of the trachea inside the chest.

In the cold the arterioles near the skin constrict (vasoconstriction). Less blood flows through the skin capillaries, and the blood goes through shunt vessels deeper in the skin, so less heat is lost by radiation. Evaporation and sweating cool the body, and vasodilation increases heat loss, so all three work against heat conservation.

In the sliding filament model the thin (actin) filaments slide past the thick (myosin) filaments towards the centre of the sarcomere, so the two sets overlap more. The filaments do not break, shorten or fuse; option C is the common mistake, but the myosin filaments keep their length and only the sarcomere shortens.

For every cycle the pump uses ATP to move three Na⁺ out of the neuron and two K⁺ in, both against their concentration gradients. Because more positive charge leaves than enters, the inside becomes more negative, which helps to set up the resting potential of −70 mV. Options C and D reverse the direction: Na⁺ is already about 10 times more concentrated outside, so moving it out is what needs energy.

Trypsin is a protease from the pancreas (released as trypsinogen and activated in the small intestine); it breaks proteins into smaller polypeptides. Lipase acts on fats, and maltase and amylase act on carbohydrates, so none of them digests protein.

Monoclonal antibodies are a single type of antibody made by identical immune cells (clones). Each binds one specific antigen, so they give precise targeting and recognition. Their main advantage is therefore greater specificity and consistency.

A is the opposite of the truth: a monoclonal antibody targets only one type of antigen, not many. B and D (cost and shelf life) are not advantages given for mAbs in the book.

The superior and inferior venae cavae bring deoxygenated blood from the body into the right atrium. The pulmonary vein brings blood from the lungs into the left atrium, the pulmonary artery carries blood from the right ventricle to the lungs, and the aorta carries blood away from the left ventricle.

Glomerular filtration takes place in the renal (Bowman's) capsule: the high pressure in the glomerular capillaries forces water and small solutes into the Bowman's capsule, where they collect as filtrate. The loop of Henle and the collecting duct are tempting because water is reabsorbed there, but that is reabsorption, not filtration. The ureter only carries urine from the kidney to the bladder.

In active immunity the body makes its own antibodies, and this happens when the immune system meets an antigen, either in an infection (natural) or in a vaccine (artificial). So active immunity is developed by exposure to antigens.

A, B and D (plasma, blood transfusion, injected antibodies) all give ready-made antibodies from another person or animal. That is passive immunity: the body has not made the antibodies itself.

PCR makes thousands to millions of copies of a chosen DNA segment in vitro, so it is a DNA amplification technique. The amount of DNA doubles with every cycle.

D is the only correct choice. B is tempting because primers hybridise to the template in the annealing step, but that is one step, not the aim of the technique. C is wrong because PCR only makes the DNA more abundant, which can simplify sequencing, but it does not read the sequence. A is wrong because nothing is degraded.

A rise in ocean temperature stresses corals, so they expel the symbiotic algae (zooxanthellae) that live in their tissues and turn white. This is coral bleaching.

Warming does not reduce sunlight. Salinity and evaporation are not the effects the book links to a rise in ocean temperature; the book's effects are coral bleaching, shifting ecosystems, smaller fish, marine heatwaves, melting ice and low oxygen.

Synthetic biology is the design and construction of new biological parts, devices and systems (such as enzymes, genetic circuits and cells), or the redesign of existing ones.

Observing natural ecosystems, studying fossils and classifying organisms describe ecology, palaeontology and taxonomy. They do not build anything new.

Bracket iii covers exactly the thick (myosin) filaments, from one end to the other. The part of a sarcomere that is as long as the thick filaments is the A band. It is not the H zone, because the H zone is only the gap in the middle of the A band where thin filaments do not reach. It is not the I band, because brackets ii and iv cover the regions of thin filaments only, on either side of a Z line. It is not the sarcomere, because that is bracket i, from Z line to Z line.

A plasmid has an origin of replication (ori), so it replicates independently inside the host cell and makes many copies of the gene inserted into it. That is the property that makes it a vector.

A is wrong because plasmids are found mainly in bacteria, not only in viruses. B is wrong because a plasmid can carry foreign genes once they are joined to it. C is wrong because plasmids do not destroy viral DNA.

For grouped data the mean is \bar{x}=\dfrac{\Sigma fx}{\Sigma f}, where f is the frequency and x is the class mark. Each class mark is multiplied by its frequency, the products are added, and the sum is divided by the total frequency.

Option A divides by a single f instead of the total \Sigma f and uses \Sigma x without the frequencies, which is the formula for ungrouped data (\Sigma x/n) written wrongly. Options C and D are not formulas for the mean.

How emphysema is caused (1.5 marks)

Emphysema is a lung disease that results from damage to the walls of the alveoli. Smoking is the main cause. Other causes are marijuana, vaping and e-cigarettes, cigar smoke, air pollution and dust. The alveolar walls break down, so there are fewer and larger air spaces, and a blockage may develop that traps air inside the lungs.

Impact on the body (1.5 marks)

• With fewer alveoli the respiratory surface is smaller, so less oxygen moves into the bloodstream.
• The person has long-term cough, wheezing, shortness of breath (even on climbing steps), tightness in the chest and increased mucus.
• There is a constant feeling of not getting enough air, ongoing fatigue and heart problems.

Phase identified (1 mark)

The phase is inspiration (inhalation). In the figure the dotted outline of the chest wall lies in front of the solid outline, so the chest has moved up and out, and the diaphragm has flattened and moved down into the abdomen.

Explanation (2 marks)

1. The diaphragm contracts and flattens (moves down). The external intercostal muscles contract and the internal intercostals relax, so the rib cage moves up and forward.
2. Both movements increase the inner space of the thoracic cavity, so the pressure in the thorax and lungs falls below atmospheric pressure.
3. Air therefore enters the lungs and the alveoli are inflated. Inspiration is the active phase of breathing.

The diagram is a nephron: A is at the closed end of the tubule, B is on the first part of the tubule that leads away from it, and C is at the long loop that dips down.

a. Role of calcium ions (1.5 marks)

b. Change in the sarcomere (1.5 marks)

The sarcomere shortens: the thin filaments slide towards the centre, so the Z lines come closer, the I band becomes shorter and the H zone disappears. The A band, which is the length of the thick filaments, stays the same.

a. Difference (2 marks)

b. One application (1 mark)

Synthetic microbes: the human insulin gene inserted into E. coli makes insulin that is harvested and purified for medical use. (Bio-sensing devices, gene editing such as golden rice, and biological computers are also accepted.)

Median (1 mark)

Mean (2 marks)

Definition (1 mark)

Monoclonal antibodies (mAbs) are antibodies of a single type, produced by identical immune cells (clones) and engineered in laboratories as a form of medication. Each mAb binds one specific antigen. They were discovered by Köhler and Milstein in 1975.

Two advantages (2 marks)

• Specific targeting: each mAb recognises only one type of antigen (for example on a cancer cell or a virus), so the treatment is aimed precisely at the target.
• Uses the body's own defence: mAbs make use of the immune system to combat disease (immunotherapy), and the same specific mAbs, joined to radioactive or fluorescent labels, can also be used to detect and image the disease.

What is a sprain? (1 mark) A sprain is an injury to a ligament. The ligament is stretched too far from its normal position. The ligaments most often injured are those of the ankle, knee and wrist.

Symptoms (1 mark) The joint becomes painful and swollen and its movement is reduced.

Management (1 mark)

• Rest the injured joint.
• Treat it conservatively with icing and physical therapy.
• Use dressings, bandages or ace-wraps to immobilise the sprain and give support.

Synapse (1 mark)

A synapse is the junction between the axon terminal of one neuron (presynaptic neuron) and the dendrite of another neuron (postsynaptic neuron), where information is passed from one neuron to the next. The neurons do not touch: a gap, the synaptic cleft, lies between them. The axon ends in rounded synaptic knobs containing synaptic vesicles full of neurotransmitter. The impulse crosses the cleft as a chemical, so it travels in one direction only.

Labelled diagram (2 marks)

The skin gives a physical, a chemical and an immune barrier. Any three points (1 mark each):

Osteoporosis is a common disease in which bone mass is reduced and the risk of fracture is increased.

Causes (any points, 1 mark in all)

• Bone resorption (by osteoclasts) becomes greater than bone deposition (by osteoblasts).
• Bone mass reaches its peak at 20 to 25 years and falls after that in both sexes.
• A low bone mass before the onset, for example from too little calcium in the twenties and thirties, means there is less tolerance for later loss.

Symptoms (1 mark) Bones become weak and thin and break easily, so fractures are more likely.

Baroreceptors (high-pressure, 1.5 marks)

These mechanoreceptors in the walls of the aorta and the carotid sinus sense blood pressure and tell the brain. When pressure is high, parasympathetic nerves cause vasodilation and slow the heart, so pressure falls. When pressure is low, sympathetic impulses increase and cause body-wide vasoconstriction, so pressure rises. This negative feedback is the baroreflex.

Volume receptors (low-pressure, 1.5 marks)

These receptors in the atria (and carotid arteries) detect a decrease in blood volume and signal the hypothalamus. It increases the production of antidiuretic hormone (ADH), so the kidney retains water, blood volume rises and blood pressure rises.

Nerve impulse. A nerve impulse is information or a signal about a stimulus that is carried from receptors to the CNS and from the CNS to the effectors. In technical terms it is a wave of electrochemical changes (an action potential) travelling along the length of a neuron.

Generation in a neuron

The anterior lobe of the pituitary gland secretes six hormones:

1. Growth hormone (GH, somatotrophic hormone, STH)
2. Thyroid-stimulating hormone (TSH)
3. Adrenocorticotropic hormone (ACTH)
4. Follicle-stimulating hormone (FSH)
5. Luteinising hormone (LH; interstitial cell stimulating hormone, ICSH, in males)
6. Prolactin (luteotrophic hormone, LTH)

One example of each: a frog (ectotherm) and a human (endotherm).

Complement system (1 mark)

The complement system is a group of over fifty small proteins found in the blood. They are made by the liver and circulate in an inactive state. When a foreign particle enters the body they are activated one after another, each activated protein activating the next.

Two mechanisms (2 marks)

• Membrane attack complex (MAC): activated proteins form complexes that make holes in the cell wall and plasma membrane of the bacterium. Water and salts diffuse in, so the bacterium swells and bursts.
• Help to phagocytes and inflammation: complement proteins help phagocytes to clear the foreign particles, and they promote inflammation, which attracts more phagocytes to the site of infection.

Ocean acidification (a fall in the pH of seawater because dissolved \ce{CO2} forms carbonic acid and releases \ce{H+}) harms marine life in three main ways, one mark each.

• Calcifying organisms are weakened. Lower pH reduces the carbonate ions (\ce{CO3^2-}) needed to build calcium carbonate, so corals, oysters, clams and sea urchins make thin, weak shells and skeletons. Oyster larvae die in large numbers and coral reefs risk erosion and collapse.
• Food chains are disrupted. Plankton such as pteropods cannot form shells properly and decline. Fish, whales and seabirds that feed on them then have less food.
• Behaviour of marine animals changes. High \ce{CO2} impairs the ability of fish to smell, navigate and avoid predators, so they find less food and are less successful in finding mates and reproducing.

Plasmid (1 mark)

A plasmid is a small circular DNA molecule found naturally in bacterial cells. It lies outside the chromosome (extrachromosomal) and replicates independently.

Plasmid as a vector (2 marks)

A vector is a vehicle that carries foreign DNA (the gene of interest) into a host cell for multiplication and expression. A plasmid does this because the gene of interest is cut and joined into it with the same restriction enzyme and DNA ligase, and the recombinant plasmid is then put into a host bacterium. It is suitable because it has:

• an origin of replication (ori), so it copies itself and the inserted gene many times (it is often present in high copy number);
• an antibiotic resistance gene as a selectable marker, to find the bacteria that took up the plasmid;
• unique restriction sites where the foreign DNA can be inserted;
• a promoter, so that the inserted gene is expressed; and a small size, so it is easily purified and inserted into cells.

Selective reabsorption is the process by which certain useful substances that were filtered out of the blood at the glomerulus (glucose, amino acids, vitamins, inorganic salts and some water) are taken back into the blood. Its significance (three points):

1. Saves useful substances. About all of the glucose and amino acids filtered are returned to the blood, so they are not lost in urine. That is why normal urine contains no glucose.
2. Conserves water. Most of the filtered water is reabsorbed (about 99.5% overall), which prevents dehydration and keeps the blood volume steady.
3. Keeps the body fluids balanced. Because only certain substances are reabsorbed, the wastes and excess salts stay in the tubule and leave in urine, so the composition of the blood is kept normal.

1. Bile production: bile emulsifies fats in the small intestine and helps carry waste away.
2. Storage and control of glucose: excess glucose is converted to glycogen for storage and returned to the blood as glucose when needed.
3. Urea formation: poisonous ammonia is converted to urea, which is excreted in the urine.
4. Detoxification: it clears the blood of drugs and other poisonous substances.
5. Plasma proteins: it makes albumin, globulins and fibrinogen (and so regulates blood clotting).

Extinction is the complete disappearance of a species. Any two species with a short discussion earn the marks (1.5 each).

1. Golden toad (Incilius periglenes)

It lived in Costa Rica's Monteverde Cloud Forest. It is believed to have gone extinct in the late 1980s because climate change brought drought and shifts in temperature that made the forest less hospitable, and disease further decimated the population.

2. Bramble Cay melomys (Melomys rubicola)

A small rodent of Bramble Cay, a tiny island off the coast of Australia, and the first mammal considered to go extinct due to climate change. Rising sea levels and storm surges wiped out its habitat.

Homologous structures have a common ancestral origin; analogous structures only have a similar function. Any three points of difference earn the marks.

Drug addiction (1.5 marks)

• Drugs are chemicals that interfere with the way neurons send, receive and process information. Some activate neurons because their structure mimics a natural neurotransmitter (for example nicotine stimulates postsynaptic receptors like acetylcholine).
• Narcotics bind to painkilling sites in the brain. With constant use they build up and block the production of endorphins, the brain's natural painkillers, so the person depends on the drug.
• The parts of the brain affected are the brain stem, limbic system and cerebral cortex. Depressants such as alcohol slow down the CNS; stimulants increase heart rate, concentration and breathing rate. Stopping suddenly causes withdrawal symptoms (anxiety, irritability, depression, headache, hallucinations).

Drug tolerance (1.5 marks)

• Tolerance is a diminished response to a drug: with repeated use the body adapts to its continued presence.
• The nervous system responds less to the same dose, so higher doses are needed to get the same alertness or effect (for example caffeine and nicotine).
• The person keeps taking more of the drug, and the harmful effects on the body continue (for nicotine: cardiovascular disease, insomnia, headache), so tolerance and addiction go together.

When the hypothalamus detects a rise in blood temperature, the body loses heat by two mechanisms.

1. Vasodilation (1.5 marks). The muscles in the walls of the arterioles near the skin relax, so the arterioles dilate and more blood flows through the skin capillaries. Heat passes from this blood to the surroundings by radiation.

2. Sweating (1.5 marks). Sweat glands secrete a salty solution onto the skin. When the sweat evaporates it takes heat from the body, which cools the skin and the blood. It is less effective in humid air.

Antivirals (1.5 marks)

A virus multiplies only inside a host cell, by absorption, penetration, replication of its RNA or DNA and exit. Antivirals work by inhibiting the ability of the virus to multiply and spread in the body. The step that is blocked most easily is the replication of the viral nucleic acid inside the host cell.

Antiretrovirals (1.5 marks)

Retroviruses such as HIV use RNA as genetic material and need reverse transcriptase to replicate. Antiretrovirals are a subset of antivirals that inhibit the replication of retroviruses. Different classes of antiretrovirals act on different stages of the HIV life cycle. They control HIV infection and reduce the risk of progression to AIDS; they keep the virus down rather than cure the infection.

Negative feedback

In a negative feedback loop a change in a given direction causes a change in the opposite direction. If a level is too high, the body does something to bring it down; if it is too low, the body raises it. The response opposes the stimulus, so the controlled condition returns to its set point and homeostasis is kept. Negative feedback is the predominant mechanism of homeostasis.

The loop

Example: blood glucose

Body temperature is controlled in the same way: the hypothalamus detects a rise or a fall in blood temperature and the effectors (blood vessels, sweat glands, muscles) bring it back to 37 degrees Celsius.

Plants depend on suitable temperature, rainfall and soil quality. Climate change alters these conditions. Any three impacts, one mark each.

Animals rely on stable ecosystems for food, shelter and mates, and climate change disrupts these. Any four points, one mark each.

Location (3 marks)

• The pancreas is a gland lying in the upper abdomen, behind the stomach, in the curve of the duodenum.
• It has two kinds of tissue: exocrine tissue (acinar cells) that makes digestive juice, and endocrine tissue, patches called the islets of Langerhans scattered through the gland.
• The islets have no duct; they release their hormones directly into the blood, so the pancreas is both an exocrine and an endocrine gland.

Function as an endocrine gland (3 marks)

Recombinant DNA technology joins the gene of interest to a vector, puts the recombinant DNA into a host and grows the host to make the product. Human insulin (two chains, A of 21 and B of 30 amino acids, joined by disulfide bonds) is made in Escherichia coli as follows.

Steps

Label: esophagus, cardia, fundus, body, lesser curvature, greater curvature, pyloric region (antrum, canal and sphincter) and the duodenum.

• Cardia: around the opening from the esophagus.
• Fundus: rounded part above and to the left of the cardia.
• Body: large central part where food is churned.
• Pylorus: lower part leading to the duodenum; the pyloric sphincter controls the passage of chyme.

Origin and receptors (1 mark)

B cells are differentiated in the bone marrow. Each carries specific B cell receptors (BCRs) on its membrane, so it can bind one particular antigen. There are millions of B cell types in the body, each specific for one antigen.

Activation and differentiation (2 marks)

Result (1 mark)

• Plasma cells secrete antibodies in bulk into the blood (more than 10 million molecules per hour) but live only a few days. The antibodies neutralise viruses and toxins, promote phagocytosis (opsonisation), activate complement and take part in ADCC.
• Memory B cells are long-lived. If the same antigen enters again they divide into more plasma cells and memory cells, giving a faster and stronger secondary response.

This stimulation of B cells and the secretion of antibodies by the plasma cells is the humoral (antibody-mediated) immune response.

Chemical digestion in the stomach is carried out by the gastric juice (about 2 to 3 litres a day), which contains mucus, hydrochloric acid, intrinsic factor, pepsinogen and the hormone gastrin.

Activation of pepsin

Protein digestion is the main chemical digestion here. Salivary amylase is stopped by the acid, so carbohydrate digestion pauses until the small intestine; the stomach digests very little fat. The churned food becomes the thick liquid chyme, which passes through the pyloric sphincter in small amounts.

The form of nitrogenous waste depends on how much water the animal can spare. Ammonia needs the most water, urea less, and uric acid the least.

Ammonotelic animals (2 marks)

• Excrete ammonia, which is highly toxic (it raises the pH of body fluids and interferes with membrane transport) but very soluble and diffuses rapidly across cell membranes.
• One gram of nitrogen needs about 500 mL of water to dilute it, so it suits aquatic animals, especially in fresh water: most fishes, protozoans, sponges, cnidaria, echinoderms and crustaceans.

Ureotelic animals (2 marks)

• Convert ammonia into urea at a cost of energy. Urea is about 100,000 times less toxic than ammonia and needs only about 50 mL of water per gram of nitrogen.
• They are some marine animals and all terrestrial ones with less fresh water: mammals including man, many amphibians and some marine animals such as sharks.

Uricotelic animals (2 marks)

• Excrete uric acid, the least toxic. It can precipitate from solution, carries four nitrogen atoms per molecule and needs only about 1 mL of water per gram of nitrogen.
• They live where water is very short: terrestrial invertebrates (land snail, many insects) and egg-laying vertebrates (reptiles such as snakes and lizards, and birds).

About 70% of the carbon dioxide is carried as bicarbonate ions.

1. Carbon dioxide diffuses from the tissues into the blood and enters the red blood cells.
2. Inside the red cell it combines with water to form carbonic acid, speeded up by the enzyme carbonic anhydrase.
3. Carbonic acid is unstable and dissociates into hydrogen ions and bicarbonate ions. Haemoglobin buffers the hydrogen ions (forming haemoglobinic acid) so the pH stays near 7.4, and oxygen is released to the tissue.
4. Bicarbonate ions diffuse out into the plasma while chloride ions move into the red cell (chloride shift).
5. In the lungs the process is reversed: bicarbonate and hydrogen ions form carbon dioxide and water, and the carbon dioxide diffuses into the alveoli.

About 23% of the carbon dioxide is carried by haemoglobin. The question calls it carboxyhaemoglobin as the book does in one place; the compound formed with carbon dioxide is properly called carbaminohaemoglobin.

• Carbon dioxide combines with the globin (protein) part of haemoglobin in the red blood cells.
• In the tissues the partial pressure of \ce{CO2} is higher than in the blood, so carbaminohaemoglobin forms.
• In the lungs the partial pressure of \ce{CO2} is higher in the blood than in the alveoli, so carbaminohaemoglobin releases its carbon dioxide.

Only about 7% of the carbon dioxide is carried dissolved in the blood plasma. It dissolves directly in the plasma as carbon dioxide. This is an inefficient way to carry carbon dioxide, but it does occur.