The epiglottis is a flap-like cartilaginous structure that closes over the opening of the trachea (glottis) during swallowing, preventing food from entering the airway.

The kidneys help regulate blood pH by excreting hydrogen ions (H⁺) and reabsorbing bicarbonate ions (HCO₃⁻), maintaining the acid-base balance of the blood.

Villi are finger-like projections found in the small intestine that increase the surface area for efficient absorption of digested nutrients.

Oxygen moves from the alveoli (higher concentration) into the blood (lower concentration) by diffusion. This concentration gradient is essential for efficient oxygen uptake.

Layer X is the palisade mesophyll, which consists of elongated, column-shaped cells packed with chloroplasts to maximise light absorption for photosynthesis. Cell type C shows this typical elongated shape with many chloroplasts.

Inspired air (P) has a higher oxygen concentration (21%) and lower carbon dioxide concentration (0,04%). Expired air (Q) has less oxygen (16%) and more carbon dioxide (4,0%) because oxygen is used and carbon dioxide is produced during cellular respiration.

Tubular secretion is the process where substances such as hormones, urea, creatine, and certain drugs are actively transported from the blood into the renal tubule for excretion in urine.

Urine is formed in the kidneys, passes through the ureters to the urinary bladder for temporary storage, and is then expelled through the urethra.

When a person drinks a lot of water, the blood becomes dilute (low solute concentration). Osmoreceptors detect this, and less ADH is released. The renal tubules become less permeable to water (or impermeable), so less water is reabsorbed. This results in a large volume of dilute urine with low urea concentration.

The cilia beat in a coordinated manner to move mucus, along with trapped dust, dirt, and pathogens, upwards towards the pharynx where it can be swallowed or coughed out. This helps keep the airways clean.

Breathing (ventilation) is the mechanical process of moving air into (inhalation) and out of (exhalation) the lungs.

Aldosterone is secreted by the adrenal cortex and increases the reabsorption of sodium ions (Na⁺) from the renal tubules back into the blood, while promoting the excretion of potassium ions (K⁺).

Pre-molars and molars have broad, flat surfaces with ridges that are adapted for grinding and crushing food, increasing the surface area for chemical digestion.

The pharynx (throat) is the common passageway for both air (from the nasal cavity to the larynx) and food (from the mouth to the oesophagus).

Secretion is the process by which cells or glands release substances such as enzymes, hormones, or saliva for use elsewhere in the body.

Chyme is the acidic, semi-fluid mixture of partially digested food and gastric juices that leaves the stomach and enters the small intestine.

The distal convoluted tubule is the coiled section of the nephron that follows the loop of Henle and leads into the collecting duct. It is involved in the regulated reabsorption of ions and water.

Osmosis is the passive movement of water molecules through a selectively permeable membrane from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration).

Chlorophyll is located in the thylakoid membranes (A) which are stacked into grana, and also in the lamellae (B) that connect the grana. Both are part of the chloroplast's internal membrane system.

The surface of a villus is made up of simple columnar epithelium, which includes goblet cells (B) that secrete mucus. "Compound columnar" (A) is not a correct description of the villus surface.

Neither circular muscle nor the epiglottis keeps the trachea open. The trachea is kept open by C-shaped rings of cartilage. Circular muscle would constrict the trachea, and the epiglottis is a flap that closes the airway during swallowing.

Convoluted tubules (A) are part of the nephron, which is the functional unit of the kidney, not the blood circulatory system. Renal calyces (B) are part of the urine-collecting system, not the blood circulatory system. The blood circulatory system of the kidney includes the renal artery, renal vein, and the network of capillaries (glomerulus and peritubular capillaries).

The renal calyx is the cup-shaped structure that surrounds the renal papilla and collects urine as it drains from the collecting ducts.

The renal capsule is the outer fibrous layer that surrounds the kidney and contains adipose tissue, which cushions and protects the kidney from mechanical shock.

The renal pelvis is the funnel-shaped cavity that collects urine from the major calyces and channels it into the ureter.

The renal medulla (D) contains the vasa recta and other blood vessels that give it a darker, reddish-brown appearance. The renal cortex (A) has fewer blood vessels and more nephron tubules, making it appear lighter in colour.

Homeostasis ensures that internal conditions such as temperature, pH, water balance, and blood glucose concentration remain within narrow limits suitable for optimal enzyme activity and cell function.

Chemoreceptors in the carotid bodies (and aortic bodies) detect changes in blood CO₂ concentration and pH, sending signals to the respiratory centre in the medulla oblongata.

The intercostal muscles and diaphragm are the effectors that increase the rate and depth of breathing to expel excess CO₂. The heart may also increase its rate to transport blood gases more rapidly.

High CO₂ levels lower blood pH, which can denature enzymes and disrupt metabolic processes. Reduced aerobic respiration means less ATP is produced, impairing cellular functions and potentially leading to cell death.

The organelle shown has thylakoid membranes stacked into grana and a stroma, which are characteristic features of a chloroplast, the site of photosynthesis.

These internal membrane structures (thylakoids and grana) and the fluid stroma are unique to chloroplasts and are not found together in other organelles. Starch granules are also visible as stored carbohydrate products of photosynthesis.

The light-dependent phase (B) occurs in the thylakoid membranes and requires light to split water (photolysis), producing ATP, NADPH, and oxygen. The light-independent phase (A) occurs in the stroma, uses CO₂ and the products of the light phase to synthesise glucose through the Calvin cycle.

Photosynthesis is essential for life on Earth because it converts light energy into chemical energy (glucose), forms the base of nearly all food chains, and releases oxygen as a by-product, which is needed for aerobic respiration.

The extensive internal membrane system (thylakoids and grana) provides a large surface area for the light-dependent reactions of photosynthesis, while the stroma contains enzymes for the light-independent reactions.

Greenhouses provide economic and social benefits by creating employment, supporting local food production, and reducing dependence on imported vegetables.

By using renewable energy sources such as solar and geothermal heat, greenhouses minimise reliance on fossil-fuel-generated electricity, lowering operational costs and environmental impact.

Heaters actively add heat, while insulation reduces heat loss by conduction and convection, helping maintain the optimal temperature for plant growth.

The temperature difference is 53°C. Expressed as a percentage of the original outside temperature (25°C), this is a 212% increase.

Light intensity is a limiting factor for photosynthesis. Initially, more light provides more energy for the light-dependent reactions, increasing the rate. However, once all chlorophyll and enzymes are saturated, further increases in light intensity have no effect.

At high light intensities, the rate of photosynthesis plateaus because the amount of chlorophyll and the number of enzymes involved in the light-independent reactions become limiting factors.

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm, where glucose is broken down into pyruvic acid.

Oxygen is the final electron acceptor in the electron transport chain, allowing the process to continue and producing water.

Carbon dioxide is released during the Krebs cycle as a waste product of cellular respiration.

Catabolic reactions involve the breakdown of larger molecules into smaller ones, releasing energy. Glycolysis breaks glucose down into pyruvic acid, so it is catabolic.

Process B is oxidative phosphorylation (electron transport chain), which produces the majority of ATP during aerobic respiration. Glycolysis (A) produces only a net gain of 2 ATP per glucose molecule.

The Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix. It requires oxygen indirectly (as the final electron acceptor). Acetyl-CoA enters the cycle, and through a series of reactions, CO₂ is released, and hydrogen atoms are transferred to NAD⁺ and FAD to form NADH and FADH₂, which carry electrons to the electron transport chain.

During intense exercise, oxygen supply to muscles cannot meet demand. Muscles switch to anaerobic respiration, where pyruvic acid is converted to lactic acid. The accumulation of lactic acid lowers muscle pH, causing fatigue and cramps.

The dependent variable is what is being measured in the experiment — here, the concentration of carbon dioxide recorded by the data logger.

A control is a standard against which the experimental results are compared. It ensures that any changes observed are due to the independent variable (presence of organisms/light) and not other factors.

In the light, both photosynthesis and respiration occur. The CO₂ produced by the snail and pondweed during respiration is used up by the pondweed for photosynthesis, keeping the CO₂ concentration constant.

After day 5, the tube was covered with black paper, blocking light. Photosynthesis stopped, but respiration continued, releasing CO₂ and causing its concentration to increase.

Without light, the pondweed could not produce oxygen. The snail and pondweed continued to respire, using up the remaining oxygen and releasing CO₂, which formed carbonic acid, lowering the pH and making the water uninhabitable.

The investigation shows that in the presence of light, photosynthesis and respiration balance each other, keeping CO₂ levels stable. In darkness, photosynthesis ceases, and respiration causes CO₂ to accumulate.

The oesophagus is the muscular tube that connects the pharynx to the stomach, propelling food by peristalsis.

The hepatic portal vein carries nutrient-rich blood from the small intestine to the liver for processing and storage.

Structure E is the small intestine. Its adaptations (length, folds, villi, rich blood supply, muscle layers, mucus) all serve to maximise the surface area and efficiency of digestion and absorption of nutrients.

Structure B is the stomach. Physical digestion mechanically breaks food into smaller pieces and mixes it with gastric juices. Chemical digestion uses enzymes (e.g., pepsin) and acid to chemically break down nutrients into absorbable forms.

Structure F is the gall bladder. Even if the gall bladder is removed, the liver continues to produce bile, which flows directly into the duodenum to emulsify fats, allowing their digestion and absorption to continue.

The diagram shows the breakdown of proteins into peptides and amino acids, which is digestion. Proteases are the enzymes that catalyse this hydrolysis.

Pepsin is a protease produced in the stomach as pepsinogen and activated by hydrochloric acid. Other proteases are produced by the pancreas and small intestine.

Assimilation is the phase where absorbed nutrients (amino acids) are used to build new proteins and other substances needed by the body's cells.

Egression (egestion) is the elimination of undigested waste. The colon absorbs water and salts, the rectum stores faeces, and the anus expels them. This prevents toxic build-up and maintains homeostasis.

Mastication (chewing) physically breaks food into smaller particles, increasing the surface area for enzyme action and lubricating the food with saliva to form a bolus for swallowing.

Salivary amylase begins the chemical digestion of starch in the mouth, breaking it down into maltose, a disaccharide that has a sweet taste.

Diffusion is the passive movement of gases (O₂ and CO₂) from an area of higher concentration to an area of lower concentration across the alveolar membrane.

The alveolus has a very thin, moist surface and is surrounded by a dense network of capillaries. These features minimise the diffusion distance and maintain a steep concentration gradient for efficient gas exchange.

Most oxygen (about 98%) is transported bound to haemoglobin as oxyhaemoglobin. A small amount dissolves directly in the blood plasma.

A bullet wound to the chest can cause a pneumothorax (collapsed lung) by allowing air into the pleural cavity. This disrupts the negative pressure needed for lung inflation, causing the lung to collapse and severely impairing ventilation.

The renal cortex contains the glomerulus (A), Bowman's capsule (B), and the proximal and distal convoluted tubules (C). The loop of Henle (D) and collecting duct (E) extend into the medulla.

Network F is the peritubular capillary network. Tubular reabsorption returns useful substances (glucose, amino acids, water, ions) to the blood, while tubular secretion removes waste products (urea, creatine, drugs) from the blood into the tubule for excretion.

The ascending limb of the loop of Henle is responsible for the active reabsorption of sodium ions (Na⁺) without water, creating the hypertonic medullary gradient necessary for water reabsorption in the collecting duct.

Without proper sodium reabsorption in the ascending limb, the medullary osmotic gradient cannot be maintained. This reduces the driving force for water reabsorption from the collecting duct, leading to excessive water loss in urine (polyuria) and dehydration.