Life Processes

Salivary amylase is an enzyme secreted by the salivary glands and mixed with food in the mouth. Its substrate is starch, a complex carbohydrate. The enzyme begins hydrolysing starch into simpler sugars such as maltose, although digestion continues elsewhere in the alimentary canal. Therefore option A is correct; fats require lipases, proteins require proteases, and minerals do not need enzymatic digestion.

Hydrochloric acid secreted in the stomach creates a strongly acidic medium and also helps activate pepsinogen into pepsin. Pepsin functions effectively in this environment and begins the digestion of proteins into smaller peptide fragments. Salivary amylase is inactivated by the acid, while maltase and lactase act mainly in the small intestine. Hence option A is the only correct answer.

Bile is produced by the liver and stored in the gall bladder. It contains no digestive enzyme, but its bile salts emulsify large fat globules into many tiny droplets. This greatly increases the surface area on which lipase can act, so fat digestion becomes more efficient. Therefore, option A is correct; the other choices describe enzyme-driven digestion or an impossible transformation.

Villi are numerous finger-like projections lining the inner wall of the small intestine. They greatly increase the available surface area and contain a network of blood capillaries and a lacteal. Consequently, digested nutrients can pass efficiently into the blood or lymph. Option A is correct; teeth chew food, the trachea carries air, and guard cells regulate stomatal openings in leaves.

Villi increase the inner surface area of the small intestine and bring absorbed nutrients close to blood capillaries and lymph vessels. If their number or size is reduced, the available area for absorption falls, so fewer digested nutrients enter the circulation. Option A is therefore correct. Chewing occurs in the mouth, air entry belongs to respiration, and clotting is a blood-protection process.

Peristalsis is the rhythmic, wave-like contraction and relaxation of muscles in the wall of the oesophagus. These involuntary waves push the swallowed food bolus downward, even when a person is not upright, until it reaches the stomach. Thus option A is correct. The oesophagus does not add light energy, filter food into blood, or direct food toward the lungs; those suggestions confuse separate body functions.

In aerobic respiration, glucose is broken down in the presence of oxygen through a series of reactions, ultimately producing carbon dioxide and water. Oxygen acts as the final acceptor in the energy-releasing pathway, allowing glucose to be oxidised completely and releasing much more energy than anaerobic respiration. Hence A is correct; light, chlorophyll and closed stomata are not requirements for this process in animal cells.

During strenuous exercise, the oxygen supply to working muscles may become insufficient for aerobic respiration. Muscle cells then obtain some energy by anaerobic breakdown of glucose, producing lactic acid. Its temporary accumulation is associated with fatigue, pain or cramps. Therefore, option A is correct. Glucose is the starting fuel, starch is a storage carbohydrate, and bile is a digestive secretion, so none of those is the substance described.

Yeast can release energy without oxygen by anaerobically breaking down glucose. In this fermentation pathway, one glucose molecule is converted into ethanol and carbon dioxide, with a small amount of energy released. Thus option A is correct. Lactic acid is the typical product in anaerobic muscle respiration, oxygen is a reactant in aerobic respiration, and starch, bile and protein are not the products of yeast fermentation.

Alveoli are numerous tiny air sacs at the ends of bronchioles. Their walls are only one cell thick, they provide a very large combined surface area, and they are surrounded by dense blood capillaries. These features shorten the diffusion distance and maintain efficient exchange: oxygen enters the blood while carbon dioxide leaves it. Therefore A is correct; digestion, urine formation and bile production occur in other organs.

In double circulation, blood passes through the heart twice during one complete journey: once between the heart and lungs, and once between the heart and the rest of the body. The four-chambered human heart keeps oxygenated and deoxygenated blood largely separate and pumps oxygen-rich blood efficiently to tissues. Therefore A is correct; the other options contradict digestion, blood composition or gas exchange.

The governing concept is pressure in the circulatory system. Arteries carry blood away from the ventricles, where it is pumped at relatively high pressure. Their thick, elastic and muscular walls withstand this pressure and help maintain blood flow. Veins return blood at much lower pressure and therefore have thinner walls. Options B, C and D describe functions that blood vessels do not perform. Thus, option A is correct.

The governing concept is one-way blood flow. Veins carry blood toward the heart, but the pressure inside them is relatively low, especially in vessels returning blood from the lower parts of the body. Valves open when blood moves toward the heart and close if it tends to reverse direction. Therefore, option A is correct. The other options are unrelated because valves neither change blood colour, make food, nor produce oxygen.

The governing concept is exchange between blood and body tissues. Capillaries have extremely narrow diameters and walls only about one cell thick, so the diffusion distance is very small. Oxygen and nutrients can move from blood to cells, while carbon dioxide and other wastes move into the blood. Thus option A is correct. Colour change, chewing and bone hardening are not functions of capillary walls.

The governing concept is the light-dependent requirement of photosynthesis. Chlorophyll absorbs light energy, which powers the reactions that produce energy-rich molecules and ultimately helps convert carbon dioxide and water into glucose and oxygen. At night, sunlight is absent, so these light-driven reactions usually stop in most plants. Water and xylem do not suddenly disappear, and chlorophyll is not always destroyed. Thus option A is correct.

The governing concept is stomatal regulation. Guard cells surround each stoma and change their turgidity when water enters or leaves them. When they become turgid, the pore generally opens; when they lose water and become flaccid, it closes. This regulates carbon dioxide entry and limits water loss through transpiration. Therefore option A is correct, while root length, flower colour and seed size are not directly controlled by guard-cell movement.

The governing concept is photolysis during the light-dependent reactions of photosynthesis. Light energy absorbed by chlorophyll splits water molecules into components; the released electrons and hydrogen ions support the reactions, while oxygen is liberated as a by-product and diffuses out of the leaf. The oxygen does not come from glucose or starch, and bile is unrelated to plants. Therefore option A is correct.

The governing concept is diffusion across a respiratory surface. In the lungs, the moist lining of the alveoli allows oxygen and carbon dioxide to dissolve before crossing the very thin alveolar-capillary membrane. This supports rapid diffusion down concentration gradients: oxygen enters the blood and carbon dioxide leaves it. A moist surface does not sweeten food, strengthen bones or clot blood. Therefore option A is correct.

The governing concept is efficient gaseous exchange by diffusion. Alveoli provide a large moist surface, and the surrounding capillary network keeps blood in close contact with the air across a very thin barrier. Oxygen therefore diffuses into the blood, while carbon dioxide diffuses from the blood into the alveolar air for exhalation. Close capillaries do not digest food, produce urine or make light. Hence option A is correct.

The governing concept is that mitochondria are the main sites of aerobic cellular respiration, where food molecules are broken down to release usable energy in the form of ATP. A cell with high energy demands, such as a muscle cell, generally needs more mitochondria to produce sufficient ATP. Vacuoles mainly store materials, the cell wall provides support, and the nuclear membrane encloses the nucleus; none is the primary site of energy release.