Life Processes

Photosynthesis requires carbon dioxide as a raw material for producing carbohydrates. Stomata provide the main pathway by which atmospheric carbon dioxide enters the leaf. If they remain closed, carbon-dioxide diffusion into the leaf decreases; after the available internal supply is used, photosynthetic rate and food formation may fall. Therefore A is correct. Oxygen does not become food, xylem transports water, and villi belong to the intestine.

Roots absorb water from the soil, mainly through root hairs. The xylem is the plant vascular tissue specialized for conducting water and dissolved mineral ions from roots toward stems and leaves. Cohesion, adhesion, root pressure, and especially transpiration pull help support this upward movement. Therefore A is correct. Phloem transports organic food, whereas alveoli and nephrons are structures of animal respiratory and excretory systems.

Leaves are the main sites of photosynthesis, so they produce sugars such as sucrose. These dissolved organic substances must be distributed to roots, stems, growing buds, fruits, and storage organs. Phloem sieve tubes and associated cells transport this food from source tissues to sink tissues, and movement can occur in different directions according to demand. Hence A is correct. Xylem carries water, stomata exchange gases, and urine formation is an animal function.

The governing concept is holozoic nutrition in a unicellular organism. Amoeba has no specialised mouth, so its flexible cell membrane extends outward to form temporary projections called pseudopodia. These projections surround and engulf a food particle, forming a food vacuole where digestion occurs. Therefore, option A is correct; stomata belong to plants, villi line the small intestine, and alveoli are lung structures, so the other options are unrelated.

The governing concept is the alimentary canal, a continuous digestive tube open at both ends. Food enters through the mouth, then moves by swallowing and peristaltic contractions through the oesophagus, stomach, small intestine and large intestine. Undigested material finally leaves through the anus. Thus option A correctly describes its tube-like continuity. Blood is carried by vessels, air by the respiratory tract, and urine is formed in the kidneys, so B, C and D are incorrect.

The governing concept is mechanical digestion and the beginning of chemical digestion. Teeth break food into smaller pieces, increasing the surface area on which digestive enzymes can act. The tongue mixes these pieces with saliva, and salivary amylase begins starch digestion. Therefore option A is correct. Chewing does not convert food directly into urine or blood, and it is not a process of removing oxygen from food, so B, C and D are unsuitable.

The governing concept is enzyme-assisted carbohydrate digestion. Salivary amylase acts on starch, a complex carbohydrate, and begins breaking it into simpler sugars such as maltose while food is still in the mouth. This is why option A is correct and why carbohydrate digestion starts before food reaches the stomach. Amylase does not convert fat into bile, produce oxygen from protein, or filter blood; those claims describe unrelated processes.

The governing concept is the protective and digestive role of gastric hydrochloric acid. Food may carry microbes into the stomach, and the strongly acidic medium destroys or inhibits many of them. The acid also provides the conditions needed for pepsin to digest proteins. Thus option A is correct. Oxygen binding is performed by haemoglobin, light capture by chlorophyll, and urine storage by the urinary bladder, not by stomach acid.

The governing concept is protection of the stomach lining during digestion. Gastric glands secrete mucus, which forms a coating over the inner stomach wall. This layer acts as a barrier and reduces direct contact between hydrochloric acid, pepsin and the living epithelial cells. Therefore option A is correct. Chlorophyll is a plant pigment, haemoglobin transports oxygen in blood, and bile salts assist fat digestion in the small intestine; none protects the stomach wall from acid.

The governing concept is emulsification of fats. Fat does not mix readily with the watery digestive contents, so bile salts divide large fat globules into many tiny droplets. This greatly increases the total surface area available to lipase, allowing the enzyme to digest fat more effectively. Therefore option A is correct. Bile does not change protein into acid, convert glucose into oxygen, or transport blood; those are biologically incorrect functions.

The governing concept is completion of digestion followed by absorption in the small intestine. It receives bile, pancreatic juice and intestinal juice, which help break carbohydrates, proteins and fats into simpler, absorbable substances. Its inner lining contains numerous villi with thin walls and blood capillaries, so digested nutrients can pass efficiently into the circulation. Thus option A is correct; water storage, lung circulation and urine formation are not its primary functions.

The governing concept is adaptation of structure for efficient absorption. Villi are numerous finger-like projections on the inner lining of the small intestine. Together they greatly increase the available surface area, allowing more digested nutrients to contact the absorptive lining at the same time. Their thin epithelium and nearby blood capillaries further support transfer into the blood. Therefore option A is correct; villi do not make acid, pump blood, or fill lungs with air.

The governing concept is water reabsorption in the large intestine. As undigested material passes through it, the large intestine normally removes part of the remaining water and helps form relatively solid faeces. If this absorption is greatly reduced, excess water stays in the faecal matter, making it loose or watery; prolonged loss can contribute to dehydration. Hence option A is correct. The other options concern oxygen production, heart anatomy and plant stomata, not intestinal water balance.

The governing concept is surface area and diffusion. Each alveolus has a thin moist wall, and millions of alveoli together create a very large exchange surface. Oxygen can therefore diffuse efficiently into surrounding capillaries, while carbon dioxide diffuses out. Option A is correct; digestion, urine storage and transport to leaves are not functions of lung alveoli.

The governing concept is rapid diffusion across a thin respiratory surface. Alveoli contain air, while the surrounding capillaries carry deoxygenated and oxygenated blood. Their dense network keeps blood close to a large alveolar surface, maintaining a short diffusion distance and concentration gradients. Thus option A is correct; the other options refer to digestion, plant transport or excretion.

The governing concept is haemoglobin-mediated oxygen transport. Haemoglobin in red blood cells binds oxygen in the lungs and carries it to tissues. If haemoglobin is low, the blood may deliver less oxygen, so aerobic cellular respiration can produce less energy for normal activities; this may cause tiredness. Option A is correct, while B, C and D are unrelated.

The governing concept is complete double circulation. The right side of the four-chambered heart receives deoxygenated blood and sends it to the lungs, while the left side receives oxygenated blood and pumps it to the body. Septa prevent major mixing, supporting efficient oxygen delivery. Option A is correct; B, C and D assign digestive or excretory functions incorrectly.

The governing concept is pressure-resistant blood transport. The ventricles pump blood into arteries with strong pressure, so arterial walls need thick muscular layers to withstand that force. Their elasticity allows them to expand during a heartbeat and recoil between beats, helping maintain smoother flow. Option A is correct; B, C and D describe functions of other systems or organisms.

The governing concept is one-way movement of blood under relatively low pressure. Veins return blood toward the heart, and skeletal-muscle movement can help push that blood forward. Because venous pressure is low, blood could fall backward, especially in limbs; valves close to stop reverse flow. Therefore A is correct, while B, C and D describe unrelated processes.

The governing concept is efficient exchange between blood and body cells. Capillaries have extremely thin walls, generally only one cell thick, so oxygen and dissolved nutrients can diffuse from the blood into tissues, while carbon dioxide and other wastes can diffuse back into the blood. Their extensive network also provides a large exchange surface. Therefore option A is correct; thick walls would hinder diffusion, and the other choices describe unrelated functions.