Life Processes

The dark period is used to destarch the leaf: stored starch is consumed in respiration. During the subsequent light period, photosynthesis still requires carbon dioxide as a raw material. Since carbon dioxide is absent, little or no new carbohydrate is produced, so iodine will not show the usual blue-black starch colour, or it will show it only weakly. Therefore option B is correct; the other statements contradict photosynthesis and plant physiology.

Water is a raw material in photosynthesis, not merely a transport medium. In the light-dependent reactions, water molecules undergo photolysis, producing electrons and hydrogen ions needed for energy conversion; oxygen is released as a by-product. Therefore option A is the major result associated with water use in photosynthesis. Urea, bile, and haemoglobin are products or substances of animal physiology and are not formed as the stated result of this plant process.

Saliva has two relevant functions in the mouth. Its water and mucus moisten, soften, and lubricate food, helping it form a bolus that can be swallowed. Saliva also contains salivary amylase, which begins the digestion of starch into simpler sugars. A reduction in secretion therefore makes swallowing harder and reduces initial starch digestion. Thus option A is correct; the other options misidentify saliva's role or describe impossible effects.

Hydrochloric acid creates the strongly acidic environment needed for the activation and effective action of pepsin, the stomach enzyme that begins protein digestion. However, acid could damage living stomach cells if it directly contacted the lining. A mucus layer, supported by protective secretions, coats the wall and helps prevent chemical injury. Therefore option A correctly explains the complementary roles; the other options confuse digestive, circulatory, plant, or impossible functions.

Stomach hydrochloric acid provides the low-pH environment required for pepsin to become active and digest proteins. If acid is absent, pepsin activation and its activity are greatly reduced, so protein digestion in the stomach becomes incomplete or weak. Digestion may continue later through other enzymes in the small intestine, but the stomach stage is impaired. Hence option A is correct; the remaining choices describe unrelated or biologically impossible events.

Bile is important even though it contains no digestive enzyme. 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. Bile does not convert fat into oxygen; starch and urine are unrelated to this function, and it cannot change blood colour.

The liver produces bile, and bile salts help emulsify fats in the small intestine. Emulsification does not chemically digest fat, but it divides large droplets into smaller ones so lipase can work over a larger surface. Without bile, fat digestion and subsequent absorption are most affected. Thus option B is correct; vitamins and minerals generally do not require enzymatic digestion, while starch uses amylase.

Villi are finger-like projections that greatly enlarge the inner surface of the small intestine. Digested nutrients pass through their thin lining, and the network of blood capillaries carries absorbed sugars, amino acids, minerals, and other soluble substances away. Therefore option A correctly combines absorption with transport. Urine formation, lung gas entry, and stomatal movement occur in different organs or structures.

The wave-like muscular movement is called peristalsis. Alternating contraction and relaxation of circular and longitudinal muscles in the alimentary canal produces a pressure wave that pushes food forward, even when a person changes posture. This maintains one-way movement from the oesophagus through the intestine. Therefore option A is correct; peristalsis does not make blood, produce bile, or open alveoli.

The large intestine absorbs a substantial amount of water and electrolytes from the undigested residue. This process helps convert the residue into more solid faeces. If water absorption is insufficient, excess water remains in the intestinal contents, so the faeces stay loose or watery and may lead to diarrhoea. Thus option A is correct; the other choices describe unrelated processes or plant transport.

During intense running, the oxygen supply may not meet the muscles’ immediate energy demand. Muscle cells then partially break down glucose anaerobically, producing a small amount of ATP and lactic acid rather than completing oxidation to carbon dioxide and water. Lactic acid accumulation is associated with fatigue or cramps. Therefore option A is correct; photosynthesis, bile, and nephrons do not provide this pathway.

Alveoli are adapted for rapid diffusion because they have very thin walls, a moist surface, and a rich capillary supply. Thickening the alveolar wall increases the distance that oxygen and carbon dioxide must cross, so diffusion becomes slower and less efficient. Consequently, less oxygen may enter the blood in a given time. Therefore option A is correct; the other effects do not follow from wall thickening.

The governing concept is efficient gaseous exchange in the lungs. Alveoli have very thin walls and are surrounded by a dense network of capillaries. Oxygen diffuses from the alveolar air into the blood, while carbon dioxide diffuses from the blood into the alveoli to be exhaled. Their close contact shortens the diffusion distance, so option A is correct. The other options describe functions of digestion or excretion, not alveolar capillaries.

The governing concept is the role of haemoglobin in transport and aerobic respiration. Haemoglobin in red blood cells binds oxygen in the lungs and carries it to body tissues. If its amount is low, less oxygen reaches cells, so aerobic respiration may produce less usable energy for muscles. This explains tiredness and makes option A correct. The other choices incorrectly assign digestive, urinary or plant functions to human organs.

The governing concept is complete separation of oxygenated and deoxygenated blood in double circulation. The right side of the human heart sends deoxygenated blood to the lungs, while the left side sends oxygen-rich blood to the body. The septum and four chambers prevent significant mixing, allowing tissues to receive blood with a high oxygen concentration for efficient respiration and energy release. Thus option A is correct; the other choices are not heart functions.

The governing concept is adaptation of blood-vessel structure to pressure. Arteries receive blood directly from the ventricles, so blood flows through them at relatively high and pulsating pressure; their walls are therefore thick, muscular and elastic. Veins return blood at lower pressure and have thinner walls, wider lumens and valves to prevent backflow. Hence option A gives the main reason. Blood colour is not a permanent artery–vein distinction, and the other options are irrelevant.

The governing concept is one-way movement of blood in veins. Venous blood returns to the heart at relatively low pressure, and body movements help push it upward, especially from the legs. Valves open toward the heart and close when blood tends to move backward. Without these valves, gravity and low pressure could permit backflow and pooling of blood. Therefore option A is correct; the other choices involve unrelated organs or plant tissue.

The governing concept is diffusion across exchange surfaces. Capillaries form a fine network beside body cells and have walls only about one cell thick. This short distance allows oxygen and dissolved nutrients to move from blood to tissues, while carbon dioxide and other wastes move into the blood. Their thin walls are therefore best suited for exchange, making option A correct. Pumping is done by the heart, urine is stored in the bladder, and fat emulsification involves bile.

The governing concept is clot formation during blood clotting. When a blood vessel is damaged, platelets collect at the wound and help form a temporary plug; clotting proteins then strengthen it, limiting blood loss. If platelet numbers are very low, this initial plug forms poorly or slowly, so bleeding may continue even after a small injury. Therefore option A is correct. The remaining options do not describe platelet function or consequences of low platelet count.

The governing concept is transport of raw materials for photosynthesis. Xylem carries water and dissolved minerals upward from roots to stems and leaves. Water is a reactant in photosynthesis, and mineral ions support chlorophyll formation and enzyme activity. If xylem is blocked, the leaf receives less water and minerals, so photosynthetic food production may fall. Thus option A is correct; phloem transports food, while the other choices concern unrelated animal functions.