Life Processes

The governing concept is the dependence of photosynthesis on stomatal gas exchange. Stomata provide the main pathway for carbon dioxide to enter the leaf. If they stay closed, carbon-dioxide availability falls even though the roots supply sufficient water. The reduced carbon dioxide limits glucose and starch formation, so photosynthesis decreases. Option A gives both linked effects. Xylem does not carry food, chlorophyll does not convert oxygen into starch, and plants have no kidneys.

The governing concept is the source of oxygen in photosynthesis. Water and carbon dioxide are both raw materials, but light energy splits water molecules during the light-dependent reactions. The oxygen atoms from this photolysis are released as molecular oxygen, while hydrogen contributes to later carbohydrate formation. Therefore option A is correct. Carbon dioxide supplies carbon for glucose, whereas glucose and starch are products or storage forms rather than the original oxygen source.

The governing concept is holozoic nutrition in Amoeba. Amoeba has no mouth, teeth or alimentary canal. It extends pseudopodia around a food particle and encloses it, forming a food vacuole. Digestive enzymes then act inside the vacuole and convert complex food into simpler soluble substances that can be absorbed. Thus option A gives the correct sequence; the other options incorrectly describe structures of plants or higher animals.

The governing concept is chemical digestion in the mouth. Saliva contains salivary amylase, an enzyme that begins breaking starch into simpler sugars, mainly maltose, during chewing. If the salivary glands malfunction, this first stage of starch digestion is reduced or absent, although digestion may continue later in the small intestine. Therefore option A is correct. Fat, minerals and water are not the food components initially digested by salivary amylase.

The governing concept is the role of gastric hydrochloric acid in protein digestion. Hydrochloric acid creates the acidic medium needed for pepsin to become active and function efficiently on proteins. If acid secretion is low, pepsin activity can decrease, so the initial breakdown of proteins in the stomach may be impaired. Option A is correct. Amylase acts mainly on starch, bile is produced by the liver, and villi are intestinal structures, so B, C and D are incorrect.

The governing concept is emulsification during digestion. Bile does not chemically digest fat because it contains no fat-digesting enzyme. Instead, its bile salts divide large fat globules into many tiny droplets, increasing the surface area available to lipase. Therefore, option A is correct. Options B, C, and D describe impossible conversions; bile neither turns fat into blood, oxygen, nor starch.

The governing concept is absorption in the small intestine. Digestion only breaks complex food into soluble substances; the body receives usable nutrients after those substances pass through the intestinal wall into blood or lymph. Villi provide a large surface area and a short route for absorption. Without them, absorption falls and less nutrient material reaches cells for respiration and energy release. Thus A is correct; the other options concern unrelated organs or false processes.

The governing concept is peristalsis, the involuntary, wave-like contraction and relaxation of muscles in the alimentary canal. After swallowing, these waves push the food bolus through the oesophagus toward the stomach, even without gravity being the main force. If peristalsis stops, forward transport is impaired and food may remain stuck or move very slowly. Therefore A is correct; B, C, and D are biologically unrelated and impossible outcomes.

The governing concept is the role of pancreatic juice in chemical digestion. It contains enzymes such as amylase for carbohydrates, trypsin and other proteases for proteins, and lipase for fats. These enzymes normally act in the small intestine. If pancreatic juice cannot reach it, digestion of all three major food components is reduced. Hence A is correct; water and oxygen are not digested, while urine formation belongs mainly to excretion.

The governing concept is alcoholic fermentation by yeast. In the absence of sufficient oxygen, yeast breaks down glucose and forms ethanol and carbon dioxide. When yeast is mixed with dough, the released carbon dioxide becomes trapped as bubbles in the dough’s gluten network. These bubbles expand during resting or heating, making the dough rise. Therefore A is correct; blood clotting, urine formation, and stomatal closure do not use yeast fermentation.

The governing concept is diffusion across the respiratory surface. Alveoli normally have very thin, moist walls, so oxygen and carbon dioxide travel across a short distance between air and blood. If the walls become thick, the diffusion distance and resistance increase. Consequently, oxygen enters the blood and carbon dioxide leaves it more slowly, reducing the efficiency of gas exchange. Thus A is correct; the other options concern unrelated or impossible effects.

The governing concept is diffusion at the respiratory surface. A moist surface allows oxygen and carbon dioxide to dissolve before crossing the membrane, while a thin surface provides a short diffusion distance. These features permit rapid movement of oxygen into blood and carbon dioxide out of blood, supporting efficient respiration. Hence A is correct. Making food sweet, changing blood colour, and dissolving bones have no connection with the structure of alveoli.

The governing concept is that gas exchange requires a moist respiratory surface. In terrestrial organisms, an exposed moist surface would lose water rapidly through evaporation. Keeping the surface inside the body, as in lungs, protects it while still allowing oxygen to enter and carbon dioxide to leave. Therefore, option A is correct; the other options do not describe the function of a respiratory surface.

The governing concept is aquatic respiration through gills. Fish cannot use gaseous oxygen from the atmosphere in the same way as land animals; they obtain oxygen dissolved in water. Continuous water flow brings fresh water over the thin gill surfaces, where oxygen diffuses into the blood and carbon dioxide diffuses out. Thus option A is correct, while the other choices describe unrelated processes.

Double circulation means that blood passes through the heart twice in one complete circuit: once between the heart and lungs, and once between the heart and the body. The four-chambered heart keeps oxygenated and deoxygenated blood largely separate, so tissues receive blood with a high oxygen concentration. This supports efficient aerobic respiration and ATP production for high energy needs. Option A is correct.

Normally, the right side of the heart carries deoxygenated blood to the lungs, whereas the left side sends oxygenated blood to the body. If the two sides mix excessively, the blood pumped to tissues contains less oxygen than normal. Cells then receive less oxygen for aerobic respiration, reducing efficient energy release and potentially causing fatigue. Therefore option A is correct; the other choices are biologically unrelated.

Arteries carry blood away from the heart, and each ventricular contraction forces blood into them at relatively high pressure. Their thick muscular walls prevent damage and their elastic fibres allow them to expand during systole and recoil between beats. This elasticity helps maintain smoother, continuous blood flow. Hence option A is correct; digestion, urine filtration and air storage are not arterial functions.

Veins return blood to the heart at relatively low pressure, especially from the lower parts of the body. Valves open toward the heart and close when blood tends to reverse direction. Without these one-way valves, gravity and muscle relaxation could allow pooling or backward flow, making venous return less efficient. Therefore option A is correct; the other options confuse circulation with unrelated processes.

Capillaries are microscopic vessels that connect the smallest arteries with the smallest veins. Their walls are usually only one cell thick, so oxygen, carbon dioxide, nutrients and wastes can diffuse across a short distance. Their extensive network lies close to body cells and provides a large exchange area. Thus option A is correct; the other choices describe no function of capillaries.

Platelets are cell fragments that gather at an injured blood vessel and help initiate clot formation. The clot creates a temporary seal, reducing blood loss until repair proceeds. If platelet numbers are low, the clotting response may be slower or weaker, so bleeding can continue for longer after an injury. Therefore option A is correct; platelets do not transport oxygen or perform the processes in the other options.