Life Processes

Saliva has both a physical and a chemical role in digestion. Its water and mucus moisten and lubricate food, making it easier to form a bolus and swallow. Salivary amylase then begins the chemical digestion of starch into simpler sugars in the mouth. Therefore option A correctly gives both effects. Saliva does not convert fat into bile, form blood or urine, make oxygen, or direct food into the lungs.

The governing concept is the location and function of the first digestive enzymes. Salivary glands release saliva into the mouth, and saliva contains salivary amylase, which begins the digestion of starch before food reaches the stomach. If these glands malfunction, this initial starch digestion is directly affected first. Thus option B is correct. Most fat digestion occurs later with bile and lipase, major protein digestion begins in the stomach, and water is absorbed rather than digested.

The stomach secretes hydrochloric acid, which creates a strongly acidic medium. This medium helps activate pepsin from its inactive precursor and provides suitable conditions for pepsin to digest proteins into smaller peptides. The acid also helps destroy many microbes entering with food, although the enzyme-supporting role answers the question directly. Therefore option A is correct; the other options describe unrelated plant or excretory processes.

The governing concept is protection during digestion. The stomach releases hydrochloric acid and digestive enzymes, but its inner lining is protected by a mucus layer. If mucus is deficient, acid can directly irritate and erode the lining, possibly causing inflammation or ulcers. Therefore option A is correct. The other options concern blood oxygen, bile production, or plant transport tissues and are unrelated to mucus protection in the stomach.

The governing concept is emulsification of fats. Bile contains no digestive enzyme, but its bile salts break large fat globules into many tiny droplets. This increases the surface area on which lipase can act and also helps provide an alkaline medium in the small intestine. Thus option A is correct. Bile does not digest starch directly, alter haemoglobin, or transform urine into blood.

The governing concept is the division of functions in the small intestine. Digestive juices from the pancreas and intestinal glands complete the breakdown of carbohydrates, proteins and fats. The lining has numerous villi with thin walls and blood capillaries, giving a large surface for rapid nutrient absorption. Hence option A is correct; the other choices describe neither the main role nor the structure of the small intestine.

The governing concept is peristalsis. The wall of the oesophagus contains circular and longitudinal muscles that contract and relax in a coordinated wave. This wave pushes a bolus of solid, semi-solid, or liquid food toward the stomach, so movement does not depend only on gravity. Therefore option A is correct. Blood cells and stomata have no role in transporting food through the human food pipe.

The governing concept is water reabsorption during digestion. The large intestine removes a considerable amount of water and some salts from undigested material, helping faeces become more solid before elimination. If this absorption is greatly reduced, excess water remains in the intestinal contents, so the faeces may be loose or watery. Thus option A is correct; the other options describe impossible changes or unrelated functions.

The governing concept is anaerobic respiration in muscles. During intense running, the oxygen supply may temporarily be insufficient for the muscles’ energy demand. Muscle cells then partially break down glucose without enough oxygen, producing lactic acid and releasing less energy. Accumulation of lactic acid is associated with fatigue and cramps in the school-level explanation. Therefore option A is correct; bile, starch and urea have different biological roles.

The governing concept is alcoholic fermentation by yeast. In the absence of oxygen, yeast breaks down glucose only partially. The products are ethanol and carbon dioxide, and a small amount of energy is released. This process is used in baking and brewing. Therefore option A is correct. Oxygen and water are associated with aerobic respiration, whereas urea, bile, starch and saliva are not the products of yeast fermentation.

The governing concept is adaptation for efficient diffusion. Alveoli are numerous tiny air sacs with very thin, moist walls, creating a large surface area. A dense network of blood capillaries surrounds them, maintaining a concentration gradient so oxygen diffuses into blood and carbon dioxide diffuses into the alveolar air. Thus option A is correct. Digestion, urine formation and bile production occur in other organs.

Haemoglobin is the oxygen-carrying pigment present in red blood cells. It binds oxygen in the lungs and transports it to body tissues. If haemoglobin is deficient, the oxygen-carrying capacity of blood decreases, so cells receive less oxygen for aerobic respiration and energy release. Therefore, option A is correct. B concerns digestion, while C and D describe unrelated plant or digestive structures.

A four-chambered heart has two atria and two ventricles. The right side receives and pumps deoxygenated blood to the lungs, while the left side receives and pumps oxygenated blood to the body. The septum keeps these two streams separate, allowing efficient double circulation and a high oxygen supply to tissues. Therefore, option A is correct; digestion, urine formation, and stomata are unrelated to heart chambers.

Arteries carry blood away from the heart. When the ventricles contract, blood enters them under relatively high pressure. Their thick muscular walls prevent bursting, and their elastic fibres allow them to expand and recoil, helping maintain a smooth flow between heartbeats. Thus option A is correct. The other choices refer to feeding, urinary storage, or plant gas exchange and have no connection with arterial structure.

Veins return blood from body tissues to the heart, and the pressure inside them is comparatively low. In many veins, especially those of the limbs, blood must move against gravity. Valves open when blood flows toward the heart and close if it starts to reverse, thereby preventing backflow and supporting one-way circulation. Therefore option A is correct; valves do not colour blood, manufacture oxygen, or digest food.

Capillaries connect the smallest arteries to the smallest veins and form dense networks close to body cells. Their walls are only about one cell thick, so oxygen, carbon dioxide, nutrients, and wastes can pass across them over a short distance by diffusion. Their narrow diameter also slows the blood flow, allowing time for exchange. Hence option A is correct; the other statements misidentify their structure or function.

Platelets are small cell fragments in the blood that gather at an injured site and help initiate clot formation. A clot forms a temporary seal over a damaged blood vessel, reducing blood loss and supporting repair. If platelet numbers are low, clotting becomes slower or inadequate, so bleeding may continue for longer after an injury. Therefore option A is correct; the other choices are unrelated effects involving oxygen, digestion, or plant stomata.

Xylem and phloem are different vascular tissues with different transport roles. Xylem mainly carries water and dissolved mineral ions from the roots toward the aerial parts, driven by transpiration pull and related forces. Phloem transports sugars and other organic substances produced in leaves to regions that use or store them; this is translocation. Therefore option A is the correct comparison. The other options incorrectly apply animal functions or gas exchange to plant tissues.

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. Evaporation creates transpiration pull, which helps lift water and dissolved minerals through the xylem. However, rapid or excessive transpiration may remove more water than the plant can absorb, causing wilting. Thus option A correctly gives both a benefit and a drawback; the other options mix unrelated digestive, urinary, or respiratory functions.

A nephron is the structural and functional unit of a kidney and performs the main steps of urine formation. Blood is filtered in the glomerulus, while useful substances such as appropriate amounts of water, salts, and glucose are selectively reabsorbed in the tubule. Remaining wastes form urine. Therefore option A is correct. The other choices describe breathing, digestion, or the pumping role of the heart, not kidney function.