Life Processes

Excretion removes metabolic wastes produced by normal cellular activity. The kidneys filter blood and eliminate nitrogenous wastes, especially urea, in urine. The lungs remove carbon dioxide and some water vapour produced during respiration. Thus both organs contribute to waste removal but handle different wastes, making option A correct; the other options incorrectly assign digestive, pumping, or bile-producing roles.

The governing concept is excretion in plants. Plants carry out metabolism, so they produce wastes even though they lack specialised excretory organs such as kidneys. Gases may diffuse out through stomata, while substances such as resins, gums and some wastes can be stored in vacuoles, leaves, bark or old tissues and removed when these parts are shed. Therefore, option A is correct; the other choices incorrectly apply animal organs or deny plant metabolism.

The governing concept is the iodine test for starch and the role of light in photosynthesis. Iodine turns blue-black when starch is present. A plant kept in darkness cannot receive the light energy required to drive photosynthesis, so the leaf does not produce enough stored starch for the test. Thus option A is correct. Excess water, absence of xylem and the number of guard cells do not explain the missing iodine colour in this experiment.

The governing concept is the need for chlorophyll in photosynthesis. A non-green region generally lacks chlorophyll or contains too little of it to absorb light effectively. Without this pigment, the light-dependent energy capture needed for food formation is greatly reduced, so starch is not detected there by iodine. Option A is therefore correct. Leaf thickness, roots and blood are not the required factors stated by this controlled observation.

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 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.

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.

Stomata are pores controlled by guard cells that allow carbon dioxide entry but also permit water vapour to escape. When water availability is low, continued high transpiration can reduce cell water content and cause wilting. Closing the stomata limits diffusion of water vapour and conserves the remaining water, although it may also reduce carbon dioxide uptake and photosynthesis. Therefore option A is correct.

The governing concept is transport in vascular tissues. Xylem carries water and dissolved minerals mainly upward, from roots toward stems and leaves, driven by transpiration and related forces. Phloem translocates food, especially sucrose made in leaves, from a source to sinks such as roots, fruits, or growing tissues. Because sources and sinks can vary, phloem movement may be upward or downward. Therefore, option A is correct; the other choices confuse plant tissues and their functions.

The governing concept is selective reabsorption during urine formation. Filtration at the glomerulus is not perfectly selective, so small useful substances such as glucose, required salts, and much of the water may enter the filtrate along with wastes. The nephron returns needed materials to the blood while retaining excess water and wastes for excretion. Therefore option A is correct. The other options misunderstand the roles of blood, lungs, and bile.

The governing concept is osmoregulation, or maintenance of the body’s water balance. When the body is dehydrated, the kidneys conserve water by increasing its reabsorption from the nephron filtrate into the blood. Consequently, less water is lost and urine becomes smaller in volume and more concentrated. Option A is correct. Complete stoppage would worsen dehydration, while digestion and oxygen production are not nephron functions.

The governing concept is excretion of nitrogenous waste. During protein metabolism, the liver forms urea from toxic ammonia. Healthy kidneys filter urea from the blood and eliminate it in urine. If kidney function fails, filtration and removal decline, so urea accumulates in the bloodstream, a condition associated with harmful waste buildup. Option A is correct; the lungs, heart, and stomach do not perform this renal excretory role.

The governing concept is detoxification of nitrogenous waste. Breakdown of amino acids can produce ammonia, which is highly toxic and cannot safely accumulate in body tissues. In the liver, ammonia is converted into urea through the urea cycle. Urea is much less toxic, dissolves readily in water, and can be transported in blood to the kidneys for urinary excretion. Thus option A is correct; the remaining options confuse urea with nutrition, respiration, or photosynthesis.