Life Processes

Human digestion begins in the mouth, where food is mechanically broken into smaller pieces by the teeth and mixed with saliva. Salivary amylase begins the chemical digestion of starch there. The stomach and small intestine continue digestion later, whereas the large intestine mainly absorbs water and forms faeces. Thus, option A is the only correct answer.

Saliva contains the enzyme salivary amylase, also called ptyalin. This enzyme acts on starch, which is a complex carbohydrate, and begins converting it into simpler sugars in the mouth. Fat and protein digestion mainly starts later with other enzymes, while vitamins do not require enzymatic digestion in this way. Hence option A is correct.

The stomach lining contains gastric glands that release hydrochloric acid, or HCl. It creates an acidic medium needed for the action of pepsin and also helps kill many harmful microbes entering with food. Sulphuric and nitric acids are not normal stomach secretions, and citric acid is associated with citrus fruits rather than gastric juice. Therefore option A is correct.

Mucus secreted by the stomach forms a protective coating over its inner lining. This barrier reduces direct contact between the stomach wall and hydrochloric acid or digestive enzymes, preventing self-digestion and irritation. The heart pumps blood, plants release oxygen during photosynthesis, and kidneys form urine; none of these is the function of gastric mucus. Thus option A is correct.

Bile juice is produced by the liver, the largest gland in the human body. It is stored and concentrated in the gallbladder, then released into the small intestine. Bile contains no digestive enzyme, but its salts emulsify fats into tiny droplets, increasing the surface area for lipase. The stomach, lungs, and kidneys do not produce bile, so option A is correct.

The main digestive role of bile is emulsification of fats. Bile salts divide large fat globules into many tiny droplets, increasing the available surface area so that lipase can digest the fat more efficiently. Bile does not directly convert starch into glucose, bind oxygen in blood, or filter urine. Therefore option A accurately describes its principal function.

Most absorption of digested nutrients occurs in the small intestine. Its inner lining has numerous villi and microvilli that greatly increase surface area, while thin walls and a rich blood supply allow glucose, amino acids, minerals, and water-soluble vitamins to enter the blood. The stomach has limited absorption, and the mouth and food pipe mainly transport food. Hence option A is correct.

The finger-like projections lining the small intestine are called villi; each projection contains a network of blood capillaries and a lacteal. Villi greatly increase the internal surface area and help absorb digested nutrients efficiently. Stomata are openings in leaves, the trachea carries air, and a nephron is the functional unit of a kidney. Therefore option A is correct.

Respiration is a life process that releases usable energy from food. During respiration, complex food molecules such as glucose are broken down, usually with oxygen and sometimes without it, and the released energy supports movement, growth, repair, transport and other cellular activities. Therefore, obtaining energy from food is the correct purpose. Urine formation, blood filtration and chewing belong to excretion, kidney function and digestion, not respiration.

Mitochondria are called the powerhouse of the cell because they are the main sites of aerobic respiration in most eukaryotic cells. They break down food molecules and capture much of the released energy in ATP, which powers cellular activities. The nucleus stores genetic material, ribosomes make proteins, and the cell wall provides support. Thus mitochondria, option A, are the correct structure.

Gas exchange in human lungs takes place mainly in the alveoli, millions of tiny balloon-like air sacs at the ends of bronchioles. Their walls are only one cell thick and are closely surrounded by blood capillaries, giving oxygen a short diffusion path into blood and carbon dioxide a path out. The stomach and small intestine digest food, while the heart pumps blood, so option A is correct.

The heart is a muscular organ that functions as the central pump of the circulatory system. Its rhythmic contractions push blood through pulmonary circulation to the lungs and systemic circulation to the rest of the body, delivering oxygen and nutrients and carrying away wastes. The kidneys filter blood, the liver processes substances, and the lungs exchange gases; therefore option A is correct.

The human heart has four chambers: the right atrium and right ventricle receive and pump deoxygenated blood toward the lungs, while the left atrium and left ventricle receive and pump oxygenated blood to the body. Septa help prevent mixing of oxygenated and deoxygenated blood. The stomach, kidney and liver do not have this four-chambered pumping arrangement, so option A is correct.

The defining function of an artery is to carry blood away from the heart under relatively high pressure. Arteries usually have thick, elastic and muscular walls to withstand and regulate this pressure. Most arteries carry oxygenated blood, although the pulmonary artery carries deoxygenated blood to the lungs, so direction—not oxygen content—is the rule. Veins return blood, while urine formation and digestion are unrelated; hence A is correct.

Veins are blood vessels whose main function is to return blood from body tissues to the heart. They generally carry deoxygenated blood, except for pulmonary veins, which bring oxygenated blood from the lungs to the left atrium. Veins usually have thinner walls and valves that help prevent backflow, especially in the limbs. Arteries carry blood away, while oxygen production and food breakdown are different processes; therefore A is correct.

The governing concept is blood clotting, or coagulation, which prevents excessive blood loss after an injury. When a blood vessel is damaged, platelets gather at the site and help form a temporary plug. They also release substances that support the clotting reaction and formation of fibrin threads. Therefore, option A is correct. Red blood cells carry oxygen, white blood cells defend against pathogens, and plasma is the liquid part of blood; these are not the main clot-forming particles.

The governing concept is transport in plants. Roots absorb water and dissolved mineral salts from the soil, and these materials must move upward to the stem and leaves. Xylem vessels and tracheids conduct water and minerals mainly from roots toward aerial parts, supported by root pressure, cohesion and transpiration pull. Thus option A, xylem, is correct. Phloem transports organic food, stomata regulate gas exchange and water loss, while villi occur in the small intestine of animals.

The governing concept is translocation of food in plants. Leaves prepare sugars by photosynthesis, but growing roots, stems, flowers, fruits and storage organs may not produce enough food themselves. Phloem tissue transports dissolved sugars and other organic nutrients from source regions, usually leaves, to sink regions where they are used or stored. Therefore option A is correct. Xylem carries water and minerals, guard cells control stomatal openings, and an alveolus is a gas-exchange structure in lungs.

The governing concept is transpiration, the loss of water vapour from the aerial parts of a plant, especially leaves, through stomata. Water absorbed by roots reaches the leaves, and some of it evaporates from moist leaf surfaces before diffusing out through open stomata. Hence option A gives the correct definition. The other choices describe impossible or unrelated processes: roots do not normally make food, flowers do not drink oxygen, and seeds do not form blood.

The governing concept is the human excretory system and maintenance of internal balance. The kidneys receive blood, filter it through millions of nephrons, selectively reabsorb useful substances and remove excess water, salts and nitrogenous wastes in urine. Thus option A is the best answer because kidneys are the principal excretory organs. Lungs also remove carbon dioxide and water vapour, and skin can release sweat, but eyes and ears are not the main organs of excretion.