Ans: Aerobic respiration occurs in the presence of oxygen and produces CO₂, water, and 38 ATP molecules. Example: respiration in most animals. Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP. Anaerobic respiration occurs without oxygen and produces less energy. In yeast: glucose → ethanol + CO₂ + 2 ATP. In human muscles: glucose → lactic acid + 2 ATP (causes muscle cramps). Aerobic is more efficient (produces more ATP).

Ans: Mouth: teeth mechanically break food; salivary amylase starts starch digestion. Oesophagus: peristalsis moves food to stomach. Stomach: HCl creates acidic medium, kills bacteria; pepsin digests proteins; mucus protects lining. Small intestine (major digestion): bile (from liver) emulsifies fats; pancreatic juice (amylase, lipase, trypsin) digests starch, fats, proteins; intestinal juice completes digestion. Villi absorb digested food into blood. Large intestine: absorbs water; remaining waste egested via anus. The liver also produces bile stored in gall bladder.

Ans: Kidneys filter waste (urea, uric acid, excess salts) from blood through millions of nephrons. In each nephron: glomerular filtration (blood filtered in Bowman's capsule), selective reabsorption (glucose, amino acids, water reabsorbed), and secretion of remaining waste as urine. Urine flows: kidney → ureter → bladder → urethra. Dialysis (artificial kidney): patient's blood is passed through a machine with a semipermeable membrane that removes urea and waste products — mimics kidney function for patients whose kidneys have failed.

Ans: Water transport (xylem): Water absorbed by root hairs by osmosis → moves through root cortex → enters xylem → transported upward through stem to leaves. Driving force: transpiration pull (evaporation of water from leaves creates suction). Root pressure also contributes. Food transport (phloem): Food (sucrose) made in leaves is transported to all parts of plant — up and down — through phloem by a process called translocation. Sucrose is loaded into phloem by active transport; water follows by osmosis creating pressure that pushes food along.

Ans: Double circulation means blood passes through the heart twice in one complete circulation — once for pulmonary circulation (heart to lungs and back) and once for systemic circulation (heart to body and back). This ensures complete separation of oxygenated and deoxygenated blood, allowing efficient oxygen supply to all body tissues. It is especially important in warm-blooded animals (mammals, birds) that need high energy for maintaining body temperature.

Ans: Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in presence of sunlight and chlorophyll). It occurs in two stages: Light reactions (in thylakoids): light energy splits water (photolysis), releases O₂, produces ATP and NADPH. Dark reactions/Calvin cycle (in stroma): CO₂ fixed using ATP and NADPH to make glucose. Factors affecting photosynthesis: light intensity (more light → more photosynthesis up to a point), CO₂ concentration, temperature (optimal ~25°C), water availability, chlorophyll content.

Ans: The small intestine is very long (~6–7 m) giving ample time for digestion. Its inner wall has millions of tiny finger-like projections called villi (and microvilli) that massively increase the surface area for absorption. It receives secretions from three sources: bile (liver), pancreatic juice (pancreas), and intestinal juice — together completing the digestion of all food types. Nutrients pass from villi into blood capillaries and lacteals (for fats).

Ans: Chlorophyll, present in the chloroplasts of green plant cells.

Ans: Bile, secreted by the liver, makes the food in the small intestine alkaline so pancreatic enzymes can act, and it emulsifies fats, breaking large fat globules into smaller ones.

Ans: Salivary amylase. It breaks down starch into sugar (maltose).

Ans: The pulmonary vein carries oxygenated blood from the lungs to the left atrium of the heart.

Ans: Ethanol and carbon dioxide, with release of energy.

Ans: Blood in veins flows at low pressure and has to move towards the heart, often against gravity. Valves allow flow only towards the heart and prevent backflow. Arteries carry blood at high pressure, so they do not need valves.

Ans: The amount of dissolved oxygen in water is much lower than the amount of oxygen in air. So aquatic organisms must breathe faster to obtain enough oxygen.

Ans: Arteries carry blood away from the heart; veins carry blood to the heart. Arteries have thick, elastic walls; veins have thin walls. Arteries have no valves; veins have valves. Arteries carry oxygenated blood (except the pulmonary artery); veins carry deoxygenated blood (except the pulmonary vein).

Ans: HCl creates an acidic medium that activates the enzyme pepsin and helps in protein digestion. It also kills germs that enter with food.

Ans: Inhalation: the intercostal muscles contract and lift the ribs outward, and the diaphragm contracts and flattens. The chest cavity expands, pressure falls and air rushes into the lungs. Exhalation: the ribs move down and in, and the diaphragm relaxes and rises, the chest cavity decreases in size, pressure rises and air is pushed out. The lungs always contain some residual air to give time for oxygen to be absorbed.

Ans: (1) Ultrafiltration: blood enters the glomerulus, and water, salts, glucose, amino acids and urea are filtered into Bowman's capsule. (2) Selective reabsorption: useful substances such as glucose, amino acids, salts and much of the water are reabsorbed into the blood by the tubule. (3) Tubular secretion: remaining wastes are added to the filtrate. The urine collects in the collecting duct, ureter and bladder and is expelled through the urethra.

Ans: Photosynthesis occurs in the chloroplasts of green plant cells, mainly in leaves. Step 1: chlorophyll absorbs light energy. Step 2: light energy is converted into chemical energy and water molecules are split into hydrogen and oxygen. Step 3: carbon dioxide is reduced to carbohydrates using the hydrogen. Equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (in sunlight, with chlorophyll). CO₂ enters through the stomata, water comes from the roots through xylem, and the glucose is stored as starch. Oxygen released is used for respiration.

Ans: Air enters through the nostrils, passes through the pharynx, larynx, trachea (supported by cartilage rings so it does not collapse), bronchi, bronchioles and finally the alveoli in the lungs. The alveoli are thin-walled balloon-like sacs surrounded by a network of capillaries, giving a large surface area. Oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be breathed out. Oxygen is carried by haemoglobin in red blood cells, which has a high affinity for oxygen, to all the tissues. Carbon dioxide is more soluble in water and is carried mainly dissolved in the blood plasma.