Ans: Raw materials for photosynthesis: (1) Carbon dioxide (CO₂) — absorbed from air through stomata. (2) Water (H₂O) — absorbed from soil by roots. (3) Sunlight — absorbed by chlorophyll. Photosynthesis takes place in the chloroplasts present in the cells of green parts of the plant, mainly the leaves. Chlorophyll, the green pigment, is located in the chloroplasts. The overall reaction: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂.

Ans: The human excretory system consists of two kidneys, two ureters, the urinary bladder, and the urethra. Kidney structure includes millions of nephrons. Urine formation in a nephron occurs in three stages: (1) Glomerular filtration — blood is filtered under pressure in the Bowman's capsule; glucose, amino acids, salts, urea, and water pass into the tubule. (2) Selective reabsorption — useful substances (glucose, amino acids, most water, some salts) are reabsorbed back into blood from the tubule. (3) Tubular secretion — remaining waste (urea, extra ions) is further added to the filtrate. The final concentrated urine flows to the ureter and then to the bladder.

Ans: Double circulation means blood passes through the heart twice in one complete journey through the body. (1) Pulmonary circulation: Deoxygenated blood from right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary vein → left atrium (oxygenated). (2) Systemic circulation: Oxygenated blood from left ventricle → aorta → all body organs → venae cavae → right atrium (deoxygenated). Advantage: Oxygenated and deoxygenated blood never mix, ensuring efficient O₂ delivery to all tissues. This system is present in all mammals and birds (warm-blooded animals with high energy needs).

Ans: Translocation is the movement of food (mainly sucrose) produced in leaves by photosynthesis to all other parts of the plant through the phloem. It can occur in both upward and downward directions unlike xylem transport which is only upward. Translocation is important because: leaves can only photosynthesize during daylight, but all plant parts need energy continuously; roots, fruits, seeds, and growing tips receive their nutrition through phloem translocation. Energy for active loading of sugars into phloem is provided by ATP.

Ans: Photosynthesis is the process by which green plants make food (glucose) using CO₂ and water in the presence of sunlight and chlorophyll. Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Conditions: Sunlight (provides energy), Chlorophyll (pigment in chloroplasts that absorbs light), CO₂ (from air through stomata), Water (absorbed by roots). Two stages: Light reactions (in thylakoid membranes) — water is split using light energy, O₂ is released; ATP and NADPH are produced. Dark reactions/Calvin cycle (in stroma) — CO₂ is fixed using ATP and NADPH to form glucose. Significance: Primary source of food and O₂ for all life on Earth.

Ans: The human heart is a muscular organ about the size of a fist located in the thoracic cavity. Structure: Four chambers — right atrium, right ventricle, left atrium, left ventricle. Separated by septa. Valves: Tricuspid (between right atrium and right ventricle), Bicuspid/Mitral (between left atrium and left ventricle), Semi-lunar valves in pulmonary artery and aorta. Functioning: Right side: Deoxygenated blood from body enters right atrium → right ventricle → pulmonary artery → lungs. Left side: Oxygenated blood from lungs enters left atrium → left ventricle → aorta → body. Double circulation: Blood passes through the heart twice per circuit — once for the pulmonary circuit (lungs) and once for the systemic circuit (body). This keeps oxygenated and deoxygenated blood separate and maintains high efficiency.

Ans: Aerobic respiration: Occurs in the presence of oxygen. Glucose is completely oxidised. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP (large amount of energy). Takes place in mitochondria. Example: In most plants and animals. Anaerobic respiration: Occurs in the absence of oxygen. Glucose is incompletely broken down. Two types: (a) In yeast/bacteria: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP (fermentation). (b) In muscle cells (during vigorous exercise): C₆H₁₂O₆ → 2C₃H₆O₃ (lactic acid) + 2 ATP. Lactic acid causes muscle cramps. Less energy is produced compared to aerobic respiration.

Ans: The epiglottis is a flap that covers the opening of the windpipe (trachea) during swallowing, so that food does not enter it.

Ans: Ultrafiltration takes place in the glomerulus and the Bowman's capsule.

Ans: In large organisms, most cells are deep inside the body and far from the surface, so diffusion over such distances is too slow to supply oxygen. Their energy needs are also higher, so transport systems and respiratory organs are required.

Ans: During vigorous exercise, oxygen supply to the muscles is insufficient, so pyruvate is converted to lactic acid by anaerobic respiration. The build-up of lactic acid causes cramps.

Ans: Autotrophic nutrition: organisms make their own food from simple inorganic substances (CO₂ and water) by photosynthesis using sunlight; found in green plants and some bacteria; chlorophyll is needed. Heterotrophic nutrition: organisms depend on other organisms for food and break down complex organic substances; found in animals, fungi and most bacteria; chlorophyll is absent.

Ans: The stomach walls contract and relax to mix food with gastric juice. The gastric glands release hydrochloric acid, the enzyme pepsin and mucus. HCl makes the medium acidic for pepsin to act on proteins and kills bacteria. Pepsin breaks proteins into smaller molecules. Mucus protects the inner lining of the stomach from the acid. Food leaves the stomach in small amounts through the sphincter muscle into the small intestine.

Ans: A nephron has a cluster of thin-walled capillaries called the glomerulus, enclosed in the cup-shaped Bowman's capsule, followed by a long tubule that leads into a collecting duct. Blood is filtered in the glomerulus; water, glucose, amino acids, salts and urea pass into the capsule. As the filtrate flows along the tubule, glucose, amino acids, salts and a large part of the water are selectively reabsorbed into the blood. Excess wastes are added by tubular secretion. The remaining fluid is urine, which collects in the collecting duct, ureter and urinary bladder and is expelled through the urethra. The amount of water reabsorbed depends on the water in the body and the quantity of dissolved waste.

Ans: Xylem (tracheids and vessels) carries water and minerals from the roots to all parts. At night root pressure moves water; during the day transpiration pull created by loss of water through stomata is the major force. Phloem carries food (sucrose) made in leaves to other parts in both directions by translocation, with sieve tubes and companion cells; it needs energy from ATP. Differences: xylem has dead cells, one-way upward flow, carries water and minerals; phloem has living cells, two-way flow, carries food.