Life Processes

Nutrition is the process of obtaining food and using it for growth, repair, and energy. Autotrophic nutrition (plants) involves making food from inorganic substances using sunlight — photosynthesis: 6CO2 + 6H2O -> C6H12O6 + 6O2. This occurs in chloroplasts. CO2 enters through stomata (guard cells control opening/closing). Water is absorbed by roots and transported through xylem. Light energy is converted to chemical energy stored in glucose. Heterotrophic nutrition (animals, fungi) involves consuming other organisms. Human digestion is a complex process: the mouth breaks down starch (salivary amylase), the stomach digests proteins (pepsin in HCl, pH ~2), the small intestine completes digestion (bile from liver emulsifies fats, pancreatic enzymes: trypsin for proteins, lipase for fats) and absorbs nutrients through villi (finger-like projections that increase surface area). The large intestine absorbs water.

Respiration is the process of breaking down glucose to release energy (ATP) for cellular activities. It is NOT the same as breathing (which is just inhaling and exhaling air). Aerobic respiration occurs in the presence of oxygen: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy (ATP). It takes place in mitochondria and produces a large amount of energy (38 ATP molecules per glucose). Anaerobic respiration occurs without oxygen. In yeast: glucose -> ethanol + CO2 + energy (used in brewing and baking). In muscle cells during heavy exercise: glucose -> lactic acid + energy (causes muscle cramps). Anaerobic respiration produces much less energy (only 2 ATP). The first step of both types is glycolysis (breakdown of glucose into pyruvate in the cytoplasm), which does not require oxygen.

In humans, the circulatory system transports materials. The heart has 4 chambers: right atrium and right ventricle (handle deoxygenated blood) and left atrium and left ventricle (handle oxygenated blood). Double circulation: blood passes through the heart twice in one complete cycle — pulmonary circulation (heart -> lungs -> heart) and systemic circulation (heart -> body -> heart). This ensures oxygenated and deoxygenated blood do not mix, providing efficient oxygen delivery. Arteries carry blood away from the heart (thick walls, no valves), veins carry blood towards the heart (thin walls, have valves to prevent backflow), and capillaries are the thinnest vessels where actual exchange occurs. In plants, xylem transports water and dissolved minerals upward from roots to leaves through transpiration pull (unidirectional, does not need energy). Phloem transports food (sucrose, amino acids) from leaves to all parts in both directions through translocation (requires ATP).

Excretion is the removal of metabolic waste products from the body. In humans, the excretory system consists of two kidneys, two ureters, a urinary bladder, and a urethra. Each kidney contains about one million nephrons — the basic functional units. A nephron has a Bowman's capsule (cup-shaped structure surrounding a cluster of capillaries called the glomerulus), a proximal convoluted tubule, a loop of Henle, and a distal convoluted tubule leading to a collecting duct. Blood filtration occurs in the glomerulus (ultrafiltration under high pressure), and useful substances (glucose, amino acids, water, salts) are reabsorbed in the tubules. The remaining fluid (urine) contains water, urea, uric acid, and excess salts. If kidneys fail, artificial filtration by a machine (dialysis or haemodialysis) is needed. In plants, excess water is removed by transpiration, oxygen by stomata, and some wastes are stored in cellular vacuoles, falling leaves, or bark.