Chapter 5 Life Processes

Nutrition is the process by which organisms obtain food for energy and growth. Plants are autotrophs — they prepare their own food through photosynthesis using sunlight, carbon dioxide, and water. Animals are heterotrophs, meaning they depend on other organisms for food. Heterotrophic nutrition is further divided into: holozoic nutrition (ingestion of whole food like in humans), saprophytic nutrition (breaking down dead organic matter, as in fungi and bacteria), and parasitic nutrition (living on or inside a host and absorbing nutrients from it, like tapeworms). Some organisms like Euglena can perform both autotrophic and heterotrophic nutrition depending on the conditions. Understanding these modes of nutrition explains the diverse ways life sustains itself across different environments and ecological niches.

Photosynthesis is the process by which green plants and some microorganisms convert light energy into chemical energy stored as glucose. The general equation is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. The four raw materials required are carbon dioxide (from air through stomata), water (absorbed from soil through roots), chlorophyll (the green pigment in chloroplasts), and sunlight. The process occurs in two stages — the light reaction (in the grana of chloroplasts, where water is split and oxygen is released) and the dark reaction or Calvin cycle (in the stroma, where CO₂ is fixed into glucose using ATP and NADPH). Factors affecting photosynthesis include light intensity, CO₂ concentration, water availability, and temperature. Leaves are specially adapted with a large surface area, thin structure, and stomata to maximise photosynthesis.

Respiration is the process by which organisms break down food to release energy. Aerobic respiration occurs in the presence of oxygen and takes place in the mitochondria. Glucose is completely broken down to carbon dioxide and water, releasing a large amount of energy (36–38 ATP molecules). The equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy. Anaerobic respiration occurs in the absence of oxygen, releasing much less energy (only 2 ATP). In yeast, it produces ethanol and CO₂ (fermentation), while in our muscle cells during intense exercise it produces lactic acid. The buildup of lactic acid causes muscle cramps. Anaerobic respiration is less efficient but allows organisms to function temporarily without oxygen, which is crucial during vigorous physical activity.

The human circulatory system transports nutrients, oxygen, hormones, and waste products throughout the body. It consists of the heart, blood vessels (arteries, veins, capillaries), and blood. The heart is a four-chambered muscular organ divided into the right atrium and right ventricle (which handle deoxygenated blood) and the left atrium and left ventricle (which handle oxygenated blood). Deoxygenated blood from the body enters the right atrium, goes to the right ventricle, and is pumped to the lungs for oxygenation (pulmonary circulation). Oxygenated blood returns to the left atrium, enters the left ventricle, and is pumped to the rest of the body (systemic circulation). Arteries carry blood away from the heart under high pressure, veins return blood to the heart, and capillaries enable exchange of substances with body cells.

Excretion is the process of removing metabolic waste products from the body to maintain internal balance (homeostasis). In humans, the main excretory organ is the kidney. Each kidney contains millions of tiny filtering units called nephrons. Blood under pressure enters the nephron through the glomerulus — a cluster of capillaries inside the Bowman's capsule. Filtration occurs here, allowing water, glucose, urea, salts, and amino acids to pass through while retaining blood cells and proteins. As the filtrate passes through the renal tubule, useful substances like glucose, water, and salts are selectively reabsorbed back into the blood. The remaining waste, now called urine (containing urea, excess salts, and water), is collected in the renal pelvis, flows to the ureter, stored in the urinary bladder, and expelled through the urethra.