Life Processes

The basic functions an organism must perform to stay alive are its life processes:

• Nutrition — getting and using food for energy and growth.
• Respiration — releasing energy from food.
• Transportation — moving materials around the body.
• Excretion — removing waste.

Autotrophs (green plants) make their own food by photosynthesis: 6CO₂ + 6H₂O → (light, chlorophyll) C₆H₁₂O₆ + 6O₂. Heterotrophs depend on others. In humans, digestion starts in the mouth (amylase), continues in the stomach (HCl + pepsin), and is completed in the small intestine, where villi absorb the nutrients.

Respiration releases the energy stored in glucose as ATP. With oxygen (aerobic, in the mitochondria) it releases much more energy than anaerobic respiration. In humans, gas exchange happens at the alveoli of the lungs.

In humans, the heart (four chambers) pumps blood in double circulation — blood passes through the heart twice each cycle, keeping oxygenated and deoxygenated blood separate. Arteries carry blood away from the heart; veins (with valves) bring it back; haemoglobin in red cells carries oxygen.

In plants, xylem carries water and minerals upward (helped by transpiration), and phloem translocates food made in the leaves.

Excretion removes harmful wastes. In humans the kidneys filter the blood; their filtering unit is the nephron. Useful substances are reabsorbed and urine is formed. (When kidneys fail, dialysis does the filtering artificially.)

Plants excrete in simpler ways — releasing gases through stomata, storing wastes in leaves/bark that fall off, or as gums and resins.