Biology

Respiration Energy from respiration Respiration is the chemical process in cells that releases energy from glucose. Cells use this energy for: muscle contraction; protein synthesis; cell division; active transport; growth; nerve impulses; and maintaining body temperature. Respiration is not the same as breathing. Breathing moves air into and out of the lungs; respiration releases energy in cells. Aerobic respiration Aerobic respiration uses oxygen to break down glucose and releases a large amount of energy. ...

Gas exchange in humans Gas-exchange surfaces A gas-exchange surface needs: a large surface area, providing many sites for diffusion; a thin surface, giving a short diffusion distance; a good blood supply, maintaining concentration gradients; and good ventilation, replacing air and maintaining concentration gradients. Alveoli provide these features. Their large total surface area gives many diffusion sites, their walls are one cell thick, capillaries carry away oxygen and bring carbon dioxide, and ventilation continually refreshes the air. ...

Diseases and immunity cover how pathogens cause disease, how the body defends itself, and how vaccines provide protection.

Transport in humans Circulatory system The human circulatory system contains: vessels to carry blood; the heart, a muscular pump; and valves to maintain one-way blood flow. Single and double circulation In a fish, there is single circulation: heart → gills → body tissues → heart Blood passes through the heart once per complete circuit. Pressure falls as blood passes through the gill capillaries. In a mammal, there is double circulation: ...

Transport in plants involves the movement of water, minerals and food substances throughout the plant. This guide covers xylem and phloem function, root hair structure, transpiration and translocation according to IGCSE Biology syllabus (2026-2028).