Every cell must take in useful substances such as oxygen and glucose, and get rid of waste such as carbon dioxide and urea. These substances cross the cell membrane in three different ways.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. The particles move down a concentration gradient and the cell uses no energy. Oxygen diffuses into respiring cells and carbon dioxide diffuses out. The steeper the concentration gradient, the faster the rate of diffusion.
Osmosis is the diffusion of water through a partially permeable membrane. Water moves from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution). A plant cell in a concentrated sugar solution loses water, so its mass falls. In pure water it gains water and its mass rises.
Active transport moves substances from a lower concentration to a higher concentration, which is against the concentration gradient. It needs energy from respiration. Root hair cells use it to take in mineral ions from the soil, and cells in the gut use it to absorb glucose.
