Substances move into and out of cells in three main ways. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. Oxygen, carbon dioxide and dissolved substances such as glucose can move this way. Diffusion is passive, which means the cell does not use any energy from respiration for it.
Osmosis is the diffusion of water through a partially permeable membrane, such as a cell membrane. Water moves from a dilute solution to a more concentrated solution. A cell placed in a very dilute solution gains water and swells, and a cell in a concentrated solution loses water and shrinks.
Active transport is different. It moves substances from a region of lower concentration to a region of higher concentration, which is against the concentration gradient. This needs energy from respiration. Root hair cells use active transport to take up mineral ions from the soil, even though the ions are more concentrated inside the root hair cell than outside it.
