Diffusion is the movement of particles from a higher concentration to a lower concentration through a semi-permeable membrane e.g. a cell membrane. This is not the only way materials can enter and exit a cell. Osmosis is the movement of water particles through a membrane from a higher to lower concentration. Active transport is movement across a membrane against the Gradient(to a higher concentration) this is achieved from enzymes in the cell wall and membranes. Because the movement is against the Gradient it needs energy which comes in the form of ATP which is thanks to mitochondria
