Every cell is surrounded by a plasma membrane. The cell membrane is selectively permeable- only certain substances can cross it freely. The structure of the membrane allows materials to move across it by 2 main means: active and passive transport. Password transport involves two processes: Diffusion and Osmosis. Passive transport allows some substances such gases and water to move into and out of the cell without using energy. Active transport includes processes such as "ion pumps", "exocytosis" and "phagocytosis". Molecules such as sucrose's are moved against their concentration gradi ...view middle of the document...
Cells use diffusion to take in things from their environment and rid themselves of waste. Diffusion takes place down a concentration gradient. Water, carbon dioxide and other small ions can diffuse freely through cell membranes. Concentration gradients occur in all organisms. Where they do, simple diffusion will transport things around. Nutrients being carried in blood for example will diffuse into those cells that need them. Diffusion is also vital for getting oxygen into cells and carbon dioxide out of them, for example, in humans. Facilitated diffusion occurs when substances move down the concentration gradient by "carrier proteins" in the cell membrane.Osmosis is also a type of passive transport involving the diffusion of water molecules across a selectively permeable membrane. Water will move by diffusion from an area where there is more of it to an area where there is a less of it. This results in the movement of water from a dilute (less concentrated) solution to a stronger (more concentrated) solution. It is the way water enters and leaves a cell. Osmosis is a special case of the diffusion of water. Osmosis occurs when water diffuses across a semi permeable membrane. Semi-permeable membranes will let small molecules pass through it easily, but will hold back larger molecules like sucrose's. It acts as if it has tiny holes in it, too small for the larger molecules to squeeze through.
