Biological membranes

• All cells and organelles are surrounded by a partially permeable membrane composed of phospholipids with protein molecules between them. The main function of the membrane is controlling the movement of substances in and out of the cell/organelle. It also contains receptors for other molecules such as hormones and enables cells to stick together.
• Diffusion: the passive movement of small, non-polar lipid soluble molecules such as co2 and oxygen from an area of high concentration to an area of low concentration.
• Facilitated diffusion: requires a channel protein in the cell membrane to transport polar molecules, charged and water soluble molecules across the membrane.
• Osmosis: the diffusion of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane.
• Active transport: all types of molecules through carrier proteins from an area of low concentration to an area of high concentration- requires ATP.
• The rate of gas exchange by diffusion becomes more rapid as: surface area of the surface increases, diffusion distance decreases, diffusion gradient becomes more steep.