2.5

• All cells and organelles are surrounded by a partially permeable membrane composed of a sea of phospholipids with protein molecules between the phospholipid molecules.
• The main function of the membrane is controlling the movement of substances in and out of the cell/organelle.
• The membrane also contains receptors for other molecules such as hormones and enables adjacent cells to stick together.
• The fluidity of the membrane and the mosaic arrangement of the protein give the structure of the membrane its name – fluid mosaic model.
• The movement of molecules through cell membrane depends on the properties of the molecule as well as the requirements of the cell.
• Diffusion is the passive movement of small, non-polar lipid soluble molecules such as carbon dioxide 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 is the diffusion of water molecules from an area of low solute concentration to an area of high solute concentration through a partially permeable membrane.
• Osmosis is 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 can transport all types of molecules through carrier proteins from an area of low concentration to an area of high concentration.
• Exocytosis and endocytosis transport large particles and bulk transport.
• In exocytosis, vesicles containing large particles are fused with the cell surface membrane.
• The rate of gas exchange by diffusion becomes more rapid as surface area of the surface increases, diffusion distance decreases, and diffusion gradient becomes more steep.