Summary of Membrane Transport
|Method |Uses energy |Uses proteins |Specific |Controllable |
|Simple (Lipid) Diffusion |N |N |N |N |
|Osmosis |N |N |Y |N |
|Facilitated Diffusion |N |Y |Y |Y |
|Active Transport |Y |Y |Y |Y |
|Endocytosis/ Exocytosis |Y |N |Y |Y |
The Structure of the Cell Membrane
The cell membrane (or plasma membrane) surrounds all living cells. It controls how substances can move in and out of the cell and is responsible for many other properties of the cell as well. The membranes that surround the nucleus and other organelles are almost identical to the cell membrane. Membranes are composed of phospholipids, proteins and carbohydrates arranged in a fluid mosaic structure, as shown in this diagram.
The phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing outwards, and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer. This hydrophobic layer acts as a barrier to all but the smallest molecules, effectively isolating the two sides of the membrane. Animal cell membranes also contain cholesterol linking the fatty acids together and so stabilising and strengthening the membrane.
The proteins usually span from one side of the phospholipid bilayer to the other (intrinsic proteins), but can also sit on one of the surfaces (extrinsic proteins). They can slide around the membrane very quickly and collide with...