The plasma membrane is often described as a fluid mosaic model. This is because of the random placement of various protein within the flexible phospholipid layers. It is also selectively permeable meaning some things can pass through it while others can not.
There are two major categories of proteins found in the plasma membrane. Integral proteins span across the entire membrane passing from one side of the phospholipid bilayer to the other. This is often refered to as a transmembrane protein. Peripheral proteins are found on the outside of the membrane and are bound to integral proteins.
Cholesterol is also found in the membrane and serves to maintain stability in temperature changes.
There are two main categories of transport in cells:
- Passive Transport - Movement that does not require energy.
- Diffusion - Movement of material from high concentration to low (down their concentration gradient). These are small, non-polar molecules like oxygen and carbon dioxide.
- Osmosis - Diffusion of water. Water will move from a hypotonic solution (lower concentration of solutes than the other side) to a hypertonic solution (more solutes). If the two solutions are isotonic (same concentration) there will be no movement.
- Facilited diffusion - The use of channel proteins or carrier proteins to move material down their concentration gradients. Channel proteins allow charged molecules to pass through such as Na+/K+. Aquaporins are a type of channel protein that help water move across more efficiently. Carrier proteins change shape when specific molecule (e.g. glucose) bind to it and are transferred througth the membrane.
2. Active Transport - Requires ATP to move material against its concentration gradient (area of low concentration to high).
- Transport proteins - These may transfer ions, monosaccharides or amino acids. The Na+/K+ pump is one example.
Vesicular transport - Use vesicle/vacuoles to move large molecules/food across the membrane.
- Exocytosis - Vesicle fuses with the cell membrane and material is released from the cell.
- Endocytosis - Plasma membrane forms a vesicle around the material and brings it into the cell. Phagocytosis is specific to this occurring with solids, pinocytosis is when this happens with liquids or dissolved material.