Fluid Mosaic Model
Cells are enclosed by a double-layered fluid membrane called the lipid bilayer. It consists of a mosaic of components which include proteins, glycoproteins, glycolipids, phospholipids, saturated and unsaturated fatty acids, and cholesterol. The phospholipid heads are polar, allowing them to orient themselves towards the watery cytoplasm and extracellular space. The hydrophobic tails orient towards each other.
Proteins associated with the membrane may be peripheral (at the surface) functioning as enzymes and signaling molecules, or integral (running through the layer), functioning as transport molecules. Cholesterol in the cell membrane helps to modulate its fluidity and rigidity. For example, when exposed to cold, phospholipids have the tendency to cluster closer together. If they stack too tightly, the permeability of the membrane will be reduced such that transport is hindered. Cholesterol retain membrane fluidity by preventing over-tightening and crystallization of lipids in the bilayer. Cholesterol also helps to prevent the cell from being too permeable. Under normal conditions if there were no cholesterol, phospholipids would be too far apart, creating huge gaps in the membrane.
Types of Cellular Transport
For the cell to stay alive it must allow molecules to go in and out of it. There are essentially two general types of cellular transport, i.e. passive and active transport. Under passive transport, substances move in and out of the cell by simple diffusion (movement from high to low concentration), a process that does not require energy. This works very well for small molecules such as water, carbon dioxide, and oxygen. Carbondioxide and oxygen pass directly and unimpeded through the phospholipid layer since they are uncharged. Water diffuses through integral membrane proteins called aquaporins. Diffusion aided by integral proteins such as aquaporins is called facilitated diffusion or facilitated transport.
Active transport requires the help of energy in the form of ATP to move molecules from one side of the cell to the next, against their concentration gradient. An example of this is the sodium-potassium pump. In this pump, three sodium ions first position themselves inside the integral protein. ATP then binds to the protein and an enzyme breaks off a phosphate group, releasing a burst of energy that causes the protein to undergo a conformational change. This change causes the protein to open on the side of the extracellular space, releasing sodium to the outside. Two potassium ions then enter the pump, binding the protein and causing it to undergo another shape change which results in the release of phosphorus from the protein, and transfer of potassium into the cytoplasm.
This means that for every three sodium that is pumped out of the cell, two potassium is pumped in. This sets up an electrical gradient in the cell where the outside is more positive than the inside. This electrical gradient is important for many cellular activities including the transfer of nerve impulses.
Another type of transport is bulk transport where large substances such as lipid droplets and food particles can be trapped and transferred across the cell membrane. As shown in the diagram below, the process involves engulfing of solid by the cell membrane which then pinches off into the cytoplasm where it can then be digested and metabolized. Endocytosis may be facilitated by receptors on the cell membrane which only allows receptor-specific substances to enter the cell. This transport is known as receptor-mediated endocytosis. A similar mechanism to endocytosis that is used for transporting fluids is known as pinocytosis. Exocytosis works in the opposite direction, expelling substances from the cell.