This week we are examining the plasma membrane, what its composition is and some of its properties. Biological membranes are primarily organized based on the chemistry of phospholipid molecules. Even in the rare cases (like mitochondria) where proteins actually outnumber phospholipids, the membrane retains its basic organization. That is, it has a hydrophobic interior sandwiched between two hydrophilic surfaces. This structure is created by the orientation of the phospholipid bilayer – that is, two layers of phospholipid molecules. Each layer is arranged with its hydrophilic ‘head group’ on the outside and the hydrophobic ‘tail’ on the inside.
In addition to the phospholipids, plasma membranes are also comprised of cholesterol molecules that contribute to the rigidity of the membrane and a variety of membrane proteins. Amongst the membrane proteins are:
1. Transport Proteins (channel and carrier proteins)
2. Enzymatic Proteins
3. Cell-Cell Recognition Proteins
4. Receptor Proteins
We discussed the function and some examples of each of these major types of proteins, with special attention paid to transport proteins. Why transport proteins? Because one of the most basic functions of the plasma membrane is to keep things on one side or the other of the membrane. Because the membrane is quite good at this, transport proteins are required to move things from one side to the other in a controlled manner.
In discussing the movement of molecules, we defined diffusion and provided some examples of how it works. In general, diffusion is due to the random motion of particles leading to a net movement of particles from regions of high concentration to regions of low concentration. After this process has acted for some time, equilibrium is reached. This does not mean that particles cease moving, but that there is no further net change in concentration likely. See a video animation of this process provided by McGraw Hill at: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html
The same principle may operate across a membrane if it is permeable to the solute in question. Again, the rule is that molecules (the solute) moves from regions of high concentration to regions of low concentration.
Osmosis, or the diffusion of water, operates by the same principle. The only difference is that osmosis describes the movement of water molecules(solvent) through a membrane rather than that of particles (solute). A good demonstration of this can be seen in the microscopic video presented below. In this film, blood cells (RBCs and WBCs) are observed under conditions of changing solution ‘tonicity’. Watch the lower left corner of the image to see what solution the cells are being exposed to. In isotonic solutions (ones of equal solute concentration), there is no net movement of water. In a hypertonic solution, water moves out of the cell and the cells shrink. In a hypotonic solution, water moves into the cells and ruptures them.
Once we understand these concepts, we can appreciate the difference between passive and active transport. Passive transport is when solutes are allowed to move ‘with’ or ‘down’ the concentration gradient – this requires no input of energy. Active transport is the movement of solute molecules ‘against’ or ‘up’ the concentration gradient and therefore requires energy input to achieve.