This course is part of a series taught by Kevin Ahern at Oregon State University on General Biochemistry. For more information about online courses go to http://ecampus.oregonstate.edu/ for the rest of the courses see http://www.youtube.com/playlist?list=PL347B70A1CC0D91C6
Also check out the free textbook "Biochemistry Free and Easy" at: http://biochem.science.oregonstate.edu
1. An interesting transporter is the Na+/Ca++ exchange pump (not shown in a figure in class, but described). It uses movement of Na+ in to cells to be a driving force for pumping Ca++ out. Remember than Ca++ stimulates muscular contraction. If Ca+ is not pumped OUT, its concentration in muscle cells remains high, stimulating contraction. Digitoxigenin is a compound from foxglove that binds the Na+/K+ ATPase, preventing development of a Na+ gradient. As a consequence, digitoxigenin increases Ca++ concentration, since Ca++ pumping requires a Na+ gradient. Digitoxigenin is used as a heart stimulant.
2. Nerve cells use the gradient of Na+ and K+ built up by the Na+/K+ pump to transmit signals. In nerve transmission, special "gates" open and close to allow Na to diffuse into nerve cells and K to diffuse out of nerve cells.
3. The first step in nerve transmission involves opening of Na+ gates. These allow Na+ to diffuse into the cell, since Na+ concentration is higher outside of cells than inside. Movement of the positively charged sodium ion causes a change in the electrical potential of the cell near the Na+ gate. To compensate for the voltage change, the K+ gates open and Na+ gates close, allowing K+ to flow out of the cell. This results in an overcompensation of the voltage. The K+ gates close and the region where the original movement of ions occurred recovers. During this time, no nerve signal can be transmitted at that point.
4. The nerve signal is transmitted as a consequence of the initial movement of Na+ into the cell. Before it can be pumped out, some of the sodium diffuses down to the next Na+ gate and the change in the voltage environment causes it to open and trigger the same events as occurred in the last step. Thus, the signal moves from one junction to another to another, ultimately arriving at the end of the axon.
5. Tetrodotoxin is a neurotoxin because it inhibits the action of nerve cells. It is found in the puffer fish and it blocks the Na+gates.
6. Channels (gates) are made by protein molecules in the membranes of cells. Channels are generally very specific for what they will allow to pass through them. Glucose channels, for example are fairly specific for glucose. Sodium and potassium channels are very specific for each respective ion.
7. Ion specificity is accomplished by two mechanisms. The first is physical. If an ion is too big to fit in a channel, it is excluded. This is the case of the sodium channel, which excludes potassium ions because they are too big.
8. The second mechanism of specificity is energy. An example is the potassium channel, which excludes sodium ions. In this case, the channel allows larger ions (potassium) to pass through, but blocks smaller ions, like sodium ions.
9. The mechanism of exclusion of the potassium channel relates to the energies of solvation of each ion. For potassium ions, the energy of desolvation of the ion as it enters the channel is overcome by the energy of resolvation as it enters the channel. Thus, entry of potassium ions is energetically favored. This is due to the geometry of the potassium channel closely matching the dimensions of the potassium ion.
10. When sodium ions try to enter the channel, their energy of desolvation is greater than is realized by their resolvation in the channel. Thus, they do not enter. Their energy of resolvation not as favorable due to their ion sizes not matching the dimensions of the potassium channel.
11. Movement of ions across a membrane is accomplished by controlled gates. Two control mechanisms include voltage controlled gates (open/close with voltage changes) and gates controlled with both voltage and a ball/chain. The latter are found in nerve cells.
12. As noted, the first gate that opens in nerve transmission is the sodium gate. Movement of sodium ions in creates the action potential that begins the transmission of the nerve signal (see above). The "ball" at the sodium gate quickly plugs the hole after the sodium ions start leaking in.
13. After the nerve has "fired" the gradient must be restored and this is the job again of the Na/K ATPase.