lab simulating protein synthesis CHNOPS - Yumpu
I would be very grateful if anyone might be able to answer a few of my
questions about the absolute fundamentals of cell membranes and their associated transport proteins, as I'm struggling to understand this area of Biology.
Protein Synthesis Simulation Activity
I’m going to assume that you mean ion channels. But the answer is
‘yes and no’. Ion channels, at least those selective for cations, have a pore that represents the ion permeation pathway through the channel across the membrane. Along most of its length, there will be space for water molecules to enter. However, ion channels show selectivity for ions. The pore narrows at the ‘selectivity filter’. In many cases, in order for the ion to pass through the pore, it must give up its waters of hydration. Negative charges lining the selectivity filter can substitute energetically for the waters of hydration. In this way, selectivity is achieved as a balance between the energy cost of the loss of the waters of hydration, the energetic favourability of the ion associating with the negative charges lining the selectivity filter and the steric hindrance of the pore. No surprise that the Nobel prize awarded for this work was in Chemistry and not Physiology or Medicine! There are also channels, termed aquaporins, that are specific for water. Ion channels and aquaporins are quite different from another set of protein channels that are responsible for opening very large holes in membranes important in the transfer of newly synthesized proteins across organellar membranes, for example. These channels are very large and will, therefore, be water-filled. Note that these channels are so large that the diffusion of small molecular weight substances through them will be free diffusion – not facilitated diffusion! In addition, there are channels present at the gap junctions between cells known as connexons that allow small molecular weight substances (water, ions, sugars etc) to pass from one cell interior to the next. These play a fundamental role in the passage of the electrical impulse through cardiac muscle that forms the basis of the heart beat.
Active transporters. These use the energy from the hydrolysis of ATP to transport substances against their prevailing electrochemical gradient. The classic example to which you refer is the sodium pump (also known as the Na+-K+ ATPase). This is an ATPase present in the cell membrane that transfers 3 sodium ions out of the cell in exchange for 2 potassium ions into the cell (both against their electrochemical gradient) at the expense of the consumption of one molecule of ATP. It plays a vital role in the maintenance of a low sodium and high potassium concentration within cells. It therefore is fundamental in maintaining the driving force for many sodium-dependent secondary active transporters. In direct answer to your question, the sodium pump is not the only active transporter.
There are other ATPases responsible for transporting protons, calcium
ions, as well as larger molecular weight substrates. There are also ATPases present in the organellar membranes. For example, a calcium ATPase in the sarcoplasmic reticulum of muscle cells plays a fundamental role in relaxation of muscle following calcium-dependent contraction. In one important example, the movement of the substrate down its electro-chemical gradient is used to drive the synthesis of ATP: This is the F1F0 ATPase of mitochondria.