Other cells, further removed from the blood supply where the glucose concentration is lower, must use active transport mechanisms because there is not a sufficient concentration of glucose to provide cells with the glucose they need. This may work well in many cases.įor example, the blood concentration of glucose is sufficiently high that red blood cells can use facilitated diffusion as a means of acquiring glucose.
Transport proteins include the sodium-potassium pump, the sodium-calcium exchanger, and lactose permease, amongst many others).Īs noted, the driving force for facilitated diffusion is concentration, meaning that in facilitated diffusion, materials will only move from a higher concentration to a lower concentration and that at the end of the process, the concentration of materials on each side of a bilayer will be equal (Figure 3.28). Transporters have high specificity and transfer rates that are orders of magnitude slower. Examples would be the sodium or potassium channels of nerve cells.
They usually involve movement of water or ions. Channels largely provide openings with some specificity and molecules pass through them at close to the rate of diffusion. With respect to movement of materials through membrane proteins, there is a difference between channels (sometimes called pores) and transporters. Figure 3.25 - A uniport, a symport, and an antiport Figure 3.26 - Electroneutral and electrogenic transporters - Image by Aleia Kim When the driving force for movement through the membrane protein is simply diffusion, the process is called facilitated diffusion or passive transport and when the process requires other energy input, the process is called active transport. If the action of a protein in moving ions across a membrane results in a net change in charge, the protein is described as electrogenic and if there is no change in charge the protein is described as electroneutral (Figure 3.26). Proteins involved in moving ions are called ionophores. If two molecules are moved in opposite directions across the bilayer, the protein is called an antiport. Proteins that move two molecules in the same direction across the membrane are called symports (also called synporters, synports, or symporters). Selective movement of ions by membrane proteins and the ions’ extremely low permeability across the lipid bilayer are important for helping to maintain the osmotic balance of the cell and also for providing for the most important mechanism for it to make ATP - the process of oxidative phosphorylation.Ī protein involved in moving only one molecule across a membrane is called a uniport (Figure 3.25). This function along with movement of ions and other substances is provided by proteins/protein complexes that are highly specific for the compounds they move. As noted earlier, it is essential for cells to be able to uptake nutrients.
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