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biophysics
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if the Plasma membrane is amphipathic (hydrophobic and hydrophilic), why does it only allow hydrophobic stuff to pass thru?
An amphipathic molecule is a molecule that has both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. This means that one end of the molecule is attracted to water while the other end repels water.
The amphipathic nature of the plasma membrane allows it to act as a selective barrier between the cell and its surroundings. The hydrophilic heads of the phospholipids face outward towards the watery environment, while the hydrophobic tails face inward, creating a hydrophobic interior. This allows the membrane to regulate the movement of substances in and out of the cell.
Cholesterol is a type of lipid that is present in the plasma membrane. Its hydrophobic nature helps to maintain the fluidity of the membrane by preventing the phospholipids from packing too closely together. This is important for the proper functioning of membrane proteins and the movement of substances across the membrane.
One example is the sodium-potassium pump, which is a transmembrane protein that uses the difference in concentration of sodium and potassium ions to transport them across the membrane. This protein has both hydrophilic and hydrophobic regions that allow it to interact with the lipid bilayer and move ions against their concentration gradient.
In prokaryotic cells, the plasma membrane is typically made up of a single layer of phospholipids, making it more rigid and less fluid compared to the phospholipid bilayer of eukaryotic cells. Additionally, prokaryotic cells do not have cholesterol in their membranes, so they rely on other methods to maintain membrane fluidity, such as incorporating different types of lipids.