In contrast, the interior of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside of the cell. A phospholipid molecule Figure 2 consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon.
This arrangement gives the overall molecule an area described as its head the phosphate-containing group , which has a polar character or negative charge, and an area called the tail the fatty acids , which has no charge. The head can form hydrogen bonds, but the tail cannot.
Figure 2. A hydrophilic head and two hydrophobic tails comprise this phospholipid molecule. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. Can anything or everything enter or leave? So, what determines what can go in or out?
Is it the nucleus? The DNA? Or the plasma membrane? The plasma membrane also known as the cell membrane forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It protects and supports the cell and also controls everything that enters and leaves the cell. It allows only certain substances to pass through, while keeping others in or out. The ability to allow only certain molecules in or out of the cell is referred to as selective permeability or semipermeability.
To understand how the plasma membrane controls what crosses into or out of the cell, you need to know its composition. It consists of two very large polypeptide chains that wind themselves into a complex that stretches between other protein molecules, such as actin , and several other proteins, including the band-III type and ankyrin.
Together, these proteins appear to form a mesh or network on the inner surface of the red blood cell, which may in turn be responsible for holding the cell in its typical biconcave shape, even as it squeezes through some very, very narrow capillaries in the blood stream. Glycophorin is a protein that extends all the way through the membrane it is a transmembrane protein.
It consists of amino acids, most of which are found exposed on the outside, external side of the cell. There is also some carbohydrate about sugar molecules joined into about 16 chains on the outside. Despite the fact that there can be as many as 6 x 10 5 of these molecules in the membrane of a typical red blood cell, its function remains a mystery. In fact, red blood cells that lack glycophorin seem to function normally!
It appears that this protein plays an important part in moving carbon dioxide, carried as the HCO 3 - ion , through the cell membrane. Since these bicarbonate ions are negatively charged, and thus strongly hydrophilic, they would normally have difficulties getting through the strongly hydrophobic inner part of the membrane. Parts of the band-III protein passes through the membrane several times, forming a channel through which the anions can pass with a lot less difficulty.
This makes it possible for the red blood cells to pick up excess carbon dioxide produced in the body tissues and transport them to the lungs for release to the outside. The heads the phospho part are polar while the tails the lipid part are non-polar.
The heads, which form the outer and inner linings, are "hydrophilic" water loving while the tails that face the interior of the cell membrane are "hydrophobic" water fearing. Water is attracted to the outsides red of the membrane but is prevented from going through the non-polar interior yellow layer.
The membranes of the cell are semi-permeable. That means that while most things are effectively kept in or out , some can pass through directly.
So how do cells move things in and out? There are three methods. Diffusion : If a molecule is very small, such as oxygen or carbon dioxide, diffusion does the trick. When the concentration of O 2 outside the cell is higher than inside, O 2 molecules diffuse in, passing through the membrane like it isn't even there.
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