The plasma membrane consists of lipids, proteins and complex carbohydrates. In its main part, the membrane is formed by a huge number of lipid molecules – phospholipids. They are arranged in two layers so that their polar hydrophilic (water-attracting) ends – “heads” – are facing outward, and non-polar water-repellent (hydrophobic) ends – “tails” – are directed inward (towards each other). The phospholipid molecules located in this way form a bilayer (bilayer) membrane.
In some places, the lipid layers of the membrane are penetrated by protein molecules. Some of them pass through the entire thickness of the membrane (integral). Others are immersed in the thickness of the phospholipid membrane layers with only one end of the molecule, and the other is located outside (semi-integral). Still others lie outside the membrane, adjacent to it — these are external, or peripheral, proteins. Some proteins can also lie between phospholipid layers.
In all eukaryotic cells, the plasma membrane can be covered with additional additional integumentary structures of the supmembrane layer, which has specific functions, and various peripheral structures. In animal cells, this can be mucus, in plant cells – the cell membrane of cellulose, and in fungi – of chitin.
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