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Membranes from different sources vary in their composition. They essentially consist of lipoproteins which are special non bonded combinations of lipids with proteins. In general, membranes contain about 60% protein and 40% carbohydrate by dry weight. Carbohydrate is usually present to the extent of 1-10% of the total dry weight in glycoproteins and glycolipids. In membranes of some cell organelles the nucleic acids DNA and RNA have also been reported, but are most likely contaminations. Membranes contain three different classes of proteins, structural proteins, enzymes and carrier proteins. Structural proteins form the 'backbone' of the cell membrane.
Membranes are made of phospholipids. The particular phospholipid has a hydrophilic head and a hydrophobic tail. The hydrophic head could contact water. Whereas, the hydrophobic tail avoids water. So, membranes consists of two layers of phospholipids arranged in ways that the hydrophilic heads touch water, and the hydrophobic tails oriented toward the hydrophobic tail of the other layer of phospholipids.
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