What is the composition of the cell membrane?

The cell membrane is a semi-permeable structure with a thickness of 6 nm to 10 nm. It consists of three layers, with an outer and inner protein layer sandwiching an innermost lipid layer. 

The cell membrane is composed of lipids, proteins, and carbohydrates. Water constitutes about 29% of the total weight of the cell membrane. 

• Lipids: There are three types of lipids present in the cell membrane - phospholipids, cholesterol and galactolipids. All lipid molecules are made up of two distinct parts – a hydrophilic phosphate head and two hydrophobic fatty acid tails. Phospholipids are the most abundant and also the most important molecule in the cell membrane. They are arranged in a bilayer that makes up the basic fabric of the cell membrane. Cholesterol molecules are found scattered among the phospholipids in the membrane. 
• Proteins: Depending on their types, proteins may be firmly embedded in the membrane spanning it entirely, partially extended into the cell membrane, or loosely attached to the external or internal cellular surface. 
• Carbohydrates: Carbohydrates, which occur in the form of glycoproteins and glycolipids, are present only on the extracellular surface of the membrane where they act as unique cellular markers.