Structure of Cell Membrane
The thin pliable and elastic semipermeable membrane which envelopes the cell is called cell membrane/plasma membrane/plasmalemma. It is the semipermeable membrane surrounding the cytoplasm of a cell. It is selectively permeable, which means that it only lets certain molecules enter and exit. It can also control the amount of some substances that go into or out of the cell.
Thickness- 7.5 – 10 nrn
- Protein – 55 %
- Phospholipids – 25%
- Cholesterol- 13%
- Other lipids- 4%
- Carbohydrates- 3%.
Its basic structure is a lipid bilayer inserted with large globular proteins molecule. Phospholipids form the basic structure of a cell membrane, called the lipid bilayer. Scattered in the lipid bilayer are cholesterol molecules, which help to keep the membrane fluid consistent.
(A) Lipid bilayer
It is composed almost entirely of phospholipids and cholesterol. Phospholipids have a water-soluble portion (PO43-: hydrophilic) and a portion that is soluble only in fats (hydrophobic). Hydrophobic portion faces each other and hydrophilic parts face to surfaces of the membrane.
(B) Membrane proteins
They float the iceberg in a sea of the lipid bilayer. They are two types –
- Integral proteins: They protrude through the membrane, provide structural channels (pores) and act as carrier proteins for membrane transport.
- Peripheral proteins: They are attached to the inside of membrane and do not penetrate. They act as enzymes that catalyze a chemical reaction of the cell.
(C) Membrane Carbohydrates: They are attached either to protein (glycoprotein) or lipids (glycolipids) the ‘glyco’ portions usually protrude outwards from the surface of the membrane. The entire outer surface of the cell often has a looser carbohydrate coat called the glycocalyx.