Question

Describe the structure of phospholipids. How are they arranged in the cell membrane?

Answer 1

Hint: Phospholipids are lipids that are derived from fatty acids and alcohols like all other fatty acids. There is a phosphate group at the end of the alcohol which can be substituted with other organic molecules such as ethanolamine, choline and serine.

Complete answer:
Synthesis of phospholipids takes place on the cytosolic side of the ER membrane. Phospholipids are amphiphilic molecules consisting of a hydrophilic head which is most often negatively charged due to the presence of the phosphate group and a hydrophobic end consisting of two tails which are made up of long chain fatty acid residues. In the presence of an aqueous solution the phospholipids arrange themselves in a way which is thermodynamically most favorable. The hydrophilic head faces the aqueous solution and the tails face the inner side often forming a circular structure known as micelles. Phospholipids are the major constituents of cell membranes where they form a bilayer that is advantageous to the cells. The bilayer is composed of two layers of phospholipids with their heads facing outward and the hydrophobic tails sandwiched in between them. The bilayers can undergo phase transitions according to change in temperatures which also alters the chemical property of the bilayer.

Note: Lipid bilayers are generally few nanometers in width. Bilayers makes the cell membrane semi permeable in nature as it does not pass all substances through it. They are impermeable to all water soluble molecules, ions, large molecules such as proteins, it only allows few gases and small uncharged molecules to pass through it.