Which one of the following is not a constituent of cell membrane?

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The plasma membrane is a thin membrane that surrounds and protects a living cell. It is present in every living cells (bacteria, plants, and animals). The membrane separates the inner content of the cell (cytoplasm, organelles, nucleus) from the outer environment. Bacterial and plant cells possess a cell wall outer to the plasma membrane. The plasma membrane is semipermeable and only allows selective molecules to pass through it. The plasma membrane keeps a fixed environment inside the cell for cellular metabolism.

The plasma membrane consists of two layers of lipid and proteins scattered on it. The lipids are of three types: glycolipids, phospholipids, and sterols.

Glycolipids are carbohydrates containing lipids. The carbohydrate molecule is attached to the lipid by a glycosidic bond. Glycolipids are present in both eukaryotic cells. The glycolipids play a crucial role in cellular recognition during the immune response, A, B, O blood grouping, and cell-cell interaction. The enzyme glycosyltransferases add the carbohydrate molecule to the lipid molecule in the Golgi apparatus.

Each phospholipid molecule has a hydrophilic head (interact with water environment) and two hydrophobic tails. The head contains a phosphate group, and the tails contain fatty acids. The head and tails are joined together by a glycerol molecule. Phospholipids are synthesis in the membrane of the endoplasmic reticulum.

Sterols (C17H28O.) are a group of lipids synthesized by plants, animals, fungi, and some bacteria. Cholesterol is a sterol lipid present in the plasma membrane animal cells. Cholesterol is the precursor molecule for steroid hormone and fat-soluble vitamins. Phytosterol is present in the plasma membrane of plants, and ergosterol is present in the plasma membrane of microorganisms.

Proline is an amino acid. It is not a constituent molecule of the plasma membrane.