Cell Membrane Structure and Physiology

Structure and Chemical Composition of the Cell Membrane

The cell membrane (also known as the plasma membrane) serves as the outer boundary of the cell and consists mainly of three major classes of biomolecules:

• Lipids:

  • Phospholipids

  • Glycolipids

  • Cholesterol

• Proteins:

  • Integral proteins

  • Peripheral proteins

  • Glycoproteins

• Carbohydrates:

  • Present on the cell surface in the form of glycolipids or glycoproteins.

Phospholipid Bilayer Structure and Functions

The fundamental matrix of the plasma membrane is organized as a phospholipid bilayer.

Phospholipid Structure

Each phospholipid molecule is amphipathic and consists of:

• Head: Made of a phosphate group, forming a hydrophilic (water-loving) head.

• Tail: Consists of 2 fatty acids, forming a hydrophobic (water-fearing) tail.

Functions of the Phospholipid Bilayer

• Forms the basic structure of the cell membrane.

• Functions as a barrier that prevents water evaporation.

Role and Functions of Cholesterol

Cholesterol molecules are intercalated within the lipid bilayer and serve key biological functions:

• Plays an important role in maintaining structural integrity.

• Regulates the fluidity of the cell membrane.

• Acts as a precursor for the synthesis of many vital substances, such as steroid hormones and bile acids.

Cell Membrane Proteins

Proteins within the cell membrane are classified into two main structural types based on their location and association with the membrane:

• Integral (Trans-membrane) Proteins: Penetrate the whole thickness of the cell membrane.

• Peripheral Proteins: Attached to either the outer surface or inner surface of the cell membrane.

Functions of Cell Membrane Proteins

1. Provide the basic structure of the cell membrane.

2. Act in cell adhesion and serve as cell surface identity markers.

3. Function as ion pumps (e.g., the $\text{Na}^+/\text{K}^+$ pump).

4. Serve as receptors for drugs or hormones.

5. Function as carriers that facilitate membrane transport.

6. Act as enzymes that catalyze specific chemical reactions (e.g., adenylyl cyclase).

7. Form channels for ions that can open or close via conformational changes in the protein molecules.

Glycocalyx and Membrane Carbohydrates

Structure of the Glycocalyx

• Refers to the cell membrane carbohydrate component.

• Formed of glycoproteins and glycolipids.

• Forms a loose coat that covers the outer surface of the membrane.

Functions of the Glycocalyx

1. Act as receptors for hormones (such as insulin).

2. Play a major role in cell-cell recognition (essential for immune function).

3. Attach cells to each other.

4. Carry a negative charge, thereby repelling negatively charged molecules.