2.4.1 The Structure of Cell Membranes

Key Features of the Cell Membrane:

1. Phospholipid Bilayer:

• Composed of phospholipids arranged in two layers.
• Hydrophilic heads (water-attracting) face outward, interacting with water inside and outside the cell.
• Hydrophobic tails (water-repelling) point inward, avoiding water and creating a hydrophobic core.
• Function:
• Allows lipid-soluble molecules (e.g., oxygen, carbon dioxide) to pass through.
• Prevents passage of water-soluble molecules and ions.

2. Fluidity:

• The phospholipids can move laterally, making the membrane flexible and self-sealing.
• This property enables processes like endocytosis and exocytosis.

Components of the Cell Membrane:

1. Proteins:

• Embedded in the phospholipid bilayer and play key roles:
• Intrinsic proteins span the membrane and include channel proteins and carrier proteins, which aid in the transport of substances.
• Extrinsic proteins are located on the membrane surface, providing mechanical support or acting as receptors.
• Functions:
• Facilitate active transport and facilitated diffusion.
• Act as receptors for hormones and other signalling molecules.

2. Cholesterol:

• Found within the bilayer.
• Functions:
• Increases stability and reduces membrane fluidity at high temperatures.
• Prevents leakage of water and dissolved ions due to its hydrophobic nature.

3. Glycolipids:

• Lipids with attached carbohydrate chains.
• Functions:
• Act as cell-surface receptors for specific molecules.
• Help in cell recognition and adhesion, forming tissues.

4. Glycoproteins:

• Proteins with attached carbohydrate chains.
• Functions:
• Act as receptors (e.g., for neurotransmitters and hormones).
• Enable cell recognition and communication.

Properties of the Membrane:

1. Selectively Permeable:

• Only specific molecules can pass through, depending on size, charge, and solubility.
• Small, non-polar molecules pass easily (e.g., oxygen).
• Large, polar molecules require proteins for transport (e.g., glucose).

2. Dynamic and Flexible:

• The fluid nature allows the membrane to adapt to changes in cell shape and to repair minor damages.

3. Self-Sealing:

• The bilayer can reseal itself after small disruptions, essential for vesicle formation and fusion.