Plasma Membrane Functions - Cell: The Unit of Life | Class 11 Biology

Plasma Membrane Structure

The plasma membrane has a fluid-mosaic structure: lipids and proteins form a flexible, shifting layer rather than a rigid sheet.

• The membrane is a fluid lipid bilayer. Its lipids have water-attracting (hydrophilic) heads and water-repelling (hydrophobic) tails. The tails face inward, creating a barrier that makes it difficult for many polar or charged substances to cross directly.
• Integral proteins are embedded in or span the bilayer, while peripheral proteins sit on its surface. Some membrane proteins provide routes for substances that cannot readily pass through the lipid core.
• The membrane is selectively permeable, regulating which substances enter and leave the cell.

Transport Across the Membrane

Passive Transport

Passive transport does not require cellular energy. Substances move down their concentration gradient, from higher concentration to lower concentration.

• Simple diffusion: Small molecules such as oxygen and carbon dioxide can move directly through the membrane. For example, oxygen moves into cells while carbon dioxide moves out.
• Facilitated diffusion: Substances that cannot easily cross the lipid bilayer move down their concentration gradient with the help of membrane proteins. This process does not require energy.
  • Channel proteins form passageways for particular substances, including water and ions. Aquaporins are examples of water-channel proteins.
  • Carrier proteins bind specific molecules and change shape to move them across the membrane. Glucose is one example.

Active Transport

Active transport uses energy, commonly ATP, to move substances against their concentration gradient, from lower concentration to higher concentration.

The sodium–potassium pump is one example. In its usual cycle, it uses ATP to move three sodium ions out of the cell and two potassium ions into the cell.

Cell Recognition and Receptors

Some membrane proteins and glycoproteins act as receptors or recognition markers, helping cells identify and interact with particular substances or other cells.

• Cell-surface markers are involved in blood-group identification and must be considered when checking compatibility for transfusions.
• The CD4 receptor on certain immune cells is an example of a receptor involved in HIV’s interaction with cells.
• Cell-surface markers are also relevant to tissue and organ compatibility.

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