Cell Physiology: Cell Membrane Structure and Composition

Cell Physiology

• The cell membrane is a biological membrane that separates and protects the interior of a cell from the outside environment.

Cell Membrane Composition

• The cell membrane is composed of approximately 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrates.

Cell Membrane Structure

• The basic structure of the cell membrane is a lipid bilayer, interspersed with large globular protein molecules.
• The lipid bilayer has hydrophilic (polar) heads and hydrophobic (nonpolar) tails, which face inward and outward, respectively.

Cell Membrane Permeability

• The lipid bilayer membrane is highly permeable to lipid-soluble substances, such as oxygen, carbon dioxide, and alcohol.
• It acts as a major barrier to water-soluble substances, such as ions, glucose, and urea.

Cell Membrane Proteins

• There are two types of membrane proteins: integral proteins and peripheral proteins.
• Integral proteins protrude all the way through the membrane, providing structural channels (pores) for water-soluble substances and acting as carrier proteins for transporting substances.
• Peripheral proteins are attached to the inner surface of the membrane and function as enzymes that catalyze chemical reactions in the cell.

Carbohydrates in the Cell Membrane

• Membrane carbohydrates occur in combination with proteins or lipids in the form of glycoproteins or glycolipids.
• The carbohydrate moieties attached to the outer surface of the cell have several functions:
  • They have a negative electrical charge, giving most cells an overall negative surface charge.
  • They attach cells to one another.
  • They act as receptor substances for binding hormones.
  • They participate in immune reactions.

Cell Junctions

• There are three kinds of cell junctions: desmosomes, tight junctions, and gap junctions.
• Desmosomes are protein attachments between adjacent cells, acting like spot welds to hold together tissues that undergo stress.
• Tight junctions are tightly stitched seams between cells, preventing the movement of material between cells.
• Gap junctions are narrow tunnels that directly connect the cytoplasm of two neighboring cells, allowing the passage of ions and small molecules.

Functions of Cell Membrane

• The cell membrane maintains cell shape and structure.
• It performs a transport function, regulating the movement of materials into and out of the cell.
• It facilitates intercellular communication.
• It enables intracellular adhesion.
• It directs cell movement.