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Cell Membrane: Structure & Functions

  • Cell membrane is also called plasma membrane or plasmalemma.

Function of Cell Membrane

  • Makes the outer wall of the cell → separates intracellular and extracellular fluid.

  • Maintains shape and size of the cell.

  • Exchange of substances between inside and outside of the cell happens through the cell membrane.

Structure of Cell Membrane

  • Very thin and pliable.

  • Thickness: about 10 nm.

  • Main constituents:

    • Lipids: 40%

    • Proteins: 55%

    • Carbohydrates: 5%

Membrane Lipids (Lipid Bilayer)
  • Phospholipids

    • Most abundant lipids in cell membrane.

    • Arranged in lipid bilayer form.

    • Lipid bilayer:

      • Made up of two layers of lipids opposed to each other like a sandwich.

      • Phosphate heads of phospholipid molecules are on outer surfaces, in contact with intracellular and extracellular water.

      • Lipid tails of phospholipid molecules are deeper in central part of the bilayer, away from water.

      • The bilayer exists in fluid form (Fluid Mosaic Model of the cell membrane).

  • Cholesterol

    • Dissolved in the lipid bilayer.

    • Determine the fluidity of the membrane.

      • at modest concentration → decreases fluidity.

      • at higher concentrations → increases fluidity.

  • Permeability:

    • As lipid bilayer is formed with lipids...

      • other lipids and lipid-soluble substances → can dissolve in it → pass through lipid bilayer.

      • water and water-soluble substances → cannot dissolve in the lipid bilayer → cannot cross it.

    • Large molecules → cannot penetrate the bilayer → cannot cross it.

Membrane Proteins
  • Mostly in form of glycoproteins.

  • Types

    • Integral Membrane Proteins: Integrated tightly to the lipid bilayer

    • Peripheral Membrane Proteins: Attached loosely to lipids or integral membrane protein

  • Functions

    • Transport Proteins: Allow passage of water and water-soluble substances across the membrane

    • Receptors: Receive signals for cell-to-cell communication.

    • Second Messengers: Participate in intracellular signaling.

    • Enzymes: ... serve as enzymes!

    • Adhesion Molecules: Attach the cell to extracellular matrix or to other cells.

    • Submembrane Cytoskeleton: Provide strength and resistance to the membrane.

    • Antigen: Participate in immune reaction.

Membrane Carbohydrates
  • Mostly in form of glycoproteins or glycolipids.

  • Their carbohydrate portion usually protrudes on outer side of the cell.

  • Cover almost the entire surface of the cell → this carbohydrate coating is called glycocalyx.

  • Functions:

    • Give the cell surface negative charge → repels other negatively charged objects.

    • Neighbor cells may get attached to one another through their glycocalyx.

    • Serve as receptors.

    • Participate in immune reaciton.


Next >>

Cells and Molecules > ▶️ Phospholipids in Water


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