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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 โ†’ 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 โ†’ decreases fluidity.

  • at higher concentrations โ†’ increases fluidity.

  • Permeability:

  • As lipid bilayer is formed with lipids...

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

  • water and water-soluble substances โ†’ cannot dissolve in the lipid bilayer โ†’ cannot cross it.

  • Large molecules โ†’ cannot penetrate the bilayer โ†’ 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 โ†’ this carbohydrate coating is called glycocalyx.

  • Functions:

  • Give the cell surface negative charge โ†’ repels other negatively charged objects.

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

  • Serve as receptors.

  • Participate in immune reaciton.


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Cells and Molecules > โ–ถ๏ธ Phospholipids in Water
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