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.