Cell Membrane: Structure & Functions

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.

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.

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.

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.

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