Cell Membrane

Function

• Separates the cell from the environment AND maintains differences between them.

• Composed of a phospholipid bilayer “studded” with proteins.

Structure

• Phospholipid: a lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group.

• Phospholipid bilayer: two layers of phospholipids with their tails pointing inward.

• Cholesterol: another lipid composed of four fused carbon rings, is found alongside phospholipids in the core of the membrane.

• Proteins (integral and peripheral): stud with the phospholipid bilayer.

• Carbohydrate groups: found on the outer surface of the plasma membrane.

  • Attach to proteins → Form glycoproteins

  • Attach to lipids → Form glycolipids

Phospholipids

• Amphipathic

• Acts as a barrier: polar/charged substances cannot cross a non-polar region.
  → They require a protein carrier.

• Only extremely small/non-polar molecules can cross the membrane without a carrier.
  → Ex: O₂, CO₂, H₂O, steroid hormones (i.e., lipid)

Membrane proteins

• Can go through the membrane (trans-membrane protein), sit on a polar surface, or sit in the non-polar layer.

• Functions of membrane proteins:

  • Carriers/channels for transport

  • Enzymes for the reaction

  • Receptors for hormones

  • Cell-cell recognition
    → Glycoproteins have carbohydrate chains that extend outside of the cell, which are unique to each person.
    → White blood cells will treat foreign glycoproteins as “invaders”.

Fluid Mosaic Model

• Often used to describe cell membranes.

• Lipid: consistency of oil (fluid)

• Proteins: arranged like tiles in a mosaic
  → Proteins can move left and right in the cell membrane, but trans-membrane proteins are “stuck”.

• Cholesterol is often found in membranes.

  • Increase fluidity at low temperatures and give membranes stability.