B2.1.12 Cholesterol and membrane fluidity in animal cells (HL) Notes

Cholesterol Regulates Membrane Fluidity in Animal Cells

1. Cholesterol plays a vital role in maintaining the structure and function of cell membranes in eukaryotes.
2. Making up 20–40% of the lipids in the plasma membrane, cholesterol is classified as a steroid, not a glyceride.
3. Its unique structure and placement within the membrane enable it to regulate membrane fluidity, ensuring the cell functions properly under varying conditions.

The Structure of Cholesterol

1. Hydrophobic Region: Most of the cholesterol molecule is hydrophobic and interacts with the hydrophobic hydrocarbon tails of phospholipids in the membrane's core.
2. Hydrophilic Region: Cholesterol has a single hydroxyl group (-OH) at one end, which is hydrophilic and aligns with the phosphate heads of the phospholipids on the membrane's surface.
3. Position in the Membrane: Cholesterol molecules are strategically positioned between phospholipids, with their hydroxyl group typically facing outward.

How Cholesterol Regulates Membrane Fluidity

1. Cell membranes exist in a dynamic state called a liquid-ordered phase, where lipid molecules are densely packed but still free to move laterally.
2. Cholesterol ensures membranes maintain this delicate balance between fluidity and stability:
   1. At High Temperatures:
      1. Cholesterol stabilizes membranes, preventing excessive fluidity and ensuring the membrane retains its structural integrity.
      2. By limiting the lateral movement of phospholipids, cholesterol reduces the risk of the membrane becoming too porous.
   2. At Low Temperatures:
      1. Cholesterol prevents the solidification of saturated fatty acid tails, ensuring the membrane remains flexible.
      2. This prevents stiffening, which could restrict cell movement and lead to damage.