Lectures on Biological Membranes and Transport
Biological Membranes: Structure, Function, and Transport
I. Composition and Basic Structure
- Primary Macromolecule: Biological membranes are primarily composed of phospholipids.
- Phospholipid Structure:
- Hydrophilic Head: Contains a phosphate group (PO_4^{3-}) which has a negative charge, making it attracted to water.
- Hydrophobic Tail: Composed of long chains of carbon and hydrogen atoms, linked by non-covalent and uncharged bonds, making it repelled by water.
- Membrane Structure: Due to their amphipathic nature (having both hydrophilic and hydrophobic regions), phospholipids spontaneously form a phospholipid bilayer.
- The hydrophilic heads face outwards (towards aqueous environments), and the hydrophobic tails face inwards (forming the core of the membrane).
- This structure creates a barrier where the center is hydrophobic, and the exterior/interior surfaces are hydrophilic.
II. Detailed Membrane Composition and Fluidity
- Beyond Phospholipids: Membranes are more complex, composed of a combination of:
- Phospholipids
- Other lipids (e.g., cholesterol)
- Proteins
- Other structures (e.g., carbohydrates).
- Structure-Function Relationship: The specific composition of a membrane directly relates to its function.
- Example (Mitochondria): Mitochondria have two membranes. The inner membrane, where cellular respiration occurs, is rich in enzymes crucial for energy production, demonstrating how specialized protein content dictates function.
- Amphipathic Nature: Many membrane components, including phospholipids and most proteins embedded within, are amphipathic.
