Phospholipids Are Amphiphatic Molecules
- Phospholipids are amphipathic molecules, meaning they have both hydrophilic and hydrophobic regions:
- Hydrophilic Head: The phosphate group forms the head, which is polar and interacts readily with water.
- Hydrophobic Tails: Two hydrocarbon tails that are non-polar and repel water.
- This dual nature allows phospholipids to form a bilayer in aqueous environments, with hydrophilic heads facing outward toward water and hydrophobic tails clustered inward, away from water.
Definition
Amphiphatic
A molecule with both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions, such as a phospholipid.
The Hydrophobic Core and Permeability
- The hydrophobic core formed by the phospholipid tails limits the membrane’s permeability to certain substances:
- Hydrophilic Particles: Ions and polar molecules, such as glucose, cannot easily cross due to their affinity for water.
- Large Molecules: Substances like glycogen and proteins are too large to pass through without assistance.
- Think of the hydrophobic core as a security gate: only small, non-polar molecules like oxygen (O₂) and carbon dioxide (CO₂) can pass freely.
- Larger or polar molecules need "keys" like transport proteins.
Interaction with Aqueous Solutions
- Cell membranes are surrounded by aqueous solutions on both sides:
- External Solution: Contains water and dissolved solutes.
- Internal Solution (Cytoplasm): Also a liquid environment with dissolved solutes.
- Water and small solutes move randomly.
- Solutes near the membrane surface may attempt to penetrate the hydrophilic phosphate heads.
- However, upon encountering the hydrophobic core, they are repelled back into the surrounding aqueous solution.
- Water molecules, despite being polar, are small enough to pass through the membrane at a slow rate.
- In contrast, larger molecules like glucose need transport proteins to cross.
- Why can water pass through the membrane more easily than glucose?
- How does the amphipathic nature of phospholipids contribute to membrane function?
