Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 7.2 Membrane Structure Results in Selective Permeability
Biological membranes do more than hold things in—they control what gets in and out. This selective permeability is critical for maintaining the internal conditions a cell needs to survive and function.
1. What Is Selective Permeability?
- Selectively permeable membranes allow some substances to cross more easily than others.
- This regulation supports nutrient uptake, waste removal, gas exchange, and ion balance.
- The lipid bilayer and embedded proteins work together to control permeability.
2. The Lipid Bilayer as a Barrier
- Phospholipid bilayer has a hydrophobic interior:
- Nonpolar molecules (O₂, CO₂, hydrocarbons) cross easily.
- Polar molecules (e.g., glucose, water) and ions cannot cross unaided.
- Water crosses slowly unless aided by transport proteins.
3. Transport Proteins: Helping Molecules Cross
- Channel proteins form hydrophilic tunnels.
- Example: Aquaporins transport billions of water molecules per second.
- Carrier proteins bind specific substances and change shape to transport them.
- Each transport protein is highly specific to its cargo.
4. Carbohydrates and Membrane Identity
- Glycolipids and glycoproteins serve as cell identity markers.
- These markers vary by species, individuals, and cell types (e.g., blood types).
5. Membrane Sidedness: Built with Direction
- Membranes have an asymmetrical structure.
- Asymmetry is established during synthesis:
- In the ER, carbohydrates are attached to proteins.
- In the Golgi apparatus, these carbohydrates are modified.
- Vesicles preserve this orientation during delivery to the membrane.
In a Nutshell
- Selective permeability allows cells to regulate their internal environment.
- The lipid bilayer permits small nonpolar molecules but blocks ions and large polar ones.
- Transport proteins enable efficient, specific transport.
- Membrane carbohydrates aid in cell recognition, and structural asymmetry is built-in during formation.