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?
A membrane is selectively permeable when it allows some substances to pass through more easily than others.
This selective access is vital for nutrient uptake, waste removal, gas exchange, and ion regulation.
The lipid bilayer and embedded proteins together determine what can cross the membrane and how.
2. The Lipid Bilayer as a Barrier
The phospholipid bilayer is hydrophobic at its core, meaning:
Nonpolar molecules like O₂, CO₂, and hydrocarbons cross easily.
Polar molecules (e.g., glucose, water) and ions struggle to pass through unaided.
Even water crosses slowly, unless transport proteins assist.
3. Transport Proteins: Helping Molecules Cross
Channel proteins form hydrophilic tunnels for specific molecules or ions.
Example: Aquaporins, which facilitate water transport—up to 3 billion water molecules per second per channel.
Carrier proteins bind their cargo and change shape to shuttle it across.
Each transport protein is highly specific, only moving certain substances (e.g., a glucose transporter won’t carry fructose).
4. Carbohydrates and Membrane Identity
Glycolipids and glycoproteins on the outer surface of the plasma membrane act as cell identity markers.
These carbohydrate tags vary among species, individuals, and cell types (e.g., blood type A, B, AB, O).
5. Membrane Sidedness: Built with Direction
Membranes have an asymmetrical structure—the inside and outside faces differ in composition.
This asymmetry is established during synthesis:
In the ER, carbohydrates are added to proteins.
In the Golgi, those carbohydrates are modified.
Vesicles transport the proteins to the plasma membrane, where the orientation is preserved.
In a Nutshell
The selective permeability of membranes enables the cell to regulate its internal environment.
The lipid bilayer lets nonpolar molecules pass but restricts ions and polar substances.
Transport proteins enable specific, rapid transport of essential molecules.
Membrane carbohydrates help distinguish cells, and membrane asymmetry is established during formation and trafficking.