Membrane Permeability (College Board AP® Biology): Study Guide
Selective permeability
Cell membranes separate the internal environment of the cell from the external environment
The structure of biological membranes means that they are selectively permeable:
A selectively permeable membrane allows some substances to cross while blocking the passage of other substances
Selective permeability allows cell surface membranes to control which substances can cross between the external and internal environment
The selective permeability of membranes allows for the formation of solute concentration gradients, e.g.:
a membrane may prevent the passage of a substance, resulting in a higher concentration on the supply side of the membrane
a membrane may contain proteins that actively pump a substance from one side to the other, allowing a high concentration to be built up on one side
Membrane structure determines the permeability of the cell membrane
The fatty acid tails of phospholipids are nonpolar, meaning that charged or polar substances cannot pass directly through the membrane
Phospholipids are tightly packed together, blocking the passage of larger molecules
Transport proteins, such as carrier proteins and channel proteins, allow the passage of large and/or polar substances, and ions, across membranes; the types of these substances that can cross are determined by the number and type of transport proteins that are present
Features of substance | Membrane transport | Example(s) |
|---|---|---|
Small, non-polar | Can pass freely between the phospholipids | N2, O2, and CO2 |
Large, polar / ions | Can only move across the membrane via transport proteins | Glucose and sucrose Na+ and Ca2+ |
Small, polar | Can pass between phospholipids in small volumes Water relies mainly on aquaporins to cross cell membranes | H2O |
The role of cell walls
Cell walls of plants, prokaryotes, and fungi are composed of complex carbohydrates, e.g. cellulose in plants
Cell walls have a structural role, providing cells with some rigidity
While cell walls are freely permeable to small molecules, they can also provide a permeability barrier for some substances:
Large molecules may be unable to cross
The rigid nature of cell walls limit the volume of water that can be taken up by a cell, preventing cells from bursting due to water uptake
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