2.4 Membrane Permeability
- Syllabus
- 2025
- Topic
- 2.4
- Level
- —
Learning objectives
2.4A—Explain how the structure of biological membranes influences selective permeabilityExplain how the structure of biological membranes influences selective permeability.• Plasma membranes separate the internal environment of the cell from the external environment. Selective permeability is the result of the plasma membrane having a hydrophobic interior.• Small nonpolar molecules, including N , O2 2 , and CO2 , freely pass across the membrane. Hydrophilic substances, such as large polar molecules and ions, move across the membrane through embedded channels and transport proteins.• The nonpolar hydrocarbon tails of phospholipids prevent the movement of ions and polar molecules across the membrane. Small polar, uncharged molecules, like HO2 or NH3 (ammonia), pass through the membrane in small amounts.2.4B—Describe the role of the cell wall in maintaining cell structure and functionDescribe the role of the cell wall in maintaining cell structure and function.• Cell walls of Bacteria, Archaea, Fungi, and plants provide a structural boundary as well as a permeability barrier for some substances to the internal or external cellular environments and protection from osmotic lysis.
The Membrane Interior Selects What Can Cross
Selective permeability arises because phospholipid hydrocarbon tails create a hydrophobic membrane interior. A substance crosses the bilayer easily only when its size and chemical properties are compatible with that interior; otherwise it needs a membrane protein or crosses only slowly.
| Substance class | Examples | Passage across the membrane |
|---|---|---|
| Small, nonpolar molecules | N2, O2, CO2 | Pass freely through the lipid bilayer |
| Small, polar, uncharged molecules | H2O, NH3 | Pass through in small amounts |
| Ions and large polar molecules | Charged or strongly hydrophilic substances | The hydrophobic core blocks direct passage; embedded channels or transport proteins provide a route |
Because different substances have different access, the plasma membrane can separate the cell's internal environment from the external environment while still allowing regulated exchange. A channel or transporter does not remove the hydrophobic barrier; it creates a specific hydrophilic pathway through it.
Selective does not mean completely impermeable. Small polar molecules can cross in limited amounts, while ions and large polar molecules generally require proteins.
Cell Walls Resist Osmotic Lysis
Bacteria, Archaea, Fungi, and plants have a cell wall outside the plasma membrane. The wall provides a structural boundary, acts as a permeability barrier to some substances, and helps the cell keep its shape.
When water enters a cell in a dilute external environment, the cell contents push outward. A cell wall resists excessive expansion, so the rising internal pressure does not simply stretch the plasma membrane until it ruptures. This mechanical resistance protects against osmotic lysis.
The cell wall is not a replacement for the plasma membrane and is not an absolute seal. It adds support and restricts some substances; the plasma membrane remains the cell's selectively permeable boundary.