Selective permeability depends on molecule properties and membrane proteins, separating direct bilayer diffusion from protein-mediated transport; This selectivity explains why different substances need different crossing mechanisms.
Coverage 2025–2025 · Updated 15 Jul 2026
Membrane permeability depends on both the lipid bilayer and transport proteins. Simple diffusion is not protein-selective; facilitated diffusion and active transport are selective because specific proteins recognize particular particles.
For simple diffusion, crossing rate follows physical properties such as particle size and hydrophobicity. Channels, carriers and pumps add binding sites or pores that admit selected ions or molecules under defined gradient and energy conditions.
| Pathway | Main selectivity basis | Energy/direction |
|---|---|---|
| Simple diffusion | Size and hydrophobic/hydrophilic properties | Passive, down gradient |
| Facilitated diffusion | Specific channel or carrier structure | Passive, down electrochemical gradient |
| Active transport | Specific pump/carrier binding and coupling | Can move against gradient using energy |
A small non-polar gas crosses the lipid core, whereas glucose usually requires a specific carrier and sodium requires a selective channel or pump.
Selective permeability is not biological intention. It emerges from molecular interactions and the transport proteins present in that membrane.
This objective is assessed through multiple choice, commonly using Identify.
Identify
Build the answer around this relationship: Selective permeability depends on both solute properties and membrane components.
Saying facilitated diffusion requires ATP because it uses a protein.
Representative question
Which molecule is paired with the component of the cell membrane that allows it to pass through the membrane?
Molecule
Component of the
cell membrane
insulin
aquaporin
glycogen
channel protein
oestradiol
phospholipid bilayer
carbon dioxide
pump protein
The SL membrane model is a decision system. The bilayer forms because phospholipids are amphipathic, and the hydrophobic core creates selective permeability. Small non-polar molecules diffuse directly; water moves by osmosis and often through aquaporins; ions and polar molecules use channels or transporters; pumps use ATP for movement against gradients. Proteins and glycocalyx components add transport, recognition, and model evidence.
