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 2024–2024 · Updated 15 Jul 2026
A membrane is selectively permeable because each solute’s size, charge, polarity and available transport pathway determine whether and how fast it crosses.
The lipid core favors small non-polar solutes, while channels and carriers add controlled routes for particular polar or charged substances. Selectivity is therefore a property of the whole membrane, not lipids alone.
Judge a crossing claim using: solute chemistry, membrane core, matching protein, and gradient/energy condition.
A glucose molecule is polar and large, so it usually needs a specific carrier even when a small non-polar gas can cross directly.
Selective permeability is not the same as biological ‘choice’ or intention; it follows physical interactions and protein specificity.
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.
