The hydrophobic core of a lipid bilayer limits passage of polar, charged, or large substances while separating aqueous compartments; This barrier property explains why controlled transport proteins are necessary for many solutes.
Coverage 2017–2018 · Updated 15 Jul 2026
The bilayer’s hydrophobic interior slows ions and most polar molecules, while small non-polar molecules cross more readily.
Crossing depends on how well a solute interacts with the tail region and on its size. The membrane can therefore separate concentration gradients without blocking every substance equally.
Use three checks: charge/polarity, size, and whether a transport protein is available. A matching pathway can change the prediction.
Oxygen can diffuse through the tail core, but a sodium ion needs a channel or carrier because its charge is incompatible with the hydrophobic interior.
‘Barrier’ does not mean ‘impermeable’. Selectivity comes from different crossing rates, not from stopping all movement.
This objective is assessed through multiple choice, commonly using Identify.
Identify
Build the answer around this relationship: The bilayer core is hydrophobic because fatty acid tails point inward.
Treating polar glucose as able to pass directly through the phospholipid core.
Representative question
Neural pathways in living brains can now be mapped by tracking the movement of water molecules inside axons. What keeps water molecules inside axons?
Plasma membrane
Hydrogen bonding
Pump proteins
Synapse
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.
