How does a cell choose what crosses its boundary?

Oxygen can enter a cell while some small charged particles need a special passage because the membrane's chemistry makes crossing easier for some substances than others. This selective permeability depends on the lipid barrier and transport proteins, rather than the cell consciously choosing individual particles.

The lipid bilayer is two layers of molecules with water-interacting heads facing the watery surroundings and oily tails facing inward. That oily interior is a chemical barrier. Small uncharged oxygen molecules can enter and pass through it relatively readily. A charged ion interacts strongly with water and crosses the oily region poorly, even though the ion may be small.

A channel protein provides a water-compatible passage through this barrier. Its structure favors particular ions, so an open matching channel can permit passage without making the whole membrane leaky. A route and a driving difference are separate requirements. In the pictured oxygen comparison, higher outside concentration means more oxygen molecules are available to cross inward than outward, giving a net inward transfer.

A net inward movement is a balance of crossings, not a ban on outward motion. Random movement can continue in both directions even at equal concentrations, when no concentration-driven net transfer remains. Electrical forces also influence real ions; this drawing leaves them aside to show how a suitable route and a concentration difference work together.