5.4 Cell Transport
The plasma membrane is differentially permeable, which means that some substances move freely across the membrane but others are restricted.
Substances can enter cells in three ways.
1. Passive transport
2. Active transport
3. Bulk transport
Passive Transport: No Energy Required
Simple diffusion occurs when the solute (a substance dissolved in a liquid solvent) moves from a higher concentration to a lower concentration.
Simple diffusion occurs until equilibrium is reached.
In the example below, the dye molecules can cross the membrane and are moving from the right side to the left side.
RESIZE B and remove copyright from B
Simple diffusion is passive because it does not require energy.
Small, uncharged molecules such as oxygen, carbon dioxide, and some ions cross cell membranes by simple diffusion.
Larger molecules that are not soluble in lipids cross membranes by facilitated diffusion.
Facilitated diffusion is also passive transport but these substances require some help (a carrier molecule) to get them across the lipid center section of the cell membrane.
Membrane proteins assist the movement of the molecule across the membrane. They work like doorways that can be opened to let substances (like glucose) across the membrane.