4. The Main Event: Controlling Traffic Across the Membrane
One of the plasma membrane’s most vital roles is to act as a semipermeable barrier. This means it allows some substances to pass through freely while restricting the passage of others, giving the cell precise control over its internal composition.
4.1 Passive Transport: Going with the Flow
Passive transport processes do not require the cell to expend energy. Instead, they rely on the natural tendency of molecules to move down their concentration gradient—from an area of higher concentration to an area of lower concentration.
- Simple Diffusion: This is the direct movement of small, nonpolar molecules (like oxygen and nitrogen) and small, uncharged polar molecules (like water, carbon dioxide, and glycerol) across the lipid bilayer. The molecules simply slip between the phospholipids.
- Facilitated Diffusion: This process is required for ions and larger polar molecules that cannot cross the lipid bilayer on their own. It is faster than simple diffusion and relies on the help of membrane proteins.
- Ion Channels: These proteins form small aqueous pores, or tunnels, through which specific ions can pass.
- Carrier Proteins: These proteins bind to a specific molecule and then undergo a shape change to shuttle it across the membrane.
4.2 Active Transport: Moving Against the Current
Active transport is the process of moving molecules against their concentration gradient, from a region of lower concentration to one of higher concentration. Because this is an uphill battle, it requires the cell to expend energy, typically from the hydrolysis of ATP.
The classic example of active transport is the Na⁺-K⁺ pump. This carrier protein acts as an antiport system, meaning it moves two different substances in opposite directions.
- For every one molecule of ATP it uses, the pump moves three sodium ions (Na⁺) out of the cell and two potassium ions (K⁺) in.
While this pump plays a role in establishing an electrical potential across the membrane, its primary function is to maintain constant cell volume. It achieves this by decreasing the intracellular ion concentration and thus the osmotic pressure, which prevents an excessive flow of water into the cell that could cause it to swell and burst.