Cell Membranes and Transport

The cell's selective boundary

The plasma membrane surrounds the cell and controls exchange with the environment.

It is not a simple wall. It is a dynamic boundary made mostly of phospholipids and proteins. Some substances pass through easily, while others need help or energy.

The phospholipid bilayer

Phospholipids have water-loving heads and water-avoiding tails.

In water, they arrange into a bilayer: heads face outward toward watery environments, and tails point inward away from water. This structure creates a barrier that blocks many charged or polar molecules.

Proteins embedded in the membrane provide many of the membrane's specific functions.

Selective permeability

The membrane is selectively permeable.

Small nonpolar molecules, such as oxygen and carbon dioxide, can cross relatively easily. Charged ions and many large polar molecules cannot simply slip through the lipid interior.

That selectivity lets cells maintain different internal conditions from the outside environment.

Passive transport

Passive transport does not require direct cellular energy input.

In diffusion, molecules move from an area of higher concentration to an area of lower concentration. In osmosis, water moves across a membrane, often toward the side with more dissolved solute.

Facilitated diffusion also moves substances down a gradient, but it uses membrane proteins such as channels or carriers.

Active transport

Active transport moves substances against a gradient.

That means the cell must spend energy directly or indirectly. Pumps can use ATP to move ions, while other transport systems use an existing gradient to drive movement of another substance.

This is how cells can maintain ion differences that are essential for nerve impulses, muscle contraction, and many signaling systems.

Bulk transport

Some materials are too large for channels or carriers.

Cells can use vesicles for bulk movement:

• Endocytosis brings material into the cell.
• Exocytosis releases material out of the cell.

These processes are especially important for secretion, membrane recycling, and uptake of larger particles.

The key idea

Membranes are active control surfaces.

They define the cell, but they also connect the cell to the world around it by deciding what crosses, when, and how.