Cell Membrane & Transport: How Things Get In and Out

The membrane is a smart fence, not a wall

The cell membrane decides what enters and what leaves the cell. It looks simple but does five jobs at once: it’s a barrier, a checkpoint, a signal receiver, an identity tag, and a transport hub. Two structural features make all of this possible — and both are tested directly on the CBE.

The phospholipid bilayer

Every cell membrane is built from two layers of phospholipids. Each phospholipid has:

• A hydrophilic head (loves water) — faces outward toward the watery environment.
• Two hydrophobic tails (avoid water) — face inward toward each other.

The result is a sandwich: heads outside, tails inside. This bilayer alone blocks anything large or charged. Small uncharged molecules (oxygen, carbon dioxide, water) slip through; ions and big molecules cannot.

Transport proteins fill the gaps

The bilayer alone can’t move ions, sugars, or amino acids. That’s where transport proteins come in. They span the bilayer and act as channels or pumps:

• Channel proteins — open passages for specific molecules to flow through (passive).
• Carrier proteins — bind a molecule on one side and release it on the other.
• Pumps — use ATP to move molecules against their concentration gradient.

This combination — phospholipid bilayer + transport proteins — is what makes the membrane selectively permeable. That phrase shows up verbatim on the CBE.

Diffusion vs. osmosis vs. active transport

The three solution words you must memorize

• Hypotonic — lower solute outside the cell. Water rushes in. Cell swells (and may burst).
• Hypertonic — higher solute outside the cell. Water rushes out. Cell shrinks.
• Isotonic — equal solute. No net water movement. Cell stays the same size.

Check yourself

Practice with CBE-style practice questions

• Which feature gives the membrane selective permeability?
• Diffusion vs. osmosis: which best describes water across a semi-permeable membrane?
• What is the function of the cell membrane?