Before the Stethoscope: physiology logo Before the Stethoscope FIELD NOTES · CELLS · 02
Interactive physiology · chapter one

Meet the membrane: more than a wall.

Explore the membrane’s moving parts, follow different substances across it, and notice how a tiny boundary helps a cell stay connected to its surroundings. Select a structure in the diagram—or use the labels below it.

A fluid mosaic, up close

Tap or click a highlighted structure to learn what it does.

EXPLORE · SELECT · NOTICE
Interactive cell membrane diagramA phospholipid bilayer with cholesterol, channel and carrier proteins, a peripheral protein, glycoproteins and glycolipids. Click a structure to explore it. OUTSIDE THE CELLINSIDE THE CELL Glycoprotein Glycolipid Channel protein Carrier protein Peripheral protein Cholesterol Phospholipid bilayer
Hydrophilic heads face watery spaces.Hydrophobic tails face inward.
Protein or lipidCholesterol / carbohydratePeripheral protein
The foundation

Phospholipid bilayer

Each phospholipid has a water-attracting head and water-avoiding tails. In a watery environment, they arrange themselves into two layers, with the tails tucked inside.

Why this mattersThe bilayer creates a flexible boundary. Small non-polar molecules can pass through more easily than charged or strongly polar substances.
Look for the pattern: the membrane is selective, fluid and organised. Its parts work together; no single structure explains everything a cell can do.
Follow the movement

Choose a molecule. Predict its route.

Pick a substance to see how its size, charge and gradient shape its journey across the membrane.

Passive transport

Simple diffusion through the bilayer

Oxygen moves from higher to lower concentration. Because it is small and non-polar, it can dissolve in the hydrophobic interior of the membrane.

Down its concentration gradientNo ATP directly required

Across the bilayer

Watch the selected route.

MODEL · NOT TO SCALE
OUTSIDEINSIDE O₂ Na⁺

The particle moves down its concentration gradient through the lipid bilayer.

Pause and check

Can you read the membrane?

Question 1 of 3 · 0 correct

Choose an answer to see what your reasoning reveals.
Use the diagram as a map, not a verdict.

A model helps us predict how molecules may move. Real cells have changing gradients, many interacting proteins and conditions that a simple picture cannot show.

Bring the mechanism back to a person

Understanding a cell is one part of learning medicine. What questions might you ask before deciding what a membrane process means for a particular patient?

Before the Stethoscope
Science with a human centre.
Educational activity · not personal medical advice · Return to the series