Cell Membrane & Transport
Every cell is surrounded by a membrane that decides what gets in and what stays out. The secret to how it works is one idea: concentration gradients. Learn the structure of the membrane and the three ways molecules cross it.
Concentration gradients are like hills
Picture concentration as elevation. Where molecules are packed together in high numbers, they are sitting at the top of a hill. Left alone, they tumble downhill — from high concentration to low — for free, just like water flows downhill. That is passive transport. Moving molecules uphill, against their natural tendency, requires a push. That push is powered by ATP. That is active transport.
⬇ Downhill — Passive transport
High → Low concentration. No energy needed — the gradient itself does the work, just like releasing a ball at the top of a hill. The cell simply opens a path and molecules flow on their own.
- Simple diffusion — small molecules slip straight through the lipid core
- Facilitated diffusion — channel proteins open a hydrophilic gate
- Osmosis — water moves down its own concentration gradient
⬆ Uphill — Active transport
Low → High concentration. Molecules must be pushed against their natural tendency — like rolling a boulder uphill. Protein pumps use the energy from splitting ATP to force this movement.
- Na⁺/K⁺ pump — maintains the voltage across nerve and muscle cells
- H⁺ pump — concentrates acid in the stomach
- Calcium pumps — allow muscles to relax after each contraction
Click a tab to explore each transport type
Phospholipid Structure
The cell membrane is a phospholipid bilayer — two sheets of phospholipid molecules arranged tail-to-tail. The hydrophilic (water-loving) phosphate heads face outward toward aqueous environments, while the hydrophobic (water-fearing) fatty-acid tails face inward, forming an oily core that blocks most polar molecules.
- Each phospholipid has a polar, charged head and two non-polar fatty-acid tails
- The bilayer is ~7 nm thick and fluid at body temperature
- Membrane proteins float in the bilayer — the fluid mosaic model (Singer & Nicolson, 1972)
- Cholesterol is embedded between phospholipids, regulating membrane fluidity
Transport types at a glance
| Type | Protein? | ATP? | Direction |
|---|---|---|---|
| Simple diffusion | No | No | High → Low |
| Facilitated diffusion | Yes | No | High → Low |
| Active transport | Yes | Yes | Low → High |
| Osmosis | Aquaporin | No | High H₂O → Low H₂O |
Key concepts
Selectively permeable
The membrane is not a wall — it's a selective filter. Small, non-polar molecules cross freely; large or charged molecules need protein help or active pumping.
Downhill is free
Moving from high to low concentration (down the gradient) requires zero energy — it happens spontaneously, like water flowing downhill. Both forms of diffusion exploit this.
Uphill costs ATP
Pushing molecules from low to high concentration — against their gradient — is like rolling a boulder uphill. Protein pumps use ATP hydrolysis to supply that force.
When transport fails
Cystic fibrosis is caused by a misfolded CFTR chloride channel — mucus becomes dangerously thick because Cl⁻ cannot exit cells normally. Digitalis (a heart drug) works by blocking the Na⁺/K⁺ pump in cardiac muscle, slowing the heart. Cholera toxin forces CFTR channels open, causing catastrophic water loss through osmosis into the gut. Membrane transport is medicine.
Quick recap
- 1Phospholipid bilayer — Two leaflets of phospholipids, hydrophilic heads outward, hydrophobic tails inward. ~7 nm thick.
- 2Fluid mosaic model — Membrane proteins float freely within the bilayer, giving it a mosaic appearance.
- 3Simple diffusion — Small non-polar molecules (O₂, CO₂) dissolve into and cross the lipid core. No protein, no ATP.
- 4Facilitated diffusion — Polar molecules and ions use channel or carrier proteins. Still down the gradient — no ATP.
- 5Active transport — Pumps (e.g. Na⁺/K⁺ ATPase) use ATP to move ions against their gradient — essential for membrane potential.