Cell BiologyExplorer14 minLesson 4 of 7

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.

The big idea

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.

HIGH concentrationLOW concentrationDownhill — FREE (no ATP needed)Uphill — costs ATP

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
EXTRACELLULARINTRACELLULARHydrophilic head(phosphate — loves water)Hydrophobic tails(fatty acids — repel water)Membrane protein(fluid mosaic model)~7nm
Phospholipid Structure

Click a tab to explore each transport type

Bilayer

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

TypeProtein?ATP?
Simple diffusionNoNo
Facilitated diffusionYesNo
Active transportYesYes
OsmosisAquaporinNo

Key concepts

  • Selectively permeable

    The membrane is not a wall — it is 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.

Real world

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

  1. 1

    Phospholipid bilayer — Two leaflets of phospholipids, hydrophilic heads outward, hydrophobic tails inward. ~7 nm thick.

  2. 2

    Fluid mosaic model — Membrane proteins float freely within the bilayer, giving it a mosaic appearance.

  3. 3

    Simple diffusion — Small non-polar molecules (O₂, CO₂) dissolve into and cross the lipid core. No protein, no ATP.

  4. 4

    Facilitated diffusion — Polar molecules and ions use channel or carrier proteins. Still down the gradient — no ATP.

  5. 5

    Active transport — Pumps (e.g. Na⁺/K⁺ ATPase) use ATP to move ions against their gradient — essential for membrane potential.