Diffusion, Osmosis & Active Transport
Cambridge IGCSE Biology — Complete Revision Guide
The Big Idea: Cells move substances across their membranes in three main ways: two that don't need energy (diffusion and osmosis), and one that does (active transport).
Quick Summary
This chapter covers how substances move into and out of cells. You need to understand three transport mechanisms, what controls how fast they work, and why each one matters to living organisms.
Diffusion
Random movement of particles from high to low concentration. No energy needed. Happens in all directions but net movement is one way.
Osmosis
Diffusion of water specifically, across a partially permeable membrane. Water moves toward salt/sugar. No energy needed.
Active Transport
Particles move against concentration gradient (uphill). Requires energy from respiration. Cells use proteins to pump things in.
Diffusion in Biology
What is Diffusion?
Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration. This happens because all particles move randomly all the time—they bounce around. On average, more end up moving one way than the other.
Definition: Diffusion
The net movement of particles down a concentration gradient, driven by their random motion. Energy comes from the particles' own movement (Brownian motion), not from the cell.
Key point: Diffusion is always passive—no cell energy (ATP) is used. The particles have kinetic energy from heat, and they use that to bump around.
Diffusion Across Cell Membranes
The cell membrane is partially permeable. This means it lets some molecules through easily, but blocks others. The main rule:
- Small molecules (oxygen, carbon dioxide, glucose) can pass through.
- Large molecules (proteins) and ions (depending on charge) are blocked or move slowly.
The membrane is made of a lipid bilayer—a double layer of fat-like molecules with embedded proteins. Small molecules can slip between or through the lipids. Larger ones get stuck.
Why Diffusion Matters to Living Cells
Cells rely on diffusion for:
- Gas exchange: Oxygen diffuses in for respiration; CO₂ diffuses out as waste.
- Nutrient uptake: Glucose and amino acids diffuse into cells.
- Waste removal: Urea diffuses out of cells into the blood.
Analogy: Diffusion is like a crowd leaving a concert
People push toward exits randomly—not organized, just each person moving independently. More people move out (from high crowd density) than in (from low crowd density). Eventually, the crowd spreads evenly through the streets. No one is "guiding" people; they just naturally spread out.
Energy Source: Brownian Motion
Brownian motion is the random, continuous movement of particles caused by their thermal energy (heat). At the atomic scale, particles never stop moving—they collide with each other constantly. This is the energy source for diffusion.
You don't need to understand the physics in detail, but remember: diffusion doesn't require the cell to use ATP; the heat energy already in the particles drives it.
Examples of Diffusion in Living Organisms