What is diffusion, and how does it move substances into and out of cells?
Define diffusion, describe how substances move down a concentration gradient, and explain its importance in living organisms
A simple, scaffolded answer to the N(A)-Level Biology outcome on diffusion. What diffusion is, how it moves substances down a concentration gradient without energy, the factors that affect its rate, and why it matters in the body.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
This outcome wants a clear definition of diffusion and an explanation of how it moves substances into and out of cells without using energy. You should be able to describe a concentration gradient, state which way particles move along it, and give biological examples such as oxygen entering the blood. You should also know the factors that change how fast diffusion happens.
The answer
What diffusion is
Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated, that is, down a concentration gradient. Particles are always moving randomly. Over time this random movement spreads them out until they are evenly mixed. The word net means the overall movement, because particles move both ways but more move from high to low.
Why no energy is needed
Diffusion is a passive process. The particles move on their own because of their natural random motion, so the cell does not have to use any energy from respiration. This is different from active transport, which does need energy.
The concentration gradient
A concentration gradient is the difference in concentration between two regions. The bigger the difference, the steeper the gradient, and the faster diffusion happens. When the concentrations become equal, there is no longer a gradient, so there is no more net movement, although the particles keep moving randomly.
Factors affecting the rate
Diffusion happens faster when:
- the concentration gradient is steeper (a bigger difference between the two regions);
- the surface area is larger (more room for particles to cross);
- the distance is shorter (a thinner barrier to cross);
- the temperature is higher (particles move faster).
Examples in context
Example 1. The smell of food spreading. When food is cooking, the smell particles spread from the kitchen (high concentration) throughout the house (low concentration) by diffusion, even with no fan. This everyday example shows particles moving from high to low concentration on their own, the same idea that moves gases in the lungs.
Example 2. Carbon dioxide leaving a working muscle. A muscle cell that is respiring makes carbon dioxide, so its concentration inside the cell is high. The blood carries less, so carbon dioxide diffuses out of the cell into the blood down the gradient, ready to be carried to the lungs and breathed out.
Try this
Q1. Define diffusion. [2 marks]
- Cue. The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
Q2. State whether diffusion needs energy from the cell, and give a reason. [2 marks]
- Cue. No; it is passive, because the particles move by their own random motion, so no energy from respiration is used.
Q3. Explain why the alveoli in the lungs have a very large surface area. [2 marks]
- Cue. A large surface area lets more oxygen diffuse into the blood at the same time, so gas exchange is faster.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksDefine diffusion. Then explain how oxygen moves from the air in an alveolus into the blood.Show worked answer →
Definition: diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until the particles are evenly spread.
For the alveolus: the air in the alveolus has a higher concentration of oxygen than the blood arriving in the capillary. So oxygen diffuses down the concentration gradient, across the thin alveolus wall and capillary wall, into the blood. The blood carries the oxygen away, which keeps the gradient steep so diffusion continues.
What markers reward: the full definition with the phrase higher to lower concentration (or down a concentration gradient), and an application that names the gradient (more oxygen in the alveolus than in the blood) and the direction of movement.
Original3 marksState three factors that increase the rate of diffusion.Show worked answer →
Three factors that increase the rate of diffusion are: a steeper concentration gradient (a bigger difference in concentration), a larger surface area, and a shorter distance to travel (a thinner barrier). A higher temperature also increases the rate, so it is an acceptable fourth answer.
What markers reward: any three correct factors. Be precise: it is a steeper gradient, not just a high concentration; and it is a shorter distance, not just a small cell. Vague answers like more particles do not score.
Related dot points
- Define osmosis as the movement of water across a partially permeable membrane and explain its effects on animal and plant cells
A scaffolded answer to the N(A)-Level Biology outcome on osmosis. What osmosis is, the partially permeable membrane, and what happens to animal and plant cells in dilute and concentrated solutions, including turgid, flaccid and plasmolysed states.
- Define active transport, explain why it requires energy, and describe examples such as mineral uptake by root hairs
A scaffolded answer to the N(A)-Level Biology outcome on active transport. Moving substances against the gradient using energy from respiration, how it differs from diffusion, and examples such as root hair mineral uptake and glucose absorption.
- Explain how gas exchange happens in the alveoli and describe how they are adapted for fast diffusion
A scaffolded answer to the N(A)-Level Biology outcome on gas exchange. How oxygen and carbon dioxide diffuse between the alveoli and the blood, and the four adaptations that make the alveoli efficient at this.
- Describe the parts of the human digestive system, the process of physical and chemical digestion, and the absorption of digested food
A scaffolded answer to the N(A)-Level Biology outcome on human digestion. The main parts of the digestive system, physical and chemical digestion by enzymes, and how digested food is absorbed in the small intestine.
- Identify the parts of animal and plant cells, state the function of each part, and describe the differences between animal and plant cells
A simple, scaffolded answer to the N(A)-Level Biology outcome on cell structure. The parts of animal and plant cells, what each part does, and the differences between the two cell types, with a clear comparison table.