Transport in Cells » Diffusion
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.1.3.1
- What diffusion is and why it needs no energy from the cell
- Examples of diffusion in living things: oxygen, carbon dioxide and urea
- How concentration gradient, temperature and surface area affect the rate of diffusion
- How to explain a change in the rate of diffusion in an exam
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Unlock This CourseWhat is diffusion?
Cells need to take things in and get rid of things they do not need. Oxygen has to get in. Waste has to get out. Many of these substances move across the cell membrane by a process called diffusion.
Think about someone opening a bottle of perfume at one end of a room. After a short time, you can smell it at the other end. The perfume particles have spread out from where there are a lot of them to where there are few of them. That is diffusion.
Particles in a gas, or particles dissolved in a solution, are always moving about at random. Because there are more particles in the crowded area, more of them move out of it than move in. Overall, they spread out. We call this the net movement. Some particles do move the other way, but more move down than up.
Key terms:
- Diffusion: the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
- Concentration: how many particles of a substance there are in a certain space.
- Concentration gradient: the difference in concentration between two areas.
Diffusion in living things
Diffusion is how substances move into and out of cells across the cell membranes. The cell does not have to do any work for this to happen. Particles simply move from a higher concentration to a lower concentration, a bit like a ball rolling downhill.
You need to know three examples.
🌬 Oxygen
In gas exchange, oxygen diffuses into cells from the air or blood, where there is lots of it. It is used up inside cells, so there is less of it there.
☁ Carbon dioxide
Carbon dioxide is made by cells and builds up inside them. It diffuses out of the cells, where there is more of it, into the blood or air, where there is less.
💧 Urea
Urea is a waste product made in cells. It diffuses from the cells into the blood plasma. The plasma carries it to the kidney, where it is removed from the body. This is called excretion.
Notice the pattern. Each substance moves from where there is more of it to where there is less of it. That is the only direction of net movement in diffusion.
What affects the rate of diffusion?
The rate of diffusion is how fast a substance spreads out. You need to be able to explain how three factors change it.
1. The concentration gradient
The bigger the difference in concentration, the faster the diffusion. If one side is packed with particles and the other side has almost none, lots of particles move across every second. As the two sides become more alike, the net movement slows down.
Living things keep the gradient steep. For example, cells keep using up oxygen, so there is always less oxygen inside them than outside.
2. The temperature
The higher the temperature, the faster the diffusion. Particles gain energy when they are warmed, so they move around faster. They spread out more quickly and so the substance diffuses faster.
3. The surface area of the membrane
The larger the surface area of the membrane, the faster the diffusion. A bigger surface gives more room for particles to pass through at the same time. Think of a wide doorway letting a crowd out faster than a narrow one.
📈 Steeper gradient
Faster diffusion
🌡 Higher temperature
Faster diffusion
📐 Larger surface area
Faster diffusion
Explaining diffusion in an exam
Examiners want you to use the correct words and give a reason. A good answer has three parts: the factor, what it does to the rate, and why.
Worked example
Question: A drop of food colouring is put into a beaker of warm water and another into cold water. Explain why the colour spreads out faster in the warm water.
Answer: The warm water has a higher temperature, so the particles have more energy and move faster. This means the colouring particles spread out from the higher concentration to the lower concentration more quickly.
Common mistakes
Saying that particles "want" to move, or that the cell "pulls" them in. Particles just move at random, and the net movement is down the concentration gradient. Another mistake is to say that particles stop moving once both sides are equal. They keep moving, but there is no net movement.
Skills: drawing diffusion
Exam questions may ask you to draw or read a diagram of diffusion. Draw many dots on one side of a line (the membrane) and few on the other, with an arrow pointing from the crowded side to the less crowded side. The arrow always points down the concentration gradient.
Exam-style question
Urea is a waste product made in cells. It moves out of the cells into the blood plasma by diffusion.
(a) What is meant by diffusion? [2 marks]
(b) Explain why urea diffuses out of the cells into the blood plasma. [2 marks]
(c) Name one other substance that moves into or out of cells by diffusion. [1 mark]
(d) The temperature of a person's body rises. Suggest what happens to the rate of diffusion of urea, and why. [2 marks]
Model answer
(a) The net movement of particles (1) from an area of higher concentration to an area of lower concentration (1).
(b) There is a higher concentration of urea inside the cells than in the plasma (1), so there is a net movement of urea down the concentration gradient out of the cells (1).
(c) Oxygen or carbon dioxide (1).
(d) The rate increases (1) because the particles gain energy and move faster (1).
Exam tip
In part (b), say where the concentration is higher and where it is lower. Just writing "it moves from high to low" without naming the places loses the mark.