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Diffusion, Osmosis and Active Transport

Diffusion moves particles down a concentration gradient, osmosis moves water through a partially permeable membrane and active transport uses energy from respiration to move substances against the gradient. You will also learn about factors affecting diffusion, surface area to volume ratio, exchange surfaces and the potato osmosis practical in AQA GCSE Biology Higher, specification references 4.1.3.1, 4.1.3.2, 4.1.3.3, 1.2, 1.5, 8.2.3, 2.1 and 2.2.
  • GCSE
  • AQA
  • Biology
  • Higher
  • Spec 4.1.3.1 · 4.1.3.2 · 4.1.3.3 · 1.2 · 1.5 · 8.2.3 · 2.1 · 2.2
  • 9 min read
  • 11 revision boxes

Introduction

1 box
1 min read

Three ways substances move in and out of cells

A cell membrane holds the cell together, but it does not seal it off. Membranes are partially permeable: they have very small holes in them, so some substances can pass through and others cannot.
Big molecules such as starch and proteins are simply too large to fit through those holes. Very small molecules do get through, including oxygen, glucose, amino acids and water. That is why a cell can take in what it needs and get rid of waste without falling apart.
There are three processes that move substances across the membrane: diffusion, osmosis and active transport. Keep one difference in mind as you work through them, because it is the thread that ties the page together: only active transport needs energy.

Learning

7 boxes
Definition

Diffusion

Diffusion is the spreading out of the particles of any substance in solution, or the particles of a gas, giving a net movement from an area of higher concentration to an area of lower concentration.
The particles are only moving about randomly, so they cross in both directions. But if there are a lot more particles on one side, more of them happen to move away from that side than towards it, so there is a net (overall) movement from higher to lower concentration. After a while the particles end up evenly spread out.
It happens in solutions and in gases, because in these the particles can move about. The simplest case is one gas diffusing through another: spray perfume in the corner of a room and the perfume particles gradually spread until you can smell it everywhere.
Perfume particles start clustered in one corner of a box of air and then spread out evenly through the whole box, showing net movement from higher to lower concentration.
In your body, oxygen and carbon dioxide are moved by diffusion in gas exchange, and the waste product urea diffuses out of cells into the blood plasma so it can be excreted in the kidney. Small dissolved substances cross the partially permeable cell membrane this way.
3 factors

Factors affecting the rate of diffusion

  • Concentration gradient

    The bigger the difference in concentration between the two sides, the faster the rate of diffusion.

  • Temperature

    A higher temperature gives a faster rate of diffusion, because the particles have more energy so move around faster.

  • Surface area of the membrane

    The larger the surface area of the membrane, the faster the rate of diffusion, because more particles can pass through at once.
2 min read

Surface area to volume ratio and exchange surfaces

A single-celled organism has a relatively large surface area to volume ratio. Diffusion alone transports enough molecules into and out of the cell to meet its needs.
As an organism gets bigger, its volume grows faster than its surface area, so multicellular organisms have a smaller surface area to volume ratio. Their outer surface cannot supply the whole inside, so they need specialised exchange surfaces and a transport system. Calculate surface area to volume ratio by dividing surface area by volume, then compare organisms.
A small single cell with arrows reaching its whole surface, next to a large block of many cells where arrows only reach the outer layer, showing that larger organisms have a smaller surface area to volume ratio and need exchange surfaces and a transport system.
An exchange surface works better with a large surface area and thin membrane, giving a short diffusion path. In animals, an efficient blood supply helps; for gas exchange, ventilation also helps. Examples are the small intestine and lungs in mammals, gills in fish and roots and leaves in plants.
Root hair cells have projections that stick into the soil. Each root branch is covered in millions of these microscopic hairs, giving the plant a large surface area for absorbing water and mineral ions.
2 min read

Osmosis

Osmosis is the movement of water molecules across a partially permeable membrane from a more dilute (less concentrated) solution to a more concentrated solution. If it helps, remember that 'more dilute' just means less concentrated.
It is a special case of diffusion. Water molecules are small enough to pass through the holes in a partially permeable membrane, but bigger molecules such as sucrose cannot fit through, so they stay where they are.
Water molecules move about randomly all the time, so they actually cross the membrane in both directions. But the dilute side has lots of water molecules and the concentrated side has fewer, so there is a steady net flow of water from dilute to concentrated. The concentrated solution therefore gets more dilute: the water behaves as if it is trying to even up the concentrations either side of the membrane.
Water molecules crossing a partially permeable membrane in both directions, with an overall net movement from the dilute solution into the concentrated sucrose solution, while the larger sucrose molecules are blocked by the membrane.
"\"5 steps\""

Required practical: osmosis in potato cylinders

If a cylinder has gained mass, water has been drawn into it by osmosis. If it has lost mass, water has been drawn out.
The independent variable is the concentration of the solution and the dependent variable is the mass of the cylinder. Keep the volume of solution, temperature and time the same for a fair test.
Calculate percentage change in mass to compare cylinders with different starting masses. A gain gives a positive percentage change and a loss gives a negative one. Plot percentage change in mass against solution concentration and interpret the trend.
Cylinders not fully dried carry excess water and give a mass that is too high. Water evaporating from a beaker changes the solution concentration. Repeat the experiment and calculate a mean percentage change at each concentration.
  1. Cut a potato into identical cylinders.
  2. Prepare beakers of sugar (or salt) solutions: pure water, a concentrated solution such as 1 mol/dm³ and concentrations in between, such as 0.2, 0.4 and 0.6 mol/dm³.
    Two beakers, one of pure water and one of concentrated sugar solution, each holding an identical potato cylinder for comparing mass change.
  3. Measure and record the initial mass of each cylinder.
  4. Put one cylinder in each beaker and leave them for twenty-four hours.
  5. Remove the cylinders, dry them with a paper towel and measure their masses again.
Percentage change in mass = ((final mass − initial mass) / initial mass) × 100.
TODO — content team to supply
2 min read

Active transport

Active transport moves substances the 'wrong' way: from a more dilute solution to a more concentrated solution, against the concentration gradient. Because it works uphill, it needs energy, and that energy comes from respiration.
Plant root hairs rely on it. The concentration of mineral ions is usually higher inside the root hair cells than in the soil around them, so if the minerals simply followed the concentration gradient they would move out of the cells. Active transport lets the plant pull mineral ions in from a very dilute soil solution instead, which it needs for healthy growth. Water enters a root hair cell by osmosis, but the minerals are taken in by active transport.
It happens in your gut too. When there is a higher concentration of glucose and amino acids in the gut than in the blood, they diffuse into the blood on their own. Sometimes the gut has the lower concentration, so the gradient is the wrong way round, and active transport moves the sugar molecules from the gut into the blood anyway. The glucose is then carried to your cells and used for respiration.
Glucose molecules being moved by active transport across the gut wall from a low concentration inside the gut into a higher concentration in the blood, using energy from respiration.
"\"1 min read\""

Comparing diffusion, osmosis and active transport

All three processes move substances into and out of cells, but they differ in what moves, which way it goes and whether the cell has to spend energy.
Key examinable difference: only active transport uses energy.
Process
What moves
Direction
Partially permeable membrane required?
Energy from respiration needed?
Diffusion
Particles
Higher to lower concentration, down the concentration gradient
No
No
Osmosis
Water molecules only
More dilute to more concentrated solution
Yes
No
Active transport
Substances
More dilute to more concentrated solution, against the concentration gradient
No
Yes

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A drop of ink is added to cold water, then to warm water

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What happens to the mass of potato in a concentrated salt solution?

Explain what will happen to the mass of a piece of potato added to a concentrated salt solution. [2 marks]
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What is the purpose of active transport in the gut?

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Common questions

Does osmosis move dissolved substances as well as water?
No. Osmosis is the movement of water molecules only through a partially permeable membrane. Larger dissolved molecules such as sucrose cannot pass through the membrane.
Why does active transport need energy?
Active transport moves substances against the concentration gradient, from a more dilute solution to a more concentrated one. Energy from respiration is needed to move them this way.
Why do multicellular organisms need specialised exchange surfaces?
As an organism gets bigger, its volume grows faster than its surface area, so its surface area to volume ratio becomes smaller. Specialised exchange surfaces provide a large surface area and a short diffusion path.