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Transport in cells

Use diffusion, osmosis and active transport in unfamiliar contexts.

Start here

The key idea

Particles move by diffusion from high to low concentration.Osmosis is the movement of water through a partially permeable membrane.Active transport uses energy to move substances against a concentration gradient.

Three ways substances cross membranesDirection and energy use distinguish each transport process.
Three ways substances cross membranesDirection and energy use distinguish each transport process.diffusion: high to lowosmosis: water onlyactive: uses energy
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.It is a passive process, meaning it does not require energy from respiration.In living organisms, substances such as oxygen, carbon dioxide and urea move by diffusion across cell membranes.The rate of diffusion increases with a steeper concentration gradient, higher temperature and a larger surface area.

2

Osmosis

Osmosis is the movement of water molecules from a dilute solution (high water concentration) to a more concentrated solution (low water concentration) across a partially permeable membrane.Like diffusion it is passive and moves down a water concentration gradient.Osmosis is important for keeping cells hydrated and for water uptake by plant roots.

3

Active transport

Active transport moves substances from a more dilute solution to a more concentrated solution, that is against the concentration gradient.Because this is the opposite direction to diffusion, it requires energy released by respiration.Examples include the absorption of mineral ions into root hair cells and the absorption of glucose from the gut into the blood when glucose levels are already high in the blood.

4

Surface area and exchange

Single-celled organisms have a large surface area to volume ratio, so enough substances can diffuse across the membrane to meet their needs.Larger organisms have a small surface area to volume ratio and so need specialised exchange surfaces, such as alveoli and villi.These surfaces are adapted with a large surface area, thin walls (short diffusion distance) and a good blood supply to maintain a steep concentration gradient.

Key terms

Definitions to learn

Diffusion

The net movement of particles from a higher to a lower concentration, down a concentration gradient.

Osmosis

The movement of water across a partially permeable membrane from a dilute to a more concentrated solution.

Active transport

Movement of substances against the concentration gradient using energy from respiration.

Partially permeable membrane

A membrane that lets some molecules (e.g. water) pass through but not others.

Concentration gradient

The difference in concentration of a substance between two regions.

Surface area to volume ratio

The surface area of an organism divided by its volume; smaller organisms have a larger ratio.

Worked example

A plant cell is placed in a concentrated sugar solution and becomes flaccid. Explain why.

1

The solution outside has a lower water concentration.

2

Water moves out of the cell through the partially permeable membrane by osmosis.

3

The vacuole shrinks and the cell becomes flaccid.

Final answer

Water leaves the cell by osmosis, so it loses turgor.

Exam habit

Use 'osmosis' only for water. For all other particles write 'diffusion' or 'active transport'.State the direction and explain the concentration gradient in every answer.

Watch out

Osmosis refers only to water. Use diffusion for other particles.

Examiner tips

How to score full marks

  • 1Use the word 'osmosis' only for water — never for glucose, ions or oxygen.
  • 2Always state the DIRECTION of movement and the concentration gradient (high to low, or against the gradient) in every answer.
  • 3If the question says a process needs energy / respiration, the answer is active transport, not diffusion or osmosis.
Quick-fire quiz

Test yourself

Pick an answer — you'll see instantly if it's right.

Which process moves particles from a HIGH to a LOW concentration without using energy?

A plant cell is placed in pure water. What happens?

Active transport moves mineral ions into root hair cells. Which statement is correct?

What is the term for the movement of water through a partially permeable membrane?

A student says 'glucose enters cells by osmosis'. What error have they made?

Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Why does a root hair cell use active transport to absorb mineral ions?
Mark scheme
  1. 1.Compare ion concentrations in soil and cell.
  2. 2.State the energy requirement.
It absorbs ions from a lower concentration in the soil to a higher concentration in the cell using energy.
2Give two factors that increase the rate of diffusion.
Mark scheme
  1. 1.Think gradient, temperature and surface area.
Any two from: steeper concentration gradient, higher temperature, larger surface area.
3A potato cylinder gains mass in dilute solution. Explain why.
Mark scheme
  1. 1.Identify movement of water.
  2. 2.State the direction of the water concentration gradient.
Water enters the potato cells by osmosis because the surrounding solution has a higher water concentration.
4Define diffusion.[1 mark]
Mark scheme
  1. 1.Recall the precise definition including direction.
The net movement of particles from an area of higher concentration to an area of lower concentration (down a concentration gradient) (1).
5Name the process by which water moves into a plant root cell.[1 mark]
Mark scheme
  1. 1.Movement of water across a membrane.
Osmosis (1).
6Explain why active transport requires energy but diffusion does not.[3 marks]
Mark scheme
  1. 1.Compare the direction of movement relative to the concentration gradient.
Active transport moves substances against the concentration gradient / from low to high concentration (1), so energy from respiration is needed (1); diffusion moves substances down the concentration gradient so happens passively without energy (1).
7Describe two adaptations of an effective exchange surface and explain how each increases the rate of exchange.[4 marks]
Mark scheme
  1. 1.Link each feature to the factors that affect rate of diffusion.
Large surface area (1) gives more space for substances to diffuse across (1); thin walls / short diffusion distance (1) so substances cross more quickly (1); good blood supply / ventilation (1) maintains a steep concentration gradient (1). (Any two adaptations with explanation.)
8A piepiece of potato is placed in pure water and gains mass; an identical piepiece placed in concentrated sugar solution loses mass. Explain these results in terms of osmosis.[4 marks]
Mark scheme
  1. 1.Compare water concentration inside the potato cells with each solution.
In pure water the water concentration is higher outside than inside the cells (1), so water moves into the cells by osmosis across the partially permeable membranes (1), increasing the mass. In concentrated sugar solution the water concentration is lower outside than inside the cells (1), so water moves out of the cells by osmosis (1), decreasing the mass.
9State what is meant by the term 'net movement' in the definition of diffusion.[2 marks]
Mark scheme
  1. 1.Think about the direction of overall particle movement.
Particles move randomly in all directions (1) but the overall / net movement is from higher to lower concentration because more particles move down the gradient than up it (1).
10Explain why the rate of diffusion is faster in smaller organisms than in larger organisms.[3 marks]
Mark scheme
  1. 1.Link organism size to surface area to volume ratio.
Smaller organisms have a larger surface area to volume ratio (1); so a greater area of membrane is available relative to the volume of the cell (1); meaning enough substances can diffuse in and out to meet the cell's needs more quickly (1).
11Root hair cells absorb nitrate ions from the soil even though the concentration of nitrate ions is lower in the soil than inside the root hair cell. Name the process involved and explain why it can work against the concentration gradient.[3 marks]
Mark scheme
  1. 1.Identify the transport process.
  2. 2.Explain the energy source.
Active transport (1); this process uses energy released by respiration (1) to move the nitrate ions from a region of lower concentration in the soil to a region of higher concentration inside the cell (1), against the concentration gradient.
12A student places red blood cells into a solution that is less concentrated than the cells' cytoplasm. Predict and explain what happens to the cells.[3 marks]
Mark scheme
  1. 1.Identify the relative water concentrations.
  2. 2.Predict the direction of osmosis.
  3. 3.State the consequence for the cell.
The solution outside is more dilute / has a higher water concentration than the cytoplasm (1); water moves into the cells by osmosis (1); the cells swell and may burst / lyse (1).
13A student investigating osmosis records the following data: potato cylinders in 0.0 mol/dm³ solution gained 8% mass; in 0.2 mol/dm³ gained 3% mass; in 0.4 mol/dm³ lost 2% mass; in 0.6 mol/dm³ lost 7% mass. Estimate the concentration at which there is no net movement of water and explain your reasoning.[3 marks]
Mark scheme
  1. 1.Identify where mass change crosses zero.
  2. 2.Link to equilibrium water concentration.
The mass change crosses zero between 0.2 and 0.4 mol/dm³, estimated at approximately 0.3 mol/dm³ (1); at this concentration the water concentration of the solution equals the water concentration of the potato cells (1); so there is no net movement of water by osmosis in either direction (1).
14Compare and contrast diffusion and active transport as methods of moving substances across cell membranes.[4 marks]
Mark scheme
  1. 1.Give similarities.
  2. 2.Give differences including direction, energy and examples.
Both move substances across cell membranes (1); diffusion moves substances down the concentration gradient and requires no energy (1); active transport moves substances against the concentration gradient and requires energy from respiration (1); diffusion is used for oxygen and carbon dioxide exchange; active transport is used for glucose absorption from the gut and mineral ion uptake in roots (1).
15Evaluate the statement: 'All substances enter and leave cells by diffusion.' Use your knowledge of diffusion, osmosis and active transport in your answer.[5 marks]
Mark scheme
  1. 1.Assess what is correct about the statement.
  2. 2.Identify the two exceptions and why they matter.
The statement is incorrect because not all substances use diffusion (1); water moves by osmosis, which is a specific type of diffusion only applicable to water across a partially permeable membrane (1); some substances such as mineral ions are absorbed against the concentration gradient by active transport, which requires energy (1); however diffusion is correct for substances such as oxygen, carbon dioxide and urea (1); overall the statement oversimplifies transport and ignores both osmosis and active transport (1).
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