K325 / 2027

Lesson 5 of 7 / Movement of substances

Diffusion, osmosis or active transport?

Which clues tell you which process is happening?

I can choose the process using the substance, membrane and direction of movement.

  • I can explain diffusion and its biological applications.Syllabus

    Syllabus K325, Topic 2(a). Diffusion and its biological applications.

  • I can explain osmosis, tissue responses and investigations.Syllabus

    Syllabus K325, Topic 2(b). Osmosis, tissue responses and investigations.

  • I can explain energy-dependent uptake in roots and villi.Syllabus

    Syllabus K325, Topic 2(c). Energy-dependent uptake in roots and villi.

About 3 min

Make a guess. You are not marked.

A root hair cell takes in water and mineral ions at the same time. How many different transport processes are involved?

  1. One: osmosis, because both cross a membrane.
  2. Two: osmosis for water, active transport for ions.
  3. One, because both enter the same cell together.
Show the answer

Two: osmosis for water, active transport for ions.

Each substance has its own process. Name the substance first, then decide using its gradient.

The key idea

Name the substance first. Then check the membrane, the gradient and whether energy from respiration is needed.

Explore the idea

What changes between diffusion, osmosis and active transport?

Compare one feature

Diffusion

HigherLowerParticle concentration

Net movement down the concentration gradient of the substance.

Osmosis

HigherLowerWater potential

Net movement of water from higher water potential to lower water potential.

Active transport

LowerHigherParticle concentration

Moves a substance against its concentration gradient, from lower to higher concentration.

Arrows show the net direction of transport. The narrow green band represents a partly permeable membrane; the rounded green shape represents a carrier. Diffusion and osmosis do not mean that individual particles move in only one direction.

Explanation

For a water question, look for a partially permeable membrane and a difference in water potential. Together, these identify osmosis. State that water moves from higher to lower water potential.

For a substance such as oxygen or carbon dioxide, compare its concentrations. Net movement from higher to lower concentration is diffusion. A membrane is not required for diffusion, although diffusion can occur across one.

For movement across a cell membrane from lower to higher concentration, identify active transport. Include the energy supplied by respiration. It is not enough to say that a substance moves into a cell.

Several processes can happen in the same cell at once. A root hair cell can gain water by osmosis while taking up mineral ions by active transport. Explain each substance separately.

Step by step
  1. 1

    What is moving?

    Use the exact substance named: water, oxygen, glucose or mineral ions.

  2. 2

    What gradient is given?

    Use water potential for osmosis and the concentration of the named substance for diffusion or active transport.

  3. 3

    Does the mechanism fit?

    Check the membrane requirement and whether energy from respiration is needed.

Worked example

Two processes, one root hair cell

Soil water has a higher water potential than the cell contents, but a lower mineral ion concentration. Explain how both water and mineral ions can enter.

One way to explain it

Water enters by osmosis, from higher water potential in the soil water to lower water potential inside, through the partially permeable cell membrane. Mineral ions enter by active transport against their concentration gradient, using energy from respiration.

Why this answer works
  • Explain water and mineral ions separately.
  • Use the appropriate gradient for each.
  • Mention energy only for active transport.

Watch out for this

A student says: "Osmosis is the movement of any particles across a membrane." What is wrong with this?

Show the answer

Osmosis specifically concerns water through a partially permeable membrane. A reference to a membrane alone cannot identify the process.

Check your understanding

Water and mineral ions both enter a root hair cell. Which statement is sufficient to identify active transport of the ions?

  1. The cell also takes in water.
  2. The ions move into the cell rather than out.
  3. The ions move from a lower concentration outside to a higher concentration inside.
Show the answer

The ions move from a lower concentration outside to a higher concentration inside.

This is movement against the ion concentration gradient across the cell membrane, requiring active transport.

Quick recall

Card 1 of 3. Answer in your head, then check.

Which clues tell you which process is happening?

You can return to this lesson any time.