K224 / K225 / 2027

Lesson 1 of 5 / Movement of substances

Diffusion

Why does oxygen move into a respiring cell?

I can predict the net movement of particles from their concentration gradient.

  • I can define diffusion and explain nutrient uptake and gas exchange in plants and humans.Syllabus

    Syllabus K224 / K225, Topic 2(a). Define diffusion and explain nutrient uptake and gas exchange in plants and humans.

About 4 min

Make a guess. You are not marked.

Oxygen has spread out evenly in a sealed box, so the concentration is the same everywhere. What are the oxygen molecules doing now?

  1. They are slowly gathering in one corner again.
  2. Still moving in all directions, with no net movement.
  3. They have stopped moving because diffusion is finished.
Show the answer

Still moving in all directions, with no net movement.

Particles never stop moving. At equal concentration as many move one way as the other, so nothing changes overall.

The key idea

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Concentration is the amount of a substance per unit volume. Compare equal volumes: the one with more particles of that substance has the higher concentration.

Explore the idea

When the particles spread out, do they stop moving?

Higher concentrationLower concentration
Diffusion: unequal particle concentrationsFourteen particles are shown in the left half and four in the right half. Particles move in both directions, with a net movement to the right. The dotted line divides equal volumes for comparison; it is not a membrane. These are schematic snapshots of particles that remain in random motion.

The two halves are equal volumes of the same container.

Both directions. More movement to the right overall.Random movement spreads particles from the region of higher concentration towards the region of lower concentration overall. Some particles still travel the other way.

  • Particles of one substance
  • An imaginary dividing line
  • Arrows compare overall movement in each direction

Schematic snapshots, not a timed simulation. Arrow widths illustrate the comparison, not measured rates. Diffusion does not require energy from respiration or a membrane.

Explanation

Particles in a liquid or a gas are always moving in random directions. If one area has more particles than another, more particles leave the crowded area than come back into it. So overall, particles spread from where there are more of them to where there are fewer. This overall movement is called net movement.

A concentration gradient is a difference in concentration between two areas. In diffusion, particles move down the gradient, from higher to lower concentration. Diffusion does not need energy from respiration. The particles move because they already have kinetic energy.

A respiring cell uses oxygen in aerobic respiration. If there is more oxygen outside the cell than inside, oxygen diffuses into the cell through the cell membrane. Respiration also makes carbon dioxide. When there is more carbon dioxide inside the cell, it diffuses out.

Diffusion is how gases are exchanged in humans and plants. In the lungs, oxygen diffuses from the air in the alveoli into the blood, because there is more oxygen in the alveoli. In a leaf that is photosynthesising, carbon dioxide diffuses from the air spaces into the cells that use it. Oxygen made in the leaf diffuses out.

Diffusion also helps the body absorb food. Some digested nutrients, such as fatty acids, diffuse into the cells lining the gut when there are more of them outside the cells. Each substance moves down its own concentration gradient.

When the concentrations become equal, the particles do not stop moving. They keep moving both ways at the same rate, so there is no net movement. Diffusion does not need a membrane. For example, a drop of ink spreads through water by diffusion.

Step by step
  1. 1

    Name the substance

    Compare the concentration of the same substance on the two sides. Oxygen and carbon dioxide can have different gradients.

  2. 2

    Find the higher concentration

    Use the information in the question. Inside is not always higher or lower than outside.

  3. 3

    State the net direction

    Particles diffuse from higher to lower concentration, down their concentration gradient.

Worked example

Oxygen entering a cell

A cell uses oxygen faster than it is replaced. The oxygen concentration outside is higher than inside. Explain its net movement.

One way to explain it

Oxygen diffuses into the cell through the cell membrane, from a higher oxygen concentration outside to a lower oxygen concentration inside, down its concentration gradient.

Why this answer works
  • Name oxygen, rather than just "particles".
  • Compare outside with inside.
  • Link the direction to the concentration gradient.

Watch out for this

A student says: "At equal concentrations, the particles stop moving." What is wrong with this?

Show the answer

Particles continue moving randomly in both directions. Equal rates of movement mean no net movement; they do not mean no movement.

Check your understanding

Oxygen concentration is higher in the alveolar air than in the blood arriving at the lungs. What happens by diffusion?

  1. More oxygen moves from the alveolar air into the blood than in the reverse direction.
  2. Oxygen moves from the blood to the alveolar air using energy from respiration.
  3. Oxygen moves only into the blood; no molecules move back.
Show the answer

More oxygen moves from the alveolar air into the blood than in the reverse direction.

The net movement is from higher oxygen concentration in the alveolar air to lower concentration in the blood, down the oxygen concentration gradient.

Quick recall

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

Why does oxygen move into a respiring cell?

You can return to this lesson any time.