K325 / 2027

Lesson 1 of 7 / 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 explain diffusion and its biological applications.Syllabus

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

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 gas are constantly moving in random directions. When there are more particles per unit volume in one region than another, more move away from the crowded region than move back into it. The overall result is movement from higher to lower concentration. This overall movement is called net movement.

A concentration gradient is a difference in concentration between two regions. Diffusion moves particles down that gradient. It does not require energy supplied by respiration, although the particles themselves have kinetic energy.

In a respiring cell, oxygen is used in aerobic respiration. If the oxygen concentration is higher outside the cell, oxygen diffuses into it through the cell membrane. Carbon dioxide produced by respiration can diffuse out when its concentration is higher inside.

Diffusion enables gas exchange in humans and plants. In the lungs, oxygen diffuses from alveolar air into blood when its concentration is higher in the alveoli. In a photosynthesising leaf, carbon dioxide diffuses from the air spaces into cells that are using it. Oxygen produced can diffuse out down its own gradient.

Diffusion also contributes to nutrient absorption. Some digested nutrients, such as fatty acids, can enter absorbing cells when their concentration is higher outside and the membrane permits them to cross. Each substance moves according to its own concentration gradient.

When concentrations become equal, particles do not stop moving. They still move in both directions, but at equal rates overall. There is then no net movement. Diffusion can also happen without a membrane, such as when particles spread through water.

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. Oxygen moves from the blood to the alveolar air using energy from respiration.
  2. More oxygen moves from the alveolar air into the blood than in the reverse direction.
  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.