Lesson 1 of 4 / Movement of substances
Diffusion
Why does oxygen move into a respiring cell?
In this lesson: Predict the net movement of particles from their concentration gradient.
About 4 min
The key ideaDiffusion 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?
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
Name the substance
Compare the concentration of the same substance on the two sides. Oxygen and carbon dioxide can have different gradients.
- 2
Find the higher concentration
Use the information in the question. Inside is not always higher or lower than outside.
- 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.
Is this true? "At equal concentrations, the particles stop moving."
Particles continue moving randomly in both directions. Equal rates of movement mean no net movement; they do not mean no movement.