3.1.1B1. Characteristics of Living Organisms and Cells
Diffusion in Biology — Combined Science 0653 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 12 of 14 Unit quiz
Section 1 of 6What diffusion is
Definition
- Definition Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
- It is a passive process, so it needs no energy from the cell.
Why it happens
- Why it happens Particles of gases and of dissolved substances are in constant random motion.
- More particles leave the crowded region than enter it,
- so there is a net movement towards the less crowded region until the particles are evenly spread.
Particles involved
- Particles involved Diffusion occurs in gases and in solutions, for example oxygen in air and glucose dissolved in water.
Section 2 of 6Diffusion across a cell membrane
1Gradient exists
- Gradient exists A substance is more concentrated on one side of the membrane than the other.
2Membrane filters
- Membrane filters The cell surface membrane is partially permeable, letting small molecules such as oxygen, carbon dioxide and water pass through easily.
3Net movement
- Net movement Small molecules move down their concentration gradient through the membrane,
- while large molecules such as starch and proteins cannot cross.
4Gradient falls
- Gradient falls The difference in concentration decreases until the concentrations are the same on both sides, when net movement stops.
Section 3 of 6Examples in living organisms
| Substance | Moves from | Moves to | Purpose |
|---|---|---|---|
| Oxygen in animals | Air in the alveoli | Blood in the capillaries | Needed for aerobic respiration |
| Carbon dioxide in animals | Respiring cells and blood | Alveoli, to be breathed out | Removing a waste product |
| Carbon dioxide in plants | Air, through the stomata | Leaf cells | Used in photosynthesis |
| Oxygen in plants | Leaf cells in the light, or air at all times | Air, or respiring cells | Released from photosynthesis, and taken in for respiration |
| Digested food (glucose) | Small intestine | Blood | Supplies cells with energy |
Section 4 of 6Factors that affect the rate
Rate of diffusion
Concentration gradient
- Concentration gradient A steeper gradient means faster diffusion.
Temperature
- Temperature Particles move faster when warmer, so diffusion is faster.
Surface area
- Surface area A larger area, such as the folded lining of the intestine, allows more particles to cross in a given time.
Distance
- Distance A shorter diffusion distance, like the thin wall of an alveolus, makes diffusion quicker.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Diffusion | The net movement of particles from a region of higher concentration to a region of lower concentration. |
| Concentration gradient | The difference in concentration between two regions. |
| Partially permeable membrane | A membrane that lets some substances through but not others. |
| Passive process | A process that does not require energy from respiration. |
| Net movement | The overall movement once movements in both directions have been compared. |
Section 6 of 6Exam technique
1
Saying particles 'know' where to go. Their motion is random; the net movement is just a result of statistics.
2
Writing that diffusion needs energy from respiration. It is passive.
3
Forgetting 'net' and 'down a concentration gradient' in the definition.
4
Forgetting that plants also need oxygen for respiration all the time, not only carbon dioxide for photosynthesis.
5
Saying that large molecules like starch diffuse freely through the cell membrane. Only small molecules cross easily.
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