In 1827 the Scottish botanist Robert Brown looked through his microscope at tiny particles from inside pollen grains floating in water. They jiggled about without stopping. He wondered at first whether this was a sign of life, so he tried specks of non-living material too. They jiggled in the same way. Brown couldn't explain it. The explanation came in the early 1900s, with Albert Einstein's work in 1905 and experiments by Jean Perrin soon after: the specks were being knocked about by water molecules that are always moving at random.
The story is often told as "Brown saw pollen grains move". That's not quite right. He was watching much smaller particles that came from inside the pollen grains. Random movement of particles is the start of diffusion, and diffusion is where this lesson begins.
This lesson covers transport in cells for AQA GCSE Biology Topic 1 (Cell biology). It's also useful for Combined Science. The potato practical has its own deck, so here the focus is on the three processes and on exchange surfaces.
Diffusion
Diffusion is the spreading out of particles of a gas, or of a substance in solution, resulting in a net movement from an area of higher concentration to an area of lower concentration. "Net" matters: particles move in every direction, but more move down the concentration gradient than up it.
Three factors affect the rate of diffusion:
- Concentration gradient: the steeper it is, the faster the diffusion.
- Temperature: particles move faster when it's warmer.
- Surface area of the membrane: more surface lets more particles cross at once.
Examples in the body include oxygen and carbon dioxide in gas exchange, and urea diffusing from cells into the blood plasma so the kidneys can excrete it.
Osmosis
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. The membrane lets water through but not larger molecules such as sugar.
An animal cell in pure water takes in water and may burst, because it has no cell wall. In a concentrated solution it loses water and shrinks. A plant cell in pure water becomes turgid: its cell wall stops it swelling further.
Active transport
Active transport moves substances from a more dilute solution to a more concentrated one, against the concentration gradient. It needs energy from respiration, which is why cells that do a lot of it often have many mitochondria.
Two examples from the specification:
- Root hair cells absorb mineral ions from very dilute soil water. Plants need these ions for healthy growth.
- The small intestine absorbs sugar into the blood even when the blood already has a higher sugar concentration.
In 1957 the Danish scientist Jens Christian Skou described an enzyme in crab nerve membranes, and he linked it to the pumping of sodium and potassium ions against their gradients. The Nobel committee called it the first discovery of an ion-transporting enzyme, and he shared the 1997 Nobel Prize in Chemistry for it. You don't need the details for GCSE, but it's active transport in action.
Surface area to volume ratio
For a cube, surface area = 6 × side² and volume = side³.
| Side | Surface area | Volume | SA:V |
|---|---|---|---|
| 1 cm | 6 cm² | 1 cm³ | 6:1 |
| 2 cm | 24 cm² | 8 cm³ | 3:1 |
| 3 cm | 54 cm² | 27 cm³ | 2:1 |
As size goes up, SA:V goes down. A single-celled organism has a large ratio, so diffusion across its membrane meets its needs. A large animal doesn't, so it needs exchange surfaces and a transport system.
Exchange surfaces
An effective exchange surface has a large surface area, a thin membrane for a short diffusion path, an efficient blood supply (in animals) and ventilation (for gas exchange in animals). Look for these in the alveoli, the villi of the small intestine, fish gills, root hairs and leaves.
Common exam mistakes
- Leaving out "net" or "partially permeable" from a definition.
- Saying osmosis or diffusion needs energy. Only active transport does.
- Saying root hairs take in water by active transport. Water enters by osmosis; mineral ions need active transport.
- Calling an animal cell turgid. Turgid describes plant cells.
- Getting SA:V backwards. Bigger organisms have a smaller ratio.
Frequently asked questions
What is the difference between diffusion, osmosis and active transport?
Diffusion moves particles down a concentration gradient, osmosis is the diffusion of water through a partially permeable membrane, and active transport moves substances against a gradient using energy from respiration.
Does osmosis need energy?
No. Osmosis is passive, like all diffusion. Only active transport needs energy from respiration.
Why do root hair cells need active transport?
The soil water is usually more dilute in mineral ions than the cell, so the ions must be moved in against the concentration gradient.
How do you work out surface area to volume ratio?
Calculate the surface area and the volume, divide the surface area by the volume, and write the answer as a ratio such as 3:1.
Try it
Say each answer before you flip the card, then try the cube calculations without the table. Then put yourself in Brown's place: specks of dead material are dancing in water and nobody knows why. What would you have tested next?
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