GCSE BIOLOGY REVISION: ACTIVE TRANSPORT
Please note: Text in colour is what the AQA GCSE biology specification requires an understanding of.
Please note: There are links with this content to the topic ‘cell specialisation.’
Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient).
Try to understand the meaning of active transport rather than viewing it as the opposite of diffusion. It is better to fully understand concepts rather than just trying to remember definitions.
This requires energy from respiration. Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth.
Active transport works against the concentration gradient. Diffusion is passive flow (requires no energy for it to happen), something that is active requires energy. The source of energy comes from respiration.
By definition: Active transport is the process that is required to move molecules against a concentration gradient. The process requires energy.
Often examiners will ask for an explanation of active transport, it is important to use the key words highlighted in bold. Definitions should have no mention of diffusion. Often candidates make the mistake in using diffusion in their definitions. For example:
‘Active transport is the diffusion of molecules against a concentration gradient.’ This is wrong, do not write this.
Why is this wrong?
Diffusion is the net movement of molecules down a concentration gradient, from an area of higher concentration to an area of lower concentration. To use diffusion in a definition for active transport you are contradicting yourself by saying that the molecules are and are not moving down a concentration gradient. So, if you are going to prioritise anything from this page remember: DO NOT MENTION DIFFUSION WHEN DEFINING ACTIVE TRANSPORT. You may be asked to explain the difference between the two, in which case you still must not mention diffusion for defining active transport, the two are very different.
It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration.
Glucose in the gut needs to be absorbed, all of it. Active transport is required because the concentration of glucose in the intestine is lower than the intestinal cells, therefore movement of glucose is against the concentration gradient.
Students should be able to:
• describe how substances are transported into and out of cells by diffusion, osmosis and active transport
• explain the differences between the three processes.
A table to summarise the three:
Process
Substances moved
What happens?
Diffusion
Oxygen, water, food, waste including carbon dioxide and urea
The net movement of substances from a high concentration to a lower concentration down a concentration gradient.
Osmosis
water
Water molecules are moved from an area of dilute solution to an area of concentrated solution through a partially permeable membrane down the concentration gradient
Active transport
Mineral ions, glucose, sucrose
Substances are moved from an area of lower concentration to an area of higher concentration, against the concentration gradient
Always remember: have I mentioned?
The direction of flow
Is a membrane involved?
Is it passive (does not require energy) or active (requires energy)?
Down or against the concentration gradient?
What is moving?
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