Supplying the cell
For an organism to function, substances must move into and out of cells. Three processes contribute to this movement - diffusion, osmosis and active transport.
Active transport
Substances are transported passively down concentration gradients. Often, substances have to be moved from a low to a high concentration - against a concentration gradient.
Active transport is a process that is required to move molecules against a concentration gradient. The process requires energy.
For plants to take up mineral ions, ions are moved into root hairs, where they are in a higher concentration than in the dilute solutions in the soil. Active transport then occurs across the root so that the plant takes in the ions it needs from the soil around it.
In animals, glucose molecules have to be moved across the gut wall into the blood. The glucose molecules in the intestine might be in a higher concentration than in the intestinal cells and blood - for instance, after a sugary meal - but there will be times when glucose concentration in the intestine might be lower.
All the glucose in the gut needs to be absorbed. When the glucose concentration in the intestine is lower than in the intestinal cells, movement of glucose involves active transport. The process requires energy produced by respiration.
Comparing diffusion, osmosis and active transport
In animals, plants and microorganisms, substances move into and out of cells by diffusion, osmosis and active transport.
| Process | Description | Substances transported | Energy required |
|---|---|---|---|
| Diffusion | Substances move from a high to a lower concentration down a concentration gradient | Carbon dioxide, oxygen, water, food substances, wastes, eg urea | No |
| Osmosis | Water moves from a high to a lower concentration across a partially permeable membrane and down a concentration gradient | Water | No |
| Active transport | Substances more from low to higher concentration up a concentration gradient | Mineral ions into plant roots, glucose from the gut into intestinal cells, from where it moves into the blood | Yes |