Substances are transported passively down concentration gradients. Often, substances have to be moved from a low to a high concentration - against (up) a concentration gradient.
Active transport is a process that moves molecules against a concentration gradient. The process requires energy.
For plants to take up mineral ions, ions are moved by active transport into root hairs, where they are in a higher concentration than in the dilute solutions in the soil. Active transport or diffusion then occurs across the root so that the plant transports 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.
In animals, plants and microorganisms, substances move into and out of cells by diffusion, osmosis and active transport.
|Process||Description||Substances moved||Energy required|
|Diffusion||Substances move from a high to a low concentration down a concentration gradient||Carbon dioxide, oxygen, water, food substances, urea||No|
|Osmosis||Water moves from a high to a low concentration across a partially permeable membrane and down a concentration gradient||Water||No|
|Active transport||Substances move against a concentration gradient||Mineral ions into plant roots, glucose from the gut into intestinal cells, from where it moves into the blood||Yes|