Both diffusion and osmosis passively transport substances down concentration gradients. However, there are times when substances need to be transported against a concentration.
Active transport is a process that transports substances against a concentration gradient. In other words, from a low concentration to a high concentration.
Carrier proteins embedded in the cell membrane use energy to transport the substances in and out of the cell. As active transport requires energy, it is an active process.
In plants, root hairs line the surface of the roots. They have a higher concentration of mineral ions than the dilute solutions in the soil. For plants to take up mineral ions, active transport occurs. This is because the plant cells are transporting the ions from a region of low concentration to a region of high concentration.
So the substances are transported from the soil (dilute concentration) to the cytoplasm of the root hair cells (concentrated solution). As this process is active transport, it requires energy and the cells use this energy to transport the mineral ions through carrier proteins.
In animals, food molecules (such as glucose) can be absorbed by the intestinal cells into the blood by diffusion. The lumen is the cavity of the small intestine. At times, the concentration of sugars such as glucose is greater in the intestinal cells and bloodstream than in the lumen.
This means that the glucose molecules must be transported against a concentration gradient, by active transport. Once the glucose molecules are absorbed by the cell, they are transported into the bloodstream and carried around the body.
So, the intestinal cells tend to have a lot of mitochondria which provide the energy required for active transport.