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 to allow the plant to take the nutrients it needs for 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. At this point it will diffuse from high concentration in the intestine to a lower concentration in the blood. This doesn’t require energy.
However, there will be times when glucose concentration in the intestine might be lower. When this is the case, movement of glucose involves active transport. The process requires energy produced by respiration.