what exactly is active transport i am aware that it is the movement of substances across membranes that requires an expenditure or energy. but when is it used? when does it all occur?
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1. uptake of mineral salts from the soil by the root hair cell
2. for absorption of food substances from the ileum unto the blood
It is used whenever transport of a molecule is energetically or kinetically unfavourable:
1. Whenever a molecule has to be transported against its concentration gradient.
2. Whenever a molecule has to be transported down its concentration gradient faster than the gradient is steep enough to drive it.
The classic examples are pumps in cell membranes, like the K+ pump and the Ca2+ pump. Cremesupreme is right about the other examples.