<em>Enzymes are chemical substances or biological molecules that catalyze biological reactions by lowering the activation energy threshold required to start such reactions.</em>
The activation refers to the minimum energy that must be conquered by the reactants in a chemical reaction in order to cross over to the product's side.
Enzymes work by lowering this energy and hence, making the crossing over to the product's side faster than normal. They are not themselves used up in reactions and can always be recovered and reused.
The exchange of a river's flow and sediment conveyor downstream of a dam often causes the greatest sustained environmental impacts. Yet even complex changes in the number and timing of water flows impact aquatic and riparian life, which can explain the biological web of a river system.
Cell transport is useful because it's, essentially, the movement of cells across the blood/veins. Active transport uses energy, while passive transport does not.
