Why do some molecules need moving across a cell membrane in the form of energy

Because that molecules are moving against a concentration gradient and they can not freely pass trough the membrane.

Explanation:

Cell membranes are constituded by lipid bilayers with an hidrophobic nature. This means that water or molecules disolved in water can not freely pass the membrane. All these molecules are in a bigger concentration outside the cell, so the transport inside cells will be against a concentration gradient and this type of transport requieres energy.

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explanation:

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