Why does the cell sometimes have to expend energy to move individual molecules across the cell membrane?

Normally, molecules move naturally down a concentration gradient [From an area of high concentration to an area of low concentration] by a process called diffusion. However, sometimes the concentration of the needed substance is higher in the cell than out of it, so the cell uses a different process called Active Transport. Unlike Diffusion, Active transport requires energy, as it is going against a concentration gradient.

Cells expend energy to move cells across the cell membrane as they use Active transport rather than diffusion.

thymine. in rna, thymine is replace by uracil

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