Question

What determines the direction in which a substance diffuses across a membrane?

Answer 1

The Concentration of the Substance.

Explanation:

Diffusion taqes place along a Concentration Gradient.


that is from higher to lower concentration.
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if You want to know more about diffusion and exchange of materials.
REFER HERE
https://www.cliffsnotes.com/study-guides/biology/biology/the-biology-of-cells/movement-through-the-plasma-membrane