Why do we need active transport if we have Diffusion?

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Diffusion is very useful when it comes to the movement of molecules down a diffusion gradient. That means from somewhere of a high concentration, so where there's lots of molecules in a given space, to somewhere of a low concentration, so the opposite. Imagine you just ate a huge meal. There are loads of glucose molecules in your intestines, but in your cell walls there's very few, so the glucose can diffuse down the concentration gradient, from your intestines to your cell walls and hence your blood. However as this happens, the concentration in your cell walls increases and the concentration in intestines decreases, until they are even... But only half the glucose has been absorbed, half is still in your intestines! in order to absorb the rest, we need to work against the concentration gradient. Active transport can continue to bring glucose into the cell wall even when the concentration in the small intestine is lower, so it can work against the concentration gradient and allow the rest of the glucose to be absorbed.

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