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What is active transport and how is it used in the absorption of glucose?

Active transport is the movement of molecules or ions against their concentration gradient, using energy in the form of ATP, across a plasma membrane.

In glucose absorption, there is an initially high concentration of glucose in the lumen of the gut as carbohydrates break down. This means there is a concentration gradient allowing the diffusion of glucose into the cells. Once the glucose is at equilibrium, it then needs to be taken up by active transport:

1) Sodium ions (Na+) are actively pumped out of the cells of the small intestine and into the blood via Sodium/Potassium (Na+/K+) pumps.
2) This creates an Na+ concentration gradient, where there is a higher concentration of Na+ in the lumen of the small intestine than inside the cells.
3) The Na+ then re-enters the cells of the small intestine via diffusion through a sodium-glucose transporter protein (alongside glucose).
4) The glucose concentration inside the cell increases and a concentration gradient is created between the inside of the cells and the blood. This allows glucose to move via facilitated diffusion into the blood. 

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