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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.