Considering the fact that the amount of glucose in the intestine is more than 5mM (i.e more than the normal blood plasma concentration), why would there still be an active transport of glucose by SGLT1? Why would the tissue spend ATP in order to take up glucose when the transport would in condition NOT be against the concentration gradient?
May be this active transport leads to the accumulation of higher Glucose concentrations in the brush border epithelial cells (higher than in the blood) , allowing free facilitated diffusion across the basement membrane. This may not be possible if the epithelial cells just rely on diffusion and equilibration with the lumen concentration. Lack of too much of facilitated diffusion channels on the apical surface could also help in better control over the process (even though with some energy expenditure!!!!)
active transport leads to the accumulation of higher Glucose concentrations in the brush border epithelial cells CAN'T be higher than in the blood because free motion must to allow the base of membrane
Glucose was such a treasure in human evolution that is was essential to get every glucose molecule out of the upper samll intestine before bacteria in the large intestine can get a hand on. And of course, how can you get glucose out of the cell when even fasting glucose levels are around 4.5 mM - you better utilize a gradient. Since SGLT1 also takes Na+-Ions this increases osmotic gradients and water absorption.
In ancient time glucose was much more precious than today. And the glucose you could get from food you did not need to synthetize - which costs ATP. Well, if bacteria break-down sugars they produce short chain fatty acids which are absorbed and which are only to a tiny extend converted back in liver into Glucose.
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