Here is a little info on how glucose is transported in cells in the digestive system and in the red blood cells where it is used for energy.
There are 3 different ways of transporting glucose: “Facilitated Diffusion”, “Primary Active Transport”, and “Secondary Active Transport”.
(There are different glucose transporters (GLUT1, 2, 3, 4, etc) but I will save that for another day. Also saving secondary active transport for another day.)
I thought of a simple analogy to make it easy to understand, and hopefully you won’t be bored with the discussion.
Imagine a playground.
On the slide, you only go one way - down the slide. And it takes work, you have to climb up the ladder to be able to go down the slide.
On the see-saw, you can go either up or down. And it doesn’t take any work, you just sit and the see-saw moves up and down toward the heavier side.
The epithelial cells in the small intestine need to bring glucose into the body, and they must prevent the reverse flow of glucose from body back into the intestine. Why? Imagine if glucose could go either way in your stomach. When you ate a donut, the glucose would flow into your blood, from the higher area of concentration to the lower area (from your gut to your blood). But a few hours later when your blood sugar was high and your gut was empty, the glucose would flow back. It can’t use the see-saw, it would keep going back and forth. So your intestine has to use the slide. It has to use active transport.
Active transport ensures that glucose doesn’t leak the wrong way. The intestine uses “Primary Active Transport”. It is a one-way process. Think of the slide, it takes work (energy) and it only goes one way, from intestine to blood.
On the other hand, red blood cells and most of the tissues in your body move glucose by facilitated diffusion instead of active transport.
While the digestive system experiences a constantly changing amount of glucose from eating or not eating, glucose concentration in the blood is more regulated. The blood glucose in the blood is normally higher than the concentrations inside the cell (intracellular).
Glucose is transported across the red blood cell membranes by a uniporter, which is membrane protein that is involved in facilitated diffusion.
As soon as glucose enters the cell, it is converted into other chemicals needed for the cell for energy reproduction or biosynthesis. So in essence it “goes away”. Think about sitting on a see-saw. You take in glucose (gain weight, go down) and then you lose weight (the glucose is used up) and you go back up.
(Please don’t confuse this with losing weight on a diet, I am speaking of “losing weight” as an analogy for the glucose being used by the cells!)
Since the concentration of glucose inside the cells continually drops lower than the 4.5-5 mM glucose level normally maintained in blood, facilitated diffusion ensures a constant flow of glucose into the red blood cells. It would be a waste of energy and totally unneeded for red blood cells to expend energy climbing up the slide (active transport). For red blood cells, the transport for glucose uses the see-saw. It uses facilitated diffusion.
If it makes sense, great! If not, please read through it a 2nd time and if I need to clarify or do better, let me know and I will try.
As always, have fun on the biochemistry playground!