Biochemistry - transporting glucose

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 slide:
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.

The See-saw:
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.

Helpful? Confusing?

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!


This is really interesting @Eric – lots of great stuff for me to chew on.

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Hi @Eric. I find all these posts about metabolism so interesting. Is this your professional field? If not, how did you learn so much? Do you have a book you could recommend?

No, I it is not a profession, I am just interested in it. Mainly from athletics and wanting to learn about how to fuel myself so I could run better. And how refueling works, and how your body used fuel for exercise and things like that.

There are a few topics I want to share in this field. Mostly I have just learned by reading and researching as much as I can.

Honestly, I found all of the diabetes references so tuned into the disease, that they did not look at the bigger picture of how things are supposed to work. For me, it is better to first know how it “should” work, and then look at fixing what is broken by the disease.

(all the diabetes sports books waste time with things like -“if you have low blood sugar during exercise, stop and drink juice and test again in 15 minutes…”) :roll_eyes:

So I have not been reading the diabetes books everyone talks about. I have been reading books like “Biochemistry” (a very difficult book for me to understand),

and these books on sports nutrition. These are specific to exercise, but the overall view of nutrition and metabolism is super helpful.

I am getting another book soon, it’s in the mail. I can share info about it when I get it. It is about metabolism and nutrition, written by a doctor, but it’s arriving next week so I can’t yet say anything about if it is any good.

If I were to make one recommendation, it would depend on your area of interest. For endurance sports, one of the most knowledgeable people in the world is Asker Jeukendrup. Anything by him is very interesting to me.

For more basic information on nutrition for recreational athletics and sports, I like the Nancy Clark book. It is not for the highly competitive interests, but more general.

I am still looking for a general and easy to understand book on metabolism and nutrition! One that is not too difficult, but also not too much baby-talk. So when I find one, I will pass it along. Hopefully the one that is on its way will be good.


Thanks - very nice. Biochem is an area where I am sorely deficient. In school it just all seemed like black magic. The utter complexity is something I find immensely overwhelming. Just the one little tiny piece of glucose moving out of the small intestine is so complicated. I do appreciate the analogies.

I was wondering where the insulin fits into this picture?

I will post about this in more detail on another post. These things are easier to take in little bits.

A glucose molecule is unable to pass through a cell membrane via simple diffusion because it is too large. Instead, cells need to assist the glucose, which is why it is called “facilitated diffusion”.

A simple analogy is that insulin is like a “key” that unlocks the door for the glucose transporters.

At low levels of insulin, there are not many glucose transporters on the membrane.

Increased insulin causes the glucose transporters to fuse with the plasma membrane and “open” the door.

I will post more about this.


This is exactly what I’m interested in. I’d be really interested to hear your review of the book you’re yet to receive! A while back I was really interested in the immune system (given that my immune system is so crazy!) and found some books on the topic that were either meant for general readers or medical students that weren’t so difficult to understand that they were completely over my head. It’d be great if there were something similar about how metabolism works.


this is the easiest analogy for me to understand. i am so so far from being a science wizard that it is embarrassing.

so thanks for making this understandable, even for dummies like me :wink:

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This was what I was told 50+ years ago, with pictures in the book to show the key!!