Thursday, 13 March 2008

What carries glucose from the intestine to other parts of the body?

Basically, blood. But here's a short summary of the glucose's travels around the body.

We start with carbohydrates that are ingested and reach your small intestine. Only monosaccharides (like glucose) can be absorbed, however, so the body must break larger carbohydrates down. This the body does via several enzymes, some from the pancreas and the saliva, and some sitting on the top of the intestinal cells. Monosaccharides are water soluble and therefore ready to be absorbed.

But here's a snag - simple diffusion isn't an option: glucose is too big to get into the cell easily. The intestine solves the problem by means of a cotransport with sodium. Sodium is actively pumped out the cell into the surrounding space, which lowers the intracellular sodium concentration. This decrease of sodium within the cell causes sodium from the intestine to try to move inward, which the cell allows... for a price. It has to take a glucose molecule with it. (Technically, the transport protein won't transport sodium until it also combines with some other appropriate molecule, like glucose.) Of the two other monosaccharides, galactose's transport is the same, but fructose uses facilitated transport.

So, right now we've got the monosaccharides into the intestinal cell. From there, they are free to leave the cell and enter the blood stream - this exit is by means of another transport protein (facilitated transport). They then travel to the liver by the portal blood stream. The liver grabs a lot of it, and the rest is circulated around the body in the systemic blood stream. There's no carrier protein for the monosaccharides; they just float freely within the blood.

From now on, let's talk just of glucose, and not of fructose or galactose, etc.. We're entitled to do this, since 80% of the monosaccharides absorbed at the small intestine are glucose. Furthermore, almost all of the fructose and galactose molecules are rapidly converted by the liver into glucose. Thus glucose is the final common fate of all the carbohydrates that we eat.

And how does the glucose gets from the blood into the cells? Again we are confronted with the awkward problem of glucose's size, and so again we must make use of carrier proteins embedded within the cell's membrane. However, this time the process isn't active - facilitated diffusion is used.

But what stops glucose from simply diffusing right out again once the concentration gradient changes? Well, once inside the cell, glucose is immediately phosphorylated to create glucose-6-phosphate (G-6-P). As with DNA, this phosphorylation locks the glucose within the cell, as the cell membrane is impermeable to G-6-P.

And from there? Well, glucose can then either be used immediately for energy creation (glycolysis), or it can be stored in the form of glycogen, which is a large polymer of glucose. All cells are capable of storing some glycogen, but the bulk of it is found in the liver (which uses it to regulate blood glucose levels) and muscle (which uses it to power itself).

So, now you know! :)

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