What does sucrose (usual table sugar) look like?


Here is that table sugar again.  Remember, it is a disaccharide, a combination of one glucose molecule and one fructose molecule, strongly linked together through a chemical bond.  That link is very stable and sucrose does not break down when boiled or baked.  That brings up another interesting problem when we talk about sugars.    Disaccharides are not taken up in our bodies! They must be broken down to the individual small sugars before they can go over the cell membranes in the intestines.  Even the small sugars cannot of themselves cross from the gut to the circulation.  There are special transporters in the intestine that carry these from the digested material in our gut into our bodies.


Well, if the bond between the small sugars is so strong, how do we split sucrose into its two simple sugars?  We use enzymes!

Enzymes are very specialized proteins designed to greatly speed-up specific reactions.  They are responsible for  driving forward the many and complex reactions in our bodies.  Enzymes get their forms from codes found in our genes that have been inherited  in an endless chain starting from our very early ancestors.  When we speak of "very early ancestors" we do NOT refer to "cavemen" but to animals that lived "up in the trees" and who nourished themselves largely by eating fruit.  Our metabolism is based on eating sugars and starch, not meat.  Many of our body's organs "burn" only sugar!



Now, back to sucrose-eating.  When that table sugar reaches the beginning of the small intestine it meets an enzyme called "sucrase".  This is a protein formed on the inside of the intestine.  It recognizes sucrose and splits the "chain" or bond between glucose and fructose.  This "sugar-clipper" is specially designed to fit the link between glucose and fructose in sucrose alone.  It does not attack other sugars. 









The small sugars released by sucrase can then be taken up by transport systems in the small intestine's wall and are  moved to the circulation.  Once in the blood they are carried to all of the body's tissues and are used to supply energy.


Jens eats candy.


We can look at a drawing of the intestines to get a more detailed description of the uptake process.  Here is "little Jens", about to devour a candy stick.  Which organs are going to be involved in handling the sugar in that candy?

Four major organs are involved here.  The esophagus is "just" a tube connecting the mouth and stomach.  The sugars from that candy are not altered until they enter the small intestine.  Here they are split into glucose and fructose and are then taken up over the small intestine's membranes and end up in the circulation. 










Go now to the next figure for details.

There are several points that you should note here:

1.  Disaccharides ("double sugars") like sucrose are not split before entering the small intestine.  Sugars are not digested in the stomach.   Enzymes that split disaccharides are only found in the upper reaches of the small intestine (called the duodenum).  No transport of sugars to the blood can occur before enzymes split them.    

2.  The released small sugars (glucose and fructose in the case of sucrose) are absorbed in the small intestine.  They do NOT reach the large intestine.  This is quite important for us!  They are then sent to the blood stream and distributed through the body.



Next: Small and Large