Starch.

Plants have two ways to store sugars. 

1.  They can make the "single" and "double" sugars discussed above.  These are stored in fruits and can be picked and eaten by animals who wander by.  This is a good way for a plant that is "stuck in the soil" to get its seeds spread around in the neighborhood.   The disadvantage is that fruits are large, wet (read juicy) and rot easily.  They are nice for the community but do not help to provide energy to seeds that are to become new plants. 

2.  What about a form of sugar that does not dissolve in water, is stable over long periods of time without refrigeration and is so compact that it can be packed in small seeds.  Enter starch!  This is a pure chain of glucose molecules  (or a polysaccharide) and has all of the characteristics that a plant could ask for.  It is easily stored in seeds and roots and quickly split to a simple sugar when energy is needed.  We have taken advantage of this and developed plants that produce large seeds that we use to make flour and which are the basis for all baking.  Much of the world's population live on rice, another seed crop which contains starch.  Root crops such as potatoes also are "storage bins" of starch. 

Let's look a little closer at starch.  As you can see in the next figure, starch is just a simple chain of glucose molecules, linked together  by a chemical bond.  We have an enzyme that can split that link, called amylase.  It is produced in the pancreas and flows  into the small intestine in the same area where we find sucrase and lactase.    Amylase clips off glucose groups "two and two".  This gives the maltose that I mentioned earlier.  Another enzyme, maltase, rapidly cleaves this "double sugar" and sets loose glucose from starch in the small intestine. 

So, starch is an excellent source of sugar for us.  But, it is not sweet, it is dry and not particularly "good to eat" as it comes from plants.  However, breakfast food, breads, spaghetti and cakes made from flour are delicious!

Why can't I eat grass?

Grass and leaves, pine needles and wood; all are made largely of sugar.  But, you cannot smell it, taste it or digest it.  That must be a strange sugar!  Well, it is not, really.  The stuff is called cellulose and is almost identical to starch.  It is made entirely of glucose.

 Let's look at this.  The only real difference between starch and cellulose is the placing of the link between the glucose groups in the chain.  While starch has what we call alpha bonds that hang below the sugar elements, cellulose has beta bonds.  These are buried behind the glucose groups.  The "starch-splitting" enzyme amylase cannot reach in between the glucose groups in cellulose and clip them up.  We have no intestinal  enzyme that can do this for us, so cellulose just passed through our digestive system without being split. 

Well, we need a little roughage (or fiber) to keep things going down there, so cellulose in fiber and in small amounts is not wasted on us, but nourishment is it not!

By the way, cows and horses and even termites do not have their own enzymes to split cellulose either.  They have extra stomachs where bacteria do that job for them.  These "bugs" have cellulose-splitting enzymes and they digest all that grass and wood.  The glucose produced is then used by the animals. 

Here is a check list over all those enzymes:

        Sucrase splits sucrose to glucose and fructose.

       Lactase splits lactose to glucose and galactose.

     Amylase splits starch to glucose.

        Maltase splits maltose to 2 glucose molecules.

            Nothing splits cellulose in animals.  Bacteria do this for them.