Re: Why is the digestion of starch to glucose necessary?

Hi Natalie,

Thanks for the interesting question. Starches are used by plants to store glucose. You probably already know that starch is a long chain (aka, a polymer) of glucose molecules, and that starches are digested by amylase enzymes. You might not know that while all glucose molecules are the same (they all have the same chemical formula), all starch molecules are not the same. While they are long chains of glucose molecules, they vary in their shapes (some of them are straight, and some are branched), and they vary in the number of glucose molecules that they contain.

These differences between starch molecules mean that it is very difficult for us to make use of them. The enzymes that carry out metabolic reactions have very specific requirements. They require the same thing every time (for example, some enzymes will use glucose and only glucose), and an enzyme that recognizes one starch molecule won't necessarily recognize different starch molecules. Similarly, the transport molecules that move nutrients from our digestive system and into our circulatory system are also very specific with respect to what they will transport. With so many different shapes and lengths of starch molecules possible, it would require a large number of different transporters and enzymes to work with them effectively. This would be very inefficient. Finally, glucose is a relatively small molecule, and this makes it much easier to manipulate than a large, unwieldy starch molecule.

In general, our metabolisms have evolved to work with certain basic units, putting these units together into polymers, and breaking these polymers apart. For carbohydrates, the basic sugar unit is usually glucose. The basic protein unit is the amino acid. The basic genetic unit is the nucleotide. These basic units can be put together into a large number of different shapes, all of which have different functions. This is why some starches are branched, and some are straight. Cellulose is another polymer of glucose where the sugars are put together differently from in starches. In order to make new cellulose, plants have to break the starches down into glucose, and then build the glucose into cellulose.

So, to answer your question, digestion of starch to glucose is necessary because enzymes and transport molecules require small, regular molecules to manipulate, and because glucose serves as the basis for most carbohydrate metabolism.

You can probably find more information about glucose metabolism in your science textbook. If not, you could visit your public library and ask for a high school-level biology or biochemistry textbook. This might be a little advanced for you, but you asked an advanced question, so you might be able to handle it. You can also search our archives for answers dealing with glucose and starch.

Keep asking questions!