unch was great: good food, good conversation. Now that the players were “full,” they were ready to play more volleyball. However, in order to play volleyball, their bodies first had to convert the food they ingested into “fuel” to burn, so they’d have the energy to contract their muscles to move their bones, and so their brains would have the energy needed to process incoming information, react, and send out orders. To harvest that energy, both their digestive systems and their mitochondria were involved, and a wide variety of enzymes had important roles to play.

Digestive System and Cellular Respiration

As food is ingested, where does it go on its way to becoming “fuel” for our bodies? First the food is chewed to break it into smaller pieces, and amylase enzymes in our saliva begin to break starches apart into sugars. Then the food is swallowed, and begins to make it way through the rest of the digestive tract.

These three sisters weren’t at this volleyball picnic, but they were at a similar picnic about 60 years ago, and they probably baked pies and made deviled eggs to bring along. Digestion of “protein foods” such as deviled eggs and hamburgers begins in our stomachs where the proteins are broken into shorter polypeptide chains. That was probably going on while these sisters were posing for this photograph!

The stomach absorbs some water and some nutrients, and the rest of the partially-digested food is passed on to the small intestine, where other enzymes further digest the food and more of the nutrients are absorbed. Then the large intestine absorbs water and some more nutrients and eliminates the “left-overs.”

All of those absorbed nutrients float along in the blood, traveling to other places in the body. Many, like vitamin A and sugar, are stored in the liver. A portion of the circulating sugar is also absorbed by brain and muscle cells where the cells’ mitochondria burn the sugar (and some fats and amino acids) as fuel via a process called cellular respiration.

However, when our bodies burn sugar for fuel, we can’t do it all in one step, or we’d end up with a bonfire, like this, in our bodies! That might be OK for fictitious, fire-breathing dragons, but for the rest of us, still more enzymes break the sugar apart in many, smaller steps and release the energy gradually. That way, the heat that is released is just enough to keep us warm, but not enough to start us on fire.

Our cells can, then, use that energy for other purposes. Muscle cells can use energy to contract and brain cells use energy to process and send the electrical impulses that keep us going. Thus, if everything is working together as it should be, then a person can play volleyball, study biology, play a musical instrument, smile at someone else, etc.

Enzymes are proteins that function as biological catalysts: they help chemical reactions to take place, but they are not part of the reaction. For example, the enzyme, maltase is the right shape to match up with a molecule of maltose sugar. Then the maltase causes the maltose to break into two molecules of glucose, which are then released, and the maltase is ready to accept another maltose and begin the whole process, again.

The first enzymes to be discovered and studied are those which are found in yeast, which the yeast use to ferment sugar and turn it into alcohol and carbon dioxide. Humans have made use of yeast and their enzymes for centuries to produce bread, beer, wine, and mead. The process of fermentation is another means of harvesting energy from food and shares its first big step, called glycolysis, with the process of cellular respiration that our cells use.

Background Information

Your Assignment
An Example of Fermentation

Most of us, fairly regularly, ingest foods that yeast enzymes helped make, such as bread, root beer and ginger ale, or beer and wine. Some people also like yogurt, a fermented product made by a bacterium called Lactobacillus acidophilus by a process which produces lactic acid rather than alcohol and carbon dioxide. If you’ve never tried making any of these before, here’s your chance. The grading criteria for this assignment are given below, and you should also refer to those as you work on the assignment. A total of 6 points is possible.

Grading Criteria

1.   Overall:
2  —  The student was very observant and took excellent notes 1  —  The student observed and took adequate notes 0  —  The student’s notes are sketchy and suggest that (s)he did not pay much attention to what was happening
2  —  The student, obviously, went beyond the minimum requirements of the assignment 1  —  The student adequately completed the assignment 0  —  The student completed considerably less of the assignment than what was required
2  —  It is evident that the student used much insight, thoughtfulness, and critical thinking when completing this assignment 1  —  The student adequately thought about the assignment – there was, perhaps, a bit of “fuzzy thinking” in a couple places 0  —  The assignment gives the appearance of being “slapped together” just to get it done, with little evidence of thoughtfulness

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Copyright © 2006 by J. Stein Carter. All rights reserved.
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