Normally I write a "Science of Science Fiction" post when I come across an inspiring science article. But a couple of days ago, my fabulous Blog Chain buddy Eric suggested I talk about how I'd use different types of digestive systems in an alien race. I'm going to take a slightly different spin on the topic and spread it out over three posts. Today, I'll provide a quick overview of how digestion works in various animals here on Earth. Tomorrow, I'll come up with some novel ways aliens might digest food, assuming their biochemistry is similar to ours. Finally, on Friday, I'll speculate how aliens much different from us might digest their food--or if they would even need food. If you have questions or suggestions along the way, please comment.
(For what it's worth, although I don't have a Ph.D., I do have a B.S. in molecular biology. I also have thirteen years experience in the enzyme industry.)
Wikipedia has a good overview article on digestion here. Basically, digestion is a process that takes food and breaks it down into its component molecules, which are then used as building materials or energy sources. Starches are broken down into simple sugars, fats into fatty acids, and proteins into amino acids. All these foods are broken down by a combination of physical and chemical processes.
The physical processes usually start first to break down big chunks of food and prepare it for chemical processing. The physical processes include cooking, breaking it up with teeth or stones in a gizzard, and changing the pH (this denatures proteins, or breaks some of the bonds that hold the protein in a particular form). Food is also moistened to prepare it for chemical processing.
Once the food has been partially broken down, chemicals (mostly enzymes) finish the job of breaking it up into the molecules used by the body. Enzymes are natural catalysts; they're proteins that speed up biological processes without being used up in the process. There are enzymes that break down starches, proteins, and lipids by severing certain bonds. Once these foods are broken down into sugars, fatty acids, and amino acids, they can be absorbed into the bloodstream and brought to the places in the body where they're needed.
Most animals carry out digestion in a dedicated tract running through their bodies. However, animals eat many different types of food, and they carry out digestion in many varied ways. Some pretreat their food with enzymes, digesting it before they even eat it. Some have multiple stomachs or chew their food several times. Any of these behaviors can be given to an alien race. But are there ways aliens might digest food not seen on Earth? Tune in tomorrow to find out!