Digestion in biochemistry refers to the hydrolysis of any large nutrient molecules. It is the process by which large, bulky molecules of little use by themselves are broken down into smaller molecules or individual ions that are immensely useful. It is different from metabolism (even though the two are sometimes used interchangeably) because digestion merely refers to the process of breaking down, it does not deal with absorption and utilization of those broken down nutrients (which is what metabolism is for).

Digestion is vital to the body's processes. In complex organisms such as humans, almost everything we eat is a complex polymer that needs to be digested (with the exception of straight glucose, but that doesn't taste too good. You will get this when you're on an IV however, if your digestion is compromised). There are three main compounds that digestion applies to:

  • Fats, the digestion of which can start in the mouth, but mostly happens in the stomach. Fats are degraded through salivary and pancreatic lipases.
  • Carbohydrates, the digestion of which begins in the mouth through salivary juices. They attack the α-1->4-glucosidic linkages that bind together long polysaccharide chains, breaking the carbohydrate down into its various parts. If you take an unsalted saltine cracker and chew it for long enough, you'll notice it suddenly becomes very sweet. This is digestion in the works, as simple carbohydrates (monosaccharides) broken down from the cracker taste sweeter than the complex carbohydrates of which it was originally made.
  • Proteins, the digestion of which begins only in the stomach. Despite what you might think, the acid in the stomach does not actually contribute significantly to breaking down the proteins. Instead, it catalyzes the reaction of pancreatic proteases, which do the real slicing and dicing of proteins into their amino acid components. The job is finished up by the intestinal peptidases.
The digestion process winds its way through quite a few organs like your liver and pancreas before it reaches the significant stage for biochemical discussions, the small intestine. Here is where the broken up components are taken up by the body and brought through the blood supply to where they're needed. They'll be synthesized into larger molecules again to serve the body's processes, completing the digestion cycle and moving into metabolism.

Di*ges"tion [F. digestion, L. digestio.]

1.

The act or process of digesting; reduction to order; classification; thoughtful consideration.

2. Physiol.

The conversion of food, in the stomach and intestines, into soluble and diffusible products, capable of being absorbed by the blood.

3. Med.

Generation of pus; suppuration.

 

© Webster 1913.

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