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Dr. Stretton note 2 - Biology 152 Digestion Lecture 2...

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Biology 152 Lecture 2- Digestion Jan 25, 2008 Digestion Most plants are photosynthetic autotrophs, that is they take carbon dioxide and water and light and synthesize carbohydrate and in the process liberate oxygen. Of course, they also synthesize their own protein, lipid, and nucleic acids. By contrast we animals cannot synthesize complicated molecules from carbon dioxide and water. We're not green, cannot absorb light and cannot photosynthesize. We have to get our energy in the form of already synthesized organic molecules, and these organic molecules are provided ultimately from photosynthesis by plants; we eat them, breaking down their molecules into carbon dioxide and water again using the oxygen plants have also provided for us. This cycle of capturing then releasing carbon dioxide is a food chain which can be either simple or complex. An example of a simple food chain is from grass to elephant; an example of a complex food chain might go from phytoplankton to zooplankton to bigger crustacean to small fish and to medium-size fish to big fish and then perhaps to you and me. Each organism in turn digests the macromolecules of the organism it eats and uses some of them to generate ATP, the universal currency of energy in biological systems. It also synthesizes its own macromolecules from the small molecular weight products of digestion, the amino acids, nucleotides and sugars. Why should this be? Why go to all this trouble? One of the unifying concepts of biology is the Unity of Biochemistry. Most of the biochemical processes that occur in organisms of various types are the same: almost everybody does glycolysis and oxidative phosphorylation, and we all do DNA makes RNA makes protein, etc, etc. Each of these processes is controlled by enzymes, and by and large all of the food we eat contains the enzymes controlling nearly all of reactions that we need to do. Why don't we just use them instead of breaking them down and then reassembling them into our own version of each enzyme? There are several reasons why this might be. Probably the simplest is that for reasons of defense you want to keep out big stuff like viruses and nucleic acids that might be infectious. So you have to have a good barrier, as we have already discussed, and it keeps big molecules like enzymes out too. But also although the biochemistry of all organisms is very similar it's not exactly identical, and basically an organism needs to have control, tight control, over its own biochemical reactions. There is strong evolutionary pressure for this. It would not be too good if your efficiency depended on exactly which food you had just eaten. If you depended on say hexokinase to metabolize glucose and you had just eaten a load of beans whose hexokinase was less efficient than that of fish enzyme, life would be very complicated indeed when you ate beans instead of fish for dinner.
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