Answer to Question 1
5
Answer to Question 2
The mechanical digestive actions include chewing, mixing by the stomach, adding fluid, and moving the tract's contents by peristalsis. The job of mechanical digestion begins in the mouth, where large, solid food pieces are torn and moistened to be swallowed easily, without choking. Nutrients trapped inside indigestible skins must be liberated by breaking these skins before being digested. Chewing bursts open kernels of corn, for example, which would otherwise be undigested. The stomach and intestines then liquefy foods through various mashing and squeezing actions and push food through the intestinal tract by waves of peristalsis.
Chemical digestion begins in the mouth, where food is mixed with an enzyme in saliva that acts on carbohydrates and another enzyme initiates a little digestion of fat, especially the digestion of milk fat. Chemical digestion continues in the stomach, where stomach enzymes and acid break down protein. Cells in the stomach release gastric juice, a mixture of water, enzymes, and hydrochloric acid. This strong acid mixture is needed to activate a protein-digesting enzyme and to initiate chemical digestion of protein. Chemical digestion progresses in the small intestine, where the liver and gallbladder contribute bile that emulsifies fat. The pancreas releases pancreatic juice, containing the alkaline compound bicarbonate, in amounts precisely adjusted to neutralize the stomach acid that has reached the small intestine. All these actions alter the intestinal environment to perfectly support the work of chemical digestion by digestive enzymes.
Methane combustion as an energy–yielding redox reaction.
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