Digestion relates to the breaking down of large molecules into smaller molecules. Food cannot be utilized by the body until it has been made small enough to be absorbed into the blood stream so it can travel to the cells that require the nutrients.
It it from the moment food starts being broken down until the moment food can no longer be broken down any further.
Mechanical digestion
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Chemical digestion
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Mouth
This is where digestion starts.
The way this occurs is different in each type of animal -
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Stomach
This is where a lot of digestion happens.
The structural and physiological adaptations of the stomach is different in each animal -
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Pancreas
The pancreas doesn't hold any food, but it produces and releases enzymes that meet with the food that's leaving the stomach and heading to the small intestines.
The physiological adaptations relating to enzymatic digestion is different for each animal group -
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Liver & gall bladder
The liver produces a secretion called bile, which gets stored in the gall bladder. When food is passing through from the stomach to the small intestine, the bile is released and mixes with the food. There are no enzymes in bile but the bile is alkaline which helps other enzymes in the small intestine to work at their fastest rate. Bile also helps to break down lipids.
The structural and physiological adaptations relating to the liver and bile duct (and bile) is different in each animal groups -
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Small intestine
The intestine has lots of folds (villi) to create a large surface area so heaps of nutrients can be moved from in the intestine into the blood capillaries (absorbed into the blood stream) in a short amount of time. Some chemical digestion still occurs in the small intestine (enzymes contained in the intestinal juice), but its job is primarily absorption.
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