The main function of the digestive system is to

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All living organisms need energy! The main function of the digestive system is to breakdown foods into small molecules that can be used by cells to release ATP – Chemical Digestion
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Chemical Digestion Occurs by means of enzymes Proteins that catalyze (speed up) chemical reactions Work by lowering the activation energy or the amount of energy needed to start a chemical reaction Specific enzymes work under optimum conditions (ex. Temperature and pH); different parts of the body will determine the conditions
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Substrates Macromolecules Lipids Carbohydrates Proteins Nucleic Acids Macromolecules are polymers made up of many monomers Monomers are put together by a dehydration reaction Remove water to form a bond Polymers are broken apart by a hydrolysis reaction Add water to break a bond
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Carbohydrates Digestion begins in the mouth Complex polysaccharides in the food are mixed with salivary amylase or ptyalin (enzymes) produced by the salivary glands Optimum pH for ptyalin is ~7.0, roughly the pH of the mouth Hydrolysis of the complex polysaccharides glycogen and plant starch by the amylase yields the reduced sugars maltose, sucrose and lactose The final digestion product is monosaccharides
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Proteins Protein digestion begins in the stomach Large protein molecules are split into shorter polypeptide chains (amino acids held together by the enzyme pepsin) The gastric pits of the stomach secrete pepsinogen:
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