14 digestion 118 - Biology 118 13 DIGESTION FOUR MAIN...

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Biology 118 13. DIGESTION FOUR MAIN STAGES: The gastrointestinal tract (GI tract) can be thought of as a ‘disassembly line’, that carries out ingestion, secretion, absorption and excretion. DIGESTION: breaking food down into smaller bits, to allow absorption. Mechanical digetion : large chunks are physically broken into smaller chunks. Chemical digestion : the chemical bonds holding large food molecules together are broken by specific enzymes. Generally, polymers are reduced to their component monomers: Proteins are hydrolyzed to component amino acids Carbohydrates simple sugars Fats fatty acids Nucleic acids (DNA, RNA) nucleotides. THE GI TRACT : the GI tract is a muscular tube. Accessory organs include the salivary glands, the liver/gall bladder, and the pancreas. The mucosa – innermost lining of the GI tract. Highly folded, to increase surface area. The muscularis externa – powerful smooth muscles, which carry out peristalsis (waves of contraction that can move materials through the tract). Mouth : mechanical digestion (chewing,) and chemical digestion – the enzyme salivary amylase, present in saliva, starts breaking starch down to simple sugars. Another enzyme present in saliva is lysozyme, which has an antibacterial function.
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