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Digestion and Absorption (Digestion (Salivary glands: Secretes saliva…
Digestion and Absorption
Digestion
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The food then enters the stomach and mixes with the HCl in the stomach killing microorganisms the stomach also contains proteases which breaks down proteins to amino acids
The Pancreas secretes pancreatic juice that contains amylase, protease and lipases
The food then travels into the small intestine which is adapted to provide a large surface area for the absorption of the products of digestion. Maltase enzymes are imbedded within the epithelium cell membrane of the small intestine. This enzyme breaks down maltose to glucose, so it is available for rapid absorption
The remains of the food then travels through the large intestine where any left water is reabsorbed causing the remains to harden into feces
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Carbohydrate digestion
Food enters the mouth and is broken up by teeth, then mixed with saliva
Salivary amylase works by catalysing the hydrolysis reactions that break the glycosidic bonds in starch to produce maltose
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Pancreatic amylase is then secreted into the small intestine and continues to hydrolyse the starch to maltose
Disaccharidase enzymes are attached to the cell membranes of epithelial cells in the ileum breaking down the disacchardies into monosaccharides like glucose which is then reabsorbed
Lipid digestion
Triglycerides are hydrolysed into a monoglyceride and 2 fatty acids and this reaction is catalysed by lipase enzymes
This involves the hydrolysis of ester bonds in lipids. The lipases used are mainly made in the pancreas- they're then transported to the small intestine when they act
Bile salts are produced in the liver and emulsify lipids meaning they cause lipids to form small droplets to increase surface area
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Many small lipids droplets have a larger surface area to volume area ratio so have a larger area for the enzyme lipase to work on
Once a lipid has been broken down by lipase, the monoglyceride and the 2 fatty acids stick with the bile salts to form tiny structures called micelles
Protein digestion
Proteins are digested by a combination of different peptidases (Proteases) These enzymes catalyse the conversion of proteins to amino acids by hydrolysing the peptide bonds between amino acids
Peptidases
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Dipeptiases
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Dipeptidases are often located in the cell surface membrane of epithelial cells in the small intestine
Absorption
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Co-transport
1) Sodium ions are actively transported out of the epithelial cell,into blood, using energy from the hydrolysis of ATP and a specific carrier protein
2) Creating a sodium ion concentration gradient from the lumen of the small intestine to the epithelial cell. So sodium ions and glucose is co-transported down the sodium ion concentration gradient via facilitated diffusion involving the specific carrier protein that only carries glucose and sodium at the same time
3) The glucose is then transported from the epithelial cell into the blood via facilitated diffusion down a concentration gradient, using a specific carrier protein
Glycerol and fatty acids
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To release the monoglyceride the micelle will break down then later reform allowing the monoglyceride to be absorbed
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Triglycerides cannot be absorbed by the epithelium directly which is why they are broken up into monoglycerides and fatty acids
Once they are across the membrane the monoglycerides the fatty acids reform into triglycerides in a vesicle which enters the Golgi apparatus
The Golgi apparatus then processes the triglycerides and combines them with proteins forming a chylomicron
The vesicle then moves to the membrane and fuses with it releasing the proteins and triglycerides through a process called exocytosis
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