Digestion basics and digestive enzymes
- Large biological molecules cannot move across cell membranes so they have to be broken down into smaller molecules to move across
- Hydrolysis reactions break down the large molecules into smaller ones by breaking the bonds using water.
- Carbohydrates are broken down into disaccharides and then monosaccharides
- Fats are broken down into fatty acids and monoglycerides
- Proteins are broken down into amino acids
- Many different digestive enzymes are used to break down foods.
- The enzymes are released to mix with the food.
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The digestion of carbohydrates
- Amylase is a digestive enzyme that catalyses the breakdown of starch. It works by catalysing the hydrolysis reactions that break the glycosidic bonds in starch to produce maltose.
- Membrane-bound disaccharides are enzymes that are attached to the cell membranes of epithelial cells that line the ileum and they also involve the hydrolysis of glycosidic bonds.
- The monosaccharides can be transported across the epithelial cell membranes in the ileum via specific transporter proteins.
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The digestion of lipids
- Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids
- This involves hydrolysis of the ester bonds in lipids
- Lipases are mainly made in the pancreas and then secreted into the small intestine.
- Bile salts emulsify lipids which causes the lipids to form small droplets. Many small droplets have a higher surface area than a large single droplet this increases the surface area lipases have to work on.
- After the lipid has been broken down by lipase, the monoglycerides and fatty acids stick with the bile salts to form micelles and they help to with lipid digestion
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The digestion of proteins
- Proteins are broken down by a combination of different peptidases and these are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between the amino acids
- Endopeptidases hydrolyse the peptide bonds within the protein
- Exopeptidases hydrolyse peptide bonds at the ends of the proteins and only removed single amino acids.
- Dipeptidases are exopeptidases that work specifically on dipeptides and they act to separate the two amino acids that make up a dipeptide by hydrolysing the bond between them and are normally found in the cell-surface membranes of epithelial cells in the small intestine
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Apsorbtion of the products of digestion
- Monosaccharide- Glucose and galactose is absorbed by active transport with sodium ions via a co-transporter protein. Fructose is absorbed via facilitated diffusion through a different transporter protein
- Monoglycerides and fatty acids- Micelles help to move move monoglycerides and fatty acids towards the epithelium. Monoglycerides and fatty acids are lipid soluble, so can diffuse directly across the epithelial cell membrane.
- Amino acids- They are absorbed in a similar way to glucose and galactose. Sodium ions are actively transported out of the epithelial cell into the ileum itself. They then diffuse back into the cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids with them
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