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Digestion Types - Coggle Diagram
Digestion Types
Lipid Digestion
- Lipases are produced in the pancreas and secreted into the small intestine.
- Bile contains bile salts. It is stored in the gall bladder and is secreted into the small intestine.
- Emulsification into smaller droplets increases the surface area for lipase action.
- Lipases hydrolyse the ester bonds in the triglycerides to form fatty acids and monoglycerides.
- There is further emulsification and further hydrolysis of triglycerides forming tiny micelles: tiny droplets consisting of fatty acids and monoglycerides surrounded by bile salts. Micelles transport the fatty acids and monoglycerides to the epithelial cell.
- Micelles come into contact with the epithelial cell, breakdown and release fatty acids and monoglycerides.
- Fatty acids and monoglycerides cross the cell surface membrane by simple diffusion because they are non-polar/hydrophobic/lipid soluble molecules.
- Once inside the cell, fatty acids and monoglycerides form triglycerides at the endoplasmic reticulum.
- The triglycerides then combine with proteins, lipoproteins and cholesterol in the golgi body, forming chylomicrons.
- Vesicles pinch off the end of the golgi body containing the chylomicrons, travel to the basal surface of the epithelial cell.
- The membranes of the vesicles containing the chylomicrons fuse with the cell surface membrane of the epithelial cell, resulting in the release of the chylomicrons out of the cell. This is the process of exocytosis.
- Chylomicrons can enter the lacteal (part of the lymphatic system), where they are carried to the blood system.
- Chylomicrons and triglycerides are hydrolysed by enzymes in the endothelial cells of blood capillaries.
Carbohydrate Digestion
Amylase is produced in the mouth and the pancreas. It hydrolyses the alternate glyosidic bonds of the starch molecule to produce the disaccharide maltose. The maltose then hydrolysed into alpha glucose by maltase which is produced by the lining or ileum.
- Food is chewed to make molecules smaller and provide a larger surface area for enzymes to work on.
- Saliva, which is mixed in with the food during chewing, contains salivary amylase. Salivary amylase starts hydrolysing any starch to maltose. Saliva also contains mineral salts to maintain a neural pH, as it is the optimum pH for salivary amylase to work.
- Food enters the stomach where conditions are acidic, which denatures the amylase and prevents further hydrolysis of the starch.
- Food is passed into the small intestine, where it mixes with pancreatic juice.
- Pancreatic juice contains pancreatic amylase and alkaline salts to maintain a neutral pH, so that amylase can function. This continues the hydrolysis of any remaining starch to maltose.
- Muscles in the intestine wall push the food along the ileum. The epithelial lining of the ileum produces maltase, which remains a part of the cell-surface membrane of the epithelial cells. This is referred to as a membrane-bound disaccharidase. The maltase hydrolyses the maltose from starch breakdown into alpha glucose.
Sucrose is found in many natural foods, especially fruits. Lactose is found in milk, and hence in milk products, such as yoghurt and cheese.
Sucrase hydrolyses the single glyosidic bond in the sucrose molecule. This hydrolysis produces the two monosaccharides, glucose and fructose.
Lactase hydrolyses the single glyosidic bond in the lact0sc molecule. This hydrolysis produces the two monosaccharides, glucose and galactose.
Protein Digestion
Proteins are large, complex molecules that are hydrolysed by a group of enzymes called peptidases. They hydrolyse peptide bonds.
Endopeptidases hydrolyse the peptide bonds between amino acids in the central region of a protein molecule, the internal bonds, forming a series of peptide molecules.
Exopeptidases hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. They release dipeptides and single amino acids.
Dipeptidases hydrolyse the bond between the two amino acids of a dipeptide. They are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.
The combined actions of endopeptidases and exopeptidases are more efficient than exopeptidases on their own. This is because endopeptidases hydrolyse internal peptide bonds to form many polypeptides. So, there are more ends for exopeptidases to work on/ there is an increased surface area for exopeptidases.
Co-Transport
- Na⁺ ions are actively transported through a Na⁺/K⁺ pump from the epithelial cell to the blood capillary. Potassium ions actively transported in.
- This establishes a concentration gradient such that there is a high concentration of Na⁺ ions in the lumen of the intestine, and a low concentration of Na⁺ ions in the epithelial cell.
- This causes Na⁺ ions to diffuse down the concentration gradient via a co-transport protein from intestinal lumen into epithelial cell. As they diffuse into the cell, they carry a glucose/amino acid molecule. The glucose/amino acid moves against the concentration gradient.
- The glucose/amino acid moves from epithelial cell to blood capillary via facilitated diffusion through a carrier protein.
The Na⁺ ion's concentration gradient drives the transport of glucose against its concentration gradient. This concentration gradient was established by the active transport of Na⁺ ions into the blood capillary, which used ATP.