GI system: L29 - Chemical digestion Flashcards
Where are enzymes located in the body?
Enzymes are secreted by the salivary glands, chief cells in the stomach, acinar cells in the pancreas and a few on the small intestines.
Describe the chemical change in composition through the digestion process of carbohydrates.
A carbohydrate molecule is a polysaccharide which gets broken down into monosaccharides such as glucose, fructose and galactose. Starch found in plants and glycogen stored in muscle tissue of animals are the main carbs in our diet. They contain monosaccharides bound together by alpha-1,4 glycosidic bonds which are broken down in chemical digestion. Carbohydrates are also ingested in disaccharides such as lactose, sucrose and maltose. We also ingest cellulose but we cannot break down the beta-1,4 glycosidic bonds, although the dietary fibre is important for keeping things moving in the lumen.
Describe the chemical change in composition through the digestion process of proteins
Proteins are made up of amino acids linked by peptide bonds which are broken down during chemical digestion. There are 9 amino acids that are essential for our diet. The intestine also has to digest an equivalent amount of endogenous proteins (such as enzymes and immunoglobulins) as well as ingested proteins.
Describe the chemical change in composition through the digestion process of lipids.
Lipids are mostly ingested in the form of triglycerides. Triglycerides consist of fatty acid molecules bound to a glycerol molecule by ester bonds. digestion of a triglyceride results in 2 free fatty acids and one monoglyceride. Contains fat soluble vitamins.
Why do we need chemical digestion?
To reduce nutrients to a size that allows them to cross the epithelial lining of GI tract.
Explain what are digestive enzymes.
They are organic catalysts specific for a certain substrate. They function optimally under certain conditions. Enzyme binds to a substrate to form an enzyme-substrate complex, enzymes then dissociate from the product in the reaction. E + S ES E + P
Explain the luminal phase of chemical digestion.
Occurs in mouth, stomach and small intestine. Initiate digestion by enzymes secreted into lumen e.g salivary glands secrete salivary amylase into lumen of mouth. Chief cells of stomach secrete pepsin into lumen of stomach. Pancreas secretes trypsin, chymotrypsin, lipase and pancreatic amylase into lumen of small intestine.
Explain the contact phase of chemical digestion.
Digestion completed by enzymes attached to brushborder (microvilli) of enterocyte (villi) in the small intestines.
Explain luminal digestion of carbohydrates.
Luminal digestion of carbohydrates occurs mostly through the action of pancreatic amylase in the small intestine. Salivary amylase in the mouth plays a minor role. Alpha-1,4 glycosidic bonds are cleaved and polysaccharides break down into disaccharides.
Explain contact digestion of carbohydrates.
Disaccharides at the brush border enzymes, break down into monosaccharides (which can then be absorbed across the brush border).
Explain luminal digestion of proteins.
Pepsinogen is secreted by chief cells which is in turn activated to break down proteins in the lumen of the stomach. Trypsin and chymotrypsin are secreted from acinar cells in the pancreas which digest proteins in the intestines. Convert proteins into polypeptides and proteoses.
Explain contact digestion of proteins.
Action of peptidases at brush border causes polypeptide breakdown into individual amino acids which can then be absorbed.
Why is there no contact digestion and why is lipid digestion more complicated?
Lipids are insoluble to aqueous environments and therefore cannot reach dissolved enzymes alone, also they can easily pass through lipid-soluble membrane.
What are the four stages of lipid digestion?
Emulsification, stabilisation, digestion and formation of micelles.
Explain the emulsification stage of lipid digestion.
Motility patterns (segmentation in small intestine, retropulsion in stomach) break down large fat droplets into small droplets. This increases surface area available for enzymes.
