Lecture 26 Flashcards
Chemical Digestion in the GIT
Why do we need chemical digestion in the GIT?
The food we eat contains nutrients mostly in their storage (large) forms
What are the three forms of nutrients we eat?
Carbohydrates, proteins and lipid/fats
What is the dietary large form of carbohydrates?
Starch (polysaccharide) and disaccharides
What is the small molecule of carbohydrates?
Monosaccharides
What does mechanical digestion do to the large forms of the nutrients?
Breaks up into smaller pieces, increasing surface area for chemical digestion
What does chemical digestion do to the large forms of the nutrients?
Breaks the large forms into their small forms to be absorbed by the GIT epithelium
How much dietaty carbohydrate do we need per day?
250-800g
What is a monosaccharide?
One saccharide molecule, e.g. glucose and fructose
What is a disaccharide?
Tow saccharide molecules, e.g. sucrose (glucose + fructose)
What is a polysaccharide?
Many monosaccharide molecules joined by bonds and the large form of carbohydrates
What are the two forms of polysarrarides?
Starch and fiber
Is starch digestible or indigestible?
Digestible
Is fibre digestible or indigestible?
Indigestible
What is the large molecule form of a protein?
Large peptides to tertiary and quaternary structures
What is the small unit of a protiein?
Amino acids and small peptides
How much dietary proteins do we need per day?
50-175g
What is the structure of a protein?
Long chains of amino acids joined by peptide bonds, peptide chains are folded into secondary and tertiary structures that may join and form quarternary structures
How many amino acids are there?
20
How many amino acids can be made in our body?
12
How many amino acids cannot be made by the body and must be in our diet?
8
What are the sources of protein?
50% diet and 50% endogenous
What is the large forms of lipids?
Triglycerides
What are the small molecule forms of lipids?
Free fatty acids, monoglycerides (glycerol + 1 fatty acid)
How much dietary lipid do we need per day?
50-150g
What are the essential lipids?
Omega 3 and 6 fatty acids, vitamins A, D, E, K
How are lipids stored?
Mostly as triglycerides (TAGs) with a glycerol backbone and 3 fatty acid chains
What are the two forms of chemicals that break down large molecules to small molecules?
Acid and digestive enzymes
What is the requirement of enzyme/substrate pairing?
Must be complementary
What is the complementary enzyme for a carbohydrate (starch)
Amylase
What is the complementary enzyme for a protein (pepsin)
Protease
What is the complementary enzyme for a Lipid
lipase
Why can starch be digested by the body but fibre cant?
Different bond formations, alpha - beta. Amylase cannot cleave the beta bond.
How do digestive enzymes work the best?
With optimal pH
What is the optimal pH of salivary enzymes?
Slightly alkaline pH
What is the optimal pH of gastric/stomach enzymes?
Acidic pH
What is the optimal pH for pancreatic enzymes?
Slightly alkaline pH
What are the two stages of chemical digestion?
Luminal and contact digestion
How does luminal digestion occur?
Chemical digestion by acid and enzymes that have been secreted into the lumen of the GIT and mixed through the food/chyme
What is the acid/enzyme active in the mouth?
Salivary amylase
What is the acid/enzyme active in the stomach?
HCl acid, Pepsin, lingual and gastric lipase
What is the acid/enzyme active in the small intestine?
Pancreatic digestive enzymes
How does contact digestion occur?
Chemical digestion by enzymes made by and attached to the brush boarder of enterocytes
What large molecules are broken down through contact digestion?
Only carbohydrates and proteins
Where in the body does contact digestion occur?
Small intestine
What are the brush boarder enzymes?
Brush boarder disaccharides and brush boarder peptidases
How does luminal digestion occur for carbohydrates?
Initiated in the mouth by salivary amylase, continued in the small intestine by pancreatic amylase
How does contact digestion occur for carbohydrates?
Brush border disaccharides in the SI break down different disaccharides into monosaccharides
How does luminal digestion occur for protein?
Initiated in the stomach by HCl acid and Pepsin, continued in the SI by pancreatic proteases
How does contact digestion occur for proteins?
Brush border peptidases in the SI break down small polypeptides into amino acids, di- or tri-peptides
Why is chemical digestion for lipids different?
They are not water-soluble, whereas digestive enzyme are and are dissolved in the fluid of the GIT
What type of chemical digestion do lipids undergo?
ONLY luminal digestion
What are the four stages of chemical digestion for lipids?
Emulsification; mixing, emulsification; stabilisation, digestion and formation of micelles
What is emulsification?
The process by which large lipid droplets are broken down into smaller, stabilised droplets to increase the surface area for digestion
What is the mixing stage of emulsification?
Motility breaks up large lipid droplets into smaller emulsification droplets. Stomach; retropulsion and small intestine; segmentation
What does the mixing stage of emulsification result in?
Droplets of about 0.5-1 micro metre
What is the stabilisation stage of emulsification?
Bile salts (& other molecules) coat the emulsion droplets. Stomach; simple emulsion (other molecules) and small intestine; complex emulsion (bile salts)
What does the stabilisation stage of emulsification result in?
Prevention of reformation of larger droplets
What are bile salts?
The primary component of bile that are made and secreted by the liver and concentrated in the gallbladder, released into the SI with the arrival of chyme.
What is the chemical nature of bile salts?
Amphipathic
What is digestion (in the digestion of fats pathway) ?
Hydrolysis; lipase converts triglycerides to 2x free fatty acids and 1x monoglyceride
What does hydrolysis of TAGs require?
Lipase and cofactor collpase
What is collpase?
Enzyme secreted by pancreas that anchors lipase to the surface of droplets
Where does hydrolysis of fats occur?
In the small intestine, at the surface of emulsion droplets
What is a micelle?
Colloidal particles formed from the aggregation of amphiphilic molecules
Why do we need to form micelles?
Products of fat digestion are insoluble in water
What is the size of a micelle?
4-7 nm
What does a micelle consist of?
20-30 molecules; bile salts, fatty acids, monoglycerides
