lecture 26 - digestion Flashcards
What are the main nutrients that undergo chemical digestion?
Carbohydrates, proteins, lipids/fats
What are the most common sources of carbohydrates in food?
Starch and glycogen
What is the molecular structure of starch and glycogen?
Long chains of glucose joined by alpha 1-4 glycosidic bonds
What are the 3 types of ingested disaccharides?
Sucrose, lactose, maltose
What monosaccharides make up sucrose?
glucose and fructose
What monosaccharides make up lactose?
glucose and galactose
What monosaccharides make up maltose?
two glucose units
What is the predominant monosaccharide ingested?
Glucose
How much protein is consumed each day?
70-100g
What are proteins essential for in the body?
Not a major source of energy but are used for Deriving the essential amino acids that cannot be synthesised in the body, for use in the body’s own protein synthesis
What are the 2 sources of protein for the body via digestions ?
50% diet and 50% endogenous sources (enzymes, etc.)
What are the endogenous sources of protein?
Enzymes or immunoglobulin secreted into the intestine
Lipids are essential for the absorption of which vitamins?
vitamin A, D, E & K - the fat soluble vitamins
What are the 3 key functions of fat in the diet?
Source of energy, absorption of fat soluble vitamins, slow gastric emptying
What is the main type of lipid ingested?
Triglycerides
What is the structure of a triglyceride?
Glycerol back bone with 3 fatty acids attached
Why do we need chemical digestion?
To reduced the size of nutrient molecules so they can be absorbed
How does mechanical digestion aid in chemical digestion?
It breaks up the food increasing the surface area available for chemical digestion
Do salivary enzymes favour alkaline or acidic pH?
alkaline
Do gastric enzymes favour alkaline or acidic pH?
acidic
Do small intestinal enzymes favour alkaline or acidic pH?
alkaline
Why can’t the body digest cellulose?
Cellulose is a polysaccharide with chains of glucose joined by beta 1-4 glycosidic bonds - which cannot be broken down like the alpha bonds found in starch and glycogen.
What are the 2 stages of chemical digestion?
Luminal digestion then contact digestion
What is luminal digestion?
Initial digestion involving enzymes secreted into the lumen from salivary glands, stomach and small intestine
What is contact digestion?
Completion of small intestine digestion using enzymes produced by enterocytes and attached to their brush border - the glycocalyx
What are polysaccharides broken down into during luminal digestion, and with what enzyme?
Amylase (salivary then pancreatic) breaks them into oligosaccharides and disaccharides
What stage of digestion are carbohydrates converted to monosaccharides?
During contact digestion, they are converted from disaccharides.
What class of enzyme sits in the brush border and breaks down disaccharides into monosaccharides during contact digestion?
brush border disaccharidases
What are the 3 key brush border disaccharidases?
Sucrase, lactase and maltase
What enzymes are involved with luminal digestion of proteins in the stomach?
Pepsin
What enzymes are involved with luminal digestion of proteins in the small intestine?
Trypsin, chymotrypsin, carboxypeptidase
What is the role of trypsin and chymotrypsin in luminal protein digestion?
Break down peptide bonds, to produce several shorter polypeptides
What is the role of carboxypeptidase in the luminal digestion of protein?
Cleaves the peptide bonds at the carboxy-terminal of proteins, to free some single amino acids and then shorten the protein
What is the purpose of contact digestion of proteins?
To convert polypeptides into individual amino acids
What class of enzyme are used in protein contact digestion in the small intestine?
Peptidases attached to the small intestine brush border
What peptidases are used in the small intestine during protein contact digestion?
Aminopeptidase, carboxypeptidase, dipeptidase
Where does the chemical digestion of fats occur?
In the lumen of the small intestine
Why do fats not undergo contact digestion?
They are lipid-soluble so will be absorbed straight through the epithelial barrier of the small intestine so need to be digested before they pass through
What are the 3 key enzymes invoked into the chemical digestion of fats?
pancreatic lipase (major), lingual lipase and gastric lipase (minor role)
What are the 4 stages of fat digestion?
Emulsification, stabilisation, digestion/hydrolysis, micelle formation
What is required for fat emulsification?
Motility - it breaks up lipid droplets into smaller droplets to increase the surface area for digestion
What is the purpose of fat emulsification?
Increases the surface area of fat for digestion
Where does simple fat emulsion occur, and via what process?
The stomach - via retropulsion
Where does complex emulsion of fats occur, and via what process ?
In the stomach - via segmentation
Where does fat stabilisation occur?
In the small intestine
What is required for stabilisation in fat digestion?
Bile salts - they stabilises emulsion droplets and further decrease their size to increase surface area
How do bile salts solubilise fats?
They have a hydrophobic side that binds to the fat as well as a hydrophilic side that allows for solubility in the aqueous luminal environment
What enzyme is required for hydrolysis of fat in fat digestion?
Lipase
What cofactor is required for hydrolysis of fat in fat digestion?
colipase
How does lipase break down fats in hydrolysis?
Converts triglycerides to mono glycerine and free fatty acids
What is the role of colipase in hydrolysis of fat digestion?
Anchors lipase to the surface of droplets so they can be digested
What is the final product in fat digestion?
Micelles
What are micelles?
Small droplets consisting of 20-30 molecules - bile salts, monoglycerides and fatty acids
What is the purpose of micelles?
To package monoglycerides and fatty acids and keep them in solution by binding them to bile salts, which are amphipathic
