Digestion and Absorption Flashcards
Define digestion
breakdown of nutrients into molecules that can be absorbed
Define absorption
movement of nutrients, water and electrolytes from the gut lumen into the internal environment
Describe the structure of the small intestinal mucosa
- surface of small intestine arranged in circular folds of Keckring
- villi project from folds and are covered in enterocytes and goblet cells
- microvilli cover the apical surface of cells creating a brush border
What are the dietary requirements?
- carbohydrates
- lipids
- proteins
- vitamins (fat and water soluble)
- minerals (trace metals)
Describe the process of digestion of carbohydrates
- mouth: salivary alpha amylase digests
- stomach: amylase on inside of bolus continues to act, amylase on surface is denatured
- duodenum: pancreatic amylase as well as brush border enzymes act on disaccharides
What are the digestable carbohydrates, and what makes them digestable?
- starch (plants)
- glycogen (animals)
they are digestable as the alpha 1-4 glycoside bonds are broken by amylase
What are the indigestable carbohydrates, and what makes them indigestable?
- cellulose (plants)
they are indigestable as the chains are linked by beta 1-4 glycoside bonds which we have no enzymes capable of breaking this down
What is produced when amylase breaks down polysaccharides?
- maltotriose
- maltose
- alpha-limit dextrin
How are the products of amylase digested?
- by oligosaccharidases
- alpha-glucosidase cleaves alpha 1-4 glycosidic bonds to remove single glucose units
- isomaltase cleaves alpha 1-6 glycosidic bonds in the alpha limit dextrin oligosaccharides
How are the products of amylase and oligosaccharidases digested?
- maltose: digested by maltase to produce glucose
- sucrose: digested by sucrase to produce glucose and fructose
- lactose: digested by lactase to produce galactose and glucose
these are final products of carb digestion
Describe how glucose and galactose are absorbed
secondary active transport
- use of sodium dependent glucose transporter 1
- located on apical membrane
Describe the absorption of fructose
facilitated diffusion via the glucose transporter 5 on apical membrane
Describe the functions of endopeptidase and exopeptidase
- endopeptidase: hydrolyse the interior peptide bond of proteins
- exopeptidase: hydrolyse one amino acid at a time from one end or the other
What are examples of endopeptidases
- pepsin
- trypsin
- chymotrypsin
- elastase
What are examples of exopeptidases?
- carboxypeptidase
- aminopeptidase
Describe protein digestion
- gastric phase: proteins denatured by HCl and pepsin
- pancreatic phase: proteins broken down into amino acids and oligopeptides by trypsin, chymotrypsin or carboxypeptidase
- mucosal phase: oligopeptides further broken down into amino acids or dipeptides/tripeptides by aligopeptidase or aminopeptidase
What are the complications o digesting lipids?
- lipids are hydrophobic so must be solubilised before digestion and absorption
- digestion begins in stomach with action of lingual and gastric lipases
- completed in small intestine with action of pancreatic enzymes
What are the 4 types of lipids and 3 main types of enzymes?
Lipids:
- fats/oils
- phospholipids
- cholesterol/cholesterol esters
- fatty acids
Enzymes:
- lipases
- phospholipases
- cholesterol esterases
Describe the digestion of triacylglycerols by lipases
- small amount digested by salivary and gastric lipases in mouth and stomach
- heat and movement of stomach mix food with lipases
- hydrolysis slow due to large aqueous/lipid interface
- as hydrolysis proceeds the rate increases due to fatty acids acting as surfactants breaking down lipid globules
- duodenum pancreatic lipases aided by bile salts
(where majority of digestion occurs)
What do bile salts do?
coat lipids to make emulsions
What happens to the products of lipid digestion?
- solubilised in mixed micelles and stored in the core
- surface of micelles are coated in bile salts
- monoglycerides and fatty acids then removed from micelles
- enter cells by diffusion
What happens when lipid products diffuse into a cell?
- absorbed fats combine with cholesterol and proteins from chylomicrons
- then released into lymphatic system as they are too big to enter vascular capillaries
Why is the recycling of bile salts so important?
we do not have enough bile salts to deal with the average meals so after use most are reabsorbed from terminal ileum into liver by enterohepatic circulation by hepatic portal vein
Describe the absorption of water in large intestines
- Na actively absorbed in exchange for K
- K reabsorbed in exchange for H+
- Cl absorbed in exchange for HCO3 and so water follows due to osmosis
- controlled by enteric nerve plexi and hormonal control
