Digestion & Absorption Flashcards
What polysaccharides are digested by humans?
glucose polymers
What disaccharides are digested by humans?
lactose and sucrose
What monosaccharides are absorbed by humans?
glucose and fructose
What digests carbohydrates in the mouth?
Salivary alpha-amylase - hydrolyses 1:4alpha linkages between glucose molecules
What digests carbohydrates in the duodenum/jejenum?
pancreatic alpha-amylase in the lumen and isomaltase, sucrase and maltase at the brush border
What triggers release of pancreatic alpha-amylase?
CCK
What linkage are amylases ineffective at?
1:6 alpha linkages
What does isomaltase do?
Break up 1:6 alpha linkages
What does sucrase do?
Breaks up sucrose into fructose and glucose
How are maltase and sucrase activated?
By pancreatic proteases
Where are glucose and fructose predominantly absorbed?
At the tips of the villi
How is glucose transported?
Via SGLT1 transporter with sodium against its concentration gradient
What is the function of sweet taste receptors on the villi?
When activated they activate the SGLT1 transported to enhance glucose uptake
How is fructose transported?
Via GLUT5 transporter down its concentration gradient into epithelial cells and via GLUT2 into the interstitium
What digests proteins in the stomach?
Pepsin
Where does pepsin come from?
Pepsinogen (the inactive form) is secreted by chief cells and is then activated by acid to pepsin
What is the action of pepsin?
Hydrolyses bonds between an aromatic amino acid and a second amino acid
What enzymes digest proteins in the duodenum and jejunum?
mostly trypsin and chymotrypsin but also elastase and carboxypeptidase A and B
Where does trypsin come from?
It is secreted by the pancreas as trypsinogen and is activated by enterokinase at the brush border membrane
Where does chymotrypsin come from?
It it secreted by the pancreas as chymotrypsinogen and is activated by trypsin
What is the action of trypsin, chymotrypsin and elastase?
act at interior bonds of peptides to produce short polypeptides
What is the action of carboxypeptidases?
act at the carboxy terminals of peptides to produce free amino acids
What happens to the short polypeptides at the brush border membrane?
They are broken down by aminopeptidases, carboxypeptidases, endopeptidases and dipeptidases
How are free amino acids absorbed?
Via 7 or or more different transporter mechanisms - 5 of which require sodium and 2 of which require chloride
How is fat digested in the mouth?
With a lingual lipase - but only with minor effect
How is fat digested in the stomach?
With a gastric lipase - but only with minor effect
How is fat digested in the duodenum?
By lipase and cholesterol esterase
How is lipase activated in the duodenum?
by colipase which is activated by trypsin
How is cholesterol esterase activated?
By bile
How is fat emulsified?
By bile salts and lecithin and contractile mixing - forms micelles
How is fat absorbed?
The bile salts on the edge of the micelles dissolve partly in the membrane allowing the lipids to dissolve across the membrane and enter the cell where they reform triglycerides and form chylomicrons which are secreted by exocytosis into the lymphatics
What are the main three short chain fatty acids produced by fermentation of dietary fibre?
acetate, proprionate and butyrate
