Digestion and absorption of nutrients Flashcards
How is digestion of carbohydrates and proteins carried out?
Intraluminal hydrolysis (before the small intestine).
Membrane digestion (in the small intestine)
Digested molecules are then absorbed into by the enterocytes.
What are the carbohydrates broken down into?
Polysaccharides
Oligosaccharides - 3 or more saccharides
Disaccharides
Monosaccharides
What are polysaccharides?
Starch, from plants.
Glycogen, from animals, more highly branched.
Glycogen and starch contains a-1,4 and a-1,6 glucose linkages.
Cellulose, from plants, contains B-1,4 glycosidic bonds
What are disaccharides?
Sucrose
Lactose
Maltose
What do the a-1,4 and a-1,6 linkages look like?
How does the small intestine absorb carbohydrates?
The small intestine can absorb only monosaccharides.
All dietary carbohydrates must be digested before absorption.
What is intraluminal hydrolysis of carbohydrates?
Starch to oligosaccharides
Salivary and pancreatic amylases
What is membrane digestion of carbohydrates?
Oligosaccharides to monosaccharides
Brush-border disaccharidases (membrane associated)
What is a-amylase?
Saliva
Pancreas
Break down polysaccharides
Luminal digestion
What are disaccharidases?
Intestine
Break down disaccharides
Membrane digestion
What are a-1,6-glucosidase isomaltase?
Break down branch points of oligosaccharides and disaccharides
Membrane digestion
What is the difference between salivary and pancreatic amylase?
Salivary amylase doesn’t do much digestion, most is digested by pancreatic amylase.
They are present in serum.
Rising levels of pancreatic amylase can be pre-diagnostic test for acute pancreatitis.
Why doesn’t carbohydrate digestion occur in the stomach?
The pH of the stomach inactivates the salivary a-amylase.
What is the process of luminal digestion?
Salivary amylase starts the process, and then is inactivated by acid.
Pancreatic amylase is also an endoenzyme that is induced by CCK and completes starch digestion.
What can amylase not act on?
Amylase is an endoenzyme that cleaves a-1,4 bonds.
Endo because it cannot cleave the terminal a-1,4 or a-1,6 links.
It also cannot act on the a-1,4 links adjacent to a-1,6 links.
Will affect the sizes of oligosaccharides produced, further breakdown by disaccharidases is required.
What are disaccharides broken down into?
Lactose is broken down by lactase into galactose and glucose
Maltose is broken down by maltase into glucose
Sucrose is broken down by sucrase into glucose and fructose.
What is the process of membrane digestion?
Disaccharidases:
Maltase cleaves internal a-1,4 links and terminal a-1,4 links.
Sucrase splits sucrose into glucose and fructose
Isomaltase cleaves a-1,6 links. a-limit dextrin
How are is fructose absorbed into the cell?
Fructose enters the enterocyte cell by facilitated diffusion through the GLUT5 transporter.
How are glucose and galactose absorbed into the cell?
Na+/K+ pump pumps Na+ out of the enterocyte and K+ in.
This causes Na+ to enter from food down the concentration gradient through the Na+/glucose SGLT1 transporter, which symports glucose and galactose.
This is secondary active transport.
How do the monosaccharides enter the blood?
Glucose, galactose and fructose are transported from the enterocyte into capillary through the GLUT2 transporter.
The blood then transports them to the liver.
How is lactose intolerance caused in babies?
Babies can have abnormal lactase levels, so they cannot absorb lactose.
When weaning, the lactase levels drop, and it can be harder to absorb lactose.
What does lactose intolerance do?
Causes cramps and bloating
Osmotic diarrhoea - lactose in large intestine, large amounts of H2O drawn in, causes diarrhoea.
What is lactase activity in a healthy individual?
Ingest lactose, increase in glucose, as lactose contains glucose.
After some time, levels decline as absorption takes place.
In large intestine, there is less sugar present, organisms do not have as much food to eat so there is less excretion.
What is lactase activity in people with lactose intolerance?
Ingest lactose, less increase in glucose levels, lactose is still present in the large intestine.
Microbes in gut have lots of food, break down lactose and excrete H2, also present in breath.
Where are proteins sourced from?
50% from the diet
50% from endogenous sources: enzymes and hormones, desquamated cells
What are proteins broken down into?
Broken down into oligopeptides and amino acids
Taken up by enterocytes
How do neonates digest proteins?
Up to 6 months old
Intact proteins are broken down by immunoglobulins by endocytosis.
Immunoglobulins are sourced from the breast milk.
What is luminal digestion of proteins?
Involves gastric and pancreatic proteases, secreted as pro-enzymes
What is HCl needed for in protein digestion?
HCl, needed to produce H+ for denaturation, produces larger surface area for digestion.
HCl act as a catalyst to activate pepsinogen to produce pepsin.
How do chief cells produce proteases?
Secrete the zymogen pepsinogen
Low pH, by HCl, converts it to pepsin
What is pepsin?
Pepsin is an endopeptidase.
It partially digests 10-15% of protein.
It has primary specificity for peptide linkages of aromatic and larger neutral amino acids - aromatic amino acids - tyrosine and phenylalanine
How are zymogens produced?
Arrival of peptides in duodenum, stimulates CCKs, squeeze on pancreas to produce zymogens.
Enteroendocrine cells by the duct contain enterokinase, which converts trypsinogen to trypsin, which then activates the other zymogens.
What are the activating agents and active forms of proenzymes?
Trypsinogen, activated by enterokinase, active form trypsin.
Trypsin then activates all the other enzymes:
Chymotrypsinogen, active form chymotrypsin.
Proelastase, active form elastase.
Procarboxypeptidase A, active form carboxypeptidase A.
Procarboxypeptidase B, active form carboxypeptidase B.
What is autodigestion of the pancreas?
Trypsin can trigger more activation of trypsinogen to form trypsin.
If trypsinogen keeps getting converted to trypsin before it reaches the duodenum, or in the fasted state, can cause autodigestion, and acute pancreatitis.
What are endopeptidases?
Have an affinity for peptide bonds adjacent to specific amino acids.
70% of polypeptides.
Trypsin, chymotrypsin and elastase produces oligopeptides with 2 to 6 amino acids.
What are exopeptidases?
Hydrolyse peptide bonds adjacent to the C-terminus.
30% of polypeptides
Carboxypeptidase A and B produces individual amino acids.
What are brush border enzymes?
Hydrolyses small peptides to amino acids in the ileum.
Dipeptidase
Aminopeptidases - cleave at ends.
Tripeptidyl peptidase - acts on tripeptides.
Dipeptidyl peptidase
Peptidyl dipeptidase
How are tripeptides absorbed?
H+ dependent co-transporter
Tripeptides are absorbed as H+ are absorbed.
Cellular peptidases further hydrolyse the peptide into amino acids.
How are dipeptides absorbed?
H+/Na+ co-transporter, absorbs dipeptides.
Cellular peptidases further hydrolyse the peptide into amino acids.
Peptides can also enter by endocytosis in the membrane.
How are amino acids absorbed?
Na+ dependent co-transporter SGLT1, amino acids go inside the enterocytes.
What is cystinuria?
An autosomal recessive hereditary disorder.
The re-absorption of cysteine from the kidneys is impaired due to the mutation in the genes encoding proximal tubule dibasic amino acid transporter - cysteine is not broken down so not reabsorbed.
