Absorption and digestion Flashcards
What types of carbohydrate may be present in the gut?
- Polysaccharides
- Disaccharides
- Monosaccharides
- Only monosaccharides are absorbed
What happens to carbohydrates of plant origin?
- These are non-digestible
- They pass into the colon where they provide nutrients for the bacteria there
What monosaccharides do we want to break our carbohydrates down into?
- Glucose
- Galactose
- Fructose
What is starch made up of?
- Amylose chains
- Amylopectin chains
Describe the structure of amylose
- Straight chains of glucose monomers
- Joined together by alpha 1-4 glycosidic bonds
What enzyme breaks down amylose?
- Amylase
- Breaks amylose down into maltose
- Maltase breaks maltose down into glucose x2
Describe the structure of amylopectin
- Branched chains of glucose
- Alpha 1-4 glycosidic bonds join glucose monomers together
- Alpha 1-6 glycosidic bonds create branches
What enzyme breaks down amylopectin?
- Amylase breaks down 1,4 bonds to give alphadextrins
- Isomaltase breaks down 1,6 bonds to give amylose
- Amylase then breaks down 1,4 bonds of amylose
- Maltase breaks maltose into glucose
What is lactose?
- Common dietary sugar found in milk
- Disaccharide of glucose and galactose
How is lactose broken down?
- Lactase (a brush-border enzyme)
- Broken down into glucose and galctose
How is sucrose broken down?
- Sucrase (a brush-border enzyme)
- Breaks down sucrose into glucose and fructose
How is glucose absorbed into intestinal epithelial cells?
- Co-transported with Na+
- Through Sodium-glucose transporter 1
- Glucose cannot bind to transporter unless Na+ has previously been bound
- Then passes into blood via Glut 2 transporter
- Galactose absorbed in the same way
How is fructose absorbed into intestinal epithelial cells?
- Through Glut 5 transporter
- Then leaves cell via Glut 2 transporter
How is an Na+ gradient maintained in intestinal enterocytes?
- Na+/K+ ATPase
- Creates low cytosolic Na+ within enterocytes
- Sets up gradient to move Na+ along with glucose/galactose into cell
Where does the glucose/galactose/fructose pass after it leaves the enterocyte via Glut 2?
- Passes into venous system
- Drains to portal system of liver
Outline how proteins are digested within the stomach?
- Chief cells secrete pepsinogen
- Pepsinogen converted to pepsin by HCl
- Protein broken down to oligopeptides and amino acids
Outline how proteins are digested within the intestinal lumen
- Pancreas releases an array of zymogens
- Enteropeptidase activates trypsinogen to trypsin
- Trypsin activates all other proenzymes
- Endopeptidases and exopeptidases break proteins down to amino acids and short peptides
- Brush border enzymes and cytosolic enzymes work on short peptides
What are the 5 major proteases of the intestine?
- Trypsinogen/trypsin
- Chymotrypsinogen/chymotrypsin
- Proelastase/elastase
- Procarboxypeptidase A/carboxypeptidase A
- Procarboxypeptidase B/carboxypeptidase B
What is the difference between endopeptidases and exopeptidases?
- Endopeptidases produce shorter polypeptides by breaking bonds in the middle of the chain
- Trypsin, chymotrypsin and elastase are all endopeptidases
- Exopeptidases cleave bonds at the end of chains
- Examples include the carboxypeptidases (A and B)
What allows peptides to be absorbed into enterocytes?
- PepT1 transporter
- Co-transports short chain peptides with Na+
How does water move out of the small intestines?
- Can move via paracellular route through tight junctions
- Can move transcellularly
- Relies on movement of Na+
What allows water to move paracellularly?
- Accumulation of Na+ in intercellular spaces
How is water secreted by enterocytes?
- Cl- moves into cell through Na+/K+/Cl- channel
- Rise in cAMP in cell causes secretion of Cl- through CFTR protein
- Causes Na+ to be drawn through tight junctions
- H2O follows into lumen of gut
What are the symptoms of vitamin B12 deficiency?
- Neurological problems
- Megaloblastic anaemia
What causes vitamin B12 deficiency?
- Lack of intrinsic factor - vit B12 needs to bind to this so it can be absorbed in the distal ileum
- Hypochlorhydria - gastric atrophy leads to inadequate HCl - cobalamin not released
- Not enough B12 in food
- Inflammation of terminal ileum e.g. due to Crohn’s disease
What causes lactose intolerance?
- Deficiency of lactase enzyme
- Diminishes after age 2
What are the symptoms of lactose intolerance?
- Diarrhoea - lactose remains in gut lumen, creating high osmotic pressure; water drawn into lumen
- Flatus and bloating - lactose fermented in colon
What are the symptoms of irritable bowel syndrome?
- Abdominal pain
- Bloating
- Flatulence
- Diarrhoea
- Constipation
- Rectal urgency
What are the risk factors for IBS?
- Female: male ratio is 2:1
- 20s-40s more affected
- Associated with psychological disorders
Which part of gluten causes coeliac disease?
- Gliadin fraction
- Found in wheat, rye, barley
What causes coeliac disease?
- Immunological response to gluten
- Damages mucosa of small intestine
- Villi flatten
- Crypts hypertrophy
- Lymphocytes found in epithelium/lamina propria
- Genetic factor
What causes coeliac disease?
- Immunological response to gluten
- Damages mucosa of small intestine
- Villi flatten
- Crypts hypertrophy
- Lymphocytes found in epithelium/lamina propria
- Genetic factor
What are the symptoms of coeliac disease?
- Malabsorption and impaired digestion
- Diarrhoea
- Weight loss
- Flatulence
- Distension
- Pain
- Anaemia (decreased iron absorption)
- Neurological symptoms (decreased calcium)
How do we investigate coeliac disease?
- Blood tests looking for IgA to smooth muscle endomysium and tissue transglutaminase
- Upper GI endoscopy
- Duodenal biopsy - mucosal pathology (reduced or absent villi)
How do we treat coeliac disease?
- Gluten free diet
