Nutrient Digestion and Absorption Flashcards
What are the primary dietary constituents
Carbohydrates
Proteins
Fat
Vitamins
Water
Minerals
How many molecules are in a oligosaccharide?
3-9 molecules
How many molecules are in a polysaccharide?
10+ molecules
What are some carbohydrates?
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides
What are the 3 main monosaccharides?
Glucose
Galactose
Fructose
What is the difference between alpha and beta glucose?
Structural isomers of glucose
The orientation of the OH group on C1 is different
Beta has OH group above ring
Alpha has OH group below ring
What is the differences and similarities between D-glucose and L-glucose?
Stereoisomers
In D-glucose the 3 hydroxyl groups are on right side with one hydrogen
In L-glucose, they are on left
D-glucose is mainly used and is narrower
What are the 3 disaccharides?
Maltose
Sucrose
Lactose
What is maltose made up of?
2 glucose molecules
What is sucrose made up of?
1 glucose and 1 fructose
What is lactose made up of?
1 galactose and 1 glucose
What are the breakdown products of monosaccharides?
Complex CHOs which are absorbed by small intestine
What bond is two monosaccharides bonded together by?
Glycosidic bond
What are disaccharides are broken down into?
Constituent monomers by brush border enzymes in the small intestine
Describe degradation of disaccharides?
Brush border enzymes break down them in small intestine
Degradation by lactase, maltase and sucrase
What are 3 main polysaccharides?
Starch
Cellulose
Glycogen
What is alpha-amylose?
Glucose linked in straight chains
What is amylopectin?
Glucose chains highly branched
Describe a starch
Plant storage form of glucose
Glucose monomers linked by alpha 1,4 glycosidic bonds - hydrolysed by amylases
Describe cellulose
Constituent of plant cell walls
Dietary fibre - requires bacteria (cellulase)
Unbranched, linear chain of glucose monomers linked by beta 1,4 glycosidic bonds
Describe glycogen
Animal storage form of glucose
Linked by alpha 1,4 glycosidic bonds
What is the differences of starch, cellulose and glycogen?
Starch and Glycogen have alpha bonds so can be hydrolysed by amylases in saliva and pancreas
Cellulose is beta bonds so requires bacteria/cellulase
What are some features of the small intestine?
Apical membrane
Microvilli
Tight junctional complex
Basolateral membrane
What is transcellular transport?
Hydrophobic molecules can transport through cell
What is vectoral transport?
Needs carriers and energy so is active directional transport
What are some glucose transporters?
SGLT1 - need 2 Na and 1 glucose
GLUT2 - enables protein facilitated glucose movement across cell membranes
Works along with the Na/K pump to create electro chemical gradient
Are glucose transporters passive or active?
Active
What are some fructose transporters in small intestine?
GLUT5 - facilitated diffusion so passive as no fructose in blood but high glucose
Describe proteins
Polymers of amino acids linked together by peptide bonds with variation in chain length
Small - 3 to 10 in length = peptides
What are the enzymes which hydrolyse proteins and peptide bonds into amino acids?
Proteases and peptidases
Describe the breakdown of a protein
Endopeptidases break down the middle of chain into 2 smaller peptides
Aminopeptidase and Carboxypeptidases then further break down
Describe transport of amino acid across membrane
Through SAAT1
Facilitated transport along by the Na/K pump
Describe transport of di and tripeptides across membrane
NHE3 - exchanger
PepT1 is a c-transporter
Works along with Na/K pump and other pumps to go across into blood
What is the acid microclimate?
pH less than 6 near brush border - microvilli
pH more than 7 further into small intestine
Affects the absorption of weak electrolytes
Where does fat digestion take place and by what enzyme?
Small intestine by pancreatic lipase
What form is fat most commonly ingested as?
Triacylglycerol
What is triacylglycerol broken down into by lipase?
Monoglyceride and 2 fatty acids
Describe triacylglycerols
Present as large lipid droplets which are insoluble in water so lipase works on surface of droplet making slow digestion
What is the process of emulsification?
Dividing large lipid droplets into smaller droplets which increases surface area and accessibility for lipase action
What does emulsification require?
Mechanical disruption of large lipid droplets into small droplets
Emulsifying agent - stops small reforming to large droplets
What are some emulsifying agents?
Bile salts + phospholipids secreted in bile
Amphipathic molecules
Explain amphipathic molecules as emulsifying agents
Non polar portions associate with non-polar interior of lipid droplet
Polar portions exposed at water surface so repel other small lipid droplets
What is absorption enchased by formation of?
Micelles - smaller than emulsion droplets
What does a micelle consist of?
Bile salt + monoglycerides + fatty acids + phospholipids
Describe the structure of micelles
Polar at micelle surface then non-polar form core
Hydrophobic fatty acids and monoacylglycerols in interior
Bile salts on exterior
What does micelle breakdown cause?
Releases small amounts of free fatty acids and monoglycerides into solution causing diffusion across plasma membrane
What is the purpose of dynamic equilibrium between fatty acids and monoglycerides in solution and micelles?
Retains most of fat digestion products in solution while constantly replenishing supply of free molecules for absorption
What happens when fatty acids and monoglycerides enter the smooth endoplasmic reticulum (sER)?
Reformed into triacylglycerol by enzymes in sER after entering epithelial cells
When inside cells what is triacylglycerol coated in?
Amphipathic protein
Explain triacylglycerol leaving epithelial cell
Transported in vesicles formed in sER membrane - processed in golgi apparatus and exocytosed at serosal membrane
What are extracellular fat droplets called?
Chylomicrons
Where do chylomicrons pass into?
Lacteals between endothelial cells - cant pass through capillary basement membrane
What is the difference between fat-soluble and water-soluble vitamins?
Fat soluble follow fat absorption paths
Water soluble absorbed by passive diffusion or carrier mediated
What does B12 deficiency lead to?
Pernicious anaemia - failure of red blood cell maturation
How is Vitamin B12 absorbed?
Binds to intrinsic factor in stomach to form complex
then absorbed via specific transport in distal ileum
How is iron absorbed?
Across brush border membrane via DMT1 into duodenal enterocytes
Either binds to ferritin or unbound
What happens to iron bound to ferritin?
Protein iron complex - intracellular iron store
What happens to iron that is unbound?
Transported across serosal membrane into blood and binds to transferrin
What does hyperaemia cause?
Increased ferritin level so more iron bound in enterocytes
What does anaemia cause?
Decreased ferratin levels and more iron released to blood
