topic 6.1- digestion and absorption Flashcards
what is peristalsis ?
peristalsis is the contraction of circular and longitudinal muscle layers
what is the function of peristalsis in the small intestine?
to mix the food with enzymes and move it along the gut.
what is the function of the pancreas?
to secrete enzymes into the lumen of the small intestine.
what are the main enzymes present in pancreatic juice?
amylase, lipase, endopeptidase
amylase
starch->maltose
lipase
triglycerides-> fatty acids + glycerol
or
triglycerides-> fatty acids + monoglycerides
nucleases
DNA and RNA -> nucleotides
maltase
maltose -> glucose
lactase
lactose -> glucose + galactose
sucrase
sucrose -> glucose + fructose
proteases/peptidases
proteins/polypeptides -> shorter peptides
dipeptidases
dipeptides -> amino acids
what do enzymes do?
they digest most macromolecules in food into monomers in the small intestine.
describe the path of digestive enzymes from the pancreas to the small intestine
- synthesised in pancreatic gland cells on ribosomes on the rER
- processed in the Golgi apparatus
- secreted by exocytosis
- flow through pancreatic duct and into small intestine
what type of reactions are macromolecules->monomers
hydrolysis
what are the two functions of the villi in the small intestine?
- to increase the surface area of epithelium over which absorption is carried out
- to absorb monomers formed by digestion as well as minerals and vitamins
draw out a labelled diagram of a villus
check
list 6 substances absorbed by the villi in the small intestine
- glucose, fructose, galactose and other monosaccharides
- amino acids
- fatty acids, monoglycerides, glycerol
- bases (from nucleotides)
- mineral ions (eg Ca, K, Na)
- vitamins
to be absorbed into the body, nutrients must pass from the —– of the small intestine to the ——- or ——- in the villi
lumen; capillaries; lacteals
what happens before nutrients pass into the lacteal/capillaries of the villi?
they must first be absorbed into epithelium cells
why can glucose not pass through the plasma membrane of the villus epithelium via simple diffusion?
because it is polar and therefore hydrophilic
describe how triglycerides are absorbed from the small intestine into the villi.
- triglycerides are digested into fatty acids and monoglycerides
- monoglycerides are absorbed into villus epithelium cells by simple diffusion
- fatty acids are absorbed by facilitated diffusion (via fatty acid transporters)
- inside the epithelium cells, fatty acids + monoglycerides form triglycerides, which cannot diffuse back out.
- triglycerides + cholesterol + phospholipids + protein -> lipoprotein particles
- these are released into the interstitial spaces of the villus by exocytosis
describe how glucose is absorbed from the small intestine into the villi
- Na-K pumps in the inwards-facing part of the plasma membrane pump Na ions by active transport from the cytoplasm to the interstitial spaces inside the villus and K ions in the opposite direction
- there is a low concentration of Na ions inside villus epithelium cells
- sodium-glucose co-transporter proteins in the microvilli transfer a Na ion and glucose molecule together from the intestinal lumen to the cytoplasm of the epithelial cell (facilitated diffusion).
- glucose channels allow the glucose to move from the cytoplasm to the interstitial spaces inside the villus and into the blood capillaries.
mouth
- mechanical digestion of food by chewing and mixing with saliva (contains lubricants and enzymes)
oesophagus
- movement of food by peristalsis from the mouth to the stomach
stomach
- churning and mixing with secreted water + acid which kills foreign pathogens in food
- initial stages of protein digestion
small intestine
- digestion of lipids, carbs, proteins, nucleic acids
- neutralisation of stomach acid
- absorption of nutrients
pancreas
secretion of lipase, amylase, and protease
liver
secretion of surfactants in bile to break up liquid droplets
gall bladder
storage and regulated release of bile
large intestine
- re-absorption of water
- further digestion by symbiotic bacteria (especially of carbs)
- formation and storage of faces
describe the 4 layers of the wall of the small intestine
- serosa (outer coat)
- muscle layers (longitudinal muscle and inside it circular muscle)
- sub-mucosa (tissue layer containing blood and lymph vessels)
- mucosa (lining of SI, with epithelium that absorbs nutrients)
describe the processs of starch digestion
Starch can exist in one of two forms – amylose (linear, only 1,4) or amylopectin (branched, 1,4+1,6)
The digestion of starch is initiated by salivary amylase in the mouth and continued by pancreatic amylase in the intestines
Amylase digests amylose into maltose subunits (disaccharide) and digests amylopectin into branched chains called dextrins (it cannot break 1,6 bonds)
Both maltose and dextrin are digested by enzymes (maltase) which are fixed to the epithelial lining of the small intestine
how and why are the products of the digestion of starch transported to the liver?
via the hepatic portal vein; excess glucose is absorbed by liver cells and converted to glycogen for storage