Intestinal Enzymes - Absoprtion Flashcards
structural adaptations of small intestine
- folds of Kerckring/ valvulae coniventes increases surface area x3
- microvilli increases surface area x20
- combined - surface area same as a tennis court
what barriers must a molecule overcome to be absorbed in small intestine?
An unstirred layer of fluid
The glycocalyx (fuzzy coat) covering the microvilli
The cell membrane
The cytoplasm of the enterocyte
The basal / lateral cell membrane
The intercellular space
The basement membrane
The membrane of the capillary of lymph vessel
The enterocyte membrane has the capacity to control the flux of solutes and fluid between the lumen and the blood. This involves several mechanisms, pinocytosis may be a major mechanism in the uptake of some proteins and other mechanisms include passive and facilitated diffusion and active transport. With passive diffusion the epithelium behaves like an inert barrier and particles traverse the cell membrane through pores in the cell membrane or through intercellular spaces. Lipid soluble substances can diffuse through lipid portions of the membrane. Whilst the tight junctions prevent the mixing of luminal and interstitial contents they are relatively leaky to ions and water in certain regions allowing some exchange between the lumen and intracellular spaces.
Digestion of proteins
Begins in stomach by action of pepsin
Short peptides digested by peptidases secreted by brush border
Controlled by local and enteric nervous reflexes in the submucosal plexus by stimuli from chyme
Pancreatic proteases and peptidases
- Secreted as inactive precursors
- Activated in the small intestine by enterokinase (an enzyme secreted by the intestinal mucosa)
- Enterokinase activates trypsin
- Trypsin then activates other protease enzymes
Precursors…
- Chymotrypsinogen
- Trypsinogen
- Procarboxypeptidase
->
Active Enzymes…
- Chymotrypsin
- Trypsin
- Carboxypeptidase
Absorption of Proteins
Absorbed through the intestinal epithelia by secondary active transport again using a Na+ co-transport system similar to that used in the absorption of glucose
Most peptide or amino acid molecules bind to the cells membrane with a specific transport protein that requires sodium binding before transport can occur
Main dietary carbohydrates
Polysaccharides
Starch from rice, noodles
Disaccharides
Sucrose (table sugar)
Lactose (milk sugar)
Maltose
Carbohydrate digestion
Carbohydrate digestion begins in the mouth – starch digested by salivary amylase
Ends in duodenum – pancreatic amylase forming glucose
Needs to be digested further for absorption
Brush border enzymes
Sucrase – sucrose end product glucose + fructose
Maltase – maltose end product glucose
Lactase – lactose end product glucose + galactose
Carbohydrate absorption occurs
in the small intestine and enters the blood via intestinal epithelia
Absorption of Carbohydrates - Secondary Active Transport
Uses the conc. gradient established from 1O active transport
Na+ will want to enter (due to conc. gradient
As it enters (passively) it will pull glucose through with it
Method used by glucose to enter the cell even if its conc. is higher on the inside
Absorption of Carbohydrates - Facilitated Diffusion
Substrate moves from area of high conc. to area of low conc.
Requires a membrane bound protein channel (transporter)
Substrate and transporter combine causing a conformational change
Substrate now crosses membrane
Bile
Secreted by liver hepatocytes
Stored & concentrated in the gallbladder
2 functions;
- fat digestion
- excretion of bilirubin & excess cholesterol
Composed of bile salts, bilirubin, cholesterol and lecithian
Bile Salts/Acids
- Synthesised by hepatocytes
- Precursor cholesterol
- Steroid nucleus conjugated with an amino acid
- Amphipathic
- Stabilise fatty emulsions in aqueous conditions
2 functions:
- Emulsification - large surface area for lipase action
- Absorption - help in the absorption of fatty acids & cholesterol
- Reabsorbed in the small intestine (diffusion & active transport)
- Pass through liver and are resecreted
- Enterohepatic recirculation of bile salts
Digestion and Absorption of Lipids
- Fats digested by pancreatic enzymes and bile salts forming micelles
- Micelles enter the cell by diffusion
- Fatty acids resulting from fat digestion enter epithelial cell
- Fatty acids are used to synthesise fats in endoplasmic reticulum
- Fats collect in clusters encased in protein to form chylomicrons
- Chylomicrons leave epithelial cell and enter lacteal
- Lymph in lacteal transports chylomicrons away from intestine
Absorption of water in large intestine
Each day around 9L of water enters the small intestine and all but 0.5L is absorbed prior to entry to the large intestine
No digestive enzymes are produced by large intestine
Feces is concentrated in large intestine as water & salt is absorbed
Small amounts of certain B complex vitamins & substantial quantities of vitamin K are synthesised by colonic bacteria; absorbed in large intestine
Vitamin K is essential for producing certain blood clotting proteins
Faecal material normally consists of 100g water and 50 solid, including undigested cellulose, bilirubin, bacteria and small amount of salt.
Process of water absorption in large intestine
- Water enters the intestines it reduces the osmolarity of the chyme
- Water will then move from lumen into intestinal epithelium
- As sodium is actively transported out of epithelial cells water follows
- Increases hydrostatic pressure
- Fluid is forced into the interstitium
- Absorbed by capillaries
- Epithelial cell sodium is replaced by diffusion from lumen
- Requires a carrier protein
Enhanced during nutrient absorption - Sodium co-transport system
Absorption of vitamins and minerals
- Vitamins A, D, E & K - fat soluble, are absorbed with the lipids
- Water soluble vitamins generally diffuse freely across the intestinal mucosa
- Vitamin B12 requires intrinsic factor
- So either gastric or ileal damage can cause B12 malabsorption
- Mineral absorption is usually proportional to the dietary intake but the absorption of iron and calcium can be regulated depending on body needs
Calcium ion absorption in large intestine
- actively absorbed into blood
- Dependent on Ca2+ binding protein in the mucosal epithelia, parathyroid hormone and vitamin D
- Parthyroid hormone activates vitamin D and the activated vitamin D in turn greatly enhances Ca2+ absorption
- Ca2+ absorption is increased by the rise in vitamin D levels which occur in response to reductions in body Ca2+ levels
iron ion absorption in large intestine
- occurs in duodenum and jejunum
- Ferrous (Fe2+) rather than ferric (Fe3+) ions are absorbed.
- Relies on transferrin which is then taken into the cell by endocytosis
- The Fe2+-transferrin attaches to a membrane receptor
- Eventually the iron is released across the basal membrane of the mucosal epithelium and is taken up by another form of transferrin in the plasma
other ions (K, Mg) mainly absorbed by diffusion
