Lecture 8 : digestion and absorption of carbohydrates, proteins & lipids Flashcards
What is the difference between transcellular and paracellular epithelial cell transport?
-
paracellular - substances diffuse between the adjacent cells of epithelium - limited by tight junctions
* transcellular -substances move across epithelial cell across either the luminal or basolateral surface, diffuse through the cytosol and** exit via the opposite membrane**
what are the main sites of absorption in the GIT and what do they absorb?
- proximal -iron, calcium fat , sugars
- middle - sugars and amino acids
- distal -bile salts
- colon - water & electrolytes
What does a carbohydrate have to be **broken down **into inorder to be digested?
- monosaccharides, they can only be absorbed in their simpliest form
where does carbohydrate digestion begin?
- it begins in the mouth/oral cavity via salivary amylase
How is starch digested?
- starch digestion begins in the mouth via salivary amylase - starch becomes maltose
- amylase gets inactivated in the stomach
- the 1:4 alpha linkages are broken down by glucoamylase
- the branched side chains are broken down by** alpha limit dextrinase**
- enzymes found in the brush border of the SI lumen help to break maltose into glucose (the monosaccharide)
what enzymes breakdown 1) **1:4 alpha linkage and 2)1:6 alpha linkage**?
- 1:4 alpha linkage -** glucoamylase**
- 1:6 alpha linkage - alpha limit dextrinase
What are examples of carbohydrate digestion disorders?
- Lactose malabsorption syndrome - defiency in lactase at the brush border
- congenital lactose intolerance -rare disorder
- sucrase- isomaltase deficiency-inherited disorder, treated with low sucrose diet
what** essential transporter** is crucial for the absorption of glucose and why?
- Na/k+ ATPase
- pumps 3 Na+ out of the cell & 2 k+ into the cell with each ATP consumed
- the sodium gradient generated in this process provides energy for the secondary active transport of glucose through specfici transporters
where is the Na+/k+ ATPase pump located on the intestine?
- on the basolateral membrane of the intestinal epithelial cells
what is the sodium & glucose co transporter called?
SGLT1 symporter
Describe the** process of glucose absorption**
- when carbohydrate is broken down into glucose, glucose needs to enter the epithelial cell to then diffuse into the blood stream
- it does this through secondary active transport through the SGLT1 transporter on the** apical membrane** of the epithelial cell of the intestine
- the sodium gradient generated by the** Na+/K+ATPase pump** is used as energy to transport Na+ and glucose in the** same direction into the cell**
- Glucose then exits the cell via** facilitated diffusion via the GLUT channel**
Describe the difference in the process of glucose absorption when glucose concentration is high vs low
-
high glucose - a GLUT2 is synthesised and moved to the **apical membrane **of the cell to allow more glucose into cell
* low glucose- just secondary active transport coupled to the NA+ gradient
How do galactose and fructose get absorbed?
galactose : same as glucose, uses secondary active transport coupled to NA+
fructose: facilitated diffusion through apical membrane via a GLUT channel and basolateral membrane
what do proteins need to be** broken down into** inorder to be absorbed?
amino acids or dipeptides
* NB - large polypeptides cannot be absorbed
what enzymes are proteins digested by?
- proteases present on the lumen of the intestine,enzymes in the brush border membrane and cytoplasmic enzymes
where does protein digestion begin?
- in the stomach, where HCL activates pepsinogen
what is the co-transporter required for protein absorption?
- SNAT - sodium neutral amino acid transporter
How are amino acids absorbed?
* similar mechanism to glucose
* uses secondary active transport with a SNAT transporter on the apical membrane
* SNAT uses the energy from the Na+ gradient generated by the Na+/K+ATPase to transport both Na+ and amino acids into the cell
* the amino acids then** exit the cell **via many specific amino acid transporters on the **basolateral membrane **
* they then diffuse into the bloodstream from the interstitial fluid through capillary pores
What are the 3 lipases that the pancreas secretes?
- triaglycerol hydrolase
- cholesterol ester hydrolase
- Phospholipase
Describe the first 2 steps of lipid absorption
hint :step 1(large fat molecules) and step 2 (smaller emulsio droplets)
- in the stomach, ingested fats form large fat gobules
- as these globules enter the duodenum, they are coated with bile salts
- the non polar regions of the bile salts** cling to the fat molecules** with their hydrophillic polar regions allow them to repel eachother & interact with water
- the fat molecules then become **emulsified fat droplets **
Describe **steps 3 and 4 **of lipid absorption
hint : step 3 - micelle formation & step 4 = diffusion across epithelial
- the dispersal of fat molecules (step 2) greatly increases the number of triaglycerides exposed to pancreatic lipases which helps them breakdown into fatty acids & monoglycerides
- these fatty acids and monoglyerides become associated with bile salts and leicithin (component of bile) to form micelles
- the micelles are then able to** diffuse into the epithelial cells**
what do the micelles contain?
non polar core - contains cholesterol & fat soluable vitamins
the polar/hydrophillic region - contains the monoglycerides and fatty acids etc
what happens to the** bile salt component of the micelle** after monoglycerides, fatty acids, cholesterol, leithcin and fat soluable vitamins are absorbed?
- the bile salt portion of the micelle** remains within the lumen** of the intestine until it reaches the terminal ileum, where it is reabsorbed and recylced by the enterohepatic circulation via the** portal vein**
what is one of the ** transporters** that allows bile salts to be recycled in the terminal ileum?
- ASBT - apical sodium- dependent bile salt transporter
Describe step 5 of lipid absorption
hint - step 5 = resynthesis of lipids in ER
- while small short chain fatty acids are** absorbed directly into the blood** stream from epithelial cells by passive diffusion, the **majority of the products of fat digestion **under go further chemical processing inside the enterocytes
- in the smooth ER, triaglycerides are reformed by the re-esterification of monoglycerides
- phospholipids are resynthesized and cholesterol undergoes re-esterification
Describe step 6 of lipud absorption
hint - step 6: chylomicron is formed and transported
- the** lipids accumulate in the vesicles** of the smooth ER to form chylomicrons
- they vesicles are released by the cell via exocytosis at the basolateral membrane of the enterocytes
- from here, they leave the intestine where they are released into the venous circulation via the thoracic duct
what is a chylomicron?
- lipoprotein particles that consist of triaglycerides, phospholipids, cholesterol & proteins
what product of lipid digestion is absorbed most slowly?
- cholesterol
what does a value of more than 7g of fat in faeces indicate? & what could this be caused by?
fat malabsorption
causes:
* bile acid deficiency
* luminal digestion reduced
