Lecture 8 : digestion and absorption of carbohydrates, proteins & lipids

Question 1

What is the difference between transcellular and paracellular epithelial cell transport?

Answer 1

• 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**

Question 2

what are the main sites of absorption in the GIT and what do they absorb?

Answer 2

• proximal -iron, calcium fat , sugars
• middle - sugars and amino acids
• distal -bile salts
• colon - water & electrolytes

Question 3

What does a carbohydrate have to be **broken down **into inorder to be digested?

Answer 3

• monosaccharides, they can only be absorbed in their simpliest form

Question 4

where does carbohydrate digestion begin?

Answer 4

• it begins in the mouth/oral cavity via salivary amylase

Question 5

How is starch digested?

Answer 5

• 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)

Question 6

what enzymes breakdown 1) **1:4 alpha linkage and 2)1:6 alpha linkage**?

Answer 6

• 1:4 alpha linkage -** glucoamylase**
• 1:6 alpha linkage - alpha limit dextrinase

Question 7

What are examples of carbohydrate digestion disorders?

Answer 7

• 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

Question 8

what** essential transporter** is crucial for the absorption of glucose and why?

Answer 8

• 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

Question 9

where is the Na+/k+ ATPase pump located on the intestine?

Answer 9

• on the basolateral membrane of the intestinal epithelial cells

Question 10

what is the sodium & glucose co transporter called?

Answer 10

SGLT1 symporter

Question 11

Describe the** process of glucose absorption**

Answer 11

• 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**

Question 12

Describe the difference in the process of glucose absorption when glucose concentration is high vs low

Answer 12

• 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

Question 13

How do galactose and fructose get absorbed?

Answer 13

galactose : same as glucose, uses secondary active transport coupled to NA+
fructose: facilitated diffusion through apical membrane via a GLUT channel and basolateral membrane

Question 14

what do proteins need to be** broken down into** inorder to be absorbed?

Answer 14

amino acids or dipeptides
* NB - large polypeptides cannot be absorbed

Question 15

what enzymes are proteins digested by?

Answer 15

• proteases present on the lumen of the intestine,enzymes in the brush border membrane and cytoplasmic enzymes

Question 16

where does protein digestion begin?

Answer 16

• in the stomach, where HCL activates pepsinogen

Question 17

what is the co-transporter required for protein absorption?

Answer 17

• SNAT - sodium neutral amino acid transporter

Question 18

How are amino acids absorbed?

Answer 18

* 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

Question 19

What are the 3 lipases that the pancreas secretes?

Answer 19

1. triaglycerol hydrolase
2. cholesterol ester hydrolase
3. Phospholipase

Question 20

Describe the first 2 steps of lipid absorption

hint :step 1(large fat molecules) and step 2 (smaller emulsio droplets)

Answer 20

• 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 **

Question 21

Describe **steps 3 and 4 **of lipid absorption

hint : step 3 - micelle formation & step 4 = diffusion across epithelial

Answer 21

• 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**

Question 22

what do the micelles contain?

Answer 22

non polar core - contains cholesterol & fat soluable vitamins
the polar/hydrophillic region - contains the monoglycerides and fatty acids etc

Question 23

what happens to the** bile salt component of the micelle** after monoglycerides, fatty acids, cholesterol, leithcin and fat soluable vitamins are absorbed?

Answer 23

• 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**

Question 24

what is one of the ** transporters** that allows bile salts to be recycled in the terminal ileum?

Answer 24

• ASBT - apical sodium- dependent bile salt transporter

Question 25

Describe step 5 of lipid absorption

hint - step 5 = resynthesis of lipids in ER

Answer 25

• 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

Question 26

Describe step 6 of lipud absorption

hint - step 6: chylomicron is formed and transported

Answer 26

• 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

Question 27

what is a chylomicron?

Answer 27

• lipoprotein particles that consist of triaglycerides, phospholipids, cholesterol & proteins

Question 28

what product of lipid digestion is absorbed most slowly?

Answer 28

• cholesterol

Question 29

what does a value of more than 7g of fat in faeces indicate? & what could this be caused by?

Answer 29

fat malabsorption
causes:
* bile acid deficiency
* luminal digestion reduced