L9*-Digestion & Absorption of nutrients-Carbs&proteins-part I

Question 1

In general, what are both carbs and proteins ? What are the 2 steps for digestion of both carbs and proteins ? and how are they absorbed ?

Answer 1

-Both Carbs and proteins are polymers.

• Digestion of both polymers are carried out in 2 steps:-
  1. Intraluminal hydrolysis (before small intestine)
  2. Membrane digestion(in the small intestine)

-The digested molecules are then reabsorbed by the enterocytes.

Question 2

Name 4 types of dietary carbohydrates.

Answer 2

1. Polysaccharides
   * Starch- from plants - contains α-1,4 and α-1,6
   linkages
   * Glycogen- from animals (more highly branched) - contains α-1,4 and α-1,6
   linkages
   * Cellulose – from plants – contains β-1, 4 glycosidic bonds
2. Oligosaccharides
3. Disaccharides (30% to 40% of dietary carbohydrates)
   * Sucrose
   * Lactose
   * Maltose
4. Monosaccharides (5% to 10% of dietary carbohydrates )

Question 3

What type of carbohydrates does the small intestine can ONLY absorb ?

Answer 3

Monosaccharides and all the other dietary carbohydrates must be digested before absorption.

Question 4

Explain the 2nd step of digestive process of dietary carbohydrates

Answer 4

2. Membrane digestion
   i). Disaccharidases found at the brush border and membrane associated.
   ii). Maltase, Sucrase & Isomaltase cleaves internal alpha-1,4 links and cleaves terminal alpha-1,4 links.
   iii). Sucrase splits sucrose into glucose and fructose.
   iv). Isomaltase cleaves alpha-1,6 links in the glycogen to form alpha-limit dextrin and then into maltose.

Question 4

Explain the 1st step of digestive process of dietary carbohydrates

Answer 4

1.Intraluminal hydrolysis
-Starch to oligosaccharides using Salivary and pancreatic amylase.
-Amylase is an endoenzyme that ONLY cleaves alpha-1,4 bonds.

i). Salivary amylase starts the process which is inactivated by the acid.
ii). Pancreatic amylase induced by CCK completes the starch digestion.

Question 5

Describe the uptake and absorption of carbohydrates and name all the transporters.

Answer 5

1. Both glucose and galactose enter the enterocyte from the lumen through SGLT1:Na/glucose transporter via secondary active transport.
2. Fructose enter the enterocyte from the lumen through GLUT5: fructose transporter via facilitated diffusion.
3. There is an increase concentrations of all glucose, galactose and fructose in the enterocyte.
4. All of them then enter the capillary/blood flow from the enterocyte through GLUT2:all monosaccharide transporter via facilitated diffusion.

Question 6

Name 3 ways to get protein via diet and other ways.

Answer 6

1. from endogenous sources
2. enzymes and hormones are proteins
3. Desquamated cells - gathers from skin > cell lysis to break open the membrane > use buffer to solubilise proteins > centrifugation, filtration to purify proteins from cell debris > analysis using western blotting.

Exception
Neonates - babies under 6 months > intact proteins > immunoglobins > breast milk > endocytosis

Question 7

In general, how are proteins digested and absorbed ?

Answer 7

-Proteins are broken down into oligopeptides & AA’s.
-Absorbed by enterocytes

Question 8

Explain the 1st step of digestive process of proteins.

Answer 8

1. Intraluminal digestion
   -involves gastric and pancreatic proteases
   -secreted as pro-enzymes

i).chief cells secrete pepsinogen, low pH(HCl) >Pepsin.
-Pepsin > Endopeptidase can partially digests 10-15% of protein

ii). 5 Pancreatic enzymes > Trypsinogen, Chymotrypsinogen, Proelastase, Procarboxypeptidase A, Procarboxypeptidase
-Also Acinar cells secreted as pro-enzymes in duodenum.

iii). Pancreatic proteolytic enzymes are either exopeptidase or endopeptidases.
-Endopeptidases(middle) have an affinity for peptide bonds adjacent to specific amino acids. Breaks into oligopeptides with 2 to 6 amino acids.
-Exopeptidases(ends) hydrolase peptide bonds adjacent to the C-terminus. Breaks into individual amino acids.

Question 9

Explain the 2nd step of digestive process of proteins.

Answer 9

2. Membrane digestion - brush border
   -Small peptides undergo further hydrolysis of AA’s.
   -Multiple exo/endopeptidases are present on both the brush border and in the cytoplasm.
   :Dipeptidase,
   :Aminopeptidase(hydrolyse peptides from amino end)
   : Tripeptidyl peptidase
   : Dipeptidyl peptidase
   : Peptidyl dipeptidase

Question 10

How are proteins absorbed ?

Answer 10

1. H+ dependent co-transporter (PepT1)
   -Tripeptides are absorbed
   -cellular peptidases further hydrolyse the peptide into amino acids

2.H+-Na+ co-transporter
-Dipeptides are absorbed
-Cellular peptidases further hydrolyse the peptide into amino acids.

3. Amino acid Na+
   -Na+ dependant co-transporter
   -AA’s > inside the enterocytes

Question 11

What is Cystinuria ? - carbs and proteins lecture

Answer 11

-Cystinuria is an autosomal recessive hereditary disorder.
-The re-absorption of cysteine from the kidneys is impaired due to mutation in the genes encoding proximal tubule dibasic amino acid transporter.