GI Digestion And Absorption

Question 1

What is digestion?

Answer 1

The breakdown of nutrients into absorbable molecules

Question 2

What is absorption?

Answer 2

Movement of nutrients, water and electrolytes from the gut lumen into the internal environment.

Question 3

What do we get from carbohydrates?

Answer 3

Starch, glycogen, cellulose and disaccharides

Question 4

What do we get lipids from? And what do we make from them?

Answer 4

Triacylglycerols/triglycerides. Phospholipids, cholesterol and cholesterol esters, free fatty acids and lipid vitamins.

Question 5

What do we get proteins from?

Answer 5

70-100g ingestedper day, 35-200g from endogenous sources e.g. dead cells in GIT and digestive enzymes.

Question 6

What are the fat-soluble vitamins?

Answer 6

A, D, E & K

Question 7

What are the water-soluble vitamins?

Answer 7

B1, B2, B3, B5, B6, B12, Folate, Biotin & C

Question 8

How is the structure of the small intestine efficient at absorption?

Answer 8

Arranged in circular folds of Keckring, to enlarge surface area. Villi project from the folds covered with epithelial cells (transport nutrients and ions to capillaries and lacteals) and goblet cells (secrete mucus). The apical surface of epithelial cells covered by microvilli (brush border).

Question 9

Carbohydrate Digestion

Answer 9

Only monosaccharides can be absorbed so first they are broken down by salivary-alpha-amylase in the mouth. Amylase which is inside the bolus continues to act in the stomach, however the outside amylase is denatured by gastric acid. In the duodenum, pancreatic amylase and brush border enzymes (maltase, sucrase and lactase) act on disaccharides to produce monosaccharides (fructose, glucose and galactose).

Question 10

Why does amylase not work on cellulose?

Answer 10

They have beta 1-4 glycosidic bonds. Amylase can only break down alpha 1-4 glycosidic bonds.

Question 11

Products of amylase on polysaccharide?

Answer 11

Maltose, maltriose, and alpha limit dextrin

Question 12

What does alpha-glucosidase do?

Answer 12

Cleaves alpha 1-4 glycosidic bonds to remove single glucose units from the non-reducing end of the polymer.

Question 13

What does isomaltase do?

Answer 13

Cleaves alpha 1-6 glycosidic bonds in the alpha-limit dextrin oligosaccharide.

Question 14

What do maltase, sucrase and lactase produce when hydrolysing oligosaccharides?

Answer 14

Maltase - produces glucose. Alpha 1-4 bond digested.

Sucrase - produces glucose and fructose. Alpha 1-2 bond digested.

Lactase - produces glucose and galactose. Beta 1-4 bond digested.

Question 15

How are the products of carbohydrate digestion absorbed?

Answer 15

• Secondary active transport via Sodium-dependent Glucose Transporter 1 (SGLT1) located on the apical membrane, transports glucose and galactose.
• Facilitated diffusion via Glucose Transporter 5 (GLUT5) transports fructose across the apical membrane.
• For basolateral membrane, all go through via Glucose Transporter 2 (GLUT2).

Question 16

Protein digestion

Answer 16

• Gastric Phase - Denaturation by HCl and Pepsin.
• Pancreatic Phase - Trypsin, Chymotrysin and Carboxypeptodase work on denatured proteins and create amino acids and oligopeptides (2-8 residues).
• Mucosal Phase - Oligopeptidase and Aminopeptidase work on Oligopeptides to create amino acids, dipeptides and tripeptides.

Question 17

What are endopeptodases?

Answer 17

Cleave large polypeptides in the middle of the chain and creates oligopeptides.

Question 18

What are exopeptidases? Name the 2.

Answer 18

Cleave amino acids one a time from either end if the chain.

• carboxypeptidases cleave from C-terminal.
• aminopeptidases cleave from N-terminal.

Question 19

Pepsin

Answer 19

Stomach. Hydrolyses links with tyrosine, D Alanine and Leucine. Creates short polypeptide chains.

Question 20

Trypsin

Answer 20

Small Intestine. Hydrolyses links with arginine and lysine.

Question 21

Chymotrypsin

Answer 21

Small intestine. Hydrolyses links with tyrosine, tryptophan, D alanine, methionine and leucine.

Question 22

Elastase

Answer 22

Small intestine. Degrades elastin.

Question 23

Lipid Digestion & Absorption.

Answer 23

1. Bile salts from liver coat fat droplets emulsified in stomach.
2. Pancreatic lipase and colipase break down fats into monoglycerides and fatty acids stored in micelles.
   3a. Monoglycerides and fatty acids move out if micelles and enter cell by diffusion.
   3b. Cholesterol is transported into cell by a membrane transporter.
3. Absorbed fats combine with cholesterol and proteins in the intestinal cells to forms chylomicrons.
4. Chylomicrons are released into the lymphatic system.

Question 24

Chylomicron

Answer 24

Core of triglycerides ane cholesterol ester covered by phospholipids and apoproteins on the outside. Packaged into secretory vesicles on the Golgi membrane and are exocytosed across basolateral membrane and enter the lacteals (lymphatic capillaries). Carried via thoracic duct and emptied into bloodstream.

Question 25

Micelle

Answer 25

Product of lipid digestion (cholesterol, monoglycerides, lysolecithim and free fatty acids) are solubilised in mixed micelles. Core contains products of lipid digestion and coated by bile salts (which are ampipathic).

Question 26

Recirculation of Bile Salts

Answer 26

There aren’t enough bile salts to digest a meal. So have to be reabsorbed in the terminal ileum back to the liver via enterohepatic circulation. 5% of bile salts lost in faeces. Cholecystokinin and secretin released stimulates bile salts to be released from gall bladder and sphinter of Oddi is opened. As bile salts find their way down the intestines, the free fat digestion products are free and can be absorbed. By the time the bile salts get to the terminal ileum, everything should have been dissolved so they can be rebsorbed.

Question 27

Absorption of Water

Answer 27

Na actively absorbed (in exchange for K), K reabsorbed in exchange for H+, Cl- absorbed (in exchange fir bicarbonate) and H2O follows due to osmosis. It is controlled by enteric nerve plexi and hormonal control (increased aldosterone = increased water absorption).

Question 28

Protein Absorption

Answer 28

• Amino acids - Na-dependent cotransport
• Dipeptides - H+-dipeptide cotransport
• Tripeptides - H+-tripeptide cotransport