Digestion and Absorption

Question 1

Define digestion

Answer 1

breakdown of nutrients into molecules that can be absorbed

Question 2

Define absorption

Answer 2

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

Question 3

Describe the structure of the small intestinal mucosa

Answer 3

• surface of small intestine arranged in circular folds of Keckring
• villi project from folds and are covered in enterocytes and goblet cells
• microvilli cover the apical surface of cells creating a brush border

Question 4

What are the dietary requirements?

Answer 4

• carbohydrates
• lipids
• proteins
• vitamins (fat and water soluble)
• minerals (trace metals)

Question 5

Describe the process of digestion of carbohydrates

Answer 5

• mouth: salivary alpha amylase digests
• stomach: amylase on inside of bolus continues to act, amylase on surface is denatured
• duodenum: pancreatic amylase as well as brush border enzymes act on disaccharides

Question 6

What are the digestable carbohydrates, and what makes them digestable?

Answer 6

• starch (plants)
• glycogen (animals)

they are digestable as the alpha 1-4 glycoside bonds are broken by amylase

Question 7

What are the indigestable carbohydrates, and what makes them indigestable?

Answer 7

• cellulose (plants)

they are indigestable as the chains are linked by beta 1-4 glycoside bonds which we have no enzymes capable of breaking this down

Question 8

What is produced when amylase breaks down polysaccharides?

Answer 8

• maltotriose
• maltose
• alpha-limit dextrin

Question 9

How are the products of amylase digested?

Answer 9

• by oligosaccharidases
• alpha-glucosidase cleaves alpha 1-4 glycosidic bonds to remove single glucose units
• isomaltase cleaves alpha 1-6 glycosidic bonds in the alpha limit dextrin oligosaccharides

Question 10

How are the products of amylase and oligosaccharidases digested?

Answer 10

• maltose: digested by maltase to produce glucose
• sucrose: digested by sucrase to produce glucose and fructose
• lactose: digested by lactase to produce galactose and glucose

these are final products of carb digestion

Question 11

Describe how glucose and galactose are absorbed

Answer 11

secondary active transport

• use of sodium dependent glucose transporter 1
• located on apical membrane

Question 12

Describe the absorption of fructose

Answer 12

facilitated diffusion via the glucose transporter 5 on apical membrane

Question 13

Describe the functions of endopeptidase and exopeptidase

Answer 13

• endopeptidase: hydrolyse the interior peptide bond of proteins
• exopeptidase: hydrolyse one amino acid at a time from one end or the other

Question 14

What are examples of endopeptidases

Answer 14

• pepsin
• trypsin
• chymotrypsin
• elastase

Question 15

What are examples of exopeptidases?

Answer 15

• carboxypeptidase

- aminopeptidase

Question 16

Describe protein digestion

Answer 16

• gastric phase: proteins denatured by HCl and pepsin
• pancreatic phase: proteins broken down into amino acids and oligopeptides by trypsin, chymotrypsin or carboxypeptidase
• mucosal phase: oligopeptides further broken down into amino acids or dipeptides/tripeptides by aligopeptidase or aminopeptidase

Question 17

What are the complications o digesting lipids?

Answer 17

• lipids are hydrophobic so must be solubilised before digestion and absorption
• digestion begins in stomach with action of lingual and gastric lipases
• completed in small intestine with action of pancreatic enzymes

Question 18

What are the 4 types of lipids and 3 main types of enzymes?

Answer 18

Lipids:

• fats/oils
• phospholipids
• cholesterol/cholesterol esters
• fatty acids

Enzymes:

• lipases
• phospholipases
• cholesterol esterases

Question 19

Describe the digestion of triacylglycerols by lipases

Answer 19

• small amount digested by salivary and gastric lipases in mouth and stomach
• heat and movement of stomach mix food with lipases
• hydrolysis slow due to large aqueous/lipid interface
• as hydrolysis proceeds the rate increases due to fatty acids acting as surfactants breaking down lipid globules
• duodenum pancreatic lipases aided by bile salts
  (where majority of digestion occurs)

Question 20

What do bile salts do?

Answer 20

coat lipids to make emulsions

Question 21

What happens to the products of lipid digestion?

Answer 21

• solubilised in mixed micelles and stored in the core
• surface of micelles are coated in bile salts
• monoglycerides and fatty acids then removed from micelles
• enter cells by diffusion

Question 22

What happens when lipid products diffuse into a cell?

Answer 22

• absorbed fats combine with cholesterol and proteins from chylomicrons
• then released into lymphatic system as they are too big to enter vascular capillaries

Question 23

Why is the recycling of bile salts so important?

Answer 23

we do not have enough bile salts to deal with the average meals so after use most are reabsorbed from terminal ileum into liver by enterohepatic circulation by hepatic portal vein

Question 24

Describe the absorption of water in large intestines

Answer 24

• Na actively absorbed in exchange for K
• K reabsorbed in exchange for H+
• Cl absorbed in exchange for HCO3 and so water follows due to osmosis
• controlled by enteric nerve plexi and hormonal control