Lecture 26: Digestion

Question 1

What are the main nutrients that undergo chemical digestion?

Answer 1

• carbohydrates (sugars)
• proteins
• lipids

Question 2

what are carbohydrates good for?

Answer 2

• important source of energy
• storage polysaccharides

Question 3

what is the structure of carbohydrates?

Answer 3

large complex chains of monosaccharides
e.g. glucose is a monosaccharide

Question 4

what are most common of sources of carbohydrates?

Answer 4

• starch
• glycogen

Question 5

what is the composition of starch and glycogen?

Answer 5

long chains of glucose joined by alpha 1-4 glycosidic bonds

Question 6

what are common forms of disaccharides? what are the composed of?

Answer 6

Sucrose
- glucose and fructose

Lactose
- glucose and galactose

Maltose
- Glucose and glucose

Question 7

what do we ingest a limited amount of?

Answer 7

monosaccharides - glucose

Question 8

what is protein like in food?

Answer 8

• ingest 70-100g per day of proteins in food
• not a major source of energy

Question 9

what are proteins needed for?

Answer 9

amino acids
- there are 20 amino acids
- 12 can be synthesised
- others essential, cannot be synthesised (e.g. histidine, leucine, lysine)

Question 10

what are the sources of protein?

Answer 10

• 50% diet
• 50% endogenous proteins, enzymes and immunoglobulins secreted into small intestine

Question 11

what is the structure of ingested proteins?

Answer 11

long chains of amino acids linked by peptide bonds

Question 12

what are lipids like in food?

Answer 12

• 100-150g/day
• not essential
• important source of energy
• has fat soluble vitamins A,D,E,K
• slows gastric emptying

Question 13

what is the main structure of lipids?

Answer 13

Mainly triglycerides
- glycerol backbone with 3 fatty acids attached

Fatty acids variable chain length
- short chain (less than 6 carbons)
- medium chain (6-12 carbons)
- long chain (12-24 carbons)

Question 14

why do we need chemical digestion?

Answer 14

• we ingest nutrients in the form of large complex molecules, but can only absorb nutrients as small molecules.
• chemical digestion reduces the size of nutrients to allow them to be absorbed

Question 15

where does chemical digestion occur?

Answer 15

at the surface of food particles
- mechanical digestions breaks up food to increase surface area available for chemical digestion

Question 16

what does chemical digestion utilise to function?

Answer 16

digestive enzymes

Question 17

what are features of digestive enzymes?

Answer 17

• extracellular
• they are organic catalysts
• they are very specific so different enzymes are needed for different substrates (e.g. amylase, protease, lipase)
• have optimal pH

Question 18

what is the optimal pH of salivary, gastric and small intestinal enzymes?

Answer 18

salivary - alkaline
gastric - acidic’
small intestinal - alkaline

Question 19

what is cellulose and its structure?

Answer 19

• structural polysaccharide of plants
• large amount of cellulose in diet
• long chains of beta 1-4 glycosidic bonds

Question 20

what are the 2 stages of chemical digestion?

Answer 20

• luminal digestion
• contact digestion

Question 21

what does luminal digestion involve?

Answer 21

initial digestion involving enzymes secreted into the lumen:
- salivary glands secrete salivary amylase
- stomach secretes pepsin
- small intestine has pancreatic enzymes (pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, lipase)

Question 22

what does contact digestion involve?

Answer 22

• occurs in small intestine
• completes digestion before absorption
• involves enzymes produces by enterocytes and attached to brush border of enterocytes

Question 23

what happens in the luminal digestion of carbohydrates?

Answer 23

• uses salivary and pancreatic amylase
• polysaccharides converted into oligosaccharides and disaccharides

Question 24

what happens in the contact digestion of carbohydrates?

Answer 24

• contact digestion of brush border disaccharidases
• disaccharides are converted into monosaccharides
• involved sucrase, lactase, maltase and others bound to brush border

Question 25

Answer 25

carbohydrate digestion summary

Question 26

what happens in the luminal digestion of proteins?

Answer 26

- involves pepsin in stomach - involves trypsin and chymotrypsin in small intestine - involves carboxypeptidase in small intestine - convert proteins into polypeptides

Question 27

what happens into contact digestion of proteins?

Answer 27

- involved peptidases - there are many different types attached to brush border - converts polypeptides into individual amino acids

Question 28

Answer 28

chemical digestion of proteins summary

Question 29

where does chemical digestion of lipids occur? what does it involve?

Answer 29

- occurs in lumen of small intestine - No contact digestion! - pancreatic lipase is main digestive enzyme - lingual lipase and gastric lipase have minor role

Question 30

what is the problem with lipid digestion?

Answer 30

- digestive enzymes are dissolved in aqueous luminal fluid - this is no problem for carbohydrates and proteins as they are soluble in water - lipids are insoluble in water so they need a more complex process

Question 31

what are the stages of lipid digestion?

Answer 31

- emulsification - stabilisation - digestion (hydrolysis) - formation of micelles

Question 32

what happens in emulsification? where does it occur?

Answer 32

1st stage of fat digestion: Motility breaks up lipid droplets into small droplets - forms an emulsion that increases surface area for digestion - simple emulsion occurs in the stomach (retropulsion) - more complex emulsion occurs in small intestine (segmentation) and bile salts stabilise droplets

Question 33

what happens in stabilisation? where does it occur?

Answer 33

2nd stage of fat digestion: - occurs in the small intestine - involves bile salts which are secreted by the liver and concentrated in the gallbladder - it is released into the small intestine with the arrival of food - has a hydrophobic and hydrophilic side - they stabilise the emulsion in the small intestine and reduce the size of the emulsion droplets to further increase surface area

Question 34

what happens in hydrolysis? where does it occur?

Answer 34

3rd stage of lipid digestion - occurs in small intestine at surface of emulsion droplets - involves lipase and cofactor colipase - both are secreted by the pancreas - colipase anchors lipase to the surface droplet - lipase converts triglycerides to monoglycerides and free fatty acids

Question 35

what happens in formation of micelles? what are micelles?

Answer 35

4th stage of fat digestion - products of fat digestion are insoluble in water, especially monoglycerides and long chain fatty acids - so they are kept in solution through the formation of micelles - micelles are small droplets consisting of 20-30 molecules of bile salts, fatty acids and monoglycerides

Question 36

steps of fat digestion

Answer 36