Lecture 26 - Chemical digestion

Question 1

Carbohydrates

Answer 1

Carbohydrates (sugars)

Important source of energy
250-800gm per day in western diet
Storage polysaccharides
Most common source is starch which is a storage polysaccharide in grains
Also glycogen which is a storage polysaccharid within meats

Large complex of monosaccharides
Glucose is a monosaccharide
Glycogen is a large complex molecule of long chains of monosaccharides

Question 2

Composition of ingested carbohydrates

Answer 2

Stach and glycogen - longs chains of glucose joined by ⍺1-4-glycosidic bonds
Glucose molecules are bound together in long chains to form a storage polysaccharide

Disaccharides
Sucrose - glucose and fructose
Lactose - glucose and galactose
Maltose - glucose and glucose

Ingest a limited amount of monosaccharides - glucose

Question 3

Proteins

Answer 3

Ingest 70-100g per day of proteins in food
Not a source of energy

Proteins are required for amino acids
There are 20 amino acids - 12 can be synthesised and the others cannot be synthesised (histidine, leucine, lysine)

Sources of protein
50% diet
50% endogenous proteins - secreted into intestine, enzymes and immunoglobulins

Question 4

Ingested proteins

Answer 4

Long chains of amino acids that are linked by peptide bonds

Question 5

Lipids

Answer 5

100-150g per day 
Not essential 
Important source of energy 
Fat soluble vitamins A,D,E and K 
Slow gastric emptying 

Mainly triglycerides
GLycerol back bone with 3 fatty acids attached

Fatty acids variable chain length
SHort chain fatty acids less than 6 carbons
Medium chain fatty acids have 6 to 12 carbons
Long chain fatty acids have 12 to 24 carbons

Question 6

Why do we need chemical digestion?

Answer 6

Ingest nutrients in form of large complex molecules - carbohydrates, proteins and lipids

Can only absorb nutrients as small molecules

Chemical digestion reduces the size of nutrients and allows them to be absorbed

Occurs at the surface of food particles
Mechanical digestion breaks up food increases surface area available for chemical direction (physically reduces the size for better chemical digestion)

Utilises digestive enzymes - responsible for releasing the nutrients and breaking them down so that we can absorb them

Question 7

Digestive enzymes

Answer 7

Are extracellular

Are organic catalysts
E+S ES complex E+product

Very specific
Need different enzymes for different substrates
Amylase, protease, lipase

Have optimal pH
Salivary enzymes = alkaline
Gastric enzymes = acidic
Small intestinal = alkaline

Large amounts of cellulose in diet
Structural polysaccharide of plants
Long chains of β1-4 glycosidic bonds - we do not have enzymes that can break down the beta glycosidic bonds but we have ones that can break down the alpha version of the glycosidic bond

Question 8

Chemical digestion - two stage process

Answer 8

Stage one - luminal digestion
Initial digestion involving enzymes secreted into the lumen …
Salivary glands - salivary amylase
Stomach - pepsin
Small intestine - pancreatic enzymes (pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, lipase)

Stage two - contact digestion
In small intestine
Completes digestion before absorption
Involves enzymes produced by enterocytes and attached to brush border of enterocytes

Question 9

Stage one - luminal digestion

Answer 9

Stage one - luminal digestion
Initial digestion involving enzymes secreted into the lumen …
Salivary glands - salivary amylase
Stomach - pepsin
Small intestine - pancreatic enzymes (pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, lipase)

Question 10

Stage two - contact digestion

Answer 10

Stage two - contact digestion
In small intestine
Completes digestion before absorption
Involves enzymes produced by enterocytes and attached to brush border of enterocytes

Question 11

Chemical digestion of carbohydrates

Answer 11

Luminal digestion
Salivary and pancreatic amylase
Polysaccharides converted into oligosaccharides (6-8 sugars) and disaccharides (2 sugars)

Contact digestion
Disaccharides are converted to monosaccharides
Involves the enzymes - sucrase, lactase, maltase - these are bound to the brush border

Question 12

Chemical digestion of proteins

Answer 12

Luminal digestion
Pepsin in stomach
Trypsin, chymotrypsin, carboxypeptidase in small intestine - secreted by pancreas (pancreatic enzymes)
Converts proteins into polypeptides

Contact digestion 
Involves peptidases (brush border enzymes) 
Many types attached to the brush border 
Convert polypeptides into individual amino acids

Question 13

Chemical digestion of lipids (fats)

Answer 13

Occurs in the lumen of the small intestine
No contact digestion
Pancreatic lipase is the main digestive enzyme
Lingual lipase and gastric lipase have minor role

The problem with lipid digestion
Digestive enzymes dissolved in luminal fluid
No problem for carbohydrates and proteins that are soluble in water
Lipids (fats) are insoluble in water - requires a more complex process

Question 14

Stages of chemical digestion of fats

Answer 14

Emulsification achieved by motility
Stabilisation achieved by bile salts
Digestion (hydrolysis) achieved by enzymes
Formation of micelles achieved by bile salts

Question 15

Emulsification

Answer 15

1st stage of fat digestion

Motility breaks up lipid droplets into small droplets
Forms an emulsion
Droplets are 0.5-1.0 µm
Increases surface area for digestion

Occurs in
Stomach - retropulsion (simple emulsion)
Small intestine - segmentation (more complex emulsion, bile salts stabilise droplets)

Question 16

Stabilisation

Answer 16

2nd stage of fat digestion

Occurs in small intestine

Bile salts
Secreted by the liver and concentrated in the gall bladder
Released into the small intestine with the arrival of food
Have hydrophobic (water hating) and a negatively charged hydrophilic side (water loving) - bile salts put themselves around the fat droplets and arrange themselves so the hydrophilic part is interacting with the aqueous environment and it prevents them from recoercing and coming back together
Stabilise the emulsion in small intestine
Also reduce the size of the emulsion droplets - further increase in surface area

Question 17

Hydrolysis

Answer 17

3rd stage of lipid digestion

Occurs in small intestine at the surface of the emulsion droplets

Involves lipase and cofactor colipase
Both secreted by pancreas
Colipase anchors lipase to surface of droplet (anchors it so that it can do digestion)

Lipase converts triglycerides to monoglyceride and free fatty acids - the problem is that these products are also insoluble in water so there needs to be a mechanism to stop them from reforming

Question 18

Formation of micelles

Answer 18

4th (final) stage of fat digestion

Products of fat digestion are insoluble in water especially monoglycerides and long chain fatty acids

Kept in solution through formation of micelles

Micelles are
Small droplets (4-6nm in diameter)
Consists of 20-30 molecules - Bile salts (amphipathic), fatty acids, monoglycerides

Question 19

Summary of protein, carbohydrate and lipid digestion

Answer 19

Carbohydrates - amylase => disaccharides => monosaccharides (there are what are actually absorbed) => absorption

Proteins - pepsin/pancreatic proteases => peptidases => amino acids => absorption

Lipids - more difficult
Emulsification => stabilisation => digestion => monoglyceride and fatty acids are packaged into micelles