Lecture 26: Chemical digestion in GIT

Question 1

What is the main reason for chemical digestion?

Answer 1

Digest food and break it up into small enough pieces to be absorbed

Question 2

How many amino acids can be made by the human body?

Answer 2

▪ 12 amino acids can be made by the body in adults

▪ Essential amino acids cannot be made by the body:
- 8 in adults

Question 3

Are lipids essential for humans?

Answer 3

▪ Most lipids are not essential
BUT
▪ Omega -3 and -6 fatty acids
▪ Fat soluble vitamins; A, D, E & K (KADE)

Question 4

What are humans source(s) of protein?

Answer 4

▪ 50% diet: plant and animal protein

▪ 50% Endogenous (body) proteins
- Secreted into intestine
- Enzymes
- Immunoglobulins

Question 5

What is the main role of amino acids in the body?

Answer 5

Source of amino acids to make body proteins (main job)

Not a major source of energy under normal conditions

Question 6

What is the main structure of lipids that we eat?

Answer 6

Mostly triglycerides (TAGs):
▪ Glycerol backbone
▪ 3 fatty acids

Question 7

What are the relative sizes of short, medium and long chain fatty acids?

Answer 7

Short chain fatty acids:
▪ < 6 carbons

Medium chain fatty acids:
▪ 6 - 12 carbons

Long chain fatty acids:
▪ 13 - 24 carbons

Question 8

In what form can carbohydrates be absorbed into the GIT?

Answer 8

monosaccharides

Question 9

In what form can proteins be absorbed into the GIT?

Answer 9

amino acids, small peptides (2 - 3 amino acids)

Question 10

In what form can lipids be absorbed into the GIT?

Answer 10

free fatty acids, monoglycerides (glycerol + 1 fatty acid)

Question 11

What are digestive enzymes?

Answer 11

Digestive enzymes are extracellular, organic catalysts

Question 12

Can cellulose (fiber) be broken down and digested in humans?

Answer 12

Cellulose can’t be digested by the body:
▪ amylase can’t break β-bonds between glucose molecules

Question 13

What is the optimal pH for salivary and pancreatic enzymes?

Answer 13

slightly alkaline pH

Question 14

What is the optimal pH for gastric (stomach) enzymes?

Answer 14

acidic pH

Question 15

Which of the following statements is CORRECT?
1. Polysaccharides are made up of many amino acids.
2. Proteins are an important source of energy in the diet.
3. Mechanical digestion increases the surface area of food
particles.
4. Fiber cannot be digested by humans because amylase is unable to break ⍺-bonds between glucose molecules.

Answer 15

3

Chippy chips

Question 16

What are the two stages of chemical digestion?

Answer 16

1. Luminal digestion: Starts chemical digestion
   - Chemical digestion by acid and enzymes which have been secreted into the lumen of the GIT and mixed through the food/chyme
2. Contact digestion: Finishes chemical digestion
   - Chemical digestion by enzymes made by and attached to the brush border of enterocytes (small intestine epithelial cells)

Question 17

What can be digested in the luminal digestion phase?

Answer 17

Carbohydrate, Protein and Lipids/Fat

Question 18

What can be digested in the contact digestion phase?

Answer 18

Further break down Carbohydrate and Protein via Brush border disaccharidases and Brush border peptidases

Question 19

What is the digestion process for carbohydrates?

Answer 19

1. Luminal digestion
   Starch (polysaccharide)
   ▪ Initiated in the mouth by salivary amylase
   ▪ Continued in the small intestine by pancreatic amylase
2. Contact digestion
   ▪ In the small intestine disaccharidases are bound to the brush border
   ▪ Different enzymes for different disaccharides:
   ▪ sucrase for sucrose, lactase for lactose, maltase for maltose

Question 20

What is the digestion process for proteins?

Answer 20

1. Luminal digestion
   ▪ Initiated in the stomach by HCl acid (hydrochloric acid) and pepsin (protease)
   ▪ Continued in the small intestine by pancreatic proteases:
   - Trypsin
   - Chymotrypsin
   - Carboxypeptidase
2. Contact digestion
   ▪ In the small intestine peptidases are bound to the brush border
   ▪ Different enzymes for different peptide bonds

Question 21

Short peptides are broken down into __________ as
result of contact digestion by ___________on the
brush border of the intestinal cells.
1. amino acids : peptidases
2. triglycerides : peptidases
3. amino acids : disaccharides
4. monosaccharides : lipases

Answer 21

1

Question 22

Are digestive enzymes water or fat soluble?

Answer 22

Digestive enzymes are water soluble and dissolved in
the fluid within the lumen of the GIT

Question 23

Does fat (lipids) undergo contact digestion?

Answer 23

NO

Fats only undergo luminal digestion (no contact digestion), which occurs in the stomach and small intestine

Question 24

What percentage of fats are digested in the stomach and what enzymes are responsible?

Answer 24

Lingual and gastric lipase digest ≈ 30% of TAGs in the stomach

Question 25

Where does a majority of lipid digestion occur and what enzyme is responsible?

Answer 25

Pancreatic lipase is the main digestive enzyme for lipids and digests TAGs in the small intestine

Question 26

What are the four stages of lipid digestion?

Answer 26

1. Emulsification: Mixing ▪Motility 2. Emulsification: Stabilisation ▪Bile salts (& others) 3. Digestion (Hydrolysis) ▪Enzymes 4. Formation of micelles ▪Bile salts

Question 27

Where are bile salts made and stored?

Answer 27

▪ Made and secreted by liver & concentrated in the gallbladder ▪ Released into small intestine with arrival of chyme

Question 28

What is special about bile salts and how do they aid lipid digestion?

Answer 28

▪ Amphipathic: have a hydrophobic (water-hating) and a negatively charged hydrophilic side (water-loving) ▪ Stabilize the emulsion droplets in small intestine ▪ Also reduce the size of the emulsion droplets ▪ Which further increases the surface area for digestion

Question 29

What is the process of emulsification?

Answer 29

Process by which large lipid droplets are broken down into smaller, stabilised droplets increasing the surface area for digestion

Question 30

What motility patterns aid in emulsifacation?

Answer 30

▪ Stomach: retropulsion (simple emulsion) ▪ Small intestine: segmentation (more complex emulsion - includes bile salts)

Question 31

What occurs during hydrolysis of lipids (stage 3)?

Answer 31

▪ Lipase converts triglycerides (TAGs) to: ▪ 2 x free fatty acids ▪ 1 x monoglyceride

Question 32

What does hydrolysis of lipids require?

Answer 32

▪ Hydrolysis of TAGs requires BOTH lipase and its cofactor colipase, both secreted by the pancreas ▪ Colipase anchors lipase to surface of droplet

Question 33

Where does hydrolysis of lipids occur?

Answer 33

▪ in the small intestine ▪ at surface of emulsion droplets

Question 34

How are the products of fat digestion kept in solution?

Answer 34

Kept in solution through formation of micelles: ▪ Consist of 20-30 molecules - Bile salts - Fatty acids (insoluble especially long chain) - Monoglycerides (insoluble)

Question 35

Bile: 1. is a lipolytic enzyme. 2. solubilises the products of lipid digestion. 3. salts reduce the surface area of lipid droplets. 4. converts trypsinogen to trypsin.

Answer 35

2