Gastrointestinal Physiology: Digestion

Question 1

What are the 3 main nutrients that undergo chemical digetion?

Answer 1

Carbohydrates, lipids and proteins

Question 2

These are made up of long chains of glucose joined by alpha 1-4 glycosidic bonds.

Answer 2

Starch and glycogen

Question 3

What are the 3 disaccharides?

Answer 3

Sucrose, lactose and maltose

Question 4

Why are lipids not essential to our bodies?

Answer 4

Because we can produce them endogenously so no need for them to be in our diet

Question 5

What are some of the properties of lipids in our body? (3)

Answer 5

• slow gastric emptying
• source of energy
• fat soluble vitamins A, D, E and K

Question 6

What is the main component of lipids?

Answer 6

Triglycerides

Question 7

This reduces the size of nutrients to allow them to be absorbed.

Answer 7

Chemical digestion

Question 8

Where does chemical digestion occur? And what does it use to break down the food particles?

Answer 8

Chemical digestion occurs at the surface of food particles.

It uses digestive enzymes secreted by SI and pancreas to break the food down

Question 9

This breaks up food and increases surface area available for chemical digestion. Usually occurs at the mouth.

Answer 9

Mechanical digestion

Question 10

These are highly specific extracellular organic catalysts that are responsible for breaking down food particles during chemical digestion.

Answer 10

Digestive enzymes

Question 11

What is the optimal pH for each of these enzymes:

a) salivary enzymes
b) gastric enzymes
c) small intestinal enzymes

Answer 11

a) alkaline
b) acidic
c) alkaline

Question 12

What does it mean by digestive enzymes being highly specific?

Answer 12

Different enzymes bind to different substrates

• amylase => starch
• proteases => proteins
• lipase => lipids

Question 13

What are the 2 stages of chemical digestion?

Answer 13

Luminal digestion and contact digestion

Question 14

This is the initial digestion which involves enzymes secreted into the lumen.

Additional question: this can occur at 3 different organs in the GI tract, what are those 3 and what enzymes specifically carryout this type of digestion?

Answer 14

Luminal digestion

Luminal digestion occurs in the stomach, the small intestine and the mouth. The enzymes in the stomach are usually pepsin, small intestine: pancreatic enzymes (pancreatic amylase, trypsin, chymotrypsin etc., and in the mouth: salivary amylase

Question 15

This is the type of digestion which occurs in the small intestine. It involves enzymes produced by enterocytes which are attached to the brush border of the enterocytes.

Answer 15

Contact digestion (via microvilli brush border enzymes)

Remember: these enzymes are produced by the epithelial cells (enterocytes) of the small intestine. Segmentation helps contact digestion occur.

Question 16

How are carbohydrates chemically digested?
- what are they broken down into
- where do they get broken down into
- what enzymes are responsible for breaking them down

Answer 16

Carbohydrate digestion occurs in two stages.

Luminal digestion occurs in the mouth and the small intestine
- luminal digestion breaks polysaccharides into oligosaccharides and disaccharides using salivary amylase and pancreatic enzymes

Contact digestion occurs in the small intestine
- contact digestion breaks disaccharides and breaks them down into monosaccharides using brushborder disaccharidases (carbohydrate enzymes)

Question 17

Where is the pancreatic amylase found?

Answer 17

in the small intestine; it is a pancreatic enzyme not a brushborder enzyme.

Question 18

How are proteins chemically digested?
- what are they broken down into
- where do they get broken down into
- what enzymes are responsible for breaking them down

Answer 18

Luminal digestion occurs in the stomach and the small intestine
- luminal digestion breaks proteins down into polypeptides using pepsin in stomach, and trypsin, chymotrypsin, and carboxypeptidase in the small intestine

Contact digestion occurs in the small intestine
- contact digestion breaks proteins into indvidual amino acids using peptidases (which are attached to the brush border)

Question 19

Where is carboxypeptidase made?

Answer 19

In the pancreas, secreted by the pancreas into SI

Question 20

How are lipids chemically digested?
- what are they broken down into
- where do they get broken down into
- what enzymes are responsible for breaking them down

Answer 20

Lipids only undergo luminal digestion in the small intestine using pancreatic lipase as the main enzyme (lingual and gastric lipase also play minor roles)

Question 21

Why do lipids have a more complex process of digestion?

Answer 21

Because the digestive enzymes are soluble in aqueous solutions, but lipids are not.

• so need to convert lipids into a water soluble form so it can be broken down by water soluble enzymes

Question 22

What are the 4 stages of lipid digestion?

Answer 22

1) emulsification
2) stabilisation
3) digestion (hydrolysis)
4) formation of micelles

Question 23

This is the process where lipids are converted into small droplets. These droplets increase surface area for digestion.

Answer 23

Emulsification

Question 24

Where does emulsification of lipids occur? and what motility patterns are associated with it in these areas?

Answer 24

Stomach - retropulsion

Small intestine - segmentation

Question 25

Where does stabilisation of lipids occur? what is responsible for stabilisation and where is it secreted from?

Answer 25

Stabilisation occurs in the small intestine

Bile salts stabilise lipid droplets

Bile is secreted from the liver and concentrated in the gallbladder

Question 26

This is the process where the lipid droplets are reduced in size by bile salts in the small intestine. These also further increase surface area for digestion.

Answer 26

Stabilisation

Question 27

Where does hydrolysis of lipids occur?

Answer 27

In the small intestine at surface of emulsion droplets.

Question 28

What are the 3 main enzymes involved in hydrolysing lipids into monoglycerides and fatty acids?

Answer 28

Pancreatic lipase, lingual and gastric lipase and colipases as well (these anchor lipases to the triglyceride surface)

Question 29

This is the process where products of fat/lipid digestion are packaged into small droplets of about 20-30 molecules which allow it to be kept in solution.

Answer 29

Formation of micelles/

Question 30

What are micelles composed of?

Answer 30

Bile salts, fatty acids and monoglycerides

Question 31

What are the 2 products of fat/lipid digestion?

Answer 31

Monoglycerides and fatty acids