Lecture 42 - Digestive System: Chemical Digestion and Absorption

Question 1

Name the 4 classes of enzymes responsible for the digestion of the 4 major groups of nutrient macromolecules

Answer 1

1. Carbohydrases break down carbohydrates (sugars)
2. Peptidases break down protein (amino acids)
3. Lipases break down fats (lipids)
4. Nucleases break down nucleic acids (nucleotides)

Question 2

What occurs during enzymatic (chemical) digestion? Why is this important?

Answer 2

It leads to the break-down of macromolecules into their component parts. It is important because large molecules are only transportable through (energy intensive) endocytosis, so efficient absorption relies on chemical digestion

Question 3

What is the function of acinar cells of the pancreas?

Answer 3

To produce a diverse array of digestive enzymes. The exocrine parts of the pancreas secrete enzymes for the digestion of every major type of macromolecule

Question 4

How are proteolytic enzymes secreted and are they normally active or inactive?

Answer 4

They are typically secreted as zymogens so that they do not digest the cell that synthesized them and only become active within the digestive tract lumen. Zymogens require the removal of a part of the protein in order to expose the active site of the enzyme. This process is irreversible

Question 5

Where are pancreatic and intestinal zymogens activated?

Answer 5

They are activated in the duodenum. Enzymes present on the duodenal enterocytes convert the pancreatic zymogen trypsinogen to trypsin. Trypsin then catalyzes the conversion of most other pancreatic zymogens to their active forms. This zymogen activation is called protein-to-protein interaction

Question 6

Name 4 types of proteolytic enzymes originating from the pancreas

Answer 6

1. Trypsinogen –> Trypsin
2. Chymotrypsinogen –> Chymotrypsin
3. Proelastase –> Elastase
4. Pro-carboxypeptidase –> carboxypeptidase

Question 7

What are “brush border enzymes” and what do they do?

Answer 7

They are intestinal enzymes associated with the microvilli of enterocytes (apical side of the membrane). These enzymes typically catalyze the breakdown of small chains into dimers and monomers (chemical digestion) that are suitable for absorption

Question 8

What is the function of bile?

Answer 8

Bile contains bile salts, which are critical for emulsification of lipids, and thus their digestion and absorption. The active components of bile are bile salts, such as cholic acid, which are amphipathic

Question 9

How does the structure of bile salts contribute to absorption?

Answer 9

Bile salts can bind to fat gobules, forming micelles which allow lipases to access the lipid molecules. Lipases chemically digest triglycerides into free fatty acids, suitable for absorption

Question 10

T/F? Bile salts are recycled

Answer 10

True - most of the bile salts that are secreted are reabsorbed within the ileum and returned to the liver for repackaging into the bile

Question 11

How does the absorption of molecules by the digestive epithelia occur?

Answer 11

It requires movement across 2 plasma membranes. Absorption into body involves the movement of solutes into enterocytes, but then also out of the enterocytes

Question 12

What is the process of lipids from micelles being processed by intracellular organelles?

Answer 12

1. Free fatty acids can rapidly diffuse from micelles across the microvilli (apical surface of the cell)
2. Inside enterocytes, triglycerides are reassembled, then packaged with phospholipids and specialized proteins into chylomicrons

Question 13

What are chylomicrons and what happens to them?

Answer 13

They contain the reassembled triglycerides along with phosopholipids and other specialized proteins. They are released via exocytosis through the basolateral membrane and they enter the lacteals of the villi and transported into the bloodstream

Question 14

What is chyle?

Answer 14

Chyle is chylomicron-rich lymphatic fluid. It enters the circulation (into left subclavian vein) via the thoracic duct

Question 15

How are monosaccharides (sugars) absorbed?

Answer 15

They are absorbed from the lumen via either facilitated diffusion or co-transport with sodium ions. They enter the intestinal capillaries that ultimately drain into the hepatic portal vein system and pass through the liver

Question 16

How might glucose (and some other monosaccharides) enter the capillaries of the villus?

Answer 16

They diffuse across the basolateral membrane and enter the capillaries of the villus

Question 17

How are amino acids transported?

Answer 17

They are transported through a combination of co-transport and facilitated diffusion

1. Sodium co-transporter on the apical side
2. Na+/K+ ATPase pump
3. “Amino acid” transporter on the basolateral side

Amino acids enter the capillaries and then pass through portal veins to the liver

Question 18

What is the lower digestive tract responsible for reabsorbing? Give an example for why this may be important

Answer 18

They are responsible for reabsorbing water, minerals (ions), and vitamins

Reabsorption is important. For example, vitamins are organic molecules that cannot be made in sufficient amounts by the body’s own cells

Question 19

How does the absorption of minerals (ie. ions) occur?

Answer 19

Ions are both secreted and absorbed throughout the digestive tract, especially the intestines through combinations of:

1. bulk transport (paracellular transport)
2. ion channels
3. co-transport

Question 20

How does the body regulate the absorption of different ions to meet its precise homeostatic needs? Name 2 examples

Answer 20

Through the action of different hormones

1. Aldosterone - increases Na+ uptake
2. PTH, calcitriol - increases Ca2+ uptake

These hormones alter the density of specific transport enzymes in enterocyte membranes

Question 21

How does water cross the leaky intestinal epithelium? What makes this happen?

Answer 21

Through osmosis via paracellular transport. An osmotic gradient is created due to the net movement of solutes into the enterocytes through facilitated diffusion and co-transport

Question 22

What dictates the mechanism of vitamin absorption? Give 3 examples

Answer 22

It depends on their chemical composition and size.

1. Vitamin C (ascorbic acid) - water soluble and small. Uptake via facilitated diffusion
2. Vitamin B12 (cobalamin) - water soluble, but large. Requires endocytosis, mediated by a protein co-factor (Intrinsic Factor)
3. Vitamin E - lipid-soluble. Transported via micelles and chylomicrons

Question 23

Where do most absorbed substances pass before they reach general circulation?

Answer 23

Nutrient-rich blood is not transported directly into the general blood supply. Most absorbed substances enter the hepatic portal vein system and pass through the liver before they reach the general circulation

Question 24

What are the 2 general patterns of metabolic activity (nutrient use)

Answer 24

The presence (or absence) of an influx of newly absorbed nutrients from the intestine alters the way the liver (and the rest of the body’s organ systems) perform basic metabolic functions

1. Absorptive - anabolic, energy stores built up, building up glucose as glycogen, glucose = ATP. Regulated by insulin
2. Post-absorptive - catabolic, energy stores retrieved, liver releases glucose, fatty acids = ATP. Regulated by Glucagon

Question 25

When does the absorptive state occur? What is the primary hormone?

Answer 25

Occurs when ingested food begins to be absorbed, and typically lasts around 4 hours. The primary hormone of the absorptive state is insulin, which promotes:

1. Storage of lipids
2. Use of blood glucose for ATP generation

Question 26

When does the post-absorptive state occur? What are the 3 hormones that regulate it?

Answer 26

Occurs when internal energy stores must be used for ATP generation. Hormones that regulate the post-absorptive state include glucagon, epinephrine, and glucocorticoids, which promote

1. Gluconeogeneisis
2. Lipolysis
3. Use of fatty acids for ATP generation