L42: Chemical Digestion and Absorption

What enzyme breaks down carbohydrates?

Carbohydrases

What enzyme breaks down proteins?

Peptidases

What enzyme breaks down fats?

Lipases

What enzyme breaks down nucleic acids?

Nucleases

What kind of cells of the pancreas produce diverse array of digestive enzymes?

Acinar cells

Name the 4 major pancreatic enzymes

Pancreatic alpha-amylase, pancreatic lipase, nucleases, and proteolytic enzymes

What are proteolytic enzymes typically secreted as and only becomes activated within the digestive tract lumen?

Zymogens

Why are enzymes secreted as zymogens?

So that they do not digest the cell that synthesized them

Zymogens require the removal of a part of a protein in order to expose the active site of the enzyme. This process is …?

irreversible

Where and how do the pancreatic and intestinal zymogens get activated?

Duodenum, mostly by protein-protein interactions

What does the enzymes present on the duodenal enterocytes convert the pancreatic zymogen trypsinogen to?

Trypsin

What does trypsin do?

Catalyzes the conversion of most other pancreatic zymogens to their active forms

What are the proteolytic enzymes that originate in the pancreas?

Trypsinogen, chymotrypsinogen, proelastase, and pro-carboxypeptidase

What intestinal enzymes are associated with the microvilli of enterocytes?

Brush border enzymes

What do brush border enzymes do?

They typically catalyze the breakdown of small chains into dimers and monomers that are suitable for absorption

What are the 4 brush border carbohydrases?

Maltase-glucoamylase, dextrinase, sucrase, and lactase

What does bile contain?

Bile salts

What are bile salts critical for?

Emulsification of lipids, and thus their digestion and absorption

What can bile salts do?

They can bind to fat globules, forming micelles which allow lipases to access the lipid molecules

What is an example of a bile salt?

Cholic acid

Bile salts such as cholic acid are … (property)

amphipathic

Lipases chemically digest triglycerides into what?

Free fatty acids

Bile salts are recycled through reabsorption in the … and transport back to the …

ileum, liver

There are other components of bile (particularly …, formed from haem) which do not participate in emulsification, but which can be reabsorbed (in the colon) or excreted

bilirubin

Where are brush border enzymes found?

Apical surface of enterocytes

Bile salts are … secreted by the liver that interact with ingested lipids

Steroids or cholesterol derivatives

Absorption of molecules by the digestive epithelia requires movement across …?

2 plasma membranes

Lipids from micelles … directly through the … membrane of enterocytes and are processed by intracellular organelles

diffuse, apical

What is chylomicrons?

Reassembled triglycerides packaged with phospholipids and specialized proteins

How are chylomicrons released?

Via exocytosis from the basolateral membrane

Where do chylomicrons enter after being exocytosed?

Lacteals of the villi

What is chylomicron rich lymphatic fluid known as?

Chyle

Chyle enters the circulation via what?

Thoracic duct (into the left subclavian vein)

Monosaccharides (sugars) are absorbed from the lumen via what?

Facilitated diffusion or co-transport with sodium ions

Monosaccharides enter the intestinal capillaries that flow into the … system and pass through the liver

hepatic portal vein

Glucose diffuse across the … membrane and enter the capillaries of the …

basolateral, villus

Amino acids are also transported via what?

A combination of facilitated diffusion and co-transport

What are vitamins?

Organic molecules that cannot be made in sufficient amounts by the body’s own cells

What kind of transport does the absorption of minerals (i.e. ions) occur through?

A mix of primary and secondary active transport

Absorption of minerals occur through combinations of what?

1. Bulk flow (paracellular transport)

2. Ion channels

3. Co-transport

What does aldosterone (hormone) do?

Increase sodium uptake

What does PTH and calcitriol (hormones) do?

Increase calcium uptake

How does water typically cross the leaky epithelium?

Through osmosis via paracellular transport

Net movement of solutes into enterocytes through co-transport and facilitated diffusion creates an …

osmotic gradient

Vitamin C (ascorbic acid) is water soluble and small. How is it transported?

Via facilitated diffusion

Vitamin E is lipid soluble. How is it transported?

Via micelles and chylomicrons

Vitamin B12 (cobalamin) is water soluble but large. How is it transported?

Via endocytosis, mediated by a protein co-factor (intrinsic factor)

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

Absorptive and post-absorptive

Is the absorptive state anabolic or catabolic?

Anabolic

Is the post-absorptive state anabolic or catabolic?

Catabolic

What is the primary hormone of the absorptive state?

Insulin

What are the hormones of the post-absorptive state?

Glucagon, epinephrine, and glucocorticoids

What does insulin promote?

Storage of lipids and use of blood glucose for ATP generation

What does glucagon, epinephrine, and glucocorticoids promote?

Gluconeogenesis, lipolysis, and use of fatty acids for ATP generation

How long does the absorptive state typically last for?

4 hours

Post-absorptive state occurs when … energy stores must be used for ATP generation

internal

What is the main source of digestive enzymes?

Pancreas