Lecture 42 - Digestive System: Chemical Digestion and Absorption Flashcards
Name the 4 classes of enzymes responsible for the digestion of the 4 major groups of nutrient macromolecules
- Carbohydrases break down carbohydrates (sugars)
- Peptidases break down protein (amino acids)
- Lipases break down fats (lipids)
- Nucleases break down nucleic acids (nucleotides)
What occurs during enzymatic (chemical) digestion? Why is this important?
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
What is the function of acinar cells of the pancreas?
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
How are proteolytic enzymes secreted and are they normally active or inactive?
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
Where are pancreatic and intestinal zymogens activated?
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
Name 4 types of proteolytic enzymes originating from the pancreas
- Trypsinogen –> Trypsin
- Chymotrypsinogen –> Chymotrypsin
- Proelastase –> Elastase
- Pro-carboxypeptidase –> carboxypeptidase
What are “brush border enzymes” and what do they do?
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
What is the function of bile?
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
How does the structure of bile salts contribute to absorption?
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
T/F? Bile salts are recycled
True - most of the bile salts that are secreted are reabsorbed within the ileum and returned to the liver for repackaging into the bile
How does the absorption of molecules by the digestive epithelia occur?
It requires movement across 2 plasma membranes. Absorption into body involves the movement of solutes into enterocytes, but then also out of the enterocytes
What is the process of lipids from micelles being processed by intracellular organelles?
- Free fatty acids can rapidly diffuse from micelles across the microvilli (apical surface of the cell)
- Inside enterocytes, triglycerides are reassembled, then packaged with phospholipids and specialized proteins into chylomicrons
What are chylomicrons and what happens to them?
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
What is chyle?
Chyle is chylomicron-rich lymphatic fluid. It enters the circulation (into left subclavian vein) via the thoracic duct
How are monosaccharides (sugars) absorbed?
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
How might glucose (and some other monosaccharides) enter the capillaries of the villus?
They diffuse across the basolateral membrane and enter the capillaries of the villus
How are amino acids transported?
They are transported through a combination of co-transport and facilitated diffusion
- Sodium co-transporter on the apical side
- Na+/K+ ATPase pump
- “Amino acid” transporter on the basolateral side
Amino acids enter the capillaries and then pass through portal veins to the liver
What is the lower digestive tract responsible for reabsorbing? Give an example for why this may be important
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
How does the absorption of minerals (ie. ions) occur?
Ions are both secreted and absorbed throughout the digestive tract, especially the intestines through combinations of:
- bulk transport (paracellular transport)
- ion channels
- co-transport
How does the body regulate the absorption of different ions to meet its precise homeostatic needs? Name 2 examples
Through the action of different hormones
- Aldosterone - increases Na+ uptake
- PTH, calcitriol - increases Ca2+ uptake
These hormones alter the density of specific transport enzymes in enterocyte membranes
How does water cross the leaky intestinal epithelium? What makes this happen?
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
What dictates the mechanism of vitamin absorption? Give 3 examples
It depends on their chemical composition and size.
- Vitamin C (ascorbic acid) - water soluble and small. Uptake via facilitated diffusion
- Vitamin B12 (cobalamin) - water soluble, but large. Requires endocytosis, mediated by a protein co-factor (Intrinsic Factor)
- Vitamin E - lipid-soluble. Transported via micelles and chylomicrons
Where do most absorbed substances pass before they reach general circulation?
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
What are the 2 general patterns of metabolic activity (nutrient use)
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
- Absorptive - anabolic, energy stores built up, building up glucose as glycogen, glucose = ATP. Regulated by insulin
- Post-absorptive - catabolic, energy stores retrieved, liver releases glucose, fatty acids = ATP. Regulated by Glucagon
When does the absorptive state occur? What is the primary hormone?
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:
- Storage of lipids
- Use of blood glucose for ATP generation
When does the post-absorptive state occur? What are the 3 hormones that regulate it?
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
- Gluconeogeneisis
- Lipolysis
- Use of fatty acids for ATP generation
