9/9/24 - Digestion and Metabolism (Reading 2) Lecture Questions

What are the steps taken to move nutrients from the Lumen of the Small Intestine into the Blood of the capillaries?

1. Nutrients enter the Lumen of the Small Intestine

2. Cross the Apical Membrane (method of transport depends on NP / P, etc.)

3. Cross the Basolateral Membrane

4. Enter the Interstitial Fluid

5. Absorbed into Blood in the Capillaries

What is the generic term for enzymes that breakdown Polysaccharides?

Where are they synthesized and secreted from?

Amylases

S/s from Salivary Glands and the Pancreas

What products are produced when Starch or Glycogen is digested by Amylases?

Can they be further degraded? If so, into what? By what enzymes?

Maltose + Limit Alpha Dextrins

Yes; both Maltose and Limit Alpha Dextrins can be further degraded into Glucose

Maltose —(Maltase, a BBE)—> 2 Glucose

Limit Alpha Dextrins —(Dextrinase)—> Glucose

Is Glucose Hydrophilic or Hydrophobic?

Hydrophilic / Polar

What are the steps taken to move Glucose from the Lumen of the Small Intestine into the Blood of the capillaries?

1. Crosses from Apical Membrane via 2˚ Active Transport (Co-Transport) with Sodium Ions — LUMEN to INNER EPITHELIAL CELL (CYTOSOL)

2. Crosses the Basolateral Membrane via Facilitated Diffusion into the Interstitial Fluid

3. Absorbed by the Blood in the Capillaries

What are the steps taken to move Galactose from the Lumen of the Small Intestine into the Blood of the capillaries?

1. Crosses from Apical Membrane via 2˚ Active Transport (Co-Transport) with Sodium Ions — LUMEN to INNER EPITHELIAL CELL (CYTOSOL)

2. Crosses the Basolateral Membrane via Facilitated Diffusion into the Interstitial Fluid

3. Absorbed by the Blood in the Capillaries

What are the steps taken to move Fructose from the Lumen of the Small Intestine into the Blood of the capillaries?

1. Cross the Apical Membrane via Facilitated Diffusion — LUMEN to INNER EPITHELIAL CELL (CYTOSOL)

2. Crosses the Basolateral Membrane via Facilitated Diffusion into the Interstitial Fluid

3. Absorbed by the Blood in the Capillaries

How much protein does the typical diet contain?

How much is actually required?

Typical diet contains 125 grams of protein per day

Only require 40 - 50 grams per day

What is the amount of protein in a typical diet?

How many grams do we actually need?

Typical diet includes 125 grams / day of protein

Only really need about 40 - 50 grams

What is the generic term for enzymes that breakdown proteins?

What are protein digestion products?

Proteases

Digestion products include:

• Individual Amino Acids

• Dipeptides (AA—AA)

• Tripeptides (AA—AA—AA)

What are the two types of Proteases?

What are their methods of action?

What are their products?

Endopeptidases:

• Split polypeptides at INTERIOR peptide bonds

• Produce SMALL peptide fragments (dipeptides / tripeptides)

Exopeptidases:

• Cleave off Amino Acids from one end of polypeptide

• Produce INDIVIDUAL Amino acids

What are the various Protease types used in Protein Digestion within the Gastrointestinal Tract?

What are some examples of these types?

1. Zymogens (Inactive Proteases)

   • Trypsinogen

   • Chymotrypsinogen

   • Procarboxypeptidase

2. Pancreatic Peptidases

   • Trypsin

   • Chymotrypsin

   • Carboxypeptidase

3. Stomach Proteases

   • Pepsinogen (inactivated)

   • Pepsin (activated)

4. Brush Border Proteases

   • Peptidases (Aminopeptidase, Dipeptidase)

   • Enteropeptidase (Enterokinase)

5. Cytoplasmic Peptidases

What does Enterokinase do to Trypsinogen?

Activates it; creates Trypsin

What is the product when Trypsin digests Proteins?

Is it considered an Exopeptidase or an Endopeptidase?

Di- / Tripeptides

Endopeptidase

What does Trypsin do to Chymotrypsinogen?

Procarboxypeptidase?

Activates it; creates Chymotrypsin

Activates it; creates Carboxypeptidase

What is the product when Chymotrypsin digests Proteins?

Carboxypeptidase?

Are they considered an Exopeptidase or an Endopeptidase?

Forms Di- / Tripeptides (Endopeptidase)

Forms Individual Amino Acids (Exopeptidase)

Of the three activated Pancreatic Proteases, which are Endopeptidases and which are Exopeptidases?

Endopeptidases — Trypsin, Chymotrypsin

Exopeptidases — Carboxypeptidase

What are the three / four(?) main steps for Absorption of Amino Acids?

1. Sodium-Potassium Pump creates a sodium ion gradient

2. Sodium ion gradient powers 2˚ Active Transport (Co-Transport) of Amino Acids with Sodium Ions across the Apical Membrane

3. Facilitated Diffusion of Amino Acids across Basolateral Membrane

4. Brush Border Enzymes ????

What are the three main steps for Absorption of Di- / Tripeptides?

1. 2˚ Active Transport with Hydrogen ions (Co-Transport)

2. Break down into Amino Acids via Cytoplasmic Peptidases

3. Facilitated Diffusion of Amino Acids across Basolateral Membrane

How many lipids does the typical diet contain?

What percent of that amount is Triglycerides?

Typical diet include about 25 - 160 grams of Lipids

About 90% are Triglycerides

What special problems to Lipids face during Digestion and Absorption?

Not water soluble, do not mix with the contents of the Stomach and Intestines, and form fat droplets

What is the generic name for enzymes used to digest Lipids?

What molecules can they act on?

Lipase

Can only act on molecules near edge of fat droplets

How can we break up big fat globules into small fat droplets to make digestion easier?

Use Bile Salts to increase the Surface Area of the droplets (EMULSIFICATION)

What is the function of Co-Lipase?

Helps load Lipases onto the fat droplets / lipids

Are Bile and Co-Lipase enzymes?

NO!! They do NOT digest lipids

Where are the different kinds of Lipases, Bile, and Co-Lipase secreted from?

Lingual Lipase — s/s from the Tongue; activated in the Stomach

Gastric Lipase — s/s from the Stomach

Pancreatic Lipase — s/s from the Pancreas

Lipoprotein Lipase (Blood) — s/s from the Liver

Co-Lipase — s/s from the Pancreas

Bile — made in Liver, secreted from / stored in Gallbladder

What are the methods of action for Bile Salts and Co-Lipase?

Bile Salts — emulsify lipids to increase surface area for lipid digestion

Co-Lipase — move Bile Salts to the side to load Lipases onto the lipids

In summary, what are the three steps of Lipid Digestion?

1. Emulsification of fat droplets by Bile Salts

2. Hydrolysis of Triglycerides in Emulsified fat droplets into Fatty Acids and Monoglycerides

3. Dissolving of Fatty Acids and Monoglycerides into Micelles to produce “mixed Micelles”

What is produced from the digestion of Triglycerides by Pancreatic Lipase?

How do these products get absorbed into the cell?

Monoglyceride (Glycerol) and Free Fatty Acids (3 Fatty Acids)

Absorbed into the cell via Simple Diffusion through the Apical Membrane

After the formation of Micelles following Lipid Digestion, what are the steps for the products to be absorbed?

1. Micelles release the Monoglycerides and Fatty Acids

2. Monoglycerides and Fatty Acids pass through the Apical Membrane via Simple Diffusion

3. Travel to the Smooth ER and are then reassembled into Triglycerides

4. Combines with other lipids and proteins within the Golgi Apparatus to form Chylomicrons (hydrophilic)

5. Chylomicrons exocytosize through the Basolateral Membrane and go around the Capillaries (too large to go into them)

6. Enter the Lacteal of the Lymphatic System and transport into blood for general circulation

7. Lipoprotein Lipases in the blood digest the Chylomicrons into Glycerol and Fatty Acids

8. Glycerol and Fatty Acids enter other cells via Simple Diffusion

What happens to Bile Salts after they serve their purpose?

What is this process called?

Bile Salts in the Ileum enter the Capillaries and travel through the Hepatic Portal Vein to return to the Liver

Process is called Enterohepatic Circulation

What are the Fat Soluble Vitamins?

How are they absorbed? Dissolved? Transported?

Vitamin A, D, E, and K

Absorbed with Lipids

Dissolve in Micelles

Transport through Simple Diffusion

What are the Water Soluble Vitamins?

What is required for their transport?

Vitamin B and C

Require special Carrier Proteins

What are the requirements for the absorption of Vitamin B12?

Must be bound to Intrinsic Factor, which is produced by Parietal Cells in the Stomach

Where is Calcium absorbed?

What does it bind to?

Where is it transported to?

How are they transported out of the cell?

Absorbed in the Duodenum and Jejunum

Binds to Brush Border Proteins (Calcium-Bind Protein)

Transported into Epithelial Cells

Transported out of the cell via the Calcium Pump across the Basolateral Membrane

How much water is absorbed from Gastrointestinal Tract? Normal intake?

G.I. Tract — 7 Liters

Normal Intake — 2 Liters

How is water absorbed?

Is this active or passive absorption?

Where does most water absorption occur?

Osmosis or something

Passive

Most water absorption occurs in the Large Intestine

Is Pepsin an Exopeptidase or an Endopeptidase?

What are the products of Pepsin degrading a Protein?

Endopeptidase

Products are smaller Peptide fragments

What are the two Brush Border Proteases?

Aminopeptidase and Enterokinase