Week 5- Chapter 66 & 71

What is the chemical reaction that builds large molecules by linking smaller units together and releasing a water molecule ($H_2O$)?

Condensation. The note defines condensation as the chemical reaction that builds large molecules (like disaccharides, triglycerides, and proteins) by linking smaller units together and releasing a water molecule.

Describe the role of digestive enzymes (hydrolases) in the process of digestion.

Digestive enzymes (hydrolases) use a water molecule to break the bonds holding large molecules together, thus breaking them down into smaller, absorbable units. Hydrolysis is the reaction that breaks down large molecules, and digestive enzymes (hydrolases) fulfill this role by utilizing a water molecule to break the bonds.

Which of the following is NOT one of the three major dietary sources of carbohydrates mentioned in the notes?
a) Sucrose
b) Fructose
c) Lactose
d) Starches

b) Fructose. The three major dietary sources listed are Sucrose, Lactose, and Starches. Fructose is a monosaccharide, an end product of digestion, not a major dietary source in its complex form.

Where does the digestion of starch begin, and what enzyme is responsible? Why does this enzyme stop working in the stomach?

Starch digestion begins in the mouth with salivary amylase (ptyalin). It stops working in the stomach because it is inactivated by the highly acidic pH (1-3) of the stomach. Salivary amylase initiates starch digestion in the mouth, but its activity is pH-sensitive, and the acidic environment of the stomach denatures it.

What is the final form of carbohydrates that is absorbed into the bloodstream?

Monosaccharides (glucose, galactose, fructose). The end goal of carbohydrate digestion is to break them down into their simplest forms, monosaccharides, for absorption.

Describe the primary transport mechanisms for glucose, galactose, and fructose absorption in the small intestine, including the specific transporters involved.

Glucose and Galactose: Use secondary active transport via the SGLT1 transporter. Sodium ($Na^+$) is actively pumped out of the cell, creating a low intracellular concentration. $Na^+$ then moves back in down its gradient, co-transporting glucose or galactose with it. Fructose: Uses facilitated diffusion via the GLUT5 transporter, moving down its concentration gradient without directly using energy. The note specifies SGLT1 for glucose/galactose via $Na^+$ co-transport and GLUT5 for fructose via facilitated diffusion.

True or False: Glucose absorption can occur efficiently without sodium.

False. The sodium gradient provides the energy to 'pull' glucose into the cell against its concentration gradient. In the absence of sodium transport, glucose absorption is virtually impossible.

What is the main proteolytic enzyme in the stomach, and what is its optimal pH for activity?

Pepsin, which is active at a very acidic pH, optimally between 1 and 3. The note highlights pepsin as the main stomach enzyme, specifically active at low pH levels.

Differentiate between endopeptidases and exopeptidases, giving an example of each mentioned in the context of protein digestion.

Endopeptidases: Cut peptide bonds within the protein chain (e.g., pepsin, trypsin). Exopeptidases: Cut amino acids off the ends of the protein chains (e.g., carboxypeptidase). This distinction is clearly made in the protein digestion section.

How are most amino acids absorbed in the small intestine? What about dipeptides and tripeptides once inside the intestinal cell?

Most amino acids are absorbed via sodium co-transport (secondary active transport). Inside the intestinal cell, any remaining dipeptides and tripeptides are broken down into single amino acids by cytoplasmic peptidases before entering the bloodstream. The note states that most amino acids use $Na^+$ co-transport, and di- and tripeptides are further broken down intracellularly.

What is the process by which bile salts break large fat globules into tiny droplets in the small intestine, and why is this process crucial for effective fat digestion?

This process is called emulsification. It is crucial because it increases the surface area for pancreatic lipase to work on, allowing for more efficient breakdown of triglycerides. Emulsification by bile salts is presented as a key step to increase surface area for enzyme action.

What are the two main end products of triglyceride digestion by pancreatic lipase?

Fatty acids and monoglycerides. Pancreatic lipase breaks down triglycerides into these two components.

After fatty acids and monoglycerides enter the intestinal cells, what happens to them, and how are they then transported out of the intestine?

Inside the intestinal cell, fatty acids and monoglycerides are re-synthesized into new triglycerides. These triglycerides are then packaged with cholesterol and phospholipids into chylomicrons. Chylomicrons are too large to enter blood capillaries, so they enter the lymphatic system (lacteals) and eventually reach the bloodstream via the thoracic duct. The note details the resynthesis, chylomicron formation, and lymphatic transport pathway.

Which type of fatty acids are an exception to the chylomicron transport pathway and are absorbed directly into the portal blood?

Short and medium-chain fatty acids. The note explicitly mentions short and medium-chain fatty acids as an exception.

List the three anatomical structures that significantly increase the absorptive surface area of the small intestine.

1. Folds of Kerckring (Plicae Circulares), 2. Villi, 3. Microvilli (Brush Border). These three structures are highlighted as key adaptations for maximizing absorption.

Explain what it means for water absorption to be "isosmotic" and how water generally moves across the intestinal membrane.

"Isosmotic" means water is absorbed in proportion to solutes, keeping the intestinal contents at the same osmolarity as the blood. Water moves across the membrane primarily via osmosis, following the absorption of solutes (nutrients and electrolytes) down its concentration gradient. The note defines isosmotic absorption and states that water follows solutes via osmosis.

What is the primary driving force for sodium ($Na^+$) absorption in the intestine?

The $Na^+/K^+$ ATPase pump on the basolateral membrane, which actively pumps $Na^+$ out of the cell, maintaining a low intracellular $Na^+$ concentration and driving its absorption from the lumen. The $Na^+/K^+$ ATPase pump is identified as the key mechanism for sodium absorption.

Describe the mechanism by which cholera toxin causes secretory diarrhea, identifying the key ion whose secretion is primarily affected.

Cholera toxin over-activates adenylyl cyclase in crypt cells, leading to high levels of cAMP. This cAMP then opens Chloride ($Cl^-$) channels in the apical membrane. As $Cl^-$ is pumped into the lumen, $Na^+$ and water follow due to osmosis, resulting in massive fluid secretion and severe, watery diarrhea. The note details the cholera toxin's effect on cAMP and $Cl^-$ secretion, leading to osmotic water movement.

Which form of bilirubin is produced from the breakdown of old red blood cells in the spleen and is transported to the liver?

Unconjugated (or Indirect) Bilirubin. This is the form of bilirubin before it undergoes processing in the liver.

A patient presents with jaundice, and laboratory tests show significantly elevated conjugated bilirubin levels with decreased urobilinogen in the urine. What type of jaundice is most likely, and what is its underlying cause?

This presentation is most consistent with Post-hepatic (Obstructive) Jaundice. The underlying cause is a blockage in the bile duct, which prevents conjugated bilirubin from being excreted into the intestine, causing it to back up into the blood. The lack of bilirubin reaching the intestine leads to decreased urobilinogen formation. Obstructive jaundice is characterized by high conjugated bilirubin and decreased urobilinogen due to bile duct blockage.