14.1. Digestion prepares Large Biomolecules for Use in Metabolism
Digestion breaks down large biomolecules into small molecules for absorption and metabolism.
• Proteins → Digested into amino acids by proteases from the stomach and pancreas.
• Polysaccharides (e.g., starch) → Cleaved into monosaccharides by α-amylase from the pancreas (and saliva).
• Lipids → Converted into fatty acids by pancreatic lipases.
• All digestive enzymes are hydrolases, meaning they cleave molecules using water.
Most digestive enzymes are secreted as inactive precursors (zymogens/proenzymes):
• Stored in granules near the cell membrane before secretion.
• Activated by proteolytic cleavage once in the intestine.
• Enteropeptidase activates trypsinogen into trypsin, which then activates other pancreatic enzymes.
• Pepsinogen (secreted in the stomach) self-activates in an acidic environment into pepsin, which then activates more pepsinogen.
Zymogen Active enzyme Site of synthesis
Pepsinogen Pepsin Stomach
Chymotrypsinogen Chymotrypsin Pancreas
Trypsinogen Trypsin Pancreas
Procarboxypeptidase Carboxypeptidase Pancreas
Proelastase Elastase Pancreas
Question 1: What do the enzymes that are needed for digestion have in common?
a. They are hydrolases.
b. They function best at low pH.
c. They are synthesized in the pancreas.
d. They start to digest themselves when you eat too much.
Answer: Digestive enzymes are all hydrolases; this means they cleave their substrate when reacting with a water molecule.
14.2. Proteases Digest Proteins into Amino Acids and Peptides
Digestion begins in the mouth:
• Chewing and saliva help create an aqueous slurry for easier enzyme action.
• Saliva contains α-amylase, which starts breaking down polysaccharides (minimal effect due to short
exposure).
Protein digestion in the intestine:
• Acidic food from the stomach enters the small intestine, triggering the release of secretin.
• Secretin stimulates NaHCO₃ release from the pancreas to neutralize stomach acid.
• Cholecystokinin (CCK) is released in response to peptides, causing the pancreas to secrete
digestive enzymes.
Pancreatic proteases break proteins into oligopeptides, which are further broken down by peptidases on intestinal cells into:
• Amino acids, dipeptides, and tripeptides, which are transported into intestinal cells.
• At least seven specific transporters exist for different amino acid groups.
• Mutations in transporters can lead to inherited disorders like Hartnup disease, affecting tryptophan absorption.
Amino acids are released into the bloodstream via antiporters for use by other tissues.
1|Page
,Question 2: What are the main functions of stomach acid for digestion? (Two are correct)
a. To denature ingested proteins.
b. To add some flavour to a bland meal.
c. To provide good conditions for pepsin.
d. To signal you have eaten too much by burning your throat through reflux.
e. To neutralize the higher pH present in the small intestine.
Answer: Breakage of ionic and hydrogen bonds within proteins happens due to low pH and causes denaturation of the protein, which enables
easier susbsequent enzymatic degradation. Pepsin needs a low pH to hydrolyze proteins into fragments that will serve as substrate for
proteases in the intestine.
Sequence of protein digestion: Place the events that enable and occur during protein digestion in the correct order.
Stimulation of cholecystokinin release from upper intestine, trypsin activates other zymogens, enteropeptidase converts trypsinogen into trypsin,
release of bile salt from gallbladder and zymogens from pancreas, and proteolytic cleavage by pepsin
Answer:
1. Proteolytic cleavage by pepsin
2. Stimulation of cholecystokinin release from upper intestine
3. Release of bile salt from gallbladder and zymogens from pancreas
4. Enteropeptidase converts trypsinogen into trypsin
5. Trypsin activates other zymogens
14.3. Dietary Carbohydrates Are Digested by Alpha-Amylases
Carbohydrates in food (e.g., pizza crust, vegetables, meat) include:
• Complex carbohydrates like starch (mainly from the crust) and glycogen (from meat).
• Simple carbohydrates like sucrose (from vegetables).
Carbohydrate digestion:
• Starch is broken down by α-amylase (from the pancreas) into:
o Maltose and maltotriose (di- and trisaccharides).
o Limit dextrin, which contains α-1,6 bonds that α-amylase cannot break.
• Further digestion by intestinal enzymes:
o α-Glucosidase → Breaks down maltose, maltotriose, and other oligosaccharides.
o α-Dextrinase → Digests limit dextrin into simple sugars.
• Disaccharide digestion:
o Sucrase → Breaks down sucrose into glucose and fructose.
o Lactase → Breaks down lactose into glucose and galactose.
Monosaccharide absorption into intestinal cells:
• Glucose & Galactose → Transported via SGLT (sodium–glucose linked transporter).
• Fructose → Diffuses through GLUT5.
• All three monosaccharides exit into the bloodstream via GLUT2 for use as energy in tissues.
Question 3: Carbohydrates are an important source of energy. Which of the following statements
are correct?
a. Sucrose is a disaccharide that consist solely of glucose.
b. Many glycosyl hydrolases are involved in digestion of (complex) carbohydrates.
c. Starch is a polymer of glucose.
d. Only one enzyme is needed to digest the carbohydrates in our diet.
Answer: Starch consists solely of glucose and is broken down by alpha-amylase into smaller fragments. These fragments are broken down into
glucose by alpha-glucosidase and alpha-dextrinase. Simpler sugars in our diet like sucrose (glucose+fructose) and lactose (glucose+galactose)
are digested to monosaccharides by sucrase and lactase, respectively.
2|Page
, 14.4. The Digestion of Lipids is Complicated by Their Hydrophobicity
Challenges of lipid digestion:
• Lipids (mainly triacylglycerols) are hydrophobic, making digestion and absorption difficult.
• Unlike proteins and carbohydrates, lipids are not water-soluble and require special processing.
Preparation for digestion:
• Stomach: Lipids are mixed and ground into an emulsion (lipid
droplets in water).
• Small intestine: Bile salts (from the gallbladder, stimulated by
cholecystokinin) further emulsify lipids, increasing enzyme access.
Lipid breakdown:
• Pancreatic lipases (secreted as zymogens) hydrolyze triacylglycerols into: free fatty acid and monoacylglycerol
• These digestion products form micelles, stabilized by bile salts, allowing transport to the intestinal epithelium.
Absorption and transport:
• Fatty-acid-binding protein (FABP) transports fatty acids and monoacylglycerol into intestinal cells.
• Inside the cells:
o Triacylglycerols are resynthesized in the smooth ER.
o They combine with proteins, phospholipids, and cholesterol to form chylomicrons.
• Chylomicrons enter the lymph and then the bloodstream, appearing as a milky substance after a lipid-rich meal.
Fate of lipids in the body:
• Chylomicrons bind to lipoprotein lipases in adipose and muscle tissue, breaking down triacylglycerols again.
• Fatty acids are transported into tissues, where they are:
o Stored in adipose tissue as triacylglycerols.
o Oxidized for energy in muscles and other tissues.
• Chylomicrons also transport fat-soluble vitamins and cholesterol.
Disorder related to lipid digestion:
• Steatorrhea: Inadequate bile salt production (due to liver disease) leads to excess fat excretion in feces.
Question 4: Which statements about chylomicrons are correct?
a. They mainly consist of triacylglycerols.
b. They are found in lymph and blood.
c. They deliver their content to muscles and fat tissue.
d. They function in the transport of fat-soluble vitamins.
e. They can be seen with the naked eye.
Answer: They mainly consist of triacylglycerols. They are found in lymph and blood. They deliver their content to muscles and fat tissue. They
function in the transport of fat-soluble vitamins
SUMMARY: Chapter 14
14.1 Digestion Prepares Large Biomolecules for Use in Metabolism
Digestion begins in the mouth, where food is homogenized into an aqueous slurry susceptible to enzyme digestion. The homogenized food then
passes into the stomach, an acidic environment. The low pH of the stomach denatures proteins, thus preparing them for degradation.
14.2 Proteases Digest Proteins into Amino Acids and Peptides
Protein digestion begins in the stomach with the action of the proteolytic enzyme pepsin. The digestion products of pepsin stimulate the release
of the hormone cholecystokinin from specialized cells in the upper intestine. Cholecystokinin stimulates the release of bile salts from the
gallbladder and digestion enzymes from the pancreas in the form of zymogens or proenzymes. Enteropeptidase converts trypsinogen into
trypsin, which, in turn, activates the other zymogens.
3|Page
Digestion breaks down large biomolecules into small molecules for absorption and metabolism.
• Proteins → Digested into amino acids by proteases from the stomach and pancreas.
• Polysaccharides (e.g., starch) → Cleaved into monosaccharides by α-amylase from the pancreas (and saliva).
• Lipids → Converted into fatty acids by pancreatic lipases.
• All digestive enzymes are hydrolases, meaning they cleave molecules using water.
Most digestive enzymes are secreted as inactive precursors (zymogens/proenzymes):
• Stored in granules near the cell membrane before secretion.
• Activated by proteolytic cleavage once in the intestine.
• Enteropeptidase activates trypsinogen into trypsin, which then activates other pancreatic enzymes.
• Pepsinogen (secreted in the stomach) self-activates in an acidic environment into pepsin, which then activates more pepsinogen.
Zymogen Active enzyme Site of synthesis
Pepsinogen Pepsin Stomach
Chymotrypsinogen Chymotrypsin Pancreas
Trypsinogen Trypsin Pancreas
Procarboxypeptidase Carboxypeptidase Pancreas
Proelastase Elastase Pancreas
Question 1: What do the enzymes that are needed for digestion have in common?
a. They are hydrolases.
b. They function best at low pH.
c. They are synthesized in the pancreas.
d. They start to digest themselves when you eat too much.
Answer: Digestive enzymes are all hydrolases; this means they cleave their substrate when reacting with a water molecule.
14.2. Proteases Digest Proteins into Amino Acids and Peptides
Digestion begins in the mouth:
• Chewing and saliva help create an aqueous slurry for easier enzyme action.
• Saliva contains α-amylase, which starts breaking down polysaccharides (minimal effect due to short
exposure).
Protein digestion in the intestine:
• Acidic food from the stomach enters the small intestine, triggering the release of secretin.
• Secretin stimulates NaHCO₃ release from the pancreas to neutralize stomach acid.
• Cholecystokinin (CCK) is released in response to peptides, causing the pancreas to secrete
digestive enzymes.
Pancreatic proteases break proteins into oligopeptides, which are further broken down by peptidases on intestinal cells into:
• Amino acids, dipeptides, and tripeptides, which are transported into intestinal cells.
• At least seven specific transporters exist for different amino acid groups.
• Mutations in transporters can lead to inherited disorders like Hartnup disease, affecting tryptophan absorption.
Amino acids are released into the bloodstream via antiporters for use by other tissues.
1|Page
,Question 2: What are the main functions of stomach acid for digestion? (Two are correct)
a. To denature ingested proteins.
b. To add some flavour to a bland meal.
c. To provide good conditions for pepsin.
d. To signal you have eaten too much by burning your throat through reflux.
e. To neutralize the higher pH present in the small intestine.
Answer: Breakage of ionic and hydrogen bonds within proteins happens due to low pH and causes denaturation of the protein, which enables
easier susbsequent enzymatic degradation. Pepsin needs a low pH to hydrolyze proteins into fragments that will serve as substrate for
proteases in the intestine.
Sequence of protein digestion: Place the events that enable and occur during protein digestion in the correct order.
Stimulation of cholecystokinin release from upper intestine, trypsin activates other zymogens, enteropeptidase converts trypsinogen into trypsin,
release of bile salt from gallbladder and zymogens from pancreas, and proteolytic cleavage by pepsin
Answer:
1. Proteolytic cleavage by pepsin
2. Stimulation of cholecystokinin release from upper intestine
3. Release of bile salt from gallbladder and zymogens from pancreas
4. Enteropeptidase converts trypsinogen into trypsin
5. Trypsin activates other zymogens
14.3. Dietary Carbohydrates Are Digested by Alpha-Amylases
Carbohydrates in food (e.g., pizza crust, vegetables, meat) include:
• Complex carbohydrates like starch (mainly from the crust) and glycogen (from meat).
• Simple carbohydrates like sucrose (from vegetables).
Carbohydrate digestion:
• Starch is broken down by α-amylase (from the pancreas) into:
o Maltose and maltotriose (di- and trisaccharides).
o Limit dextrin, which contains α-1,6 bonds that α-amylase cannot break.
• Further digestion by intestinal enzymes:
o α-Glucosidase → Breaks down maltose, maltotriose, and other oligosaccharides.
o α-Dextrinase → Digests limit dextrin into simple sugars.
• Disaccharide digestion:
o Sucrase → Breaks down sucrose into glucose and fructose.
o Lactase → Breaks down lactose into glucose and galactose.
Monosaccharide absorption into intestinal cells:
• Glucose & Galactose → Transported via SGLT (sodium–glucose linked transporter).
• Fructose → Diffuses through GLUT5.
• All three monosaccharides exit into the bloodstream via GLUT2 for use as energy in tissues.
Question 3: Carbohydrates are an important source of energy. Which of the following statements
are correct?
a. Sucrose is a disaccharide that consist solely of glucose.
b. Many glycosyl hydrolases are involved in digestion of (complex) carbohydrates.
c. Starch is a polymer of glucose.
d. Only one enzyme is needed to digest the carbohydrates in our diet.
Answer: Starch consists solely of glucose and is broken down by alpha-amylase into smaller fragments. These fragments are broken down into
glucose by alpha-glucosidase and alpha-dextrinase. Simpler sugars in our diet like sucrose (glucose+fructose) and lactose (glucose+galactose)
are digested to monosaccharides by sucrase and lactase, respectively.
2|Page
, 14.4. The Digestion of Lipids is Complicated by Their Hydrophobicity
Challenges of lipid digestion:
• Lipids (mainly triacylglycerols) are hydrophobic, making digestion and absorption difficult.
• Unlike proteins and carbohydrates, lipids are not water-soluble and require special processing.
Preparation for digestion:
• Stomach: Lipids are mixed and ground into an emulsion (lipid
droplets in water).
• Small intestine: Bile salts (from the gallbladder, stimulated by
cholecystokinin) further emulsify lipids, increasing enzyme access.
Lipid breakdown:
• Pancreatic lipases (secreted as zymogens) hydrolyze triacylglycerols into: free fatty acid and monoacylglycerol
• These digestion products form micelles, stabilized by bile salts, allowing transport to the intestinal epithelium.
Absorption and transport:
• Fatty-acid-binding protein (FABP) transports fatty acids and monoacylglycerol into intestinal cells.
• Inside the cells:
o Triacylglycerols are resynthesized in the smooth ER.
o They combine with proteins, phospholipids, and cholesterol to form chylomicrons.
• Chylomicrons enter the lymph and then the bloodstream, appearing as a milky substance after a lipid-rich meal.
Fate of lipids in the body:
• Chylomicrons bind to lipoprotein lipases in adipose and muscle tissue, breaking down triacylglycerols again.
• Fatty acids are transported into tissues, where they are:
o Stored in adipose tissue as triacylglycerols.
o Oxidized for energy in muscles and other tissues.
• Chylomicrons also transport fat-soluble vitamins and cholesterol.
Disorder related to lipid digestion:
• Steatorrhea: Inadequate bile salt production (due to liver disease) leads to excess fat excretion in feces.
Question 4: Which statements about chylomicrons are correct?
a. They mainly consist of triacylglycerols.
b. They are found in lymph and blood.
c. They deliver their content to muscles and fat tissue.
d. They function in the transport of fat-soluble vitamins.
e. They can be seen with the naked eye.
Answer: They mainly consist of triacylglycerols. They are found in lymph and blood. They deliver their content to muscles and fat tissue. They
function in the transport of fat-soluble vitamins
SUMMARY: Chapter 14
14.1 Digestion Prepares Large Biomolecules for Use in Metabolism
Digestion begins in the mouth, where food is homogenized into an aqueous slurry susceptible to enzyme digestion. The homogenized food then
passes into the stomach, an acidic environment. The low pH of the stomach denatures proteins, thus preparing them for degradation.
14.2 Proteases Digest Proteins into Amino Acids and Peptides
Protein digestion begins in the stomach with the action of the proteolytic enzyme pepsin. The digestion products of pepsin stimulate the release
of the hormone cholecystokinin from specialized cells in the upper intestine. Cholecystokinin stimulates the release of bile salts from the
gallbladder and digestion enzymes from the pancreas in the form of zymogens or proenzymes. Enteropeptidase converts trypsinogen into
trypsin, which, in turn, activates the other zymogens.
3|Page
