MCAT Biology › Pancreas and Liver
The liver primarily serves to help detoxify both endogenous and exogenous substances from the blood and intestines. Once blood from the intestines (delivered by the portal vein) or from the systemic circulation (delivered by the hepatic artery) enters the liver, it is filtered over liver cells called hepatocytes. Endogenous substances, such as bilirubin, and exogenous substances, such as drugs, are taken up by transporters on hepatocytes and undergo three phases of metabolism. The three phases allow the transported compound to be detoxified by a method of electron transfer (phase I), by addition of amino acid derivatives (phase II), and finally by exocytosis from the hepatocyte into the bile (phase III). The bile is then transported into the small intestine, and finally excreted from the body.
Amino acid derivatives are often taken from the Krebs cycle, added to sugar nucleotides, and transferred to molecules for detoxification. A common example of an enzyme responsible for this is UDP-glucuronosyl transferase.
How does phase I metabolism in the liver, conducted primarily by the cytochrome P450 system, serve to change an exogenous drug?
As we are told in the passage, phase I metabolism occurs by the transfer of electrons, commonly called an oxidation-reduction reaction. Phase I metabolism in the liver serves primarily to oxidize endogenous and exogenous molecules by passing electrons from a substrate to iron, and finally to oxygen. These oxidation reactions are carried out by the cytochrome P450 system and allow metabolites to become more soluble in urine for excretion. The other reactions listed are carried out by various transferases during phase II metabolism in the liver.
The liver primarily serves to help detoxify both endogenous and exogenous substances from the blood and intestines. Once blood from the intestines (delivered by the portal vein) or from the systemic circulation (delivered by the hepatic artery) enters the liver, it is filtered over liver cells called hepatocytes. Endogenous substances, such as bilirubin, and exogenous substances, such as drugs, are taken up by transporters on hepatocytes and undergo three phases of metabolism. The three phases allow the transported compound to be detoxified by a method of electron transfer (phase I), by addition of amino acid derivatives (phase II), and finally by exocytosis from the hepatocyte into the bile (phase III). The bile is then transported into the small intestine, and finally excreted from the body.
Amino acid derivatives are often taken from the Krebs cycle, added to sugar nucleotides, and transferred to molecules for detoxification. A common example of an enzyme responsible for this is UDP-glucuronosyl transferase.
How does phase II metabolism in the liver function to change highly reactive metabolites of phase I metabolism, or to change other endogenous molecules for excretion?
We are told in the passage that phase II metabolism occurs when a sugar nucleotide-amino acid derivative is affixed to a substance. Context clues are helpful for pinpointing the exact reaction taking place, as the passage provides us with an example of an enzyme that catalyzes the reaction, UDP-glucuronosyl transferase. We can infer from the answer choices that glucuronidation likely occurs using this enzyme. Furthermore, phase II metabolism in the liver is called conjugation, and uses various transferases (acetyltransferases, methytransferases, glucuronosyltransferases) to add function groups onto reactive metabolites from phase I metabolism and endogenous molecules, like bilirubin. Adding these functional groups serves to make the products more hydrophilic in order to be excreted in urine (if smaller) or bile (if larger).
The liver primarily serves to help detoxify both endogenous and exogenous substances from the blood and intestines. Once blood from the intestines (delivered by the portal vein) or from the systemic circulation (delivered by the hepatic artery) enters the liver, it is filtered over liver cells called hepatocytes. Endogenous substances, such as bilirubin, and exogenous substances, such as drugs, are taken up by transporters on hepatocytes and undergo three phases of metabolism. The three phases allow the transported compound to be detoxified by a method of electron transfer (phase I), by addition of amino acid derivatives (phase II), and finally by exocytosis from the hepatocyte into the bile (phase III). The bile is then transported into the small intestine, and finally excreted from the body.
Amino acid derivatives are often taken from the Krebs cycle, added to sugar nucleotides, and transferred to molecules for detoxification. A common example of an enzyme responsible for this is UDP-glucuronosyl transferase.
Phase III metabolism in the liver involves what action upon modified metabolites?
Transport
Oxidation
Methylation
Acetylation
The passage tells us that phase III metabolism typically occurs by exocytosis. Remember that exocytosis is the transport of a molecule from within the cell to outside the cell with the help of ATP and vesicle packaging. We know that phase III metabolism must involve transport of a molecule from one side of the plasma membrane to the other. Phase III of liver metabolism is the transport of proteins that have been oxidized (phase I) or conjugated (phase II) from the hepatocyte into the bile system. Remember, in contrast to smaller, hydrophilic molecules that result from phase I or II metabolism and can be excreted into the urine, larger molecules must be transported (phase III metabolism) into the bile for excretion.
Which of the following is not a function of the liver?
Produce red blood cells
Gluconeogenesis
Synthesize albumin and clotting factors
Detoxify drugs in the blood
The functions of the liver are many and varied. The liver is responsible for gluconeogenesis (making new glucose), beta-oxidation of fats, detoxification of drugs and toxins via the cytochrome P450 system, removal of dead red blood cells from circulation, and the creation of blood proteins, including albumin and clotting factors.
The bone marrow is responsible for generating new red blood cells.
Bile is essential for healthy digestion and absorbtion of fats. Where is bile made in the body and where is it stored before use?
Bile is made in the liver and stored in the gallbladder.
Bile is made in the gallbladder and stored in the liver.
Bile is made in the pancreas and stored in the gallbladder.
Bile is made in the gallbladder and stored in the pancreas.
Bile is made in the liver and stored in the gallbaldder. People can survive without a gallbladder, but having a gallbladder helps release a large amount of bile into the small intestine when it is needed most. This helps make digestion of fats much more effective.
Which of the following actions is not performed by the liver in order to regulate blood glucose levels?
Insulin storage
Glycogenolysis
Gluconeogenesis
Glycogenesis
The liver is vital in the control of blood glucose levels. It stores excess glucose by creating glycogen via the process of glycogenesis. It can also release glucose into the bloodstream by stimulating gluconeogenesis and glycogenolysis. Glycogenolysis is the process of converting glycogen back to glucose, while gluconeogenesis is the process of making glucose from non-carbohydrates.
Insulin is created in the pancreas, and is released in response to high blood sugar levels. The insulin then stimulates the liver to store glucose as glycogen.
The liver primarily serves to help detoxify both endogenous and exogenous substances from the blood and intestines. Once blood from the intestines (delivered by the portal vein) or from the systemic circulation (delivered by the hepatic artery) enters the liver, it is filtered over liver cells called hepatocytes. Endogenous substances, such as bilirubin, and exogenous substances, such as drugs, are taken up by transporters on hepatocytes and undergo three phases of metabolism. The three phases allow the transported compound to be detoxified by a method of electron transfer (phase I), by addition of amino acid derivatives (phase II), and finally by exocytosis from the hepatocyte into the bile (phase III). The bile is then transported into the small intestine, and finally excreted from the body.
Amino acid derivatives are often taken from the Krebs cycle, added to sugar nucleotides, and transferred to molecules for detoxification. A common example of an enzyme responsible for this is UDP-glucuronosyl transferase.
Metabolism of drugs and endogenous materials for excretion occurs in which cell type?
Hepatocyte
Enterocyte
Smooth muscle cell
Endothelial cell
The liver is the primary site of metabolism for all exogenous drugs and endogenous substances. Substances are delivered to the liver hepatocytes by the blood and undergo phase I, phase II, and/or phase III metabolism before being excreted into the bile or back into the blood to be passed to the kidney for excretion into the urine.
Remember that phase I metabolism is oxidation (turns lipophilic molecules into hydrophilic molecules), phase II metabolism is conjugation, and phase III metabolism is transport to the bile. All phases, however, take place in the liver by function of the hepatocytes.
The liver primarily serves to help detoxify both endogenous and exogenous substances from the blood and intestines. Once blood from the intestines (delivered by the portal vein) or from the systemic circulation (delivered by the hepatic artery) enters the liver, it is filtered over liver cells called hepatocytes. Endogenous substances, such as bilirubin, and exogenous substances, such as drugs, are taken up by transporters on hepatocytes and undergo three phases of metabolism. The three phases allow the transported compound to be detoxified by a method of electron transfer (phase I), by addition of amino acid derivatives (phase II), and finally by exocytosis from the hepatocyte into the bile (phase III). The bile is then transported into the small intestine, and finally excreted from the body.
Amino acid derivatives are often taken from the Krebs cycle, added to sugar nucleotides, and transferred to molecules for detoxification. A common example of an enzyme responsible for this is UDP-glucuronosyl transferase.
A decrease in the extraction of a drug by the liver may be observed under which of the following conditions?
Liver disease
Decrease in binding of the drug to plasma proteins
Increase in hepatic blood flow
Increasing the expression of phase I metabolic enzymes
The fraction of a drug that the liver can act on depends on how much of the drug is delivered to the liver. Often, this initial metabolism is called "first pass metabolism," and serves to determine the initial dose of each drug when it is taken orally. Decreasing blood flow, increasing binding of a drug to plasma proteins, liver disease (hepatocyte death), and genetic mutations that decrease the activity of the liver metabolism enzymes will all reduce the amount of drug available for the liver to act upon, leading to a decrease in drug extraction.
The Islets of Langerhans in the pancreas serve to secrete which product?
Insulin
Pepsin
Trypsin
Bicarbonate
The pancreas has two primary purposes, acting as both an endocrine gland and an exocrine organ. Islets of Langerhans secrete the endocrine factors, while the acinar cells produce the exocrine factors of the pancreas.
The Islets of Langerhans serve to secrete insulin, which promotes the absorption of glucose from the blood into cells throughout the body. The Islets also secrete glucagon, somatostatin, and ghrelin. Pepsin is produced by chief cells in the stomach, while trypsinogen (the zymogen of trypsin) is produced by acinar cells of the pancreas. Bicarbonate is produced in response to secretin secretion by acinar cells in the pancreas.
Essentially, pancreatic hormones are released from the Islets of Langerhans, while pancreatic digestive enzymes are released from the acinar cells.
In patients with diabetes, certain cells are either destroyed or limited in function. These cells are __________.
beta cells
alpha cells
delta cells
epsilon cells
Diabetes results from an autoimmune destruction (type I) or dysfunction (type II) of pancreatic beta cells.
The Islets of Langerhans in the pancreas are made of acinar cells, which secrete various essential hormones. Alpha cells secrete glucagon, while beta cells secrete insulin. Delta cells secrete somatostatin and epsilon cells secrete ghrelin. Beta cells are the most abundant acinar cell.