MCAT Biology : Digestive System
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Example Question #1 : Digestive System
In patients with diabetes, certain cells are either destroyed or limited in function. These cells are __________.
delta cells
alpha cells
beta cells
epsilon cells
beta 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.
Example Question #2 : Digestive System
The Islets of Langerhans in the pancreas serve to secrete which product?
Insulin
Bicarbonate
Pepsin
Trypsin
Insulin
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.
Example Question #53 : Excretory And Digestive Systems
The interaction between blood pressure and kidney function in humans requires coordination by the renin-angiotensin-aldosterone system (RAAS). This system involves the dynamic interplay of the kidneys, lungs, and blood vessels to carefully regulate sodium and water balance.
A normal human kidney has cells adjacent to the glomerulus called juxtaglomerular cells. These cells sense sodium content in urine of the distal convoluted tubule, releasing renin in response to a low level. Renin is an enzyme that converts angiotensinogen to angiotensin I (AI). AI is converted to angiotensin II (AII) by angiotensin converting enzyme (ACE) in the lung.
AII stimulates aldosterone secretion in the zona glomerulosa of the adrenal gland. Aldosterone then acts to upregulate the sodium-potassium pump on the basolateral side of distal tubule epithelial cells to increase sodium reabsorption from the urine, as well as increasing potassium excretion.
Angiotensinogen is similar to other precursor molecules, which must be cleaved before they become active. Which of the following organs secretes digestive precursor molecules that must also be cleaved by proteolytic processes before they are functional?
Thyroid gland
Pituitary gland
Goblet cells
Adrenal glands
Pancreas
Pancreas
Digestive precursor molecules are secreted by both the pancreas and chief cells of the stomach. The pancreas secretes trypsinogen, which is later cleaved to the active trypsin. Chief cells in the stomach secrete pepsinogen, which is then cleaved into the active pepsin. These precursor molecules are known as zymogens.
Example Question #1 : Digestive System
Which of these statements about the liver is FALSE?
I. Hepatocytes make bile.
II. The blood supply to the liver is exclusively through the hepatic artery, which is a branch of the celiac trunk.
III. The liver is an important storage site for glycogen.
IV. Cirrhosis is and end stage condition of fibrosis or scarring of the liver.
V. The liver is capable of converting lipids to glucose.
III
IV
I
V
II
II
Although it is true that the hepatic artery is a branch or the celiac axis, or trunk, the liver has a dual blood supply in the form of the hepatic artery and the portal vein. The latter structure brings absorbed nutrients from the small and large intestine directly back to liver cells for processing.
Example Question #2 : Digestive System
Which of the following actions is not performed by the liver in order to regulate blood glucose levels?
Glycogenolysis
Gluconeogenesis
Insulin storage
Glycogenesis
Insulin storage
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.
Example Question #1 : Pancreas And Liver
What is the primary purpose of the liver?
Release enzymes for food digestion
Absorb macro- and micronutrients
Breakdown larger food particles
Filter and detoxify blood
Filter and detoxify blood
The purpose of the liver is to filter blood delivered from the systemic circulation by the hepatic artery, and from the intestines through the hepatic vein. The liver serves to use phase I, II, and III metabolism to oxidize, conjugate, and transport toxic chemicals to the urine or bile for excretion, respectively.
The purpose of the pancreas is to release enzymes for food digestion, while the purpose of the small intestine is to absorb micro- and macronutrients. Additionally, the purpose of the stomach is to breakdown larger food particles into smaller ones.
Example Question #2 : 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?
Oxidation
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.
Example Question #3 : 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 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).
Example Question #4 : 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.
Phase III metabolism in the liver involves what action upon modified metabolites?
Methylation
Transport
Acetylation
Oxidation
Transport
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.
Example Question #5 : 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.
Metabolism of drugs and endogenous materials for excretion occurs in which cell type?
Endothelial cell
Hepatocyte
Smooth muscle cell
Enterocyte
Hepatocyte
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.
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