Carbohydrate Metabolism Overview
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Questions and Answers

What condition is associated with GLUT1 deficiency?

  • Seizures in early infancy (correct)
  • Diabetes mellitus
  • Obesity
  • Ketoacidosis
  • Which transporter is primarily responsible for glucose uptake in muscle and adipose tissue in response to insulin?

  • GLUT4 (correct)
  • GLUT1
  • GLUT2
  • GLUT3
  • What is the primary function of GLUT5?

  • Facilitate glucose entry in fetal tissues
  • Transport dietary fructose (correct)
  • Regulate blood glucose levels
  • Transport glucose into pancreatic cells
  • In which cells is GLUT2 predominantly expressed?

    <p>β-cells of the pancreas and liver (C)</p> Signup and view all the answers

    Which GLUT transporter is considered a high-affinity transporter for glucose?

    <p>GLUT1 (D)</p> Signup and view all the answers

    What is the primary function of SGLT1 in enterocytes?

    <p>Active transport of glucose and galactose (B)</p> Signup and view all the answers

    What triggers the translocation of GLUT4 to the plasma membrane?

    <p>Insulin signaling (A)</p> Signup and view all the answers

    How do monosaccharides first enter the enterocytes?

    <p>Across the brush border (apical) membrane (A)</p> Signup and view all the answers

    What distinguishes SGLTs from GLUTs in the transport of monosaccharides?

    <p>SGLTs are dependent on ATP hydrolysis while GLUTs are not (B)</p> Signup and view all the answers

    Which GLUT transporter has a specific role in transporting glucose and fructose into enterocytes?

    <p>GLUT2 (C)</p> Signup and view all the answers

    Which of the following does not express GLUT4 and does not depend on insulin for glucose uptake?

    <p>Liver (C)</p> Signup and view all the answers

    What happens to newly absorbed monosaccharides after they exit the enterocytes?

    <p>They are delivered directly to the liver through the hepatic portal vein (B)</p> Signup and view all the answers

    What is the main regulatory factor for the activity of SGLTs?

    <p>Cellular energy availability (C)</p> Signup and view all the answers

    What is the main function of GLUT3?

    <p>Serve as a glucose transporter in high-demand tissues like the brain (D)</p> Signup and view all the answers

    Which of the following cells are GLUT1 and GLUT3 not predominantly associated with?

    <p>Skeletal muscle (C)</p> Signup and view all the answers

    Which statement about GLUT1 is true?

    <p>GLUT1 is critical for glucose transport across the blood–brain barrier (A)</p> Signup and view all the answers

    Which of the following correctly describes the transport mechanisms of SGLTs and GLUTs?

    <p>SGLTs use active transport while GLUTs rely on facilitated diffusion (D)</p> Signup and view all the answers

    What is the role of the hepatic portal vein in monosaccharide transport?

    <p>It transports monosaccharides to the liver for metabolism (D)</p> Signup and view all the answers

    How many isoforms of SGLT have been identified so far?

    <p>Seven (B)</p> Signup and view all the answers

    What is the primary structural component of plants?

    <p>Carbohydrates (A)</p> Signup and view all the answers

    Which type of carbohydrate typically contains more than 10 saccharide units?

    <p>Polysaccharides (B)</p> Signup and view all the answers

    Which carbohydrate is the most abundant in complex carbohydrates?

    <p>Glucose (D)</p> Signup and view all the answers

    What role do oligosaccharides play when conjugated to proteins or lipids in cell membranes?

    <p>Cell surface modulation (A)</p> Signup and view all the answers

    What is the term for the hairlike extensions of enterocytes that increase the surface area for nutrient absorption?

    <p>Microvilli (B)</p> Signup and view all the answers

    Which term describes the folds in the mucosa that protrude into the lumen of the small intestine?

    <p>Folds of Kerckring (A)</p> Signup and view all the answers

    How are carbohydrates primarily classified?

    <p>By their structure (B)</p> Signup and view all the answers

    What is the main purpose of glycogenesis in the body?

    <p>To convert glucose into glycogen for energy storage (C)</p> Signup and view all the answers

    Which enzyme catalyzes the phosphorylation of glucose in muscle cells?

    <p>Hexokinase (B)</p> Signup and view all the answers

    How does muscle hexokinase regulate glucose entry into muscle cells?

    <p>By being negatively modulated by glucose-6-phosphate (D)</p> Signup and view all the answers

    What role does liver glycogen play in blood glucose levels?

    <p>It can be broken down to glucose and released into the bloodstream (B)</p> Signup and view all the answers

    Which metabolic pathway is primarily responsible for producing nicotinamide adenine dinucleotide phosphate (NADPH)?

    <p>Pentose phosphate pathway (B)</p> Signup and view all the answers

    What is the primary function of insulin in the regulation of blood glucose levels?

    <p>To lower blood glucose levels by promoting glucose uptake (A)</p> Signup and view all the answers

    How does glucagon function in relation to insulin?

    <p>It increases blood glucose levels by promoting glycogen breakdown (C)</p> Signup and view all the answers

    What role does GLUT4 play in glucose metabolism?

    <p>It translocates to the cell membrane in response to insulin to facilitate glucose uptake (A)</p> Signup and view all the answers

    What occurs in insulin-resistant states regarding GLUT4?

    <p>GLUT4 remains in GLUT4 storage vesicles and does not translocate effectively (D)</p> Signup and view all the answers

    Which hormones primarily influence the regulation of blood glucose levels?

    <p>Insulin and glucagon (A)</p> Signup and view all the answers

    What process is stimulated by insulin to store glucose in the excess post-meal state?

    <p>Glycogenesis (A)</p> Signup and view all the answers

    What effect do glucocorticoid hormones primarily have on blood glucose levels?

    <p>Increase hepatic gluconeogenesis (B)</p> Signup and view all the answers

    Where is GLUT4 synthesized before it translocates to the cell surface?

    <p>Ribosomes of the rough endoplasmic reticulum (A)</p> Signup and view all the answers

    What immediate physiological event triggers the release of insulin after carbohydrate intake?

    <p>Rise in blood glucose concentration (C)</p> Signup and view all the answers

    What is glycogenolysis mainly responsible for?

    <p>Breakdown of liver glycogen to release glucose (B)</p> Signup and view all the answers

    Flashcards

    Carbohydrates

    Abundant organic molecules, primarily found in plants, providing energy as sugars and starch. They also have structural roles in cells and tissues, and are components of RNA and DNA.

    Simple Carbohydrates

    Carbohydrates composed of monosaccharides and disaccharides.

    Complex Carbohydrates

    Polymers of saccharide units linked by glycosidic bonds. Oligosaccharides have 3-10 units; polysaccharides have more than 10.

    Small Intestine Structures

    The small intestine has folds (Kerckring's folds), villi (finger-like projections), and microvilli (extensions of cells) that increase its surface area for nutrient absorption.

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    Villi

    Finger-like projections in the small intestine, lined with cells (enterocytes), containing blood capillaries and lymph vessels for nutrient transport.

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    Enterocytes

    Cells lining the villi in the small intestine that absorb nutrients and transport them into the bloodstream or lymph.

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    Microvilli

    Hair-like extensions of enterocyte plasma membranes on the villi, significantly increasing the surface area for nutrient absorption.

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    Enterocyte brush border membrane

    The part of an enterocyte's membrane facing the intestinal lumen (inside of the gut).

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    Monosaccharide absorption

    Process of transferring simple sugars from the gut into the bloodstream.

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    Plasma membrane

    The thin boundary layer surrounding a cell.

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    SGLTs (Sodium Glucose Cotransporters)

    Membrane proteins that actively transport glucose and sodium together into cells.

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    GLUTs (Glucose Transporters)

    Membrane proteins that facilitate glucose transport across cell membranes by passive diffusion.

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    SGLT1 role

    Primarily responsible for absorbing glucose and galactose from the gut (intestine) into enterocytes.

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    Active transport

    Movement of molecules across a membrane that requires energy input from the cell.

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    Facilitated diffusion

    Movement of molecules across a membrane from high to low concentration, with help from carrier proteins, but no energy required.

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    GLUT1

    A glucose transporter that crosses the blood-brain barrier and is essential to the developing brain.

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    GLUT1 function

    Supplies glucose to erythrocytes, brain endothelial cells, and fetal tissues.

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    GLUT1 deficiency

    Causes seizures early in infancy due to lack of glucose to the brain, requiring a ketogenic diet.

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    GLUT2 function

    Low-affinity, high-capacity transporter of monosaccharides in pancreas, liver, intestine, & kidney. Transports glucose/fructose when levels are high.

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    GLUT2 pancreas role

    Sensitive glucose level indicator and involved in insulin release.

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    GLUT3 function

    High-affinity glucose transporter in brain and neurons, also in spermatozoa, placenta, and early embryos.

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    GLUT4 function

    Insulin-dependent glucose transporter in muscle and adipose tissue, not in liver, kidney.

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    GLUT4 translocation

    Insulin moves GLUT4 from storage vesicles to the cell membrane for glucose uptake.

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    GLUT5 function

    Specific fructose transporter, primarily in small intestine; also in kidney, brain, muscle, and adipose.

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    Blood glucose maintenance

    A coordinated effort of multiple organs (intestine, liver, kidney, muscle, adipose) to maintain normal blood glucose levels.

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    Monosaccharides transport

    GLUT2 is involved in the movement of monosaccharides from enterocytes into the portal blood. This process is important for the absorption and distribution of these crucial nutrients.

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    Glycogen Synthesis

    The process of converting glucose into glycogen, which is a storage form of glucose primarily in the liver and skeletal muscles. This allows the body to store energy for later use.

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    Glycogen Breakdown

    The process of converting stored glycogen back into glucose. This occurs mainly in the liver, releasing glucose into the bloodstream to maintain blood sugar levels.

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    Muscle vs. Liver Glycogen

    Muscle glycogen is primarily used as an energy source within the muscle itself, while liver glycogen is released into the bloodstream to maintain blood glucose levels.

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    Glucose-6-Phosphate

    A phosphorylated form of glucose, produced by the enzyme hexokinase in most cells. This molecule is a key intermediate in various metabolic pathways.

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    Hexokinase Regulation

    Hexokinase, the enzyme that converts glucose to glucose-6-phosphate, is regulated by the product of the reaction. This means that when cells have enough glucose-6-phosphate, the entry of glucose into the pathway is slowed down.

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    Blood Glucose Regulation

    The process of maintaining stable blood glucose levels through hormonal and metabolic actions.

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    Insulin's Role

    The primary hormone that lowers blood glucose by promoting glucose uptake into cells, especially muscle and adipose tissue.

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    Glucagon's Role

    The primary hormone that raises blood glucose by stimulating glycogen breakdown in the liver.

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    Glycogenesis

    The process of converting glucose into glycogen for storage in muscle and liver.

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    Glycogenolysis

    The process of breaking down stored glycogen into glucose, primarily in the liver.

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    Gluconeogenesis

    The process of synthesizing glucose from non-carbohydrate sources, such as amino acids and fatty acids, in the liver.

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    Insulin Resistance

    A condition where cells are less responsive to insulin, leading to impaired glucose uptake and potentially elevated blood glucose levels.

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    Hyperglycemia

    High blood glucose levels, often associated with diabetes.

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    What happens after a carb-rich meal?

    Insulin is released, promoting glucose uptake into cells, lowering blood glucose levels.

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    Study Notes

    Carbohydrate Metabolism

    • Carbohydrates are the most abundant organic molecules on Earth
    • They are the primary structural component of plants and provide energy in the form of starch and sugars
    • Carbohydrates provide half or more of the global food energy consumption.
    • Carbohydrates act as metabolic intermediates, RNA and DNA constituents, structural components of cells and tissues, and energy storage molecules.
    • The variety of carbohydrate functions stems from structural diversity.
    • Carbohydrate molecules are constructed from carbon, oxygen, and hydrogen in a ratio approximating a hydrate of carbon (C-H2O)n.

    Types of Carbohydrates

    • Carbohydrates are categorized into simple and complex carbohydrates
    • Simple carbohydrates include monosaccharides (one sugar unit) and disaccharides (two sugar units).
    • Complex carbohydrates include oligosaccharides (3–10 sugar units) and polysaccharides (more than 10, often thousands, of sugar units).
    • Examples of monosaccharides include glucose, fructose, and galactose.
    • Examples of disaccharides include sucrose, lactose, and maltose.
    • Examples of complex carbs include starch, glycogen, and dietary fiber.
    • A diagram of carbohydrate classification illustrates the hierarchical structure.

    Complex Carbohydrates

    • Complex carbohydrates are polymers of saccharide units linked by glycosidic bonds.
    • Oligosaccharides contain 3–10 saccharide units, and polysaccharides contain more than 10.
    • Glucose is the most abundant type of saccharide
    • Complex carbohydrates are a significant component of the human diet.
    • In the body, oligosaccharides are often conjugated to proteins and lipids in cell membranes and act as important cell function modulators.

    Structure of Carbohydrates

    • Diagrams of the structural variations between Maltose, Lactose, Sucrose and Trehalose are provided to show glycosidic bonds as Haworth projections.

    Digestion of Complex Carbohydrates

    • The breakdown of amylose and amylopectin begins in the mouth with salivary a-amylase.
    • The acidity of gastric juice halts the activity of salivary a-amylase.
    • The pancreas releases pancreatic a-amylase in the small intestine for further digestion.
    • Further digestion is completed in the small intestine with the help of enzymes within the brush border
    • Hydrolysis continues with the help of brush border enzymes including maltase, α-Glucosidase, α-dextrinase (isomaltase), yielding glucose.

    Small Intestine Structure

    • The folds of Kerckring, villi, and microvilli increase the absorptive surface area of the small intestine.
    • Enterocytes line the villi and have microvilli forming a brush border on the apical membrane, increasing surface area for nutrient absorption.
    • Blood capillaries and lacteals transport absorbed nutrients away from the enterocytes.

    Monosaccharide Absorption and Transport

    • Dietary monosaccharides cross the plasma membrane of enterocytes twice to be absorbed into blood.
    • Monosaccharides first enter the cell through the brush border (apical) side, then exit through the basolateral side to capillaries connected to the hepatic portal vein.
    • They are delivered to the liver for metabolism.
    • Two transporter families facilitate monosaccharide transport: Sodium glucose cotransporters (SGLTs) and facilitated diffusion glucose transporters (GLUTs).
    • Different GLUTs are expressed in different tissues.
    • GLUT1, GLUT2, GLUT3, and GLUT4 have specific roles.

    Maintenance of Blood Glucose Concentration

    • Maintaining normal blood glucose is crucial for homeostasis and involves multiple organs and hormones.
    • Insulin lowers and glucagon raises blood glucose levels.
    • Glucagon also breaks down liver glycogen in a process of glycogenolysis.
    • Cortisol also increases blood glucose levels via gluconeogenesis.

    Glycogenesis.

    • Glycogenesis is the synthesis of glycogen from glucose in response to high blood glucose.
    • The process of glycogen synthesis is initiated by the protein glycogenin.
    • Glycogen synthase catalyzes the addition of glucose to the growing glycogen chain.
    • Branching enzyme is required to create branches in the glycogen molecule for storage and increased solubility.

    Glycogenolysis.

    • Glycogenolysis is the process of breaking down glycogen to glucose in response to decreased blood glucose levels.
    • Glycogen phosphorylase catalyzes the process
    • The debranching enzyme is also involved in glycogenolysis.
    • Free glucose is only produced in the liver and kidneys, not in muscle.

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    Description

    This quiz explores the essential role of carbohydrates in metabolism and their various functions in living organisms. It covers the structural diversity of carbohydrates, their classification into simple and complex types, and their significance as energy sources. Test your knowledge on how carbohydrates contribute to global food energy and cellular structure.

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