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What is the primary role of the bicarbonate secreted by the pancreas?

  • To stimulate glucagon production
  • To neutralize low pH chyme from the stomach (correct)
  • To aid in bile storage
  • To emulsify dietary lipids

    • Which of the following statements accurately describes pancreatic digestive enzymes?

  • They function at the brush border of enterocytes.
  • They are produced by liver cells.
  • They are secreted into the bloodstream directly.
  • They work in the lumen of the small intestine. (correct)

    • What happens when there is an excess of glucose consumed?

  • The pancreas secretes glucagon.
  • The pancreas activates bile production.
  • The pancreas initiates passive diffusion.
  • The pancreas secretes insulin. (correct)

    • The common bile duct is responsible for the transport of bile from which organ?

    <p>Gallbladder and liver</p> Signup and view all the answers

    Which component is NOT part of the bile secreted by the liver?

    <p>Pancreatic enzymes</p> Signup and view all the answers

    What process allows nutrients to cross the plasma membrane without a transporter?

    <p>Unmediated passive diffusion</p> Signup and view all the answers

    What is the purpose of emulsifying dietary lipids?

    <p>To facilitate their interaction with digestive enzymes</p> Signup and view all the answers

    Where do the secretions from the pancreas and the bile merge before entering the small intestine?

    <p>Duodenum</p> Signup and view all the answers

    What is the primary carbohydrate that humans consume, making up 75% of our carbohydrate intake?

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

    Which of the following statements about aldose and ketose sugars is true?

    <p>Both aldose and ketose sugars predominantly have six carbons.</p> Signup and view all the answers

    What type of bond links monosaccharides together to form disaccharides?

    <p>Glycosidic bonds</p> Signup and view all the answers

    What is the structure of lactose?

    <p>Galactose + Glucose</p> Signup and view all the answers

    Which type of starch is characterized by having a branching structure?

    <p>Amylopectin</p> Signup and view all the answers

    Which carbohydrate is mainly found in dairy products?

    <p>Lactose</p> Signup and view all the answers

    Which carbohydrate is known as a product of digestion and consists of two glucose monosaccharides?

    <p>Maltose</p> Signup and view all the answers

    Which of the following statements correctly describes fructooligosaccharides?

    <p>They are non-digestible carbohydrates containing fructose.</p> Signup and view all the answers

    What type of glycosidic bond links the first carbon of galactose with the fourth carbon of glucose in lactose?

    <p>Beta 1-4 bond</p> Signup and view all the answers

    What is a key difference between glucose and fructose in terms of their structure?

    <p>Fructose has a carbonyl group at the second carbon.</p> Signup and view all the answers

    What is the first step when converting ATP to ADP?

    <p>ATP + H2O &gt;&gt; ADP + Pi</p> Signup and view all the answers

    Which pathway does glucose primarily follow during carbohydrate metabolism?

    <p>Conversion to pyruvate</p> Signup and view all the answers

    Why is the direct conversion of ATP to AMP less common?

    <p>More enzymes are available for converting ATP to ADP.</p> Signup and view all the answers

    After glycolysis, what are the potential pathways for pyruvate?

    <p>Be converted to lactate or enter mitochondria for TCA cycle</p> Signup and view all the answers

    What is produced during the second step when ATP is converted directly to AMP?

    <p>2 Pi</p> Signup and view all the answers

    What is one option for amino acid metabolism?

    <p>Transformation to pyruvate</p> Signup and view all the answers

    In anaerobic metabolism, what happens to pyruvate?

    <p>It is converted to lactate.</p> Signup and view all the answers

    Which option represents a pathway of protein metabolism?

    <p>They can enter the TCA cycle directly.</p> Signup and view all the answers

    What is the enzyme responsible for converting glucose-6-phosphate to glucose-1-phosphate in glycogen synthesis?

    <p>Glycogen synthase</p> Signup and view all the answers

    Which compound is produced when lactate is converted to pyruvate during gluconeogenesis?

    <p>NADH</p> Signup and view all the answers

    In the liver and kidney, what is the primary regulator of the glycolytic enzyme phosphofructokinase-1 (PFK-1) under fed conditions?

    <p>Fructose 2,6-bisphosphate</p> Signup and view all the answers

    During fasting, which enzyme is primarily active to regulate glycogen breakdown?

    <p>Glycogen phosphorylase</p> Signup and view all the answers

    How many ATP molecules are used in the conversion of lactate to glucose through gluconeogenesis?

    <p>6 ATP</p> Signup and view all the answers

    What type of bond does glycogen phosphorylase cleave during glycogen breakdown?

    <p>Alpha 1,4 bond</p> Signup and view all the answers

    Which of the following is NOT a precursor used in gluconeogenesis?

    <p>Glucose</p> Signup and view all the answers

    What is the function of glucose-6-phosphatase in gluconeogenesis?

    <p>Removes phosphate from glucose-6-phosphate to produce free glucose</p> Signup and view all the answers

    What physiological state is indicated by the presence of glucagon?

    <p>Fasted state</p> Signup and view all the answers

    Where does gluconeogenesis primarily occur within the body?

    <p>Liver and kidney</p> Signup and view all the answers

    What effect does fructose 2,6-bisphosphate have on fructose 1,6-bisphosphatase?

    <p>Down-regulates the enzyme</p> Signup and view all the answers

    Which cycle involves the conversion of pyruvate to alanine and back to glucose in the liver?

    <p>Glucose-alanine cycle</p> Signup and view all the answers

    What is the main role of insulin in carbohydrate metabolism?

    <p>To promote glycogenesis</p> Signup and view all the answers

    During the transition from fasting to feeding, what happens to glycogen synthase activity?

    <p>It becomes inactive due to phosphorylation</p> Signup and view all the answers

    What is the primary function of the sodium-potassium ATPase pump?

    <p>Facilitate the uptake of nutrients against their concentration gradient</p> Signup and view all the answers

    Which mechanism allows for the uptake of glucose even when its concentration is low in the small intestine?

    <p>Secondary transport via sodium-potassium pump</p> Signup and view all the answers

    What is the primary role of the large intestine in digestion?

    <p>Fermentation of non-absorbable food by GI bacteria</p> Signup and view all the answers

    Which digestive process occurs in the mouth when starting starch digestion?

    <p>Chemical digestion using salivary amylase</p> Signup and view all the answers

    Which of the following statements about starch digestion in the stomach is true?

    <p>There is no digestion of starches due to low pH</p> Signup and view all the answers

    What is the main purpose of vesicular transport in the digestive system?

    <p>Handle very large molecules</p> Signup and view all the answers

    How do symport and antiport mechanisms differ?

    <p>Symport moves two molecules together; antiport moves them in opposite directions</p> Signup and view all the answers

    What primarily limits digestion of carbohydrates in the stomach?

    <p>Low pH that denatures enzymes</p> Signup and view all the answers

    Study Notes

    Monosaccharides

    • D-Glyceraldehyde: The starting point for carbohydrate synthesis, it has one chiral carbon.

    • D-Glucose (GlcP): An aldose sugar with 4 chiral carbons, the main carbohydrate we consume.

    • D-Galactose (GalP): An aldose sugar with 6 carbons, found in dairy.

    • D-Erythrulose: An uncommon carbohydrate with one chiral carbon.

    • D-Fructose (Fruf): The second most consumed carbohydrate, found in sweeteners and added sugars. It is a ketose sugar with 3 chiral carbons.

    • Glucose and Fructose: These sugars both have 6 carbons, but fructose has a double bond to oxygen on the 2nd carbon, while glucose has a double bond to oxygen on the 1st carbon.

    Aldose and Ketose Sugars

    • Aldose sugars (glucose, galactose) have a double bond to oxygen on carbon #1.
    • Ketose sugars (fructose) have a double bond to oxygen on carbon #2.

    Other Uncommon Monosaccharides

    • Oxidized sugar derivatives: Examples include uronic acids and derivatives of other carbs. They aren't common in food.
    • Reduced sugar derivatives: Sugar alcohols with the carbonyl group reduced to an alcohol. Often added to food products as reduced or no-calorie sweeteners. Examples include D-xylitol and D-glucitol (D-sorbitol).

    Disaccharides

    • Glycosidic Bonds: Bonds between monosaccharides.
    • Lactose: A disaccharide composed of galactose and glucose, linked via a beta 1-4 bond. Found in dairy.
    • Sucrose: A disaccharide composed of glucose and fructose, linked via an alpha 1-2 glycosidic bond. Found in table and added sugars.
    • Maltose: A disaccharide composed of two glucose molecules, linked via an alpha 1,4 bond. A product of digestion.
    • Trehalose: A disaccharide composed of two glucose molecules, linked via an alpha 1,1 bond. Found in mushrooms.

    Oligosaccharides

    • Fructooligosaccharides: Non-digestible carbohydrates.
    • Raffinose: A disaccharide composed of galactose, glucose, and fructose. Galactose is linked to glucose by an alpha 1,6 bond and glucose is linked to fructose by an alpha 1,2 bond.
    • Oligofructose: A disaccharide containing three fructose molecules, linked via beta 2,1 bonds.

    Starches

    • Amylose: A long polymer of glucose linked by alpha 1,4 bonds.
    • Amylopectin: A branched polymer of glucose with alpha 1,4 and alpha 1,6 bonds.
    • Glycogen: The form in which we store glucose in the body. Similar structure to amylopectin, with alpha 1,4 and alpha 1,6 bonds.

    Digestion and Absorption

    Organ Overview

    • Pancreas: Performs both exocrine and endocrine functions.
      - Exocrine Function: Secretes bicarbonate and pancreatic digestive enzymes into the small intestine, via the pancreatic duct. - Endocrine Function: Secretes insulin to decrease blood glucose levels when glucose is high, and glucagon to increase blood glucose levels when glucose is low.
    • Gallbladder: Collects bile from liver.
    • Liver: Secretes bile into the small intestine. Bile components include bile salts/acids, cholesterol, and phospholipids.

    Transport Processes

    • Passive Transport: Does not require energy, and can be unmediated or mediated.
    • Unmediated Passive Diffusion: Does not use a transporter. Nutrients freely cross the plasma membrane or go between cells.
    • Mediated Passive Diffusion: Requires a transporter molecule. Facilitated diffusion.
    • Active Transport: Requires energy to move nutrients across a membrane, often from a lower concentration to a higher concentration. Used for most carbohydrate uptake.
    • Vesicular transport: Used for very large molecules.

    Large Intestine

    • Ferments leftover, non-absorbable food via GI bacteria.
    • Absorbs water.
    • Has villi but no microvilli.

    Starch Digestion

    • Mouth: Begins digestion of starches with mechanical (chewing) and chemical (salivary amylase) digestion.
    • Stomach: No digestion of starches due to low pH.
    • Small Intestine: Site of primary starch digestion.

    Energy Metabolism

    • Protein Metabolism: Amino acids and carbon skeletons can be turned into pyruvate, acetyl-CoA, or used directly in the TCA cycle.

    • Carbohydrate Metabolism: Glucose or glucose 1-phosphate from glycogenolysis or lactate from anaerobic metabolism enters glycolysis.

    • Glycolysis: The main metabolic pathway for glucose. Produces pyruvate.

    • Pyruvate: Can continue on to the TCA cycle and ETC (aerobic metabolism), or be converted to lactate (anaerobic metabolism).### Carbohydrate Metabolism

    • Monosaccharides that undergo glycolysis invest 2 ATP, produce 2 NADH and 4 ATP.

    • Monosaccharides can be converted to a glycolytic intermediate or used for fatty acid synthesis.

    Glycogen Metabolism

    • Glycogen synthesis:
      • Glucose is converted to glucose-6-phosphate.
      • Glucose-6-phosphate is converted to glucose-1-phosphate.
      • UTP-glucose is synthesized as a substrate for glycogen synthase.
      • Glycogen synthase adds one glucose molecule to a growing glucose polymer using UDP-glucose as a substrate.
    • Glycogen breakdown:
      • Glycogen is a polymer of glucose with alpha 1,4 and alpha 1,6 bonds at branch points.
      • Glycogen phosphorylase breaks the alpha 1,4 bond, releasing glucose-1-phosphate and the remaining glycogen molecule.
      • Debranching enzyme cleaves the alpha 1,6 bond releasing free glucose.

    Pentose Phosphate Pathway

    • Part 1
      • Glucose-6-phosphate is converted to 6-phosphogluconolactone by glucose-6-phosphate dehydrogenase.
      • This step reduces NADP to NADPH + H+.
      • 6-phosphogluconate is converted to a ribose sugar and CO2 by phosphogluconate dehydrogenase.
      • This step also reduces NADP to NADPH + H+.
    • Part 2
      • Excess sugars are returned to glycolysis.
      • Excess ribose sugars are converted back to glycolytic intermediates.

    Gluconeogenesis

    • Gluconeogenesis is the synthesis of carbohydrates from non-carbohydrate precursors.
    • Three non-carbohydrate precursors are used: lactate, alanine, and glycerol.
      • Lactate is produced from anaerobic metabolism.
      • Alanine is produced from pyruvate.
      • Glycerol is the backbone of triglycerides.
    • Lactate's path through gluconeogenesis:
      • Lactate is converted to pyruvate, reducing 2 NAD to 2 NADH + H+.
      • Pyruvate is transferred from the cytoplasm to the mitochondria.
      • Pyruvate is carboxylated to oxaloacetate using 2 ATP.
      • Oxaloacetate is converted to malate, oxidizing 2 NADH to 2 NAD.
      • Malate crosses the mitochondrial membrane into the cytoplasm and is converted back to oxaloacetate, reducing 2 NAD to 2 NADH.
      • Oxaloacetate is converted to phosphoenolpyruvate using 1 ATP.
      • 3-Phosphoglycerate is converted to 1,3-bisphosphoglycerate using 2 ATP.
      • 1,3-bisphosphate is converted to glyceraldehyde 3-phosphate dehydrogenase, oxidizing 2 NADH + H+ to NAD.
      • Glucose 6-phosphatase removes the phosphate from glucose-6-phosphate to form free glucose, which can be secreted from the liver into circulation.
      • Gluconeogenesis from lactate requires 6 ATP to produce glucose.
    • Alanine's path through gluconeogenesis:
      • Alanine is converted to pyruvate without producing reducing equivalents.
      • Gluconeogenesis from alanine requires 11 ATP to produce glucose.
    • Glycerol's path through gluconeogenesis:
      • Glycerol uses 2 ATP and produces 2 NADH, resulting in a net positive of 3 ATP.
      • The body is generally fasting during lipolysis, which breaks down stored fat into glycerol and fatty acids.
    • The liver and kidney can perform gluconeogenesis.

    Tissues that produce precursors for gluconeogenesis

    • Adipose tissue breaks down stored fat into glycerol and fatty acids.
    • Skeletal muscle converts glucose to pyruvate and then to lactate during anaerobic metabolism, especially during exercise. Skeletal muscles also break down proteins during starvation, releasing glutamine and alanine.
    • Red blood cells are purely anaerobic and constantly produce lactate.

    Substrate Cycles

    • Cori cycle:
      • Glucose in the blood is taken up by tissues, converted to pyruvate, and then to lactate via anaerobic metabolism. Lactate is secreted into the bloodstream and taken up by the liver, where it is converted back to glucose via gluconeogenesis.
      • The Cori cycle is an example of a futile cycle, starting and ending with glucose.
    • Glucose-alanine cycle:
      • Glucose in the blood is taken up by tissues and converted to pyruvate, which is then converted to alanine. Alanine is secreted into the bloodstream and taken up by the liver, where it is converted to pyruvate and then back to glucose via gluconeogenesis.

    Regulating Glycolysis

    • Insulin (fed state):
      • Insulin up-regulates 6-Phosphofructo-1-kinase (6PF-1K) by increasing the production of fructose 2,6-bisphosphate (F2,6BP).
      • F2,6BP up-regulates 6PF-1K and down-regulates fructose 1,6-bisphosphatase (F1,6-Pase), which removes a phosphate from F1,6BP (a step in gluconeogenesis).
      • Insulin signals phosphorylation of Ser-32, resulting in the production of F2,6BP from F6P by 6PF-2K.
      • F2,6BP allosterically regulates 6PF-1K and F1,6-Pase.
    • Glucagon (fasted state):
      • Glucagon down-regulates 6PF-2K by dephosphorylating Ser-32.
      • Less F2,6BP is produced, so there is less regulation of 6PF-1K and F1,6-Pase.

    Regulating Glycogen Synthesis

    • Glycogen synthase a (deP) is the active, dephosphorylated form and synthesizes glycogen by adding glucose residues to the polymer.
    • Glycogen synthase b (P) is the less active, phosphorylated form.
    • Transitioning from phosphorylated to dephosphorylated state:
      • Fasted state: glucagon is present, signaling cAMP, which regulates Protein Kinase A. Protein Kinase A phosphorylates inhibitor-1 a (P), which down-regulates protein phosphatase 1 (PP1). PP1 dephosphorylates glycogen synthase, so inhibiting PP1 keeps glycogen synthase in its inactive, phosphorylated state in the fasted state.
      • Fed state: PP1 is up-regulated, dephosphorylating glycogen synthase and making it active.
    • Fed to fasted state (insulin present):
      • Insulin down-regulates glycogen synthase kinase, which phosphorylates glycogen synthase. This keeps glycogen synthase dephosphorylated and active.
    • Transitioning to a fasted state (glucagon present):
      • Glucagon activates cAMP, which activates two kinases: phosphorylase kinase and protein kinase A. These kinases phosphorylate glycogen synthase, inactivating it.

    Regulating Glycogen Breakdown

    • Glycogen phosphorylase is the primary enzyme for glycogen breakdown.
      • In its dephosphorylated state, glycogen phosphorylase is less active.
      • In its phosphorylated state, glycogen phosphorylase is more active.
    • Fasted state: glucagon is present, signaling cAMP, which activates protein kinase A.
      • First half: protein kinase A activates phosphorylase kinase, which phosphorylates glycogen phosphorylase, activating it.
      • Second half: protein kinase A phosphorylates inhibitor 1, which inhibits PP1, slowing down the dephosphorylation of glycogen phosphorylase.

    Glycolysis & Gluconeogenesis Regulation Figure

    • The main regulated step in glycolysis is the 6PF-1K reaction, which uses ATP.
      • Up-regulation of 6PF-1K is stimulated by:
        • Fructose 2,6-bisphosphate (F2,6BP)
        • AMP
        • ADP
        • Insulin
      • Down-regulation of 6PF-1K is stimulated by:
        • ATP
        • Citrate
        • Glucagon
        • H+
    • Up-regulation of F1,6-Pase is stimulated by:
      • ATP
      • Glucagon
    • Down-regulation of F1,6-Pase is stimulated by:
      • AMP
      • ADP
      • Insulin
      • F2,6BP
    • Up-regulation of Pyruvate Kinase is stimulated by:
      • F1,6BP
      • Insulin
    • Down-regulation of Pyruvate Kinase is stimulated by:
      • ATP
      • Alanine
      • Glucagon

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