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Anatomy & Physiology II - Unit 4: Chapter 25: Carbohydrate Metabolism

Metabolism

• Definition: All chemical processes that occur within the body.
• Two Main Processes:
  • Catabolism:
  • Breakdown of complex structures into simpler ones.
  • Hydrolysis: Involves adding water to break a bond.
  • Anabolism:
  • Building of complex structures from simpler ones.
  • Dehydration Synthesis: Involves removing water to create a bond.

Use of Nutrients

• Sources: Carbohydrates, lipids, and proteins.
• Function: Nutrients are broken down and absorbed; mitochondrial processes utilize these nutrients to produce ATP (Adenosine Triphosphate).

Adenosine Triphosphate (ATP)

• Structure:
  • Composed of:
  • Adenosine: Consists of adenine (a nitrogenous base) and ribose (a 5-carbon sugar).
  • Triphosphate Chain: Contains three phosphate groups with high-energy bonds.
• Energy Storage:
  • Maximum energy is stored in the bond between the second (beta) and third (gamma) phosphate.
  • Conversion: When the gamma phosphate group is released, ATP becomes ADP (Adenosine Diphosphate).
  • Further removal of the second phosphate, if needed, results in AMP (Adenosine Monophosphate).
• Regeneration: ATP is regenerated by reattaching phosphate groups to ADP or AMP, a process that requires energy.

Where is the Energy?

• Definition of Energy: The ability to do work.
• Law of Thermodynamics: Energy cannot be created or destroyed but can change forms.
• Calories: The unit of measurement for energy in food.
• Energy Storage: Found in chemical bonds of molecules; energy is released when these bonds are broken in metabolic reactions.

Coenzymes

• Function in Metabolism: Required for redox (oxidation-reduction) reactions, primarily derived from B vitamins.
• Key Coenzymes:
  • NAD⁺ (Nicotinamide Adenine Dinucleotide)
  • Accepts hydrogen ions (H⁺) and electrons, forming NADH.
  • FAD (Flavin Adenine Dinucleotide)
  • Similar function and can be reduced to form FADH₂.

Phosphorylation

• Definition: The addition of a phosphate group to a molecule, requiring energy to form the bond.
  • Dephosphorylation: Removal of a phosphate group, which releases energy.
• Activation: Phosphorylated molecules are activated for cellular functions.

Substrate Level Phosphorylation

• Experimental Detail: Direct transfer of high-energy phosphate groups from phosphorylated substances to ADP to create ATP, occurring without oxygen (anaerobic).
• Occurrence: Happens three times per glucose molecule (2 times in glycolysis and once in the citric acid cycle).

Oxidative Phosphorylation

• Definition: Produces more ATP than substrate-level phosphorylation and occurs only in the presence of oxygen (aerobic).
• Process: Involves a chemiosmotic mechanism that couples the movement of hydrogen ions across membranes to chemical reactions, utilizing energy released from nutrient oxidation to pump H⁺ across the inner mitochondrial membrane.
• Outcome: H⁺ flows through ATP synthase, producing ATP from ADP and phosphate.

Introduction: Acetyl CoA

• Definition: A coenzyme formed as a transitional step in carbohydrate, fat, and protein oxidation.
• Role in Metabolism: Combines with oxaloacetate to initiate the Krebs cycle.

Coenzymes in Metabolism

• Notable Coenzymes:
  • NAD⁺: Reduces to form NADH.
  • FAD: Reduces to form FADH₂.
  • Coenzyme A (CoA): Acts as a carrier molecule, but does not participate in redox reactions.

Cellular (Aerobic) Respiration: Summary

• Overview: Breaks down one glucose molecule into ATP through four main steps:
  1. Glycolysis
  2. Transitional step
  3. Krebs Cycle
  4. Electron Transport Chain (ETC)
• Equation: $C<em>6H</em>{12}O<em>6 + 6O</em>2 <br /> ightarrow 6CO<em>2 + 6H</em>2O + ext{Energy}$
• Outcome: Produces 36 or 38 ATP molecules from one glucose molecule.

Stages of Cellular (Aerobic) Respiration

• Glucose Uptake: Insulin facilitates glucose entry into cells, promoting glucose uptake via facilitated diffusion.
• Glycolysis: Occurs in the cytosol, breaking down glucose to 2 pyruvate molecules, producing 2 ATP and 2 NADH; this process is anaerobic.
• Transitional Step: Remnants of glucose modified in the mitochondria for the Krebs cycle; no ATP produced here yet energy is stored.
• Krebs Cycle: Pyruvate is further broken down, generating 2 ATP and high-energy coenzymes (NADH, FADH₂).
• Electron Transport Chain (ETC): ATP production (32-34 ATP) occurs on the inner mitochondrial membrane utilizing energy from stored coenzymes.

Overall Steps of Carbohydrate Metabolism

• Pathway Summary: All food carbohydrates eventually converted to glucose; cellular respiration then processes glucose with oxygen for ATP production, yielding water and carbon dioxide as byproducts.
• Order of Steps: Glycolysis → Acetyl CoA → Citric Acid Cycle → Electron Transport Chain.

Summary of Carbohydrate Metabolism

• Stage Specifics:
  

• Products per Glucose Molecule:
• Glycolysis: 2 H₂O, 2 ATP, 2 NADH + H⁺
• Acetyl-CoA production: 2 CO₂, 2 NADH + H⁺
• Krebs Cycle: 4 CO₂, 2 ATP, 2 FADH₂, 6 NADH + H⁺
• Electron Transport Chain: 6 H₂O, 34 ATP

Glycolysis

• Definition: Pathway where glucose is broken down into energy via enzymatic reactions.
• Process Summary: 10 chemical steps producing 2 pyruvic acid molecules; occurs anaerobically in the cytosol.
• Inputs: Glucose (6 carbons)
• Outputs: 2 pyruvic acid (3 carbons each), 2 NADH + H⁺, Net gain: 2 ATP

Aerobic Phase Entry

• Process: If oxygen is present, pyruvic acid enters transitional phase leading to Krebs cycle, each glucose produces two pyruvic acid molecules, doubling the steps.

Transitional Phase

• Process: Each pyruvate is converted to acetyl-CoA in mitochondrial matrix (removal of CO₂ and addition of CoA).
• Outcome: Produces 2 NADH and doesn't create ATP.

Krebs Cycle

• Location: Mitochondrial matrix.
• Function: Produces 2 ATP, generates NADH and FADH₂ for the ETC; runs twice per glucose molecule.

Summary of Krebs Cycle Products

• Inputs: 2 acetyl-CoA
• Outputs: 1 oxaloacetic acid, 6 NADH + H⁺, 2 FADH₂, 4 CO₂, 2 ATP

Electron Transport Chain and Oxidative Phosphorylation

• Location: Inner mitochondrial membrane.
• Mechanism: Involves pumping H⁺ ions to create a gradient, driving ATP synthase to produce ATP.
• ATP Yield: Each NADH generates ~3 ATP; FADH₂ generates ~2 ATP.
• Final Step: Oxygen acts as the final electron acceptor in the ETC, forming water upon combining with excess H⁺.

Summary Table of ATP Production from One Glucose Molecule

• Glycolysis: 2 ATP
• Acetyl-CoA Production: 0 ATP
• Citric Acid Cycle: 2 ATP
• Electron Transport Chain: 28 ATP
• Total: 32 ATP

Other Types of Carbohydrate Metabolism

• Glycogenesis: Formation of glycogen from glucose, stimulated by insulin.
• Glycogenolysis: Breakdown of glycogen into glucose when blood glucose is low, stimulated by glucagon.
• Gluconeogenesis: Formation of new glucose from non-carbohydrate sources; occurs mainly in the liver; triggered by low blood sugar situations.

Summary of Carbohydrate Reactions

• Glycolysis: Converts glucose to pyruvic acid, produces ATP without oxygen.
• Glycogenesis: Converts glucose to glycogen for storage.
• Glycogenolysis: Converts glycogen back to glucose in low energy states.
• Gluconeogenesis: Synthesizes new glucose from non-carbohydrate sources to maintain blood sugar levels.

Edition Notes

• Last Updated: 7/15/25 - Checked for accuracy and clarity.
• Contact for Corrections: Stephen Taylor (stephen.taylor@dtcc.edu)
• Credits Include: Contributions from multiple individuals involved in course development and editing.