Energy, Chemical Reactions, and Cellular Respiration

3.1 Energy: Key Concepts

• Energy exists as Potential (stored, position) or Kinetic (motion).
• Potential energy converts to kinetic energy to do work.
• Example: Na+ ions have potential energy from their concentration gradient; moving down the gradient shows kinetic energy.

3.1b Forms of Energy

• Kinetic energy forms: Electric (charged particles), Mechanical (objects' motion), Sound (vibration), Radiant (electromagnetic waves; e.g., visible light $-[400 \text{ nm to } 700 \text{ nm}]$), Heat (random motion).

3.1c Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only converted (e.g., $\text{Energy cannot be created or destroyed; it can only be converted from one form to another.}$).
• Second Law: Every energy transformation loses some energy as heat (nonusable energy) (e.g., $\text{When energy is transformed, some energy becomes heat.}$).
• In the body, conversions generate heat, aiding homeostasis.

3.2 Changes in Chemical Structure (Energy Changes)

• Decomposition: AB ( \rightarrow ) A + B
• Synthesis: A + B ( \rightarrow ) AB
• Exchange: AB + C ( \rightarrow ) A + BC
• Redox (Oxidation–Reduction):
  • Oxidized = loses electrons; Reduced = gains electrons.
  • NAD$\text{+}$ accepts electrons to become NADH: $\text{NAD}^+ + 2e^- + \text{H}^+ \rightarrow \text{NADH}$
  • NADH oxidizes back to NAD$\text{+}$ to release energy, shuttling chemical energy from fuel (e.g., glucose) to the electron transport chain.

3.30 Mechanism of Enzyme Action

• Substrate binds to the enzyme's active site (enzyme–substrate complex).
• Enzyme changes shape (induced fit) to facilitate reaction.
• Product forms and is released; enzyme is then free.
• Example: Lactase acts on lactose to yield glucose and galactose; glycogen synthase polymerizes glucose to glycogen.

3.3e Reaction Rates: Enzyme Activity Factors

• Enzyme activity and reaction rates are affected by:
  • Substrate concentration
  • Temperature
  • pH

3.3 Inhibition: Competitive vs Noncompetitive

• No inhibitor: substrate binds active site.
• Competitive inhibition: inhibitor binds active site; substrate cannot.
• Noncompetitive (allosteric) inhibition: inhibitor binds allosteric site, changing enzyme shape; active site cannot bind substrate.

3.4 Cellular Respiration: Overview

• Multi-step pathway catabolizing organic molecules via enzymes to release energy and synthesize ATP.
• Couples exergonic bond-breaking to endergonic ATP formation.
• Requires oxygen for maximum ATP synthesis (final electron acceptor O$\text{2}$).
• Stages: Glycolysis ( \rightarrow ) Intermediate Stage ( \rightarrow ) Citric Acid Cycle ( \rightarrow ) Electron Transport System (GICES).
• Overall glucose oxidation ($\text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + 6\text{O}</em>2 \rightarrow 6\text{CO}<em>2 + 6\text{H}</em>2\text{O}$).

3.4a Overview of Glucose Oxidation

• Stepwise enzymatic catabolism of glucose releases energy for ATP synthesis (exergonic).
• Indirect method (oxidative phosphorylation): ATP from NADH/FADH$\text{2}$.
• Direct method (substrate-level phosphorylation): ATP formed directly in glycolysis and citric acid cycle.
• With oxygen: glucose is fully broken down.
• NAD$\text{+}$/NADH and FAD/FADH$\text{2}$ shuttle electrons to the ETC.

3.4b Glycolysis

• Location: cytosol.
• Net products per glucose: $2 \text{ ATP} + 2 \text{ NADH}$
• Stages: Steps 1–4 (ATP investment); Steps 5–10 (ATP and NADH payoff, yields 2 pyruvate).
• Overall (simplified): $\text{Glucose} \rightarrow 2 \text{ Pyruvate} + 2 \text{ ATP} + 2 \text{ NADH} + 2 \text{H}_2\text{O} + 2 \text{H}^+$

3.4c Intermediate Stage

• Location: mitochondrial matrix.
• Pyruvate transported into mitochondria, converted to Acetyl CoA by pyruvate dehydrogenase complex.
• Per glucose: 2 NADH produced (one per pyruvate).
• Reaction: Pyruvate ( \rightarrow ) Acetyl CoA + CO$\text{2}$; NAD$\text{+}$ reduced to NADH.

3.4d Citric Acid Cycle

• Location: mitochondrial matrix.
• Acetyl CoA enters; generates per Acetyl CoA: 1 ATP, 3 NADH, 1 FADH$\text{2}$.
• Per glucose (doubled): $2 \text{ ATP}$, $6 \text{ NADH}$, $2 \text{ FADH}_2$, and CO$\text{2}$ byproduct.

3.4e Electron Transport System (ETC)

• Location: inner mitochondrial membrane (cristae).
• NADH and FADH$\text{2}$ donate electrons; energy pumps H$\text{+}$ across membrane, creating gradient.
• Oxygen is final electron acceptor: $\text{O}<em>2 + 4\text{H}^+ + 4e^- \rightarrow 2\text{H}</em>2\text{O}$
• Proton gradient drives ATP synthase (ADP + P$\text{i}$ ( \rightarrow ) ATP).
• Net ATP yield (theoretical per glucose):
  • Substrate-level phosphorylation: $4 \text{ ATP}$
  • Oxidative phosphorylation: from NADH and FADH$\text{2}$ (≈$30 \text{–}34 \text{ ATP}$).
  • Total ATP: typically shown as $38$ (with $10 \text{ NADH} \rightarrow 30 \text{ ATP}$ and $2 \text{ FADH}_2 \rightarrow 4 \text{ ATP}$).

3.4f ATP Production Summary

• Total ATP per glucose: $38$ (some texts cite ≈30–32).
• ATP formed by:
  • Substrate-level phosphorylation: in glycolysis and citric acid cycle ($\text{ADP} + \text{P}_i \rightarrow \text{ATP}$).
  • Oxidative phosphorylation: in ETC, driven by NADH and FADH$\text{2}$ via ATP synthase.

3.4g Fate of Pyruvate with No Oxygen

• In absence of oxygen, pyruvate reduces to lactate in cytoplasm to regenerate NAD$\text{+}$ for glycolysis (fermentation).
• Reaction: $\text{Pyruvate} + \text{NADH} \rightarrow \text{Lactate} + \text{NAD}^+$
• Allows glycolysis to continue when ETC is unavailable.