Comprehensive Notes: ATP Production and Cellular Respiration

Learning Outcomes

  • Explain how energy is harvested from glucose via oxidation.

  • Explain how NADH is formed.

  • Explain how NADH oxidation provides the energy for the ion gradient that powers ATP synthase.

ATP and Energy Currency

  • ATP is a high-energy molecule that can be made by harvesting energy from other sources.

Mitochondrial ATP Synthesis and H+ Gradient

  • ATP Synthase uses a hydrogen ion (H+) gradient to induce allostery.

  • The H+ gradient is generated by other processes in the mitochondria (and chloroplasts in plants).

  • ATP Synthase takes in H+ ions from the gradient; H+ binding generates allostery, enabling catalysis.

Pathways to the Citric Acid (Krebs) Cycle

  • The C atoms entering the cycle can come from multiple sources:

    • Amino acids: carbons converted to pyruvate and acetyl-CoA.

    • Fatty acids: oxidized to acetyl-CoA.

    • Glucose: oxidized to pyruvate, which can be converted to acetyl-CoA.

  • These pathways funnel carbon skeletons into the Citric Acid Cycle.

  • Reference: Alberts, ECB, 5th edition, Chapter 13.

Stepwise Concept: Generation of Pyruvate (Glycolysis Overview)

  • Step 1: Generate pyruvate from glucose via glycolysis (overview).

  • Conceptual contrast:

    • DIRECT BURNING OF SUGAR (nonliving systems): large activation energy, all free energy released as heat, no stored energy.

    • STEPWISE OXIDATION OF SUGAR IN CELLS (living systems): small activation energies overcome by enzymes at body temperature; some free energy stored in activated carriers (e.g., NADH, ATP).

  • Free energy stored in activated carriers enables energy capture and transport within the cell.

  • Source: Alberts, ECB, 5th edition, Chapter 13.

Stepwise Glycolysis: Pathway Overview (Key Steps 1–5, 6–10 summarized)

  • Step 1

    • Glucose is phosphorylated by hexokinase using ATP to form glucose-6-phosphate (G6P).

    • The negative charge of the phosphate traps glucose inside the cell and prevents its passage through the plasma membrane.

  • Step 2

    • Isomerization of glucose-6-phosphate to fructose-6-phosphate (F6P).

  • Step 3

    • Phosphofructokinase (PFK) phosphorylates fructose-6-phosphate to form fructose 1,6-bisphosphate; this step helps regulate glycolysis (controls entry into glycolytic pathway).

  • Step 4

    • Cleavage of fructose 1,6-bisphosphate into two three-carbon sugars: glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP).

  • Step 5

    • DHAP is isomerized to GAP, so two molecules of GAP proceed to the oxidation/energy-generation phase.

  • Step 6

    • Oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate with NAD+ reduced to NADH and inorganic phosphate incorporated; this is where high-energy electrons are first captured as NADH.

    • Reaction example (illustrative): extNAD++extGAP<br>ightarrowextNADH+extH++ext1,3−bisphosphoglycerate.ext{NAD}^+ + ext{GAP} <br>ightarrow ext{NADH} + ext{H}^+ + ext{1,3-bisphosphoglycerate}.

  • Step 7

    • Transfer of the high-energy phosphate to ADP to form ATP (substrate-level phosphorylation).

  • Step 8

    • Conversion of 3-phosphoglycerate to 2-phosphoglycerate.

  • Step 9

    • Dehydration to phosphoenolpyruvate (PEP).

  • Step 10

    • Transfer of the high-energy phosphate from PEP to ADP to form ATP, yielding pyruvate.

  • Overall note: There are two molecules of GAP generated in steps 4–5 for each glucose; this leads to two NADH and two ATP produced during glycolysis (via steps 6, 7, and 10).

  • Visuals reference: Alberts, ECB, 5th edition, Chapter 13.

Glycolysis: Step-by-Step Details (Diagrammatic Summary)

  • Step 1: Glucose + ATP → Glucose-6-phosphate (via hexokinase); trapped inside cell.

  • Step 2: Glucose-6-phosphate ⇄ Fructose-6-phosphate (isomerization).

  • Step 3: Fructose-6-phosphate + ATP → Fructose 1,6-bisphosphate (via phosphofructokinase).

  • Step 4: Fructose 1,6-bisphosphate → Glyceraldehyde-3-phosphate + DHAP (cleavage).

  • Step 5: DHAP ⇄ Glyceraldehyde-3-phosphate (isomerization).

  • Step 6: Glyceraldehyde-3-phosphate + NAD+ + Pi → 1,3-bisphosphoglycerate + NADH + H+.

  • Step 7: 1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP.

  • Step 8: 3-phosphoglycerate → 2-phosphoglycerate.

  • Step 9: 2-phosphoglycerate → Phosphoenolpyruvate (PEP).

  • Step 10: PEP + ADP → Pyruvate + ATP.

  • Outcome (per glucose): two molecules of GAP proceed through glycolysis; NADH produced at step 6; ATP produced at steps 7 and 10; overall energy investment in steps 1–3 is recouped in steps 7–10.

  • Source: Alberts, ECB, 5th edition, Chapter 13.

Citric Acid Cycle (Krebs): Steps, Outputs, and Net Results

  • Step 1 (Condensation): Acetyl-CoA combines with oxaloacetate to form citrate (citrate synthase).

  • Step 2 (Isomerization): Citrate ⇄ isocitrate via aconitase.

  • Step 3 (Oxidative Decarboxylation): Isocitrate dehydrogenase converts isocitrate to α-ketoglutarate with release of CO₂ and formation of NADH.

  • Step 4 (Oxidative Decarboxylation): α-Ketoglutarate dehydrogenase converts α-ketoglutarate to succinyl-CoA with release of CO₂ and formation of NADH.

  • Step 5 (Substrate-level phosphorylation): Succinyl-CoA synthetase converts succinyl-CoA to succinate, generating GTP (or ATP, depending on organism).

  • Step 6 (Oxidation): Succinate dehydrogenase converts succinate to fumarate, producing FADH₂.

  • Step 7 (Hydration): Fumarate is converted to malate by fumarase.

  • Step 8 (Oxidation): Malate dehydrogenase converts malate to oxaloacetate, generating NADH.

  • Net result of one turn of the cycle: 3 NADH, 1 GTP, 1 FADH<em>2, and 2 CO</em>2.3\ \text{NADH},\ 1\ \text{GTP},\ 1\ \text{FADH}<em>2,\ \text{and } 2\ \text{CO}</em>2. (oxaloacetate is regenerated to continue the cycle).

  • Context: The cycle uses acetyl-CoA derived from pyruvate and various carbon skeletons; per turn it also releases CO₂.

NADH and FADH2: Carriers to the Electron Transport Chain

  • NADH and FADH2 carry electrons to the ETC.

  • They donate electrons, which are used to pump protons across the inner mitochondrial membrane, establishing the proton gradient.

  • The ETC culminates in the reduction of O2 to H2O (terminal electron acceptor).

  • ETC and redox reactions link catabolism of glucose to ATP production.

  • Slide reference: Alberts, ECB, 5th edition, Chapter 14.

Mitochondrial Architecture and the Proton Gradient

  • Mitochondria have two membranes: outer and inner membranes.

  • The inner membrane creates the intermembrane space and the matrix compartment.

  • A voltage gradient across the inner membrane, plus a pH gradient, constitutes the proton-motive force that drives ATP synthesis and transport processes.

  • The gradient drives:

    • ADP + Pi to ATP via ATP synthase and ADP/ATP exchange across the inner membrane (driven by the voltage component).

    • Import of pyruvate into mitochondria and phosphate import driven by the proton gradient.

  • Diagrammatic note: Intermembrane space is proton-rich; matrix is proton-poor relative to the intermembrane space.

  • Transporters mentioned include ADP/ATP exchanger and phosphate import mechanisms.

  • Source references: general pulse from slide content; Alberts text corroborates mitochondrial structure.

ATP Synthase Mechanism and Allostery

  • ATP Synthase binds H+ from the gradient, which induces allosteric conformational changes in the enzyme.

  • The turbine-like rotational mechanism couples proton flow to the synthesis of ATP.

  • Consequences of allostery:

    • Couples the energy of the gradient to ATP synthesis.

    • Lowers the activation energy of the ATP-forming reaction.

  • The entire ATP synthase is a single polypeptide complex capable of coordinated conformational changes.

  • Source: Alberts, ECB, 5th edition, Chapter 4.

Redox Reactions and Energy Harvesting

  • Redox reactions are central to harvesting energy from high-energy electrons.

  • Through a series of redox steps, NADH and FADH2 donate electrons to the ETC, powering proton pumping and the creation of the H+ gradient.

  • This redox-driven gradient ultimately powers ATP synthase to produce ATP.

Connections to Foundational Principles and Real-World Relevance

  • Central principle: Energy flow in cells is governed by oxidation of energy-rich substrates (glucose) to NADH/FADH2, transfer of electrons through the ETC, and generation of a proton gradient that drives ATP synthesis.

  • The glycolytic steps illustrate regulation (e.g., phosphofructokinase as a regulatory checkpoint) to balance energy needs and substrate availability.

  • The integration of glycolysis, the pyruvate dehydrogenase step, the Citric Acid Cycle, and the Electron Transport Chain demonstrates metabolic coupling and high-energy yield via substrate-level and oxidative phosphorylation.

  • Practical implications include understanding energy balance, metabolic regulation, and how disruptions can impact cellular energy production.

Key Equations and Data

  • Glyceraldehyde-3-phosphate dehydrogenase reaction (Step 6 of glycolysis):
    NAD++GAP→NADH+H++1,3-bisphosphoglycerate\text{NAD}^+ + \text{GAP} \rightarrow \text{NADH} + \text{H}^+ + \text{1,3-bisphosphoglycerate}

  • ATP generation in glycolysis (Steps 7 and 10): substrate-level phosphorylation of ADP to ATP.

  • Net yield per turn of the Citric Acid Cycle:
    3 NADH, 1 GTP, 1 FADH<em>2, 2 CO</em>23\ \mathrm{NADH},\ 1\ \mathrm{GTP},\ 1\ \mathrm{FADH<em>2},\ 2\ \mathrm{CO</em>2}

  • Net yield per glucose in glycolysis (conceptual from Steps 6–10 and two GAPs):

    • NADH produced: 2 NADH2\ \mathrm{NADH}

    • ATP produced: 2 ATP2\ \mathrm{ATP} (via Steps 7 and 10)

  • Conceptual note on energy flow: energy captured as activated carriers (NADH, ATP) enables subsequent ATP production via the electron transport chain and oxidative phosphorylation.

References

  • Alberts, ECB, 5th edition, various chapters as cited in the slide deck (Chapters 4, 13, 14).