Cellular Respiration, Free Energy, and Metabolic Pathways: Study Notes

Concept 7.0 – Free Energy and Chemical Equilibrium

Free Energy and Chemical Equilibrium

Understanding free energy and chemical equilibrium is essential for grasping how biological reactions proceed and how cells regulate metabolic processes.

• Free Energy (G): The energy in a system available to do work. Changes in free energy ($\Delta G$) determine whether a reaction is spontaneous.

• Chemical Equilibrium: The state in which the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant.

• Direction and Rate of Reactions: The concentrations of reactants and products, as well as their free energies, influence the direction and rate of chemical reactions.

• Equilibrium Constant (Keq): Indicates the ratio of product to reactant concentrations at equilibrium.

• Cellular Reactions: Cells prevent reactions from reaching equilibrium by removing products or adding reactants, allowing continuous metabolic activity.

Equation:

$\Delta G = \Delta H - T\Delta S$

Where $\Delta G$ is the change in free energy, $\Delta H$ is the change in enthalpy, $T$ is temperature in Kelvin, and $\Delta S$ is the change in entropy.

Concept 7.1 – Catabolic Pathways Oxidize Organic Fuels

Overview of Catabolic Pathways

Catabolic pathways break down organic molecules, releasing energy that cells capture for work, primarily through cellular respiration.

• Cellular Respiration: The process by which cells extract energy from organic molecules, such as glucose, using oxygen.

• General Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP + heat)}$

• Energy Release: Energy is released during oxidation reactions and captured in ATP.

• Types of Redox Reactions: Includes combustion, respiration, photosynthesis, and electron transport.

• NAD+ and NADH: NAD+ acts as an electron carrier, accepting electrons during catabolism and becoming NADH.

• Major Stages of Cellular Respiration: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, and Oxidative Phosphorylation.

• Mitochondrial Structure: The inner membrane contains proteins for electron transport and ATP synthesis; the matrix hosts enzymes for the citric acid cycle.

• ATP Generation: Substrate-level phosphorylation and oxidative phosphorylation are the main mechanisms.

Example: During glycolysis, glucose is split into two pyruvate molecules, producing ATP and NADH.

Concepts 7.2 / 7.3 / 7.4 – Cellular Respiration Stages

Stages of Cellular Respiration

Cellular respiration consists of several stages, each contributing to the production of ATP and the flow of energy in the cell.

• Glycolysis: Occurs in the cytoplasm; breaks glucose into pyruvate, producing ATP and NADH.

• Pyruvate Oxidation: Converts pyruvate to acetyl-CoA, generating NADH and releasing CO2.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose, producing ATP, NADH, and FADH2.

• Oxidative Phosphorylation: Includes electron transport chain and chemiosmosis; occurs in the inner mitochondrial membrane, producing most of the cell's ATP.

Key Events and Energy Changes:

• Electron carriers (NADH, FADH2) donate electrons to the electron transport chain.

• Proton gradient is established across the inner mitochondrial membrane.

• ATP synthase uses the proton gradient to synthesize ATP.

Equation for ATP Yield:

$\text{Total ATP} \approx 30-32$ per glucose molecule

Contribution to ATP Generation:

• Glycolysis: 2 ATP (substrate-level phosphorylation)

• Citric Acid Cycle: 2 ATP (substrate-level phosphorylation)

• Oxidative Phosphorylation: 26-28 ATP

Concept 7.5 – ATP Production Without Oxygen

Anaerobic Pathways and Fermentation

Cells can generate ATP without oxygen through fermentation or anaerobic respiration, though less efficiently than aerobic respiration.

• Fermentation: Glycolysis followed by conversion of pyruvate to lactate or ethanol, regenerating NAD+ for continued glycolysis.

• Anaerobic Respiration: Uses electron acceptors other than oxygen (e.g., nitrate, sulfate).

• ATP Yield: Only 2 ATP per glucose (from glycolysis).

• Limitation: Fermentation is less efficient and occurs when oxygen is scarce.

Example: Muscle cells perform lactic acid fermentation during intense exercise when oxygen is limited.

Concept 7.6 – Connections to Other Metabolic Pathways

Integration of Metabolic Pathways

Cellular respiration is interconnected with other metabolic pathways, allowing cells to adapt to changing energy demands and nutrient availability.

• Catabolic and Anabolic Pathways: Intermediates from cellular respiration can be used for biosynthesis of amino acids, nucleotides, and lipids.

• Regulation: Altering one aspect of a pathway (e.g., enzyme inhibition) can affect upstream and downstream reactions.

• Metabolic Flexibility: Cells can switch between pathways depending on nutrient availability and energy needs.

Example: Acetyl-CoA from fatty acid breakdown can enter the citric acid cycle for energy production.

Stage	Main Location	ATP Yield	Key Products
Glycolysis	Cytoplasm	2 ATP	2 Pyruvate, 2 NADH
Pyruvate Oxidation	Mitochondrial Matrix	0 ATP	2 Acetyl-CoA, 2 NADH, 2 CO2
Citric Acid Cycle	Mitochondrial Matrix	2 ATP	6 NADH, 2 FADH2, 4 CO2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	26-28 ATP	H2O, ATP

Additional info: Academic context and explanations have been expanded for clarity and completeness.