Metabolism and Energy: Thermodynamics, ATP, Enzymes, and Cellular Respiration

Metabolism and Energy

8.1. Thermodynamics and Metabolism

Metabolism encompasses all chemical reactions occurring within a cell, enabling it to maintain life, grow, and reproduce. These reactions are governed by the laws of thermodynamics.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

• Metabolism: The sum of all chemical reactions in a cell, divided into catabolic (breakdown, releasing energy) and anabolic (building, consuming energy) pathways.

• Kinetic Energy: Energy of motion (e.g., movement of molecules).

• Potential Energy: Stored energy due to position or structure (e.g., chemical bonds).

• Spontaneous Processes: Occur without energy input; increase entropy.

• Nonspontaneous Processes: Require energy input; decrease entropy locally.

8.2. Free Energy and Cellular Work

Free energy (G) is a measure of a system's capacity to do work. Changes in free energy determine whether a reaction is spontaneous.

• Gibbs Free Energy (G): The energy in a system available to do work at constant temperature and pressure.

• Spontaneous Reactions: Occur when the change in free energy ($\Delta G$) is negative.

• Exergonic Reactions: Release energy; $\Delta G < 0$.

• Endergonic Reactions: Require energy input; $\Delta G > 0$.

• Types of Cellular Work:

  • Chemical work: Synthesis of macromolecules.

  • Transport work: Pumping substances across membranes.

  • Mechanical work: Movement (e.g., muscle contraction, chromosome movement).

• Energy Coupling: Cells couple exergonic and endergonic reactions, often using ATP as an intermediary.

Equation for Free Energy Change:

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

where $\Delta H$ is change in enthalpy, $T$ is temperature (Kelvin), and $\Delta S$ is change in entropy.

8.3. ATP: Structure and Function

ATP (adenosine triphosphate) is the primary energy currency of the cell, driving most endergonic reactions.

• Structure of ATP: Composed of adenine, ribose, and three phosphate groups.

• ATP Hydrolysis: The terminal phosphate bond is broken, releasing energy.

• ATP Couples to Endergonic Reactions: The energy released from ATP hydrolysis is used to drive cellular work.

ATP Hydrolysis Reaction:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

8.4. Enzymes and Activation Energy

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy required for reactions to proceed.

• Enzyme Specificity: Determined by the enzyme's three-dimensional structure and active site.

• Activation Energy (Ea): The initial energy input required to start a reaction.

• Enzymes: Do not change the overall $\Delta G$ of a reaction, but lower Ea to increase reaction rate.

8.5. Regulation of Enzyme Activity

Cells regulate enzyme activity to control metabolic pathways.

• Allosteric Regulation: Regulatory molecules bind to sites other than the active site, causing conformational changes that inhibit or stimulate enzyme activity.

• Inhibition: Allosteric inhibitors decrease enzyme activity.

• Stimulation: Allosteric activators increase enzyme activity.

9.1. Redox Reactions and Cellular Respiration

Catabolic pathways yield energy by oxidizing organic fuels, primarily through redox reactions.

• Redox Reactions: Involve the transfer of electrons; oxidation is loss of electrons, reduction is gain of electrons.

• Cellular Respiration Equation:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{energy}$

• NAD+: An electron carrier that cycles between oxidized (NAD+) and reduced (NADH) states.

• Electron Transport Chain (ETC): Series of protein complexes that transfer electrons, ultimately producing ATP.

• Stages of Respiration:

  1. Glycolysis (cytoplasm)

  2. Pyruvate oxidation (mitochondrial matrix)

  3. Citric acid cycle (mitochondrial matrix)

  4. Oxidative phosphorylation (inner mitochondrial membrane)

• Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP to form ATP.

• Oxidative Phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis.

9.2. Glycolysis

Glycolysis is the first stage of cellular respiration, breaking down glucose into pyruvate.

• Process: One 6-carbon glucose is split into two 3-carbon pyruvates.

• ATP Investment: ATP is required in the preparatory steps to phosphorylate glucose intermediates.

• Substrate-Level Phosphorylation: ATP is generated directly in glycolysis.

• Reduction of NAD+: Electrons are transferred to NAD+, forming NADH.

9.3. Oxidation of Pyruvate and the Citric Acid Cycle

Pyruvate is further oxidized in the mitochondria, linking glycolysis to the citric acid cycle.

• Location: Pyruvate oxidation and the citric acid cycle occur in the mitochondrial matrix.

• Linking Step: Pyruvate is converted to acetyl-CoA, which enters the citric acid cycle.

• Citric Acid Cycle: Completes the oxidation of glucose derivatives, producing NADH, FADH2, ATP, and CO2.

• Cycle Nature: The cycle regenerates oxaloacetate, allowing continuous operation.

9.4. Oxidative Phosphorylation, Electron Transport Chain, and Chemiosmosis

Most ATP is produced during oxidative phosphorylation, which involves the electron transport chain and chemiosmosis.

• Electron Transport Chain: Electrons from NADH and FADH2 are transferred through protein complexes, releasing energy.

• Proton Gradient: The ETC pumps protons into the intermembrane space, creating a proton motive force.

• Chemiosmosis: Protons flow back through ATP synthase, driving ATP production.

• ATP Yield: Oxidative phosphorylation produces the majority of ATP from glucose oxidation.

Types of Phosphorylation:

Type	Location	ATP Yield
Substrate-Level	Cytoplasm, Mitochondrial Matrix	Small (2-4 ATP)
Oxidative	Inner Mitochondrial Membrane	Large (about 26-28 ATP)

9.6. Oxidation of Other Food Molecules

Cells can oxidize molecules other than glucose (such as fats and proteins) to generate ATP.

• Fats: Broken down into glycerol and fatty acids; fatty acids enter the citric acid cycle as acetyl-CoA.

• Proteins: Deaminated and converted into intermediates of glycolysis or the citric acid cycle.

Example: Beta-oxidation of fatty acids produces acetyl-CoA, NADH, and FADH2 for ATP production.

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