Central Metabolism, Thermodynamics, and Cellular Respiration Study Guide

Central Metabolism and Thermodynamics

Thermodynamics in Biological Systems

Thermodynamics governs the energy transformations that occur in biological systems. Understanding the laws of thermodynamics is essential for explaining how cells obtain and use energy.

• 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.

• Coupling Reactions: Cells drive unfavorable reactions by coupling them with favorable ones, often using ATP hydrolysis.

• "Rules of Thumb" for Reaction Favorability: Reactions tend to proceed when they result in lower free energy and increased entropy.

Example: The hydrolysis of ATP to ADP and Pi is highly exergonic and is used to drive many endergonic cellular processes.

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) is the primary energy carrier in cells. Its hydrolysis releases energy that can be used to power cellular work.

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

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Cellular Functions: ATP is used in muscle contraction, active transport, and biosynthetic reactions.

Example: ATP powers the sodium-potassium pump, maintaining ion gradients across membranes.

Catabolism and Anabolism

Metabolism consists of catabolic (breakdown) and anabolic (biosynthetic) pathways. Catabolism releases energy, while anabolism consumes energy.

• Catabolism: Breakdown of molecules to release energy (e.g., glycolysis, citric acid cycle).

• Anabolism: Synthesis of complex molecules from simpler ones (e.g., protein synthesis).

• Relation to Thermodynamics: Catabolic reactions are generally exergonic; anabolic reactions are endergonic and require energy input.

Redox Reactions and Electron Carriers

Redox reactions involve the transfer of electrons between molecules, which is central to cellular respiration and energy production.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron Carriers: Molecules like NAD+, FAD, and NADP+ shuttle electrons during metabolic reactions.

• Thermodynamic Favorability: Redox reactions proceed spontaneously when electrons move from molecules with lower to higher reduction potential.

Example: NAD+ is reduced to NADH during glycolysis and the citric acid cycle.

Glycolysis and Cellular Respiration

Glycolysis

Glycolysis is the first step in cellular respiration, converting glucose into pyruvate and generating ATP and NADH.

• Location: Cytoplasm of the cell.

• Key Steps: Glucose is phosphorylated, split into two three-carbon molecules, and converted to pyruvate.

• Products: 2 ATP (net), 2 NADH, 2 pyruvate per glucose molecule.

Example: Glycolysis provides energy for cells in both aerobic and anaerobic conditions.

Pyruvate and the Citric Acid Cycle

Pyruvate produced in glycolysis is transported into mitochondria and converted to acetyl-CoA, which enters the citric acid cycle (Krebs cycle).

• Pyruvate to Acetyl-CoA: Pyruvate is oxidized, producing NADH and releasing CO2.

• Citric Acid Cycle: Acetyl-CoA is oxidized, generating NADH, FADH2, and ATP (or GTP).

• Location: Mitochondrial matrix.

• Key Steps: Entry points for carbon, electron carriers are reduced, and CO2 is released.

Example: The citric acid cycle is central to energy production and provides intermediates for biosynthesis.

Electron Transport Chain (ETC) and ATP Synthesis

The electron transport chain uses electrons from NADH and FADH2 to create a proton gradient across the inner mitochondrial membrane, driving ATP synthesis.

• Key Players: Complexes I-IV, cytochrome c, ubiquinone.

• Oxygen: The ultimate electron acceptor, forming water.

• ATP Synthase: Uses the proton gradient to synthesize ATP from ADP and Pi.

• Flow of Electrons: Electrons move from carriers with lower to higher reduction potential.

Example: Most cellular ATP is produced by oxidative phosphorylation in the mitochondria.

Metabolic Pathways and Regulation

Metabolic pathways are interconnected and regulated to meet cellular energy demands.

• Metabolic Substrate Map: Shows how metabolites flow through glycolysis, citric acid cycle, and other pathways.

• Major Themes: Interconnectedness, regulation, compartmentalization.

• Regulation: Enzymes are regulated by feedback inhibition, allosteric control, and substrate availability.

Tables

Comparison of Catabolism and Anabolism

Process	Function	Energy Change	Examples
Catabolism	Breakdown of molecules	Releases energy	Glycolysis, Citric Acid Cycle
Anabolism	Synthesis of molecules	Consumes energy	Protein synthesis, DNA replication

Major Electron Carriers in Cellular Respiration

Carrier	Oxidized Form	Reduced Form	Role
NAD+	NAD+	NADH	Transfers electrons to ETC
FAD	FAD	FADH2	Transfers electrons to ETC

Additional info:

• Metabolic pathways are highly regulated to ensure efficient energy production and biosynthesis.

• Compartmentalization within cells (e.g., mitochondria) allows for separation and regulation of metabolic processes.

• Reduction potential determines the direction of electron flow in the ETC.