ATP: The Energy Currency of the Cell

Introduction to ATP

ATP (adenosine triphosphate) is the primary energy carrier in all living organisms. It stores and supplies the energy needed for many biochemical cellular processes.

• ATP is used to power most cellular activities, including biosynthesis, transport, and mechanical work.

• Energy is consumed or produced in metabolic pathways, usually in the form of ATP.

• Most chemical reactions in cells occur through a series of steps called a metabolic pathway.

Structure and Hydrolysis of ATP

ATP is composed of three main components: a ribose sugar, an adenine nitrogenous base, and three phosphate groups.

• Hydrolysis of ATP releases energy by breaking the bond between the terminal phosphate group, forming ADP (adenosine diphosphate) and inorganic phosphate ().

• The reaction:

• This process is highly exergonic, with a standard free energy change () of approximately .

Exergonic and Endergonic Reactions

ATP hydrolysis is an example of an exergonic reaction, which releases energy. Endergonic reactions require energy input.

• Exergonic reaction: ; energy is released spontaneously.

• Endergonic reaction: ; energy is required, nonspontaneous.

• ATP hydrolysis couples with endergonic reactions to drive cellular work.

ATP and Phosphorylation

ATP provides energy for cellular work via phosphorylation, the transfer of a phosphate group to another molecule.

• Phosphorylation changes the shape and activity of the recipient molecule, often making a nonspontaneous process possible.

• Types of work powered by ATP phosphorylation:

  • Transport work: Moving substances across membranes (e.g., active transport).

  • Mechanical work: Movement of motor proteins and cellular structures.

• Phosphorylation is a key mechanism by which ATP does cellular work.

Cellular Respiration: How Cells Make ATP

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules (such as glucose) to produce ATP.

• Occurs in several stages: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, and Electron Transport Chain.

• Overall reaction:

• Energy is released gradually, not all at once, allowing cells to capture it efficiently as ATP.

Stages of Cellular Respiration

Cellular respiration consists of four main stages, each contributing to ATP production.

• Glycolysis: Occurs in the cytosol; breaks down glucose into pyruvate, producing a small amount of ATP and NADH.

• Pyruvate Oxidation: Converts pyruvate into Acetyl CoA, generating NADH and releasing CO2.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondria; Acetyl CoA is oxidized, producing NADH, FADH2, ATP, and CO2.

• Electron Transport Chain and Oxidative Phosphorylation: NADH and FADH2 donate electrons to the chain, creating a proton gradient used by ATP synthase to generate ATP.

Electron Carriers: NAD+ and FAD

Electron carriers are molecules that transport electrons during cellular respiration.

• NAD+ (Nicotinamide adenine dinucleotide): Accepts electrons and becomes NADH.

• FAD (Flavin adenine dinucleotide): Accepts electrons and becomes FADH2.

• These carriers transfer electrons to the electron transport chain, facilitating ATP production.

Redox Reactions in Cellular Respiration

Cellular respiration involves many redox reactions, where electrons are transferred between molecules.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Electron donor is the reducing agent; electron acceptor is the oxidizing agent.

• Example: In the overall reaction, glucose is oxidized and oxygen is reduced.

Summary Table: Stages of Cellular Respiration

Additional info:

• Substrate-level phosphorylation refers to the direct transfer of a phosphate group to ADP to form ATP, occurring in glycolysis and the citric acid cycle.

• Oxidative phosphorylation uses the energy from electrons transferred through the electron transport chain to power ATP synthase.

• ATP synthase is a special protein in the mitochondrial membrane that synthesizes ATP as protons (H+) flow down their concentration gradient.