Biology Energy Processes and ATP

Energy

• Energy is essential for multiple processes in living organisms, including anabolic reactions like protein synthesis and DNA replication.
• Active transport mechanisms, such as the sodium-potassium pump, rely on energy.
• Mechanical functions, including muscle contraction and movement via flagella or cilia, require energy.
• Organisms use energy to maintain homeostasis, support bioluminescence, and transmit nerve impulses.

ATP as Energy Currency

• ATP (adenosine triphosphate) hydrolysis releases energy through the conversion to ADP (adenosine diphosphate) and AMP (adenosine monophosphate).
• Energy release from ATP hydrolysis is characterized by the values: 30.6 kJ mol⁻¹ for ATP to ADP, 30.6 kJ mol⁻¹ for ADP to AMP, and 14.2 kJ mol⁻¹ for AMP to adenosine.
• ATP is small, water-soluble, and easily transported within cells, making it an immediate energy source.
• Phosphate (Pi) is an effective leaving group, allowing ATP synthesis via ATP synthase to be reversible and efficient.
• ATP maintains a high turnover rate and acts as a link between anabolic (energy-consuming) and catabolic (energy-releasing) reactions.

ATP Synthesis

• ATP can be synthesized through glycolysis and the Krebs cycle, with oxidative phosphorylation occurring during aerobic respiration.
• Only oxidative phosphorylation necessitates oxygen, which combines with electrons and protons in the final electron acceptor stage of respiration.
• Absence of the electron transport chain (ETC) prevents chemiosmosis and ATP synthesis.

Respiration Overview

• Respiration processes utilize organic molecules as fuel, proceeding through glycolysis, link reaction, Krebs cycle, and oxidative phosphorylation.

Glycolysis

• Glycolysis breaks down glucose to form two molecules of pyruvate and occurs in the cytoplasm.
• The initial step involves the phosphorylation of glucose using 2 ATP to provide activation energy.
• The phosphorylation process involves conversions from glucose to glucose phosphate, then to fructose phosphate, and ultimately onto fructose bisphosphate.

Link Reaction

• Takes place in the mitochondrion matrix, requiring active transport of pyruvate into the matrix.
• Decarboxylation removes CO₂, and dehydrogenation removes hydrogen.
• Generates a net gain of 2 ATP and 2 reduced NAD from 2 pyruvate molecules.

Coenzyme A

• Coenzyme A, composed of a nucleoside and vitamin, carries acetyl groups (CH3-CO) into the Krebs cycle.

Krebs Cycle

• The Krebs cycle is a closed pathway of enzyme-controlled reactions in the mitochondrial matrix.
• Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), resulting in the release of CO₂ and reduction of NAD and FAD.
• Oxaloacetate is regenerated to perpetuate the cycle, and the net gain of CO₂ and reduced NAD is double that from one cycle due to two pyruvate molecules.

Oxidative Phosphorylation and Electron Transport Chain (ETC)

• This stage occurs in the inner mitochondrial membrane (cristae) and involves chemiosmosis.
• Reduced NAD and FAD donate hydrogen atoms to the ETC, splitting into protons (H⁺) and electrons (e⁻).
• Electrons move down an energy gradient, facilitating the pumping of H⁺ ions from the matrix to the intermembrane space, creating a proton gradient.
• Protons flow back into the matrix through ATP synthase, generating ATP from ADP and Pi via electrical potential energy.
• Oxygen serves as the final electron acceptor, combining with electrons and protons to form water (H₂O).