Vocabulary for module 23

exergonic reaction

A chemical reaction with a net release of energy.

endergonic reaction

A chemical reaction with a net requirement for input of energy.

Recognize chemical reactions that require ATP hydrolysis.

ATP can drive other reactions because its hydrolysis is exergonic and releases energy. Therefore, the hydrolysis of ATP is spontaneous, having a negative free energy change (-ΔG). Spontaneous reactions can drive non-spontaneous reactions if they are coupled together.

Why does ATP hydrolysis release a lot of free energy?

ATP is unstable compared to its products.

phosphoanhydride bond

Covalent bond that links two phosphate groups to one another. Two are found in ATP. Hydrolysis of one of these bonds releases 7.3 kcal/mol of energy.

Describe the role of ATP in energy-coupling reactions.

The ATP molecule is composed of a nitrogenous base called adenine, a ribose sugar and a chain of three phosphate groups. Having three phosphate groups makes ATP relatively unstable when compared to ADP and inorganic phosphate (Pi or HOPO32-), which are the products of its hydrolysis. This difference in stability between products and reactants causes ATP hydrolysis to release a large amount of energy, which can be coupled to energetically unfavorable reactions in the cell.

ATP hydrolysis

The exergonic chemical reaction in which the combination of ATP with water results in the release of an inorganic phosphate ion and ADP.

adenosine triphosphate (ATP)

The primary energy currency of the cell; composed of the nitrogenous base adenine, the 5-carbon sugar ribose and three phosphate groups.

Identify the structural features found in ATP.

a nitrogenous base, a ribose sugar, and three phosphate groups

Complete the following sentence: The hydrolysis of one mole of ATP can be used to drive reactions that have a ΔG that is...

less than 7.3 kcal/mol. Correct This answer is correct because the hydrolysis of ATP has a ΔG = -7.3 kcal/mol. It can only drive reactions that require the input of less energy than is released.

The sodium-potassium pump is an active transport pump that uses energy to pump potassium into cells and sodium out of cells. Why is ATP energy required?

to pump the Na+ and K+ ions against their diffusion gradient

Explain how ATP hydrolysis performs cellular work.

There are three main types of cellular work: chemical, mechanical, and transport. For example, ATP can be used to provide the mechanical work that powers DNA helicase to separate double-stranded DNA into single-stranded DNA during DNA replication. One of the most vital roles of ATP is the coupling of ATP hydrolysis to active transport of ions across cell membranes.

energy coupling

Transfer of energy from one chemical reaction to another process.

ATP is not the only molecule that can drive reactions. When ATP is depleted during exercise, muscle cells use phosphocreatine to drive the regeneration of ATP. Consider the following half reactions: 1. ATP + H2O → ADP + Pi ΔG = -7.3 kcal/mol 2. Phosphocreatine + H2O → creatine + Pi ΔG = -10.3 kcal/mol From these two reactions, calculate the Gibbs free energy of the following coupled reaction, catalyzed by creatine kinase: Phosphocreatine + ADP → ATP + creatine ΔG = ? Which is the correct net Gibbs free energy of the reaction?

ΔG = -3 kcal/mol This answer is correct because net ΔG = +7.3 + (-10.3) = -3 kcal/mol. The ATP is being formed, not hydrolyzed, which reverses the negative sign. The difference in Gibbs free energy is then calculated by adding the Gibbs free energy of the two half reactions.