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Chapter 8.2 Notes EQ
What is the role of ATP in Biochemical Energetics?
-Cells rely on adenosine triphosphate (ATP) for the capture and transfer of the free energy they need to do chemical work. ATP operates as a kind of “energy currency”. -ATP has another important role in the cell beyond its use as an energy currency: it is a nucleotide that can be converted into a building block for nucleic acids.
Figure 8.5 (a) ATP
How does ATP store energy?
ATP hydrolysis releases energy -An ATP molecule consists of the nitrogenous base adenine bonded to ribose (a sugar), which is attached to a sequence of three phosphate groups. ATP+H2O→ADP+Pi +free energy -The important property of this reaction is that it is exergonic, releasing free energy. Under standard laboratory conditions, the change in free energy for this reaction (ΔG) is about –7.3 kcal/mol (–30 kJ/mol). -Two characteristics of ATP account for the free energy released by the loss of one or two of its phosphate groups. • The free energy of the P— O bond between phosphate groups (called a phosphoric acid anhydride bond) is much higher than the energy of the O—H bond that forms after hydrolysis. So some usable energy is released by hydrolysis. • Because phosphate groups are negatively charged and so repel each other, it takes energy to get phosphates near enough to each other to make the covalent bond that links them together (e.g., to add a phosphate to ADP to make ATP). Some of this energy is conserved when the third phosphate is attached. -The production of light by living organisms is referred to as b ioluminescence. It is an example of an endergonic reaction driven by ATP hydrolysis that involves an interconversion of energy forms. luciferase luciferin + O2 + ATP ———→ oxyluciferin + AMP + PPi + light
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Figure 8.6 Coupling of Reactions
ATP couples exergonic and endergonic reactions -The reverse reaction, the formation of ATP from ADP and Pi, is endergonic and consumes as much free energy as is released by the hydrolysis of ATP. ADP+Pi +free energy→ATP+H2O What are coupled reactions?
-Many different exergonic reactions in the cell can provide the energy to convert ADP into ATP. -Coupling of exergonic and endergonic reactions is very common in metabolism. Free energy is captured and retained in the P—O bonds of ATP. -An active cell requires the production of millions of molecules of ATP per second to drive its biochemical machinery. Figure 8.7 Coupling of ATP Hydrolysis to an Endergonic Reaction
Summary:
An ATP molecule is typically consumed within a second of its formation. At rest, an average person produces and hydrolyzes about 40 kg of ATP per day—as much as some people weigh. This means that each ATP molecule undergoes about 10,000 cycles of synthesis and hydrolysis every day. ATP is synthesized and used up very rapidly. But these biochemical reactions could not proceed so rapidly without the help of enzymes.