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Chapter 6: Life’s Mainspring: An Introduction to Energy Overview

Life depends on a constant influx of energy. Organisms transform energy to carry out the processes of life. Within cells, energy is made available in the form of ATP molecules, and its use is made more efficient by enzymes.

6.1 Energy Is Central to Life

6.2 What Is Energy?

6.3 How Is Energy Used by Living Things?

6.4 The Energy Currency Molecule: ATP

6.5 Efficient Energy Use in Living Things: Enzymes

6.6 Lowering the Activation Barrier through Enzymes

6.7 Regulating Enzymatic Activity

6.1 Energy Is Central to Life

• Living things require a source of energy. The Sun is the ultimate source of energy for most living things on Earth.

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6.2 What Is Energy?

• Energy is the capacity to bring about movement against an opposing force.
• Two fundamental principles of energy are the first law of thermodynamics, which states that energy is never created or destroyed, but is only transformed; and the second law of thermodynamics, which states that energy transfer will always result in a greater amount of disorder in the universe.
• In every energy transaction, some energy will be lost to the disordered form of heat. Entropy is a measure of the amount of disorder in a system; the greater the entropy, the greater the disorder.
• Living things can bring about local increases in order (in themselves), through their metabolic processes, but it takes energy to do this. For most living things, the source of this energy ultimately is the Sun. Plants are able to grow by harvesting solar energy, and animals feed on plants.

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6.3 How Is Energy Used by Living Things?

• Energy is stored away in endergonic (uphill) reactions, in which the products contain more energy than the reactants. Conversely, energy is released in exergonic (downhill) reactions, in which the reactants contain more energy than the products. The linkage of simple sugars to form a complex carbohydrate is an endergonic reaction, which requires an input of energy. The breakdown of a complex carbohydrate into simple sugars is an exergonic reaction, which releases energy.

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6.4 The Energy Currency Molecule: ATP

• Adenosine triphosphate (ATP) is the most important energy transfer molecule in living things. It is the final product of the energy-harvesting process, in which energy is extracted from food. ATP drives reactions by donating the third of its three phosphate groups to those reactions, in the process becoming the two-phosphate molecule adenosine diphosphate (ADP). The energy supplied by food powers the process by which a third phosphate group is attached to ADP, making it ATP once again.

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6.5 Efficient Energy Use in Living Things: Enzymes

• Enzymes are a class of proteins that greatly increase the rate of chemical reactions in an organism. Nearly every chemical process that takes place in living things is facilitated by an enzyme.
• The sum of all the chemical reactions that a cell or larger living thing carries out is its metabolism. Many activities in living things are controlled by metabolic pathways in which a series of interrelated steps is undertaken, each one of them facilitated by an enzyme.

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6.6 Lowering the Activation Barrier through Enzymes

• Enzymes lower activation energy, meaning the energy required to initiate a chemical reaction. Enzymes are catalysts: They retain their original chemical composition while bringing about a change in the molecules they bind with, called their substrates.
• Enzymes generally take the form of globular or ball-like proteins whose shape includes a pocket into which the enzyme's substrate fits. The pocket is the active site: that portion of the enzyme that binds with and transforms a substrate.

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6.7 Regulating Enzymatic Activity

• Enzyme activity can be controlled in several ways. One of these is allosteric regulation, in which the product of an enzyme-controlled reaction binds with the enzyme, thus reducing its activity.

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