The Energy of Life
5.1 What Is Energy?
1. Energy is the ability to do work. In some organisms, photosynthesis stores solar energy in chemical bonds. Cellular respiration releases energy.
2. Energy comes from the sun, the wind, moving water, the earth, and the tides and is held in the bonds of molecules of organisms.
3. Potential energy is stored energy available to do work. Kinetic energy is action.
5.2 How Do the Laws of Thermodynamics Describe Energy Transfer?
4. The laws of thermodynamics govern the energy transformations of life.
5. The first law states that energy cannot be created or destroyed but only converted to other forms.
6. The second law states that all energy transformations are inefficient, because every reaction results in increased entropy (disorder) and the loss of usable energy as heat.
5.3 How Do Cells Metabolize Nutrients?
7. Metabolism is the sum of the energy and matter conversions in a cell. It consists of enzyme-catalyzed reactions organized into often interconnected pathways and cycles.
8. Anabolism includes reactions that synthesize molecules. These reactions diverge from a few reactants to many products and require energy.
9. Catabolism includes reactions that break down molecules, converge from many reactants to a few products, and release energy.
10. Each reaction of a metabolic pathway rearranges molecules into new compounds, which changes the amount of free energy available in chemical reactions to do work.
11. In endergonic reactions, products have more energy than reactants, and entropy decreases. These reactions are not spontaneous—they require energy input.
12. In exergonic reactions, products have less energy than reactants, entropy increases, and energy is released.
13. At chemical equilibrium, a reaction proceeds in both directions at the same rate.
14. Many energy transformations in organisms occur via oxidation and reduction reactions. Oxidation is the loss of electrons from a molecule; reduction is the gain of electrons. Oxidation and reduction reactions occur simultaneously, in pairs, and form electron transport chains.
5.4 ATP is Cellular Energy Currency
15. ATP stores energy in its high-energy phosphate bonds. Many energy transformations involve coupled reactions, in which the cell uses the energy released by ATP to drive another reaction. Other compounds that take part in cellular metabolism include cofactors, coenzymes, NADH, NADPH, FADH2, and cytochromes.
5.5 Enzymes and Energy
16. Enzymes are proteins that speed spontaneous biochemical reactions to a biologically useful rate by lowering the energy of activation.
17. In negative feedback, a pathway’s product interacts with a regulatory enzyme for the pathway, temporarily shutting down its own synthesis when its levels rise. In positive feedback, a product stimulates its own production.