The Flow of Electrons Provides Energy for Organisms

Nearly all living organisms derive their energy, directly or indirectly, from the radiant energy of sunlight, which arises from thermonuclear fusion reactions carried out in the sun. Photosynthetic cells absorb light energy and use it to drive electrons from water to carbon dioxide, forming energy-rich products such as glucose (C6H12O6), starch, and sucrose and releasing O2 into the atmosphere:

light \

(light-driven reduction of CO2)

Nonphotosynthetic cells and organisms obtain the energy they need by oxidizing the energy-rich products of photosynthesis and then passing electrons to atmos-

Potential energy

1 Nutrients in environment (complex molecules such as sugars, fats)

1 Sunlight

Chemical transformations within cells

Energy transductions accomplish work

Cellular work:

• chemical synthesis

• mechanical work

• osmotic and electrical gradients

• light production

• genetic information transfer

Heat

Increased randomness (entropy) in the surroundings

Metabolism produces compounds simpler than the initial fuel molecules: CO2, NH3, H2O, HPOf-

Decreased randomness (entropy) in the system

Simple compounds polymerize to form information-rich macromolecules: DNA, RNA, proteins

FIGURE 1-24 Some energy interconversion in living organisms. During metabolic energy transductions, the randomness of the system plus surroundings (expressed quantitatively as entropy) increases as the potential energy of complex nutrient molecules decreases. (a) Living organisms extract energy from their surroundings; (b) convert some of it into useful forms of energy to produce work; (c) return some energy to the surroundings as heat; and (d) release end-product molecules that are less well organized than the starting fuel, increasing the entropy of the universe. One effect of all these transformations is (e) increased order (decreased randomness) in the system in the form of complex macromolecules. We return to a quantitative treatment of entropy in Chapter 13.

pheric O2 to form water, carbon dioxide, and other end products, which are recycled in the environment:

(energy-yielding oxidation of glucose)

Virtually all energy transductions in cells can be traced to this flow of electrons from one molecule to another, in a "downhill" flow from higher to lower electrochemical potential; as such, this is formally analogous to the flow of electrons in a battery-driven electric circuit. All these reactions involving electron flow are oxidation-reduction reactions: one reactant is oxidized (loses electrons) as another is reduced (gains electrons).

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