Chapter 13 INTRODUCTION TO METABOLISM
Living cells and organisms must perform work to stay alive, to grow, and to reproduce. The ability to harness energy and to channel it into biological work is a fundamental property of all living organisms; it must have been acquired very early in cellular evolution. Modern organisms carry out a remarkable variety of energy transductions, conversions of one form of energy to another. They use the chemical energy in fuels to bring about the synthesis of complex, highly ordered macromolecules from simple precursors. They also convert the chemical energy of fuels into concentration gradients and electrical gradients, into motion and heat, and, in a few organisms such as fireflies and some deep-sea fish, into light. Photosynthetic organisms transduce light energy into all these other forms of energy.
The chemical mechanisms that underlie biological energy transductions have fascinated and challenged biologists for centuries. The French chemist Antoine Lavoisier recognized that animals somehow transform chemical fuels (foods) into heat and that this process of respiration is essential to life. He observed that
in general, respiration is nothing but a slow combustion of carbon and hydrogen, which is entirely similar to that which occurs in a lighted lamp or candle, and that, from this point of view, animals that respire are true combustible bodies that burn and consume themselves…. One may say that this analogy between combustion and respiration has not escaped the notice of the poets, or rather the philosophers of antiquity, and which they had expounded and interpreted. This fire stolen from heaven, this torch of Prometheus, does not only represent an ingenious and poetic idea, it is a faithful picture of the operations of nature, at least for animals that breathe; one may therefore say, with the ancients, that the torch of life lights itself at the moment the infant breathes for the first time, and it does not extinguish itself except at death.i
A portrait by Jacques Louis David of Antoine Lavoisier (1743–1794) in the laboratory with chemist Marie Anne Pierrette Paulze (1758–1836), his wife.
We now understand much of the chemistry underlying that “torch of life.” Biological energy transductions obey the same chemical and physical laws that govern all other natural processes, and many of the types of chemical reactions that occur in living organisms have been long known to organic chemists. One unique feature of cellular chemistry is its exquisitely sensitive regulation by a variety of mechanisms that respond to changes in the external and internal circumstances of the cell and organism.
In this chapter we lay out the foundational principles for understanding the reactions of metabolism that follow in Part II. We first review the laws of thermodynamics and the quantitative relationships among free energy, enthalpy, and entropy. We then review the common types of biochemical reactions that occur in living cells, reactions that harness, store, transfer, and release the energy taken up by organisms from their surroundings. Our focus then shifts to reactions that have special roles in biological energy exchanges, particularly those involving the cofactors ATP (for phosphoryl transfers) and NADH (for electron transfers). Finally, we look at the most common of the strategies for regulating biochemical reactions. Watch for examples of these principles as you read this chapter:
The chemical changes and energy transductions in living organisms follow the laws of thermodynamics.
The free-energy change is the maximum energy made available to do work when a chemical reaction occurs. If two reactions can be combined to yield a third reaction, the overall free energy change is the sum of the two. Cells accomplish energy-requiring chemical work by coupling an energy-releasing (exergonic) reaction such as the cleavage of ATP to an endergonic reaction (which requires energy input).
Although thousands of different chemical reactions occur in the biosphere, most of them fall within a small set of reaction types.
ATP is the universal energy currency in living organisms. Transfer of its phosphoryl group to a water molecule or metabolic intermediates provides the energetic push for muscle contraction, the pumping of solutes against concentration gradients, and the synthesis of complex molecules.
Oxidation-reduction reactions indirectly provide much of the energy needed to make ATP. Reduced substrates such as glucose are oxidized in several steps, with the energy of oxidation steps conserved in the form of a reduced cofactor, NADH. Energy stored in NADH is used to drive the synthesis of ATP.
To respond to changes in external circumstances, cells must regulate enzyme activities, by changing either the number of enzyme molecules or the catalytic activity of preexisting enzyme molecules.
The chemical changes and energy transductions in living organisms follow the laws of thermodynamics.
The free-energy change is the maximum energy made available to do work when a chemical reaction occurs. If two reactions can be combined to yield a third reaction, the overall free energy change is the sum of the two. Cells accomplish energy-requiring chemical work by coupling an energy-releasing (exergonic) reaction such as the cleavage of ATP to an endergonic reaction (which requires energy input).
Although thousands of different chemical reactions occur in the biosphere, most of them fall within a small set of reaction types.
ATP is the universal energy currency in living organisms. Transfer of its phosphoryl group to a water molecule or metabolic intermediates provides the energetic push for muscle contraction, the pumping of solutes against concentration gradients, and the synthesis of complex molecules.
Oxidation-reduction reactions indirectly provide much of the energy needed to make ATP. Reduced substrates such as glucose are oxidized in several steps, with the energy of oxidation steps conserved in the form of a reduced cofactor, NADH. Energy stored in NADH is used to drive the synthesis of ATP.
To respond to changes in external circumstances, cells must regulate enzyme activities, by changing either the number of enzyme molecules or the catalytic activity of preexisting enzyme molecules.