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2.4  Cell Processes

via a nonradiative electronic molecular orbital resonance effect that generates high-​energy

electrons from the photosynthetic reaction center.

Quantum tunneling of these excited electrons (see Chapter 9) occurs in a series of elec­

tron transfer reactions with a drop in electron energy coupled at each stage to a series of

chemical reactions, which results in the of pumping protons across a membrane in which

consequent electrochemical energy is used to fuel the reaction of carbon dioxide with water,

to oxygen as a by-​product as well as produce small sugar molecules, which lock up the energy

of the originally excited electrons into high-​energy chemical bonds.

On the basis of simple thermodynamics, for any process to occur spontaneously requires

a negative change in free energy, which is a (thermal) nonequilibrium condition. This is true

for all processes, including those biological. Living matter in effect delays the dispersion of

their free energy toward more available microstates (which moves toward a condition of

thermal equilibrium at which the change in free energy is precisely zero) by placing limits on

the number of available microstates toward which the free energy can be dispersed. This is

achieved by providing some form of continuous energy input into the system.

Ultimately, this energy input comes principally from the sun, though in some archaea, this

can be extracted from heat energy from thermal vents deep in the ocean. Another way to

view this is that energy inputted into the local thermal system of a living organism is utilized

to perform mechanical work in some form to force the system away from its natural tendency

of a state of maximum disorder as predicted from the second law, which is done by fueling

a variety of subcellular, cellular, and multicellular processes to regulate the organism’s stable

internal environment, a process that biologists describe as homeostasis (from the Greek,

meaning literally “standing still”).

KEY POINT 2.13

To overcome a tendency toward maximum entropy, a local thermal system requires

energy input to perform work against this entropic force. In most living organisms, this

energy ultimately comes from the sun, but may then also be provided in the form of

chemical energy from food that has been synthesized from other organisms using the

energy from the sun. This work against the sum of entropic forces in a living organism,

through various processes that constitute homeostasis, results in a more stable and

ordered internal environment.

Either way, this energy is trapped in some chemical form, typically in relatively simple

sugar molecules. However, releasing all of the energy trapped in a single molecule of glucose

in one go (equivalent to >500 kBT) is excessive in comparison to the free energy changes

encountered during most biological processes. Instead, cells first convert the chemical poten­

tial energy of each sugar molecule into smaller bite-​sized chunks by ultimately manufacturing

several molecules of ATP. The hydrolysis of a single molecule of ATP, which occurs normally

under catalytic control in the presence of enzymes called “kinases,” will release energy locally

equivalent to ~18 kBT, which is then converted into increased thermal energy of surrounding

water solvent molecules whose bombardment on biological structures ultimately fuels mech­

anical changes to biological structures.

This input of free energy into biological systems can be thought of as a delaying tactic,

which ultimately only slows down the inevitable process of the system reaching a state of

thermal equilibrium, equivalent to a state of maximum entropy, and of death to the biological

organism. (Note that many biological processes do exist in a state of chemical equilibrium,

meaning that the rate of forward and reverse reactions are equal, as well as several cellular

structures existing in a state of mechanical equilibrium, meaning that the sum of the kinetic

and potential energy for that structure is a constant.)

But how are ATP molecules actually manufactured? Most sugars can be relatively easily

converted in the cell into glucose, which is then broken down into several chemical steps

releasing energy that is ultimately coupled to the manufacture of ATP. Minor cellular