Biological forms of energy and reducing power
Readings: p.82-84, 86-98, 104-114
By the end of this topic, you should be able to:
• Differentiate betweenΔG andΔG°
• Predict whether a chemical reaction is spontaneous based on itsΔG
• Give examples of energetically favourable reactions the cell uses to drive unfavourable
reactions forward, and explain the general mechanism by which this occurs
Living organisms carry out a diverse set of tasks, such as building and maintaining
physical structures, moving, synthesizing macromolecules, maintaining electrochemical
gradients, and maintaining a constant body temperature.All of these processes require
energy. One fundamental problem living organisms face is how to obtain energy to carry
out these tasks.
When considered at their most basic level, all of the activities mentioned above involve
chemical reactions. Any chemical reaction can, in principle, proceed in one of two
directions, in which the “reactants” are transformed into “products”, or vice versa.
Whether or not a chemical reaction requires energy to proceed in a particular direction
depends on the change in Gibbs free energy (ΔG, which equals G – G ) that
takes place when the reaction occurs. TheΔG for a given reaction depends in part on the
inherent characteristics of the reaction and the molecules involved, and in part on the
molecules’concentrations and environmental factors.
If ΔG for a reaction equals zero, then the reaction is said to be at equilibrium.At
equilibrium, the forward and reverse reactions occur to the same extent and no net
reaction takes place. If ΔG is negative, (i.e., <0), the forward reaction is more
energetically favourable, or spontaneous (Fig 3-16, p.91). If all molecules begin at equal
concentration, there will be a net conversion of reactants to products without addition of
energy. This will continue until equilibrium is reached, at which time the concentration of
products will be higher than that of reactants (Fig 3-18, p.93).
However, if ΔG for a reaction is greater than zero, the forward reaction is energetically
unfavourable, and at equilibrium reactants have a higher concentration than products. To
take a specific example, the reaction of combining glucose and fructose to make sucrose
(p.95, top right panel) has a standard free energy change (ΔG°) of +5.5 kcal/mole. (Note:
the small circle inΔG° indicates that the value is calculated using the G for each
molecule under “standard” conditions, with defined pressure and concentration. TheΔG°
incorporates information about only the natures of the molecules involved, and not about
the actual conditions under which the reaction occurs in a particular system. Even though
the standard conditions are not normally met in biological systems, they are useful as a
point of reference.) If sucrose is to be made in appreciable quantities, the natural
tendency of the glucose and fructose molecules to remain apart must be overcome. One strategy to make products even when theΔG° is positive is to change the
concentrations of the reactants and products, such that the ratio of reactants to products is
high. This is frequently done in living systems, but is not always practical or desired.
An alternate strategy to drive an unfavourable reaction forward is to couple that reaction
to an energetically favourable reaction (i.e., one with a negativeΔG), such that the total
ΔG for both reactions is negative (Fig 3-17, p.92; Fig. 3-30, p.105). If the totalΔG is
negative, then both reactions will proceed spontaneously. Knowing this, the question
becomes: is there a suitable energetically favourable reaction that organisms can couple
to many different unfavourable reactions? If so, where do the reactants for the
energetically favourable reaction come from?
The vast majority of energy used to sustain life comes ultimately from the sun. Light
energy is captured by plants and stored in carbohydrates (this process, photosynthesis, is
not covered in this course). The plants can then use cellular respiration to break down the
carbohydrates, releasing energy that the plants use to carry out processes necessary for
life (Fig 3-9, p.87). Animals likewise use respiration to derive energy from sugars, fats,
and proteins obtained by eating plants and other animals.
However, the breakdown of (for example) sugar molecules like glucose is not used to
directly power chemical reactions in the cell.Amajor reason for this is that a lot more
energy is available in a glucose molecule than is needed to drive individual chemical
reactions in the cell. Most of the energy would be wasted as heat. It is more desirable to
convert the energy available in glucose into a form that can be used in small amounts, a
little at a time, as needed (Fig 13-1, p.426). The universal energy carrier in biological
systems is adenosine triphosphate (ATP; Fig 2-23, p.57, and Movie 2.3).ATP is a
nucleotide and one of the building blocks for RNA; the general structure of nucleotides is
covered in Topic 18.
Hydrolysis ofATP to formADP and inorganic phosphate (P) is energetically favourable
(ΔG° of -7.3 kcal/mol). Combining the hydrolysis ofATP with unfavourable reactions
such as synthesis of sucrose from glucose and fructose can result in an overall reaction
that is favourable (p.95). Another example of an energetically unfavou