Page 173 - Algae Anatomy, Biochemistry, and Biotechnology

156 Algae: Anatomy, Biochemistry, and Biotechnology

temperatures. Conditions that favor oxygenation over carboxylation include low CO 2 levels, high
temperatures, and high light intensities.

ENERGY RELATIONSHIPS IN PHOTOSYNTHESIS: THE BALANCE SHEET

The consumption of a mole of glucose releases 686 kcal of energy. This value represents the differ-
ence between the energy needed to break the bonds of the reactants (glucose and oxygen) and the
energy liberated when the bonds of the products (H 2 O and CO 2 ) form. Conversely, the photo-
synthesis of a mole of glucose requires the input of 686 kcal of energy. The overall equation for
each process is the same; only the direction of the arrow differs:

C 6 H 12 O 6 þ 6O 2 þ , . 6H 2 O þ 6CO 2 (3:5)

The average bond energies between common atoms are the following: C22H has 98 kcal/mol;
O22H has 110 kcal/mol; C22C has 80 kcal/mol; C22O has 78 kcal/mol; H22H has 103 kcal/mol;
C22N has 65 kcal/mol; O55O has 116 kcal/mol; C55C has 145 kcal/mol; C55O (as found in CO 2 )
has 187 kcal/mol.
The 24 covalent bonds of glucose require a total of 2182 kcal to be broken. The six double
bonds of oxygen require another 696 kcal. Thus a grand total of 2878 kcal is needed to break all
the bonds of the reactants in cellular respiration.
As for the products, the formation of six molecules of CO 2 involves the formation of 12 double
polar covalent bonds each with a bond energy of 187 kcal; total ¼ 2244. The formation of six mol-
ecules of H 2 O involves the formation of 12 O22H bonds each with an energy of 110 kcal;
total ¼ 1320. Thus a grand total of 3564 kcal is released as all the bonds of the products form.
Subtracting this from the 2878 kcal needed to break the bonds of the reactants, we arrive at
2686 kcal, the free energy change of the oxidation of glucose. This value holds true whether we
oxidize glucose quickly by burning it or in the orderly process of cellular respiration in mito-
chondria. The minus sign indicates that free energy has been removed from the system. The
details of the energy budget are just the same. The only difference is that now it takes 3564 kcal
to break the bonds of the reactants and only 2878 kcal are released in forming glucose and
oxygen. So we express this change in free energy (þ686 kcal) with a plus sign to indicate that
energy has been added to the system. The energy came from the Sun and now is stored in the
form of bond energy that can power the needs of all life.
Photosynthetic reduction of CO 2 can be summarized by the equations:
þ
2H 2 O ! O 2 þ 4H þ 4e (3:6)
þ

CO 2 þ 4H þ 4e ! (CH 2 O) þ H 2 O (3:7)
Four electrons are required to be transferred from water, through a redox span of 1.24 eV, to reduce
one molecule of CO 2 . The energy required for the reduction of 1 mol of CO 2 is therefore 4 mol
4
23
1.25 eV 1.60 10 219 JeV 21 6.02 10 mol 21 ¼ 47.77 10 J (115.10 kcal). Theoreti-
cally, the energy requirement could be satisfied by the capture of 4 mol of photons of PAR light,
say a red photon of 700 nm, which have an energy content of 4 mol 2.84 10 219 J 6.02 10 23
4
mol 21 ¼ 68.4 10 J (163.40 kcal). However, due to the thermodynamic losses during energy con-
version, the fraction of absorbed photon energy converted into chemical energy seldom exceeds 0.35.
Thus, eight moles of photons are required for the reduction of 1 mol of CO 2 (8 mol 2.84 10 219
4
23
4
J 6.02 10 mol 21 ¼ 136.82 10 J 0.35 ¼ 48.20 10 J(115.20 Kcal). Amole of glucose
(formed by the addition of six CO 2 molecules) requires 6 115.20 kcal equal to 691.20 kcal. This
value is calculated by taking in account the balance of the energy of bonds previously described.