634
CHAPTER  13         SPONTANEITY OF CHEMICAL PROCESSES
■2 Glucose and other important carbohydrates are discussed in Section 17.6.
BIOCHEMICAL ENERGY PRODUCTION
Living organisms use carbohydrates as their source of energy. Plants make their ■ rii carbohydrates through photosynthesis. Animals, on the other hand, obtain carl V-drates by eating plants or other animals. Plants and animals transform carbohyc -> into fats, which also can be used as sources of energy. The extraction of chei ■! energy from these compounds is called metabolism.
Metabolism involves highly spontaneous oxidation reactions, as illustiau  N glucose (a carbohydrate) and palmitic acid (a fat):
Glucose: C6Hl206 + 6 02 -> 6 C02 + 6 H20              AG° - -2870 kj
Palmitic acid: C]5H3lC02H + 23 02 -» 16 C02 + 16 H20         AG° = -9790 kJ
O
Palmitic acid:
[ i     hone acid
The negative standard free energy changes of these reactions arise because Iht ■ I tively weak 0=0 bond in molecular oxygen is converted into stronger 0—11 C=0 bonds in H20 and C02. Entropy also favors these reactions because ga*   ■ oxygen converts a solid into gaseous C02 and liquid H20. Not only are the pre !■■ u in more disordered phases, but there are also more molecules on the product í. the equation than on the side of the starting materials.
The large amount of energy stored in molecules such as glucose and pal miti      ' means that a little fat or carbohydrate goes a long way as a fuel for life proc However, a living cell would be destroyed quickly if all the energy stored in  ■ molecules were released in a single reaction. To utilize energy-rich molt   ' -without being destroyed, cells use elaborate chains of sequential reactions that    ■ . this stored energy to be harvested a little at a time. Part of this energy is iele<t heat that maintains our constant body temperature as it is dissipated to the sun   i ! ings. Another portion of the energy is stored in other high-energy molecules th    " body uses as "power sources" for the many reactions that occur within cells In addi tion to storing the energy produced in metabolism, these high-energy species .serve as energy transport molecules, moving to different regions of the cell wherr eneigy ľ. required for cell functions.
The most important of these energy transport molecules is adenosine triphos* phate (ATP). Some of the energy released during the oxidation of glucose is used t( drive a condensation reaction in which adenosine diphosphate (ADP) and phospliorK acid link together and eliminate water. This reaction stores chemical energy, as mdi cated by its positive standard free energy change:
ADP + PLPO„ -» ATP + FLO
AG0- +30.6 kJ
The molecular details of this reaction are illustrated in Figure 13-17. Although ATI is a complex molecule, notice that the adenosine portion does not change as this icac tion occurs. The condensation reaction merely adds an additional phosphate group h the end of an existing chain.
BIOENERCETICS
635
Diphosphate
«i : -o—»I
ľ»      ric acid
HO           OH
ADP
Triphosphate Ó        ~~0       ~~0 -^    HO—P—O—P—O—P—O OH        OH        OH
11   exact processes by which carbohydrates and fats are converted to C02 and
pend on the conditions and the particular needs of the cell. Each possible
^(e involves a complex series of chemical reactions, many of which are accompa-
by the conversion of ADP to ATP. Glucose, for example, can convert as many
í ADP molecules into ATP molecules as it is oxidized to C02 and H20:
C6H!206 + 6 02 + 36 ADP 4- 36 H3P04 -> 6 C03 + 36 ATP + 42 H,0
HO           OH
ATP
+ H,0
FIGURE 13-17
Phosphoric acid reads with ADP to produce waier and ATR Because a P—O— P linkage is weaker than a P—O—H linkage, this reaction is endoihermic by about 30 kJ/mol. Notice that the adenosine portion of the molecule remains intact during this reaction.
UPLED REACTIONS
Us
use the energy stored in ATP molecules to drive reactions that would otherwise nonspontaneous  under physiological conditions.  This is  accomplished by upling the nonspontaneous reaction with the conversion of ATP back to ADP and
osphoric acid:
ATP + H20 -» ADP + H3P04
AG0 = -30.6 kJ
bnpled reactions share a common intermediate that transfers energy from one action to the other. For example, the amino acid glutamine is synthesized in cells 'reacting ammonia with another amino acid, glutamic acid.
K The right-hand portion of the ATP molecule is adenosine. It contains adenine, an important biochemical building block introduced in Section 11.7.
O
O
HO"   ^/   \^   "OH    +    NH,
NH, Glutamic acid
H,N"   \/  "v^     OH    +    H,0
NH, Glutamine
lis reaction is thermodynamically unfavorable, AG0 = +34 kJ. The reaction is wen by coupling it with the conversion of ATP into ADP.
Glutamic acid + NH3 -> Glutamine + H20
ATP + H20 -> ADP + H3P04
Net:    Glutamic acid + ATP + NH-, -> Glutamine + ADP + H3P04
lie net energy change for the coupled process is the sum of the AG° values for the ndtvidual reactions:
AG°[xn = AG"glutamire + AGVp - 14 kJ + (-30.6 kJ) = -17 kJ
he negative value of AG°rf.n shows that the free energy released in the ATP reaction ís mote than enough to drive the conversion of glutamic acid into glutamine.
Although the coupled reactions can be represented by the net reaction, this process actually occurs in steps, in the first step of the coupled reaction, a phosphate group is transferred from ATP to glutamic acid:
636
CHAPTER  13         SPONTANEITY OF CHEMICAL PROCESSES
O                              o                 o
HO—rP—- O— ADP  +   HO
OH                                                NH3
Glutamic acid
OH
O         O
^    HO—P—O
OH
O
OH    +
NH,
Next, an ammonia molecule reacts with the phosphorylated form of glutamic ; producing phosphoric acid and glutamine:
o        O NH,   +    HO—P—O
OH
OH                       NH2
Phosphorylated glutamic acid
->   H3P04   +   H3N
jgH A catalyst is a chemical species f h of makes a reaction go faster than it would in the absence of a catalyst. Enzymes and other catalysts are discussed in Chapter 14.
Overall, one molecule of ATP is converted to ADP and phosphoric acid for t molecule of glutamine produced from glutamic acid.
Coupled biochemical reactions occur on the surface of an enzyme. As describe in Chapter 11, enzymes are huge proteins that catalyze an immense vai of biochemical reactions. The surface area of an enzyme has a particular shape accommodates the various molecules that must react in a coupled reaction.
Coupled reactions are also involved in the synthesis of ATP. Sample Prot 13-11 illustrates one of these energy-storing reactions.
SAMPLE PROBLEM 13-11      ATP-FORMING REACTIONS
One of the biochemical reactions that produces ATP involves the conversion of acetyl ph phate to acetic acid and phosphoric acid:
Although the standard free energy change of the ADP/ATP reaction is 30.6 kJ/mol under standard conditions, the concentrations of the phosphate species in cells are far from 1 M. This causes the free energy to vary from its standard value- Typically, the conversion of ATP to ADP in a living cell releases around 50 kJ/mol of energy.
O         O
II -O—P—OH   +   H20
OH
Acetyl phosphate
O
-OH
Acetic acid
+    H3P04        AG0 = -46.9 kJ/mol
Write the overall balanced equation, and show that the coupled reaction is spontaneous.
METHOD: A coupled process links a spontaneous reaction with a nonspontaneous one. 1 this case the energy released in the acetyl phosphate reaction provides the energy needed drive the conversion of ADP to ATP.
Combining the two reactions gives the overall balanced equation:
Acelyl phosphate                   Acetic acid
CH3C02P03H2 + r^O-> CH3C02H + H3PÖ4
ADP + HsP04 -> ATP + FfjQ
Net: CH3C02P03H3 + ADP -» CH3C02H + ATP
The net energy change for the coupled process is the sum of the AG" values for the individual reactions:
AG°rxn - AG°acelylphosphatc + AG°ATP = "46.9 kJ + 30.6 kJ = -16.3 kJ
The negative value of AGarxn shows that the free energy released in the acetyl phosphate reaction is more than enough to drive the conversion of ADP to ATP.
13.6
BIOENERPETICS
ERGY EFFICIENCY
ils store the energy that is released during the oxidation of glucose by converting "*P into ATP. The storage process cannot be perfectly efficient, however, because "h step in the reaction sequence must have a negative free energy change. In prac-
1 terms, this requires that some energy be released to the surroundings as heat.
The complete balanced equation for glucose oxidation coupled with ATP produc-n under normal physiological conditions is:
CeH1206 + 6 02 + 36 ADP + 36 U3P04 -> 6 C02 + 42 H20 + 36 ATP
ccording to this equation, the oxidation of 1 mol glucose yields 36 mol ATP. We i determine the overall free energy change for this process from the values for its :coupled parts:
C6Hl206 + 6 02 -> 6 C02 4- 6 H20                  AG" - -2870 kJ
36 (ADP + H3P04 -> ATP + H20)     AG0 - (36 mol)(-30.6 kJ/mol) = 4-1100 kJ
AGovcrall = -2870 kJ + 1100 kJ = -1770 kJ
jthough 1100 kJ of energy is stored in this coupled process, 1770 kJ of energy is "asted." Thus cells harness 38% of the chemical energy stored in glucose to drive e biochemical machinery of metabolism. The remaining 62% is dissipated as heat, ismg the entropy of the surroundings as the living cell organizes itself and its * mediate environment.
Fats such as palmitic acid are metabolized through pathways similar to the ones for the oxidation of glucose. The complete oxidation of 1 mol of palmitic acid "blecule liberates 9790 kJ of free energy and produces 130 ATP molecules. You "ould be able to verify that this metabolic process has about the same efficiency as ..e oxidation of glucose.
v One mole of glucose releases 2870 kJ of free energy, whereas one mole of palmitic
releases much more free energy, 9790 kJ. Although some of this extra energy
~ults from its larger molecular size, palmitic acid also releases more energy per atom
of carbon than glucose. Glucose oxidation releases about 480 kJ/mol of carbon atoms,
hereas palmitic acid releases about 610 kJ/mol of carbon atoms. Organisms convert
bohydrates into fats because fats store more energy per unit mass.
ECTION EXERCISES
13 6 1    Nitrogen-fixing bacteria react N2 with H20 to produce NH3 and 02 using ATP as their energy source. Approximately 24 molecules of ATP are consumed per molecule of N2 fixed. What percentage of the free energy derived from ATP is stored in NH3?
362   The hydrolysis of ATP to ADP has A/f - -21.0 kJ/mol, whereas AG0 - -30.6
kJ/mol at 298 K. Calculate AS0 for this reaction. What happens to the spontaneity of this reaction as the temperature is increased to 37 °C?
H 6 3    In running a mile, an average person consumes about 500 kJ of energy.
a.  How many moles of ATP does this represent?
b.  Assuming 38% conversion efficiency, how many grams of glucose must be "burned"?