Biological Physics: Energy, Information, Life

11.3. Mitochondria as factories[[Student version, January 17, 2003]] 427

generation:

a

b

distribution and utilization:

glucose

NADH NAD+

glycolysis

pyruvate pyruvate

dehydrogenase

acetyl CoA

CoA

Krebs

cycle

NAD+NADH

respiratory

chain

O 2 ,H+ H 2 O

high-μ

low-μ

protons

protons

outer side

inner side

mitochondrial membrane

F0

turbine

F1

synthase

ADP, Pi ATP othermachines

other

pumps

direct

heat

generation

torque

flagellar

motor

torque

Figure 11.8:(Schematic.) Outline of the activity of a mitochondrion, emphasizing the parallels to Figure 11.7.
(a)Metabolism of sugar generates a difference in the electrochemical potential of protons across the inner mitochon-
drial membrane. For simplicity, “NAD” represents both the carrier molecules NADH and FADH 2 (the dashed line
represents an indirect process of import into the mitochondrion). (b)The protons in turn drive a number of molecular
machines. (Although mitochondria do not have flagella, bacteria such asE. colihave a similar arrangement, which
does drive their flagellar motor.)

molecule nicotinamide adenine dinucleotide (orNAD+). The net reaction,

CH 3 –CO–COO−+HS–CoA + NAD+→CH 3 –CO–S–CoA + CO 2 +NADH, (11.14)

adds two electrons (and a proton) to NAD+,yielding NADH. (Glycolysis also generates another
molecule of NADH per pyruvate; this NADH enters the respiratory chain indirectly.)

Krebs cycle The second step also occurs in the mitochondrial matrix. A cycle of enzyme-
catalyzed reactions picks up the acetyl-CoA generated in the previous step, oxidizing further the
acetyl group and recovering coenzyme A. Corresponding to this oxidation, three more molecules
of NAD+are reduced to NADH; in addition a second carrier molecule, flavin adenine dinucleotide
(abbreviatedFAD), gets reduced to FADH 2 .The net reaction,

CH 3 –CO–S–CoA + 2H 2 O+FAD + 3NAD++GDP^3 −+P^2 i−

→2CO 2 +FADH 2 +3NADH + 2H++GTP^4 −+HS–CoA, (11.15)

thus adds eight electrons (and three protons) to the carriers FAD and NAD+.Italso generates
one GTP, which is energetically equivalent to an ATP. Different authors refer to this part of the
reaction as theKrebs cycle,orthetricarboxylic acid cycle.

Summary Reactions Equations 11.14 and 11.15 oxidize pyruvate completely: Pyruvate’s three
carbon atoms each end up as molecules of carbon dioxide. Conversely, four molecules of the carrier