Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

810 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
the electron carriers in the respiratory chain? What is their
order?
(A)
direction of rotation
14–8 Normally, the flow of electrons to O 2 is tightly
linked to the production of ATP via the electrochemical
gradient. If ATP synthase is inhibited, for example, electrons
do not flow down the electron-transport chain and
respiration ceases. Since the 1940s, several substances—
such as 2,4-dinitrophenol—have been known to uncouple
electron flow from ATP synthesis. Dinitrophenol was
once prescribed as a diet drug to aid in weight loss. How
would an uncoupler of oxidative phosphorylation promote
weight loss? Why do you suppose dinitrophenol is
no longer prescribed?
inner membrane
matrix
(B)
5
β
γ
α
β
actin filament
14–9 In actively respiring liver mitochondria, the pH in
the matrix is about half a pH unit higher than it is in the
cytosol. Assuming that the cytosol is at pH 7 and the matrix
is a sphere with a diameter of 1 μm [V = (4/3)πr 3 ], calculate
the total number of protons in the matrix of a respiring
liver mitochondrion. If the matrix began at pH 7 (equal to
that in the cytosol), how many protons would have to be
pumped out to establish a matrix pH of 7.5 (a difference of
0.5 pH units)?
14–10 ATP synthase is the world’s smallest rotary motor.
Passage of H + ions through the membrane-embedded
portion of ATP synthase (the F o component) causes rotation
of the single, central, axle-like γ subunit inside the
head group. The tripartite head is composed of the three
αβ dimers, the β subunit of which is responsible for synthesis
of ATP. The rotation of the γ subunit induces conformational
changes in the αβ dimers that allow ADP and
Pi to be converted into ATP. A variety of indirect evidence
had suggested rotary catalysis by ATP synthase, but seeing
is believing.
To demonstrate rotary motion, a modified form of
the α 3 β 3 γ complex was used. The β subunits were modified
so they could be firmly anchored to a solid support and the
γ subunit was modified (on the end that normally inserts
into the F o component in the inner membrane) so that a
fluorescently tagged, readily visible filament of actin could
be attached (Figure Q14–2A). This arrangement allows
rotations of the γ subunit to be visualized as revolutions
of the long actin filament. In these experiments, ATP synthase
was studied in the reverse of its normal mechanism
by allowing it to hydrolyze ATP. At low ATP concentrations,
the actin filament was observed to revolve in steps of 120°
and then pause for variable lengths of time, as shown in
Figure Q14–2B.
A. Why does the actin filament revolve in steps with
pauses in between? What does this rotation correspond to
in terms of the structure of the α 3 β 3 γ complex?
B. In its normal mode of operation inside the cell,
how many ATP molecules do you suppose would be synthesized
for each complete 360° rotation of the γ subunit?
Explain your answer.
14–11 How much energy is available in visible light? How
much energy does sunlight deliver to Earth? How efficient
revolutions
4
3
2
1
0
0 20 40 60 80 100
time (seconds)
Figure Q14–2 Experimental set-up for observing rotation of the
γ subunit of ATP synthase (Problem 14–10). (A) The immobilized
α 3 β 3 γ complex. The β subunits are anchored to a solid support and
a fluorescent actin filament is attached to the γ subunit. (B) Stepwise
revolution of the actin filament. The indicated trace is a typical example
Figure p14.04/14.07
from one experiment. The inset shows the positions in the revolution
at which the actin filament pauses. (B, from R. Yasuda et al., Cell
93:1117–1124, 1998. With permission from Problem Elsevier.) p14-31/14-47
are plants at converting light energy into chemical energy?
The answers to these questions provide an important
backdrop to the subject of photosynthesis.
Each quantum or photon of light has energy hv,
where h is Planck’s constant (6.6 × 10 –37 kJ sec/photon)
and v is the frequency in sec –1 . The frequency of light is
equal to c/λ, where c is the speed of light (3.0 × 10 17 nm/
sec) and λ is the wavelength in nm. Thus, the energy (E) of
a photon is
E = hv = hc/λ
A. Calculate the energy of a mole of photons (6 × 10 23
photons/mole) at 400 nm (violet light), at 680 nm (red
light), and at 800 nm (near-infrared light).
B. Bright sunlight strikes Earth at the rate of about 1.3
kJ/sec per square meter. Assuming for the sake of calculation
that sunlight consists of monochromatic light of wavelength
680 nm, how many seconds would it take for a mole
of photons to strike a square meter?
C. Assuming that it takes eight photons to fix one
molecule of CO 2 as carbohydrate under optimal conditions
(8–10 photons is the currently accepted value), calculate
how long it would take a tomato plant with a leaf
area of 1 square meter to make a mole of glucose from CO 2 .
Assume that photons strike the leaf at the rate calculated
above and, furthermore, that all the photons are absorbed
and used to fix CO 2 .