Essential Cell Biology 5th edition

492 CHAPTER 14 Energy Generation in Mitochondria and Chloroplasts
• Electron-transport chains associated with photosystems transfer
electrons from water to NADP + to form NADPH, which produces O 2
as a by-product.
• The photosynthetic electron-transport chains in chloroplasts also
generate a proton gradient across the thylakoid membrane, which
is used by an ATP synthase embedded in that membrane to generate
ATP.
• The ATP and the NADPH made by photosynthesis are used within
the chloroplast stroma to drive the carbon-fixation cycle, which produces
carbohydrate from CO 2 and water.
• Carbohydrate is exported from the stroma to the plant cell cytosol;
there it provides the starting material used for the synthesis of many
other organic molecules and for the production of the materials used
by plant cell mitochondria to produce ATP.
• Both mitochondria and chloroplasts are thought to have evolved
from bacteria that were endocytosed by other cells. Each retains its
own genome and divides by processes that resemble bacterial cell
division.
• Chemiosmotic coupling mechanisms are of ancient origin. Modern
microorganisms that live in environments similar to those thought to
have been present on the early Earth also use chemiosmotic coupling
to produce ATP.
KEY TERMS
antenna complex
ATP synthase
carbon fixation
cell respiration
chemiosmotic coupling
chlorophyll
chloroplast
cytochrome
cytochrome c oxidase
electron-transport chain
iron–sulfur center
light reactions
matrix
mitochondrion
nitrogen fixation
oxidative phosphorylation
photosynthesis
photosystem
quinone
reaction center
redox pair
redox potential
redox reaction
respiratory enzyme complex
stroma
thylakoid
QUESTIONS
QUESTION 14–11
Which of the following statements are correct? Explain your
answers.
A. After an electron has been removed by light, the
positively charged chlorophyll in the reaction center of the
first photosystem (photosystem II) has a greater affinity for
electrons than O 2 has.
B. Photosynthesis is the light-driven transfer of an electron
from chlorophyll to a second molecule that normally has a
much lower affinity for electrons.
C. Because it requires the removal of four electrons to
release one O 2 molecule from two H 2 O molecules, the
water-splitting enzyme in photosystem II has to keep the
reaction intermediates tightly bound so as to prevent partly
reduced, and therefore hazardous, superoxide radicals from
escaping.
QUESTION 14–12
Which of the following statements are correct? Explain your
answers.
A. Many, but not all, electron-transfer reactions involve
metal ions.
B. The electron-transport chain generates an electrical
potential across the membrane because it moves electrons
from the intermembrane space into the matrix.