Photosynthesis ❮ 83Answers and Explanations
12.Which of the following is an incorrectstatement
about the Calvin cycle?
A. The main inputs to the reactions are
NADPH, ATP, and CO 2.
B. The main outputs of the reactions are
NADP+, ADP, and sugar.
C. More NADPH is used than ATP during the
Calvin cycle.
D. Carbon fixation is the first step of the process.
E. The reactions occur in the stroma of the
chloroplast.
13.Which of the following is the source of the carbon
in sugar produced during photosynthesis?
A. CO 2
B. H 2 O
C. Rubisco
D. PEP carboxylase
E. Pyruvate
14.The light-dependent reactions of photosynthesis
occur in the
A. stroma.
B. mitochondrial matrix.
C. thylakoid membrane.
D. cytoplasm.
E. nucleus.- C—CAM plants open their stomata at night and
close their stomata during the day to avoid water
loss due to heat. The carbon dioxide taken in
during the night is incorporated into organic
acids and stored in vacuoles until the next day,
when the stomata close and CO 2 is needed for
the Calvin cycle. - B—The Calvin cycle uses ATP, NADPH, and
CO 2 to produce the desired sugar output of
photosynthesis. - D—Cyclic photophosphorylation occurs because
the Calvin cycle uses more ATP than it does
NADPH. This is a problem because the light
reactions produce an equal amount of ATP and
NADPH. The plant compensates for this dispar-
ity by dropping into the cyclic phase when needed
to produce the ATP necessary to keep the light-
independent reactions from grinding to a halt. - A—Transpiration is the process by which plants
lose water through their leaves. Not much else to
be said about that. ☺
5. C—One of the major problems encountered by
plants in hot and dry conditions is of photo-
respiration. In hot conditions, plants close their
stomata to avoid losing water to transpiration.
The problem with this is that the plants run low
on CO 2 and fill with O 2. The oxygen competes
with the carbon dioxide and attaches to RuBP,
leaving the plant with a lowered capacity for
growth. C 4 plants cycle CO 2 from mesophyll
cells to bundle sheath cells, creating a higher
concentration of CO 2 in that region, thus allow-
ing rubisco to carry out the Calvin cycle without
being distracted by the O 2 competitor.
6. A—The light-dependent reactions are the source
of the oxygen given off by plants.
7. D—Chemiosmosis occurs in both photosynthesis
and cellular respiration. This is the process by
which the formation of ATP is driven by electro-
chemical gradients in the cell. Hydrogen ions
accumulate on one side of a membrane, creating a
proton gradient that causes them to move through
channels to the other side of that membrane, thus
leading, with the assistance of ATP synthase, to the
production of ATP.
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