Download 4.2 Section 4.2

UNIT 2: Metabolic Processes
Chapter 4: Photosynthesis
4.2 Fixing Carbon Dioxide:
The Light-Independent Reactions
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
4.2 Fixing Carbon Dioxide: The LightIndependent Reactions
Plants convert carbon dioxide to carbohydrates by using ATP
and NADPH produced in the light-dependent reactions, along
with enzymes in the stroma.
The initial step involves converting carbon dioxide into organic
compounds in a process called CO2 assimilation.
This step occurs in the Calvin cycle.
The reactions in the Calvin cycle can be
grouped into three phases. The end organic
molecule is glyceraldehyde-3-phosphate (G3P).
UNIT 2 Chapter 4: Photosynthesis
The Calvin Cycle: Phase 1
Section 4.2
Calvin Cycle
Carbon Dioxide Fixation
• CO2 bonds with ribulose-1,5-bisphosphate (RuBP). The
resulting six-carbon compound is unstable and breaks down
into two 3-phosphoglycerate (PGA) molecules.
• The reaction is catalyzed by the enzyme ribulose bisphosphate
carboxylase (also named rubisco).
•
Plants that use this
method to fix carbon
are called C3 plants,
and the pathway is
called C3
photosynthesis.
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
The Calvin Cycle: Phase 2
Reduction
•
PGA is activated by ATP and then reduced by NADPH and
converted to glyceraldehyde-3-phosphate (G3P).
In their reduced, higherenergy state, some of the
G3P molecules leave the
cycle to become glucose,
while other G3P
molecules remain to
replenish RuBP.
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
The Calvin Cycle: Phase 3
Regenerating RuBP
•
•
ATP energy is used to break and re-form the bonds to make
RuBP from G3P.
The Calvin cycle must be completed six times in order to
synthesize one molecule of glucose.
Of the 12 G3P molecules
that are produced in six
cycles, 10 are used to
make RuBP and 2 are
used to make one glucose
molecule.
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
The Calvin Cycle
The net equation for the Calvin cycle is:
6CO2 + 18 ATP + 12 NADPH + water → 2 G3P + 16 Pi + 18
ADP
+ 12 NADP+
G3P is the starting
substance for many
carbohydrates and
some plant oils and
amino acids.
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
Adaptation to Photosynthesis
Oxygen competes with carbon dioxide for the same active
site on rubisco.
In a process called photorespiration, oxygen reacts with
RuBP, and different products are formed, thus reducing the
efficiency of photosynthesis.
In controlled lab conditions, photosynthesis is more
efficient and the oxygen inside the leaf increases. In nature,
photosynthetic efficiency ranges from 0.1 percent to
3 percent.
Continued…
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
Adaptations to Photosynthesis
Leaves prevent water loss in hot, dry conditions by closing their stomata.
Water is conserved, but carbon dioxide is prevented from entering.
Continued…
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
Adaptations to Photosynthesis
C4 and CAM plants live in hot and dry climates. They have
evolved mechanisms to reduce photorespiration and
improve the efficiency of photosynthesis.
C4 plants separate the uptake of
CO2 from the Calvin cycle in
different types of cells. CAM
plants do something similar with a
biochemical pathway. In both
cases, they increase the ratio of
carbon dioxide to oxygen for the
Calvin cycle.
UNIT 2 Chapter 4: Photosynthesis
C4 Plants
C4 plants use energy to
“pump” carbon dioxide
into the bundle-sheath
cells, where it becomes
concentrated. Included
among the C4 plants are
food crops such as corn,
sorghum, sugarcane
(shown here), and millet.
Also included are grasses
such as crabgrass and
Bermuda grass.
Section 4.2
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
CAM Plants
The opening and closing
of stomata in CAM plants,
such as pineapple and
cacti, are opposite from
most plants. The stomata
are open at night and
closed in the daytime.
When the carbon dioxide
is removed from the fourcarbon compound malate
in the daytime, it cannot
leave the cell because
the stomata are closed. In
cool climates, CAM plants
are very inefficient,
because they use energy
to drive the reactions that
store carbon dioxide.
UNIT 2 Chapter 4: Photosynthesis
The Energy Cycle in Plant Cells
Both aerobic
respiration and
photosynthesis
occur in plants
and are closely
related. Together
they represent a
plant cell’s energy
cycle.
Section 4.2
UNIT 2 Chapter 4: Photosynthesis
Section 4.2
Aerobic Respiration and Photosynthesis
UNIT 2 Chapter 4: Photosynthesis
Section 4.2 Review
Section 4.2