The Calvin Cycle - Advanced
Douglas Wilkin, Ph.D.
Barbara Akre
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Printed: December 21, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Barbara Akre
www.ck12.org
C HAPTER
Chapter 1. The Calvin Cycle - Advanced
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The Calvin Cycle Advanced
• Trace the flow of energy and materials through the Calvin cycle.
Other than being green, what do all these fruits and vegetables have in common?
They are full of energy. Energy in the form of glucose. Fruit and vegetable plants, like all plants, are autotrophs and
producers, producing energy from sunlight. The energy from sunlight is briefly held in NADPH and ATP, which is
needed to drive the formation of sugars such as glucose. And this all happens in the Calvin Cycle.
Photosynthesis Stage II: The Calvin Cycle - Making Food “From Thin Air”
During the light-dependent stage of photosynthesis, two of the three reactants (water and light) were used to produce
oxygen gas, one of the products (and essentially a waste product of this process). All three necessary conditions are
required –the chloroplast with chlorophyll pigments, and enzyme catalysts. The first stage transforms light energy
into chemical energy, stored to this point in molecules of ATP and NADPH. Look again at the overall equation
below. What is left?
Waiting in the atmosphere is one more reactant, carbon dioxide, and yet to come is the product which is food for
all life –glucose. These key players perform in the second stage of the photosynthesis, in which food is “made from
thin air.” The second stage of photosynthesis can proceed without light, so its steps are sometimes called “lightindependent” or “dark” reactions. Many biologists honor the scientist, Melvin Calvin, who won a 1961 Nobel Prize
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for working out this complex set of chemical reactions, naming it the Calvin Cycle. The Calvin Cycle has two parts.
First carbon dioxide is "fixed." Then ATP and NADPH from the Light Reactions provide energy to combine the
fixed carbons to make sugar.
The Calvin Cycle is discussed at http://www.youtube.com/watch?v=slm6D2VEXYs (13:28).
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/259
Carbon Dioxide is “Fixed”
Why does carbon dioxide need to be fixed? Was it ever broken? Life on Earth is carbon-based. Organisms need not
only energy but also carbon atoms for building bodies. For nearly all life, the ultimate source of carbon is carbon
dioxide (CO2 ), an inorganic molecule pulled into the producer from the atmosphere. CO2 , as you saw in Figure 1.1,
makes up .038% of the Earth’s atmosphere.
Animals and most other heterotrophs cannot take in CO2 directly. They must eat other organisms or absorb organic
molecules to get carbon. Only autotrophs can build low-energy inorganic CO2 into high-energy organic molecules
like glucose. This process is carbon fixation.
FIGURE 1.1
Stomata on the underside of leaves take in CO2 and release water and O2 .
Guard cells close the stomata when water is scarce. Leaf cross-section
(above) and stoma (below).
Plants have evolved three pathways for carbon fixation. The most common pathway combines one molecule of CO2
with a 5-carbon sugar called ribulose biphosphate (RuBP). The enzyme which catalyzes this reaction, ribulose-1,5bisphosphate carboxylase oxygenase (nicknamed RuBisCo), is the most abundant enzyme on earth! The resulting
6-carbon molecule is unstable, so it immediately splits into two much more stable 3-carbon phosphoglycerate
molecules. The 3 carbons in the first stable molecule of this pathway give this largest group of plants the name
“C-3.”
Dry air, hot temperatures, and bright sunlight slow the C-3 pathway for carbon fixation. This is because the stomata,
tiny openings under the leaf which normally allow CO2 to enter and O2 to leave, must close to prevent loss of water
vapor ( Figure 1.1) by transpiration. Closed stomata lead to a shortage of CO2 . Two alternative pathways for
carbon fixation demonstrate biochemical adaptations to differing environments. All three carbon fixation pathways
lead to the Calvin Cycle to build sugar.
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Chapter 1. The Calvin Cycle - Advanced
See CAM Plants at http://www.youtube.com/watch?v=xp6Zj24h8uA (8:37) for further information.
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/261
FIGURE 1.2
Even chemical reactions adapt to specific
environments. Carbon fixation pathways
vary among three groups.
Temperate
species (maple tree, left) use the C-3
pathway. C-4 species (corn, center) concentrate CO2 in a separate compartment
to lessen water loss in hot bright climates.
Desert plants (jade plant, right) fix CO2 by
CAM photosynthesis only at night, closing
stomata in the daytime to conserve water.
C-4 Plants
Plants such as corn solve the problem by using a separate compartment to fix CO2 . C-4 plants utilize a specific leaf
anatomy. These plants have both bundle-sheath cells, which are photosynthetic cells arranged into tightly packed
coverings or sheaths around the veins of a leaf, and loosely arranged mesophyll cells, which lie between the bundle
sheath cells and the leaf surface. The bundle-sheath cells form a protective covering on leaf veins. The Calvin Cycle
is confined to the chloroplasts of these bundle sheath cells in C-4 plants. Instead of direct fixation to RuBisCO in
the Calvin Cycle, CO2 is incorporated into a 4-carbon organic acid, which has the ability to regenerate CO2 in the
chloroplasts of the bundle sheath cells. Bundle sheath cells can then utilize this CO2 to generate carbohydrates by
the conventional C-3 pathway.
In these C-4 plants, fixation of CO2 occurs in mesophyll cells when it combines with a 3-carbon molecule, phosphoenolpyruvate (PEP), resulting in a 4-carbon oxaoloacetate molecule. This reaction is catalyzed by the enzyme PEP
carboxylase. Because the first stable organic molecule has four carbons (oxaoloacetate), this adaptation has the name
C-4. This 4-carbon molecule is converted into another 4-carbon molecule, malate, which is shuttled unto the bundlesheath cells, where it is broken down into CO2 and a 3-carbon pyruvate. When enough CO2 accumulates, RuBisCo
fixes it a second time, this time as part of the Calvin Cycle. The pyruvate is transported back to the mesophyll
cell where it is converted into phosphoenolpyruvate, allowing the process to continue. Compartmentalization allows
efficient use of low concentrations of carbon dioxide in these specialized plants.
See C-4 Photosynthesis at http://www.youtube.com/watch?v=7ynX_F-SwNY (16:58) for further information.
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FIGURE 1.3
In this diagram of the cross section of a
leaf, bundle sheath cells can be seen in
pink around a leaf vein, and mesophyll
cells can be seen in green surrounding
the bundle sheath cells and the outer
layers of the leaf. Also noticeable is a cutaway of two guard cells (purple) surrounding a stomata.
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/260
CAM Photosynthesis
Cacti and succulents such as the jade plant avoid water loss by fixing CO2 only at night. These plants close their
stomata during the day (by closing their guard cells) and open them only in the cooler and more humid nighttime
hours. Leaf structure differs slightly from that of C-4 plants, but the fixation pathways are similar. The family of
plants in which this pathway was discovered gives the pathway its name, Crassulacean Acid Metabolism, or CAM
( Figure 1.2). CAM photosynthesis is an adaptation to arid conditions in some plants. CAM photosynthesis is a
two-step process: part occurs during the day and part at night.
At night, when the stomata are open, CO2 can enter the leaf cell, but the light reactions of photosynthesis cannot
take place, so ATP and NADPH can’t be made. The carbon dioxide is fixed in the cytoplasm of mesophyll cells by
a PEP reaction similar to that of C-4 pathway, PEP carboxylase combines CO2 and PEP, making oxalacetate, which
is subsequently transformed into malate. But, unlike the C-4 plant, the resulting malate is not immediately passed
on to the Calvin Cycle but are stored in vacuoles for later use. The next day, after the sun comes out and the light
reactions restart making ATP and NADPH, malate is released from the vacuoles of the mesophyll cells and enters
the stroma of the chloroplasts where an enzyme releases the CO2 , which then enters into the Calvin Cycle.
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Chapter 1. The Calvin Cycle - Advanced
How Does the Calvin Cycle Store Energy in Sugar?
As Melvin Calvin discovered, carbon fixation is the first step of a cycle. Like an electron transport chain, the Calvin
Cycle, shown in Figure 1.4, transfers energy in small, controlled steps. Each step pushes molecules uphill in terms
of energy content. Recall that in the electron transfer chain, excited electrons lose energy to NADPH and ATP. In
the Calvin Cycle, NADPH and ATP formed in the light reactions lose their stored chemical energy to build glucose.
Use the diagram below to identify the major aspects of the process:
• the general cycle pattern
• the major reactants
• the products
FIGURE 1.4
Overview of the Calvin Cycle Pathway.
First, notice where carbon is fixed by the enzyme Rubisco. In C-3, C-4, and CAM plants, CO2 enters the cycle by
joining with 5-carbon ribulose bisphosphate to form a 6-carbon intermediate, which splits (so quickly that it isn’t
even shown) into two 3-carbon 3-phosphoglycerate molecules. Now look for the points at which ATP and NADPH
(made in the light reactions) add chemical energy (“Reduction” in the diagram) to the 3-carbon molecules. The
resulting glyceraldehyde-3-phosphate “half-sugars” can enter several different metabolic pathways. One recreates
the original 5-carbon precursor, completing the cycle. A second combines two of the 3-carbon molecules to form
glucose, the universal fuel for life. The cycle begins and ends with the same 5-carbon RuBP molecule, but the
process combines carbon and energy to build carbohydrates –food for life.
So –how does photosynthesis store energy in sugar? Six “turns” of the Calvin Cycle use chemical energy from ATP
to combine six carbon atoms from six CO2 molecules with 12 hydrogens from NADPH. The result is one molecule
of glucose, C6 H12 O6 .
Vocabulary
• bundle-sheath cells: Photosynthetic cells arranged into tightly packed sheaths around the veins of a leaf.
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• Calvin Cycle: The second stage of photosynthesis; results in the formation of a sugar.
• CAM photosynthesis: A photosynthetic adaptation to arid conditions in some plants; allows stomata to be
closed during the day.
• carbon fixation: The process which converts carbon dioxide in the air to organic molecules, as in photosynthesis.
• mesophyll cells: Photosynthetic parenchyma cells that lie between the upper and lower epidermis layers of a
leaf.
• RuBisCo: The enzyme that combines one molecule of CO2 with a 5-carbon sugar called ribulose biphosphate
(RuBP); the most abundant enzyme on earth.
• stomata (singular, stoma): Openings on the underside of a leaf which allow gas exchange and transpiration.
• transpiration: A process by which plants lose water; occurs when stomata in leaves open to take in carbon
dioxide for photosynthesis and lose water to the atmosphere in the process.
Summary
•
•
•
•
•
The Calvin Cycle uses the NADPH and ATP from the Light Reactions to “fix” carbon and produce glucose.
Carbon dioxide enters the Calvin Cycle when Rubisco attaches it to a 5-carbon sugar.
Most plants fix CO2 directly with the Calvin Cycle, so they are called C-3 plants.
Some plants have evolved preliminary fixation pathways, which help them conserve water in hot, dry habitats.
C-4 plants use a 3-carbon carrier to compartmentalize initial carbon fixation in order to concentrate CO2 before
sending it on to Rubisco.
• CAM plants open their stomata for preliminary CO2 fixation only at night.
• In the Calvin Cycle, the fixed CO2 moves through a series of chemical reactions, gaining a small amount of
energy from ATP or NADPH at each step.
• Six turns of the cycle process 6 molecules of carbon dioxide and 12 hydrogens to produce a single molecule
of glucose.
Review
1. Match the major events with the stage of photosynthesis (Light Reactions or Calvin Cycle) in which they
occur.
a.
b.
c.
d.
e.
f.
g.
Carbon dioxide is fixed.
Electrons in chlorophyll jump to higher energy levels.
Glucose is produced.
NADPH and ATP are produced.
NADPH and ATP are used.
Oxygen gas is released.
Water is split.
2. Explain the value of cycles of chemical reactions, such as the Calvin Cycle.
3. Define carbon fixation.
4. Explain how their various methods of carbon fixation adapt C-3, C-4, and CAM plants to different habitats.
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Chapter 1. The Calvin Cycle - Advanced
References
1. Top: User:Maksim/Wikimedia Commons; Bottom: Alex Costa. Top: http://commons.wikimedia.org/wiki/F
ile:Leaf_anatomy.jpg; Bottom: http://commons.wikimedia.org/wiki/File:Plant_stoma_guard_cells.png . Top:
Public Domain; Bottom: CC BY 2.5
2. Left to right: John Talbot (Flickr:jbctalbot); Flickr:lobo235; Jill Robidoux (Flickr:jylcat). Left to right: http:
//www.flickr.com/photos/laserstars/503948601/; http://www.flickr.com/photos/lobo235/76154752/; http://ww
w.flickr.com/photos/jylcat/562393266/ . CC BY 2.0
3. Laura Guerin. CK-12 Foundation . CC BY-NC 3.0
4. Zachary Wilson. CK-12 Foundation . CC BY-NC 3.0
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