The Calvin Cycle (Photosynthesis Stage 2):
The Reduction of CO2 to Sugar
As covered earlier, the Calvin cycle involves the stroma. The
Calvin cycle also imports the outputs from the light reaction (ATP
and NADPH), another input in the form of CO2, and involves
certain outputs (C6H12O6 which is glucose, NADP+, H+, ADP, P+)
The Calvin Cycle (occurring in the
stroma) converts CO2 to sugar by
using the energy in ATP and NADPH
from the light reaction. The Calvin
Cycle does not directly depend on
light but instead, on the products of
the light reaction: ATP and NADPH.
However the Calvin Cycle, like the light
reaction, occurs only in the light.
Calvin Cycle Detailed
Calvin Cycle Simplified
Calvin Cycle
1. Rubisco, an enzyme, catalyzes
the first step. This first step is
RuBP joining with CO2 to form
3-PGA.
2. ATP and NADPH provide the
energy to convert 3-PGA to G3P.
3. 5 out of 6 G3P go back into the
cycle, while 1 G3P exits, which is
the net product of the Calvin
Cycle.
4. ATP is used to convert G3P into
RuBP and the cycle continues.
OUTPUT: G3P is used to make
glucose.
To get one glucose molecule, the
Calvin Cycle must input 6
molecules of carbon dioxide.
Therefore, double everything on
this diagram to get one glucose
(C6H12O6) molecule.
The Calvin Cycle is the C3 Pathway
which is the metabolic pathway for
carbon fixation in photosynthesis,
which occurs in all plants.
Review: Photosynthesis Uses Light
Energy to Make Food Molecules
C3 Pathway
The Calvin Cycle is the C3 Pathway which is the metabolic pathway
for carbon fixation in photosynthesis, which occurs in all plants.
C3 plants perform the C3 Pathway and stop. C3 plants usually
require moderate sunlight, moderate temperatures, plentiful water
and have high carbon dioxide levels. They are often adapted to
the cold. Examples of C3 grasses include brome, timothy,
orchard, fescue, wheat, rice and barley.  Some of these are
known as cool season grasses. Other C3 plants include bean and
many trees and shrubs.
C3 plants may have problems in extreme hot weather because
oxygen gas levels are higher than carbon dioxide levels in the
atmosphere.
C3 plants have high carbon dioxide compensation points (Lower
Photosynthetic Efficiency).
Compensation Point – the carbon dioxide concentration at which
photosynthesis just equals cellular respiration.
When carbon dioxide levels fall below compensation points, plant
will eventually die because of a lack of carbon dioxide.
Photorespiration
Photorespiration in a C3 plant
During hot, dry sunlight conditions, plants can
close stomata to reduce water loss. This causes
no CO2 to enter the plant therefore causing low CO2
levels within the plant. Since there is a reduced
amount of CO2, oxygen gas (O2) is being used
instead.  This is photorespiration (respiration
involves oxygen). Photorespiration results in no
ATP, no NADPH, and no resulting sugar. This is a
severe loss of energy and photorespiration can
drain away as much as 50% of the carbon fixed by
the Calvin Cycle.
C3 plants have high photorespiration rates on hot
sunny days because they close their stomata and
CO2 levels decrease. Therefore, C3 plants are less
efficient in hot weather.
During the typical Calvin cycle, rubisco binds with
CO2. However, in photorespiration rubisco binds
with O2 because O2 is more readily available than
CO2.
Peroxisomes break down the products of photorespiration. Photorespiration is
probably a vestige from ancient times when the atmosphere had little or no free
oxygen to divert rubisco and sugar production. Photorespiration is not entirely
understood.
Photosynthesis vs. Photorespiration
Environmental Stress & Photosynthesis
C4 Plants Have a Pathway (The C4 Pathway) to Prevent
Water Loss and Prevent Photorespiration.
- C4 plants thrive in hot, sunny environments where C3 plants
would wilt and die.
- C4 plants perform the C4 pathway which is the C4 cycle along
with the C3 cycle (Calvin Cycle) together.
- C4 plants keep their stomata closed most of the time when
weather is dry but still produce sugar.
How can they do this? Where do they get the carbon dioxide
from? They can do this because of C4 Pathway.
C4 Plant Structure
- Bundle sheath cells (parenchyma cells with much starch)
surround the vascular bundles. The C3 cycle occurs in
these bundle sheath cells.
- C4 cycle occurs in the mesophyll cells.
The C4 Pathway (C4 & C3 Cycles Together)
1. CO2 enters the leaf by the way of the stomata.
2. CO2 diffuses into a mesophyll cell.
3. In the mesophyll, CO2 combines with a 3-carbon compound called PEP (a.k.a
phosphoenolpyruvate) forming the 4-carbon compound oxaloacetate. (Hence, the
plants have the name C4 plants). The enzyme that catalyzes this reaction, PEP
carboxylase, does not bind with oxygen and can therefore fix CO2 more efficiently than
rubisco.
Note: PEP is a compound and PEP carboxylase is an enzyme catalyzing the reaction.
4. Oxaloacetate is then converted into a 4-carbon compound called malate, which is
transported (by plasmodesmata) into a bundle sheath cell.
5. Once the malate is in the bundle sheath cell, it releases CO2, which gets incorporated
into G3P in the C3 cycle (Calvin Cycle). Because the bundle sheath cell is deep within
the leaf and little oxygen is present, rubisco can fix CO2 efficiently without being
diverted to the dead end of photorespiration.
6. After CO2 is released from malate, pyruvate is transported back to a mesophyll cell
where it is converted back into PEP.
With CO2 sequestered inside bundle-sheath cells, there is a steep carbon dioxide gradient
between the airspace in the mesophyll of the leaf near the stomates and the
atmosphere around the leaf. Thus, C4 plants can maximize the amount of carbon
dioxide that diffuses into the air space in the leaf and minimize the length of time the
stomates must remain open.
C4 Plant Examples: Corn, Sugarcane, Crabgrass, Bermuda Grass, Many native prairie
grasses (Warm Season Grasses), Sorghum, Millet and Tropical-Pasture Grasses. Many
tropical monocots are C4 plants but there are some dicots that are C4. Few trees or
shrubs are C4 plants.
C4 Plants Perform the C4 Pathway, which is the
C4 Cycle along with the C3 Cycle (Calvin Cycle).
C3 cycle occurs here.
C4 Pathway Simplified
Mesophyll Cell
C4 cycle
Bundle
Sheath
Cell
C3 cycle
C4 cycle occurs here.
2 Figures Showing C4 Pathway
C4 Plants (C4 Pathway {C4 Cycle & C3 Cycle Together})
C4 plants show little on no
photorespiration. In C4 plants, the
light reaction and carbon fixation
occurs in the mesophyll, and the
Calvin Cycle (C3 cycle) occurs in the
bundle sheath cells. In C4 plants,
bundle sheath cells lie under
mesophyll cells, deep within the leaf
where carbon dioxide is
sequestered.
C3 Plants (C3 Cycle Only)
C3 plants can have very high rates
of photorespiration under hot,
sunny conditions. Under milder
conditions, when photorespiration
is less likely to occur, the C3 plants
are more efficient than C4 plants, in
part because they expend less
energy to capture CO2.
In C3 leaves, the Calvin Cycle
(C3 cycle) occurs in the mesophyll.
These photosynthetic cells have
direct access to carbon dioxide.
CAM Plants Also Have a Way to Prevent Water Loss and
Prevent Photorespiration.
CAM Plants are Succulent Plants that have Crassulacean Acid Metabolism.
A Succulent Plant is
a plant having juicy
or watery tissues.
These plants are
often found in hot,
dry climates.
Crassulacean Acid Metabolism (CAM) – the biochemical pathways by
which the succulent genus Crassula and other plants (pineapple, cacti)
fix carbon at night and release it for photosynthesis during the day.
CAM Pathway
1. During the day, stomata are closed to prevent water loss. No CO2 is coming into the
plant during the day.
2. During the night, stomata are open to take in CO2 and water loss is minimal. (This is
the reverse of how most plants behave).
3. At night, when CO2 is rapidly absorbed, the enzyme PEP carboxylase initiates the
fixation of CO2 by reacting with PEP to form oxaloacetate.  Then, malate, a
4-carbon compound is generally produced.
4. Malate is converted to malic acid & stored in the
vacuole at night. Malic acid concentrations rapidly
increase in the leaf-cell vacuoles at night.
5. Leaf acidity decreases during the next day as malic
acids leave the vacuole and again become malate and
CO2 is released.
6. Even though stomata are closed during the day, the C3
cycle of photosynthesis takes place (powered by the
light reaction of that day) & converts the internally
released CO2 (originally taken in the night before) into
a carbohydrate eventually resulting in glucose.
During the CAM pathway, CO2 stays in the same cell but
the carbon atoms are converted into various
molecules and are shuffled around depending on the
time of day.
CAM, C4, and C3 plants all eventually use the Calvin Cycle
(C3 cycle) to make sugar from CO2.
CAM Pathway Simplified
Crassulacean Acid Metabolism (CAM)
Nighttime
Daytime
Factors Affecting Productivity
Plant Productivity – Amount of living tissue produced per unit of time by
a plant or population of plants.
Scientists can breed productivity into plants by selective breeding & biotechnology.
Productivity is a direct result of photosynthesis rates.
Productivity is affected by the environment in which the plant lives. The following items
affect plant productivity:
1) Temperature – most plants do well between 50 & 77 degrees Fahrenheit.
2) Light – can be too intense for some plants and not enough for others. Some
shade plants only need “flecks” of sunlight.
3) CO2 – levels can be too low in closed greenhouses during the winter.
4) Water – only 1% used for photosynthesis and much is lost by transpiration.
5) Nutrients – Primary (Great Amounts Needed)  Nitrogen (N), Potassium (K),
and Phosphorous (P).
Secondary (Second only to Primary)  Calcium (Ca),
Magnesium (Mg) and Sulfur (S).
Micronutrients (Needed in Smaller Amounts) Boron (B), Copper
(Cu), Iron (Fe), Chlorine (Cl), Manganese (Mn),
Molybdenum (Mo), Zinc (Zn).
Important Nutrient Facts: N is found in plant enzymes and proteins. Mg and N are needed in
chlorophyll. N, P, K are often important components of fertilizer. Ca and Mg can be
supplemented with lime. Fe is needed for chlorophyll synthesis. To split water into O2,
plants need Mn, Cl & Ca. Consequently, soils with poor amounts of nutrients can result in
plants with poorly developed photosynthetic capacities and reduced growth & lower yields.
BIO 141 Botany with Laboratory
•
This product is sponsored by a grant awarded under the President’s Community-Based Job Training Grants as implemented by the U.S. Department of
Labor’s Employment and Training Administration. The information contained in this product was created by a grantee organization and does not
necessarily reflect the official position of the U.S. Department of Labor. All references to non-governmental companies or organizations, their services,
products, or resources are offered for informational purposes and should not be construed as an endorsement by the Department of Labor. This product
is copyrighted by the institution that created it and is intended for individual organizational, non-commercial use only.