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 Simplified Calvin Cycle Detailed

Calvin Cycle

The Calvin Cycle is the C3 Pathway which is the metabolic pathway for carbon fixation in photosynthesis, which occurs in all plants.

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 OUTPUT: G3P is used to make glucose. 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.

Review: Photosynthesis Uses Light Energy to Make Food Molecules

C3 PathwayThe 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.

PhotorespirationDuring 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.

Photorespiration in a C3 plant

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 & PhotosynthesisCC44 Plants Have a Pathway (The C Plants Have a Pathway (The C44 Pathway) to Prevent Pathway) to Prevent

Water Loss and Prevent Photorespiration.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 CThe C44 Pathway (C Pathway (C44 & C & C33 Cycles Together) Cycles Together)

Note: PEP is a compound and PEP carboxylase is an enzyme catalyzing the reaction.

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.

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.

C3 cycle occurs here.

C4 cycle occurs here.

C4 Plants Perform the C4 Pathway, which is the C4 Cycle along with the C3 Cycle (Calvin Cycle).

C4 Pathway Simplified

C4 cycle

C3 cycle

Mesophyll Cell

Bundle Sheath

Cell

2 Figures Showing C4 Pathway

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 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.

C4 Plants (C4 Pathway {C4 Cycle & C3 Cycle Together})

C3 Plants (C3 Cycle Only)

CAMCAM Plants are Succulent Plants that have CCrassulacean AAcid MMetabolism.

CAM Plants Also Have a Way to Prevent Water Loss and CAM Plants Also Have a Way to Prevent Water Loss and Prevent Photorespiration. Prevent Photorespiration.

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.

A Succulent Plant is a plant having juicy or watery tissues. These plants are often found in hot, dry climates.

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.

CAM PathwayCAM Pathway

CAM Pathway Simplified4. 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, CCAM, C44, and C, and C33 plants all eventually use the Calvin Cycle plants all eventually use the Calvin Cycle (C(C33 cycle) to make sugar from CO cycle) to make sugar from CO22..

Crassulacean Acid Metabolism (CAM)

Nighttime Nighttime DaytimeDaytime

Factors Affecting ProductivityPlant 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:

Important Nutrient Facts: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.

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).

BIO 141 Botany with Laboratory

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