Packet 34

1
Photorespiration

• Packet 34
• Chapter 10

2
Introduction

• In the 1960s, it was discovered that illuminated
  plants consume and use O2 and produce CO2.
• With low CO2 levels and high O2 levels, this
  photorespiration overwhelms photosynthetic CO2
  fixation (light reactions Calvin cycle).

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Introduction II

• In a very lengthy and costly process, O2 is
  converted into CO2 and 3-phosphoglyceraldehyde.
• Photorespiration involves the use of three
  organelles
• Chloroplast
• Peroxisome
• Mitochondria
• Photorespiration also requires the use of ATP and
  NADPH.
• Reducing the number of those molecules readily
  available for the Calvin cycle (photosynthesis).

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Introduction III

• With higher levels of O2, O2 competes with CO2 as
  a substrate for the enzyme rubisco (RuBP
  carboxylase/oxygenase).
• This is the foundation of photorespiration.
• O2 binds to rubisco and starts the series of
  reactions that eventually lead to the production
  of CO2 and 3-phosphoglyceraldehyde.
• A wasteful process that undoes the some of the
  work of photosynthesis.

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Introduction IV

• Plants can be divided into three categories based
  on how they deal with photorespiration.
• C3 plants
• No mechanism developed to decrease
  photorespiration.
• C4 plants
• CAM plants
• Both C4 and CAM plants have mechanisms in place
  to decrease photorespiration.

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Photorespiration C3 Plants
7
Photorespiration in C3 Plants

• On dry, hot days in the presence of light C3
  plants close their stomata.
• This causes the plant to use O2 retain as much
  H2O.
• O2 binds to rubisco and starts the series of
  reactions.
• H2O is retained for use in the light reactions to
  fill the ATP and NADPH used as a result of
  photorespiration.

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C4 CAM Plants Introduction

• C4 and CAM plants have devised mechanisms that
  prevent/reduce the impact of photorespiration.

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C4 Plants Photorespiration

• Concentrating CO2

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Introduction I

• C4 plants occur largely in tropical regions
  because they grow faster under hot and sunny
  conditions.
• C3 plants live in cooler climates where
  photorespiration is less of a burden and less ATP
  is required to fix carbon.

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Introduction II

• On a hot bright day, when photosynthesis has
  depleted the level of CO2 at the chloroplast and
  raised that of O2, the rate of photorespiration
  reaches the rate of photosynthesis.
• However, C4 plants have leaves that are different
  anatomically to that of C3 plants and have
  devised a mechanism to reduce the impact of
  photorespiration.

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Introduction III

• Photorespiration is negligible in C4 plants
  because the concentration of carbon dioxide is
  always high in the bundle sheath cells.
• C4 plants concentrate CO2.

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C4 Plants Process I

• Phosphoenol-pyruvate (PEP) ? Oxaloacetate
• PEP carboxylase adds CO2 to PEP to produce
  Oxaloacetate
• Occurs in the mesophyll cell.

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C4 Plants Process II

• Oxaloacetate?Malate
• Malate dehydrogenase reduces oxaloacetate into
  malate.
• NADPH is used during this step.

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C4 Plants Process III

• Malate is transported from mesophyll cell into
  the bundle sheath cell.

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C4 Plants Process IV

• Malate ? Pyruvate
• Malic enzyme converts malate into pyruvate.
• Two byproducts are made.
• CO2
• The CO2 is now considered to be concentrated.
• NADPH
• Both are used in the Calvin Cycle that occurs
  within the bundle sheath cell.

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C4 Plants Process V

• Pyruvate leaves the bundle sheath cell and enters
  the mesophyll cell.

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C4 Plants Process VI

• Pyruvate?PEP
• Pyruvate-phosphate dikinase converts pyruvate
  into PEP.

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C4 Plants Process VII

• Process is repeated to concentrate more CO2.

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Additional Information on C4 Plants.

• At lower light levels and temperature, C4 plants
  will utilize the traditional C3 pathway.
• Examples of C4 plants
• Sugarcane
• Corn
• Crab grass.

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CAM Plants Photorespiration

• Storing CO2

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Introduction I

• CAM is an acronym for crassulacean acid
  metabolism.
• Examples
• Succulent plants family Crassulaceae
• Family Cactaceae
• Family Lilaceae
• Family Orchidaceae
• Many others in 25 families.

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Introduction II

• CAM plants exhibit a pathway similar to C4 plants
  and allow them to live in highly xeric
  conditions.
• CAM plants store CO2 using a variation of the
  technique used by C4 plants.

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CAM PlantsNight

• CAM plants open their stomata at night.
• If these plants, living in the xeric conditions
  opened their stomata during the daytime, would
  lose large amounts of H2O through osmosis and
  then evaporation.
• PEP carboxylase fixes carbon at night in the
  mesophyll cells
• Stomata are open at night
• Minimizes water loss and allows the entry of CO2
• Calvin Cycle occurs during the daytime

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CAM PlantsNight II

• PEP?oxaloacetate?malate
• PEP carboxylase and malate dehydrogenase fixes
  CO2 at night in the mesophyll cells.
• Malate is stored in vacuoles.

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CAM PlantsDaytime I

• During the daytime, while the stomata is closed,
  malate is converted into pyruvate in the bundle
  sheath cell.
• This allows the production of CO2.
• CO2 is used to drive the Calvin cycle in the
  bundle sheath cell.

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Review
28
Review

• Types of plants
• C3
• C4
• Concentrate CO2 in the bundle sheath cells.
• CAM
• Store CO2 in the form of malate by having the
  stomata only open at night.

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Homework Assignment

• What are some of the similarities, and
  differences, between C4 and CAM plants?