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UNIT 1:  METABOLIC PROCESSES

 

D.  Alternative Pathways of Carbon Fixation

 

C₃ Plants in Hot, Arid Climates

C₃ plants use rubisco to add CO₂ to RuBp.  The resulting compound is a 3-carbon molecule called PGA.  Rice, wheat, soybeans, and Kentucky blue grass are examples of C₃ plants.  On hot, dry days most plants close their stomata to conserve water but at the same time reduce CO₂ intake, which reduces photosynthesis rates.  If stomata are closed, O₂ builds up & CO₂ can’t enter.  This results in photorespiration – a process where rubisco accepts O₂ in the place of CO₂, since oxygen levels increase, and carbon dioxide levels decrease.  Figure 1, p. 168, demonstrates how photorespiration rates increase as photosynthesis rates decrease, with increasing temperature, and stomatal closure.  Rubisco adds O₂ to the Calvin cycle, instead of CO₂.  The product splits, and the two-carbon compound piece is exported from the chloroplast, to the mitochondria, where peroxisomes break it down and CO₂ is released.  Unlike respiration, photorespiration makes no ATP.  Unlike photosynthesis, photorespiration makes no food.  Photorespiration appears to be a feeble attempt, by the plant, to produce CO₂ when levels are low.  The problem is that important, valuable Calvin acids are “siphoned” out of the photosynthetic pathway, and the photorespiratory-produced CO₂ is not fixed since the organic acid that is involved in the “fixing” (namely RuBp) was consumed to make the CO₂ in the first place.  Photorespiration does not benefit C₃ plants at all, which would lead you to ask the question:  why does it happen?  One of the proposed explanations as to why C₃ plants photorespire, is that this mechanism is in fact a remnant trait that was once adaptive when plants grew in low O₂ atmospheric concentrations, but is now maladaptive since the environment has changed.  Modern rubisco retains some of its ancestral affinity for O₂, which makes photorespiration inevitable.

 

Some plants undergo an alternate carbon fixation pathway, which minimizes photorespiration, especially in hot, arid conditions.  These strategies have evolved as both modifications in biochemistry and in plant physiology.  The following two pathways represent solutions to the problem of maintaining photosynthesis when stomata close on sunny, hot, dry days. 

 

C₄ Plants in Hot, Arid Climates

C₄ plants are plants that undergo a different carbon fixation process than C₃ plants.  Sugar cane, corn, and crab grass are examples of C₄ plants.  Figure 2, p. 169, shows a C₄ leaf cross-section.  The differences between C₃ leaf anatomy and C₄ leaf anatomy is notable.  Click on http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html#C-4 for an illustration of both leaf types.  C₄ plants have two types of photosynthetic cells in their leaf tissue.  Bundle sheath cells surround veins, and mesophyll cells surround the bundle sheath cells.  All three cells are connected via plasmodesmata (cell-cell connections – kind of like underground tunnels connecting two buildings)  This anatomical arrangement of photosynthetic cells, together with the biochemical modifications within these cells, establishes the difference between C₃ carbon fixing and C₄ carbon fixing.  In the C₄ pathway the Calvin cycle is preceded by the incorporation of CO₂ into an organic acid in the mesophyll cell first.  The CO₂ is added to phosphoenolpyruvate (PEP) to make oxaloacetate with the help of special C₄ fixing enzyme called phosphoenolpyruvate carboxylase (PEPc).  Oxaloacetate is a 4-carbon organic acid, which is the reason why this is called the C₄ pathway.  PEPc has a much higher “grab” or affinity for CO₂ than rubisco does, especially when levels of CO₂ are low.  Basically, PEPc can “fix” CO₂ efficiently when rubisco cannot.  The oxaloacetate becomes malate, which then enters the bundle sheath cells via the plasmodesmata.  The malate loses a CO₂ and becomes a 3-carbon pyruvate.  The pyruvate moves into the mesophyll cell and is greeted by an ATP.  The ATP phosphorylates the pyruvate to recreate the PEP so that the cycle is complete.  This “pre-Calvin” cycle ensures a continuous “pumping” of CO₂ molecules into the bundle sheath cells, thereby maintaining a high concentration of CO₂ rubisco in the Calvin cycle.  Effectively, the C₄ pathway minimizes photorespiration (during low CO₂ levels), and maximizes sugar production.  Figure 2, p. 169 illustrates the C₄ pathway in detail.  The C₄ pathway uses almost twice as much ATP as the C₃ pathway to produce glucose.  However, without this pathway, photorespiration would stress the plant out so much that it would not survive!

 

CAM Plants in Hot, Arid Climates

Unlike most plants, some plants, such as cacti and pineapple plants, open their stomata at night and close them during the day to prevent water loss, since they live in very hot and dry environments.  Closing stomates during the day prevents water loss, however limits the amount of CO₂ in the leaves.  To compensate for these low CO₂ levels, the stomates open at night to let in CO₂ when temperatures are relatively lower.  CAM plants incorporate CO₂ into an acid called crassulacean acid, within the mesophyll cells, and store it in vacuoles overnight.  During the day, when ATP and NADPH are made by light reactions, the CO₂ is released from the organic acids so it can be incorporated into sugar in the Calvin cycle.  CAM stands for “Crassulacean acid metabolism”, since all the plants in the Crassulacean family, commonly known as succulent plants, undergo this CO₂ fixing process. 

 

Comparing C₃, C₄, and CAM Photosynthesis

Figure 4, p. 171, illustrates C₃, C₄, and CAM CO₂ fixation.  In C₃ plants, low levels of CO₂ cause rubisco to go “schizo” – it no longer binds to CO₂.  Instead, it bonds to oxygen.  The result is a decrease in CO₂ uptake, a siphoning off of Calvin acids, and an increase in photorespiration rates.  C₃ plants do not have a mechanism to help them fix CO₂ when it occurs in low levels.  In C₄ plants, the first part of carbon fixation and the Calvin cycle occur in separate compartments of the leaf.  In CAM plants, the two steps occur in the same compartment, but at different times of the day.  Figure 3, p. 170, illustrates a comparison of C₄ and CAM plants.  Both methods initially produce organic acids that eventually transfer CO₂ to the C₃ Calvin cycle.  In C₄ plants, these two processes occur in two different types of cells at the same time, connected by plasmodesmata (spatial separation).  In CAM plants, the two processes occur in the same compartment cell, but at different times (temporal separation): carbon fixation into organic acids during the night and the Calvin cycle during the day.

 

Homework:            1-9, p. 172