Theory

Photosynthetic organisms use light to form ATP and NADPH which are then used as energy sources to form sugars and other organic compounds from CO₂ and water.  In all photosynthetic organisms CO₂ is taken up and reduced (i.e. gain of electrons or hydrogens) to carbohydrate (CH₂O) via a cyclic process that employs the enzyme ribulose 1.5-bisphosphate (Rubisco) (Figure 1).  This pathway, which appears to have evolved in the early stages of plant evolution, is called the photosynthetic carbon reduction cycle (PCR) or alternatively the Calvin cycle (after Melvin Calvin who elucidated the pathway in the early fifties) (Figure 2). In many plant species the carbon fixed from CO₂ is assimilated during the first stage of the PCR pathway into two identical three-carbon products (3-phosphoglycerate, 3PGA) ( Figure 2).  This form of photosynthesis is termed C3-photosynthesis and is found in all major plant families or in about 300,000 species.  Typical C3 plants include: barley, sunflower, rice, tomatoes, wheat, peanuts, cotton, sugar beet, oats, and most trees (Lawlor 1993).
 

Figure 1:  Overview of photosynthetic reactions. From Callow JA. 1999.  Virtual tutorials in plant biology project.  University of Birmingham, England.
 

Figure 2: The Calvin Cycle. From Callow JA. 1999. Virtual tutorials in plant biology project.  University of Birmingham, England.

    Phase 1: To make a 6-carbon sugar, six turns of the cycle (each cycle reducing one molecule of CO₂ ) are needed to
                       produce12 molecules of  the 3-carbon phospoglycerate (PGA).

    Phase 2: The 12 PGA molecules are reduced to the sugar phosphate, glyceraldehyde 3-phosphate (G3P) utilizing ATP
                      and NADPH from the light reactions.  Two of these molecules are combined to make hexose sugars
                      which represent the net gain of carbon in the whole cycle.

    Phase 3: The other 10 molecules of G3P are used to regenerate RuBP the substrate for Rubisco.
 

Environmental pressures such as high temperatures, saline environments, limited water availability and changing concentrations of CO₂ increased the demand for more efficient mechanisms of concentrating CO₂ and supplying it to the photosynthetic carbon reduction cycle.  The most notable of these alternative pathways of carbon assimilation that evolved is C4-photosynthesis (Figure 3).  C4 refers to the first stable products of carbon assimilation, which are 4-carbon carboxylic acids produced after CO₂ is assimilated into a 3-carbon precursor, phosphoenol pyruvic acid (PEP).  In C4 plants, cellular differentiation occurs which spatially regulates carbon assimilation. The 4 carbon acids are formed in the mesophyll cells and are then transferred to special cells (bundle-sheath cells) where they are transformed to compounds that release CO₂.  Rubisco then fixes the CO₂ released in the PCR cycle (Figure 3). C4 species have the C4 pathway in addition to the C3 pathway, not as an alternative.
 

Figure 3.  Cellular organization of many C4 plants.

The leaves of C4 plants have a characteristic Krantz (wreath in German) anatomy.  They contain two interconnected cell types, thick-walled bundle-sheath cells surrounded by thin-walled mesophyll cells. CO₂ enters the leaves via the stomata and is converted by the enzyme carbonic anhydrase into bicarbonate (HCO₃-). The enzyme PEP-carboxylase (PEPcase) fixes HCO₃- into the 4 carbon compound oxalacetate, which is then converted to malate (another 4C compound).  Malate is shunted to the Bundle-sheath cells where it is split by malic enzyme to form pyruvate (3-C compound) and CO₂.  The CO₂   is then fixed by Rubisco via the normal Calvin pathway while pyruvate is shunted back to the mesophyll cells where it is converted back to PEP. Adapted from Callow JA. 1999.  Virtual tutorials in plant biology project. University of Birmingham, England.

About 3000 species of C4 plants have been recorded from some 18 families of flowering plants.  Some C4-plants of economical importance are sugar cane, maize, and sorghum.  In general, C4-plants are thought to have evolved under conditions of high temperatures and lower CO₂ concentrations (review in Black, 1994) and thus many C4-species are from the tropics.  Characteristically, C4-plants have higher rates of photosynthesis than C3-plants.  Photosynthesis in C4 plants does not saturate but increases at high light intensities and can continue at very low CO₂ concentrations.  Subsequently, these plants have rapid growth rates and higher biomass and economic yields than C3-plants.
 

The observed increases in both global temperatures and in the atmospheric concentrations of carbon dioxide (CO₂) could have profound effects on primary production for both “wild” and cultivated species.  Higher temperatures and higher CO₂ concentrations could mean higher total primary production by increasing both the geographical areas of plant growth as well as higher photosynthetic rates for individual species.  Interspecific interactions and competition would favor species adapted to high temperatures and resistant to low water levels.  These populations could subsequently “migrate” or be introduced to new habitats and succeed other plants that cannot survive such conditions.  Additionally, the increase in primary production could then modify important biogeochemical cycles such as the carbon, oxygen, and nitrogen cycles (Figure 4).
 
 

Figure 4: Photosynthesis and Respiration: The influence of plant photosynthesis and respiration on atmospheric concentrations of oxygen and carbon dioxide. Ilana Berman-Frank, Ph.D.
 

Table 1: Summary comparison between C3 and C4 plants.  Modified from Callow JA. 1999.  Virtual tutorials  in plant biology project.  University of Birmingham, England.
 

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