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.
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).
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.
other 10 molecules of G3P are used to regenerate RuBP the substrate for
Environmental pressures such as high temperatures,
saline environments, limited water availability and changing concentrations
of CO2 increased the demand for more efficient mechanisms of
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
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.
|Economically Important Species||wheat, rice, barley, potato||maize, sugar cane, millet|
|Leaf anatomy||Pallisade and spongy mesophyll, if bundle sheath present no chloroplasts||Kranz anatomy with bundle sheath containing chloroplasts|
|Chloroplasts||1 type||2 types (dimorphic)|
|Primary Carboxylase||Rubisco||PEPCase in mesophyll|
|Secondary Carboxylase||None||Rubisco separated in space (bundle sheath)|
|Primary CO2 acceptor||RuBP||PEP|
|1st stable product||3-phosphoglyceric acid (3-PGA)||oxalacetate (OAA)|
|Transpiration rate||high||~25% of C3|
|light compensation point||5 Wm-2||< 1 Wm-2|
|photorespiration rate||High (30% of net photosynthesis)||low to undetectable|
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