Rubisco and carbon fixation
It is relatively simple for plants to acquire oxygen when their rate of respiration exceeds their rate of photosynthesis, since the concentration of oxygen in air is high (about 21%). However, when the photosynthesis rate exceeds the respiration rate, it is more difficult to acquire carbon dioxide, which is present at a concentration of only about 0.04%. (Typically CO2 concentrations are expressed in parts per million or ppm. A concentration of 0.04% is equivalent to 400 ppm.) In order to capture CO2 molecules from the atmosphere, plants utilize an enzyme called ribulose bisphosphate carboxylase/oxygenase (rubisco) to catalyze the attachment of free CO2 molecules to a five-carbon receptor (ribulose 1,5-bisphosphate or RuBP) that is a component of the Calvin-Benson cycle of photosynthesis. The capture of free CO2 molecules is called carbon fixation. Because of the importance of this reaction and the slowness in acquiring the low concentration CO2, plants expend a large fraction of their resources to the production of rubisco, which makes up about 20% or more of the protein in a plant leaf.
Unfortunately (for the plant), rubisco not only functions as a carboxylase (attaching carbon to RuBP) but also as an oxygenase (attaching O2 to RuBP). RuBP that has been oxygenated is changed into other compounds unusable for carbon fixation, so plants use a metabolic pathway that recycles these compunds. Because this pathway uses O2 and releases CO2, it is known as photorespiration. Photorespiration consumes ATP and NADPH and releases some of the captured carbon. It therefore reduces the photosynthetic efficiency of the plant. Under normal conditions, rubisco has a much greater affinity for CO2 than for O2, so carbon fixation predominates over photorespiration. However, under low CO2 concentrations and high temperatures, photorespiration is favored over carbon fixation.
Water loss in C3 plants
Fig. 4 Cross section of a C3 leaf (Ligustrum sp.; common name: privet)
High temperatures also reduce the efficiency of photosynthesis in another way. Stomata in leaves (Fig. 4) allow diffusion of CO2 and O2 in and out of the mesophyll tissue in the interior of the leaf. However, water loss (i.e. transpiration) is an unavoidable consequence as well. At high temperatures (and low humidity) the rate of water loss through the leaves can increase to dangerous levels and leaves respond by closing their stomata. Once the stomata close, CO2 within the leaf becomes depleted and the CO2 concentration falls to a level where photorespiration is favored over carbon fixation. Thus the plant may become unable to store energy in the form of carbohydrates through photosynthesis even when light is available.
Adaptation in C4 plants
Fig. 5 Cross section of a C4 leaf (Saccharum = sugar cane)
Some plant species, such as certain groups of grasses, have evolved a mechanism that overcomes the problems caused by photorespiration. They separate carbon fixation spatially from the Calvin-Benson cycle by capturing CO2 with the enzyme PEPcase (forming a four-carbon organic acid) in mesophyll cells adjacent to the air spaces in the leaf. The four-carbon acid is then transported to adjacent bundle sheath cells, where the Calvin-Benson cycle occurs utilizing rubisco. Because the rubisco in the bundle sheath cells is kept in a CO2-rich environment, little photorespiration occurs. Plants with this mechanism are known as C4 plants (from the four-carbon organic acid that is formed). Plants without it are known as C3 plants (from the three-carbon intermediate that forms as carbon is fixed in those plants). The C4 process is so efficient at carbon fixation in a low CO2 environment that net photosynthesis can continue at CO2 concentrations approaching zero.
C4 plants (Fig. 5) predominate over C3 plants (Fig. 4) in areas where it is hot and water stress can be severe, such as tropical grasslands. On the other hand, C3 plants are more productive than C4 plants under low light and cool temperatures because the "extra steps" cost the C4 plants ATP. Therefore, both C3 and C4 species persist in the different climatic conditions along longitudinal gradients.
In today's experiment, we will investigate differences in the photosynthetic responses of Phaseolus vulgaris (beans, a C3 species) and Zea mays (maize or corn, a C4 species).
