17 February 2006
Reading: Chapter 8, Taiz and Zeiger's Plant Physiology
5. C4 photosynthesis
A variety of photosynthetic organisms have ways of avoiding photorespiration. Many algae actively take up bicarbonate ion from the surrounding water and thus raise the [CO2] inside their photosynthetic cells, suppressing photorespiration. On land, CO2 cannot be taken up by pump proteins because it is small and uncharged. A "metabolic pump" is used instead to concentrate CO2 inside special "bundle sheath" cells where RUBISCO does the carboxylation reaction. This mechanism is called C4 photosynthesis because the first product of carboxylation has four carbons instead of the three.
C4 photosynthesis has evolved among land plants a number of times and there are several versions of it. We will learn the NADP-malic enzyme type, which is present in maize, sugarcane, sorghum, and crabgrass, among others.
An alternative to the C4 path would be significant improvement in the carboxylating efficiency of RUBISCO. Some improvement has occurred over time (more ancient photosynthetic lineages have RUBISCO isoforms that are a little less specific fro CO2 than those in plants) but not enough to suppress the adaptive value of the C4 path. has not. It may be that changes in RUBISCO that would abolish oxygenation are not structurally possible.
a. 2 cell types, 2 carboxylating enzymes
Maize plants and other NADP-malic enzyme C4 plants have a characteristic architecture of their leaves. The photosynthetic cells are divided into 2 types: mesophyll cells and bundle sheath cells. The bundle sheath cells form a ring around the vascular bundles of the leaves. The mesophyll cells are the rest of the photosynthetic cells, more distant from the vascular bundles (See Figure 8.9 in your text for micrographs). Mesophyll and bundle sheath cells differ in their photosynthetic machinery, as described below:
Mesophyll cells Bundle Sheath cells No RUBISCO RUBISCO present PEPCase present No PEPCase PS II & PS I present No PS II. PS I is present
b. The "metabolic pump"
The C4 path is illustrated in your text in figure 8.11. To summarize, in the mesophyll cells of C4 plants, the carboxylating enzyme PEPCase combines CO2 with phosphoenolpyruvate, yielding oxaloacetate, a 4 carbon compound. This is how C4 photosynthesis gets its name. The first product of carboxylation is a 4 carbon compound. (The first stable product of carboxylation in C3 plants is 3PGA, a 3 carbon compound).
Oxaloacetate formed in the mesophyll cells is converted to malate. The malate is then transported into the bundle sheath cells. In the bundle sheath cells, the malate is decarboxylated, yielding pyruvate and CO2. The pyruvate is transported back to the mesophyll cell and phosphorylated to phosphoenol pyruvate, completing the cycle.
RUBISCO and the Calvin cycle are present in the chloroplasts of the bundle sheath cells of C4 plants. CO2 levels in the bundle sheath chloroplasts are roughly 10 times the air level because of the release of CO2 from malate. Also, oxygen is relatively low in bundle sheath chloroplasts because of the absence of PS II. The high ratio of CO2 to oxygen in the bundle sheath cells prevents the oxygenation reaction of RUBISCO and the processing of 2 phosphoglycolate is thus unnecessary. C4 plants exhibit no detectable photorespiration.
PEPCase is the acronym for the enzyme phosphoenolpyruvate carboxylase. Like RUBISCO, PEPCase "fixes" carbon dioxide into an organic molecule. Unlike, RUBISCO, PEPCase does not react with oxygen. It also has a higher affinity for CO2 than RUBISCO does, as can be seen from its lower Km value.
Km CO2 for PEPCase = 2 µM
Km CO2 for RUBISCO = 12 µM
(Km O2 for RUBISCO = 250 µM)
The different Km values of PEPCase and RUBISCO can be seen in the carboxylation rates of C3 and C4 plants when measured over a range of carbon dioxide concentrations.
d. ATP and NADPH
From the diagram of the C4 path in figure 8.11 of your text, you can see that running the Calvin cycle in bundle sheath cell chloroplasts presents some problems. There is no PS II in these chloroplasts and thus no O2 but also no NADPH. NADPH for the Calvin cycle is provided partly by the oxidation of malate to pyruvate and also by some direct transport of NADPH from the mesophyll cells, which have PS II and PS I and run regular photosynthetic electron transport. ATP for the Calvin cycle in bundle sheath chloroplasts is provided mostly by means of cyclic photophosphorylation driven by PS I, a case where this pathway is significant to photosynthesis.
e. Costs and benefits of C4 photosynthesis
C4 photosynthesis avoids photorespiration and is thus potentially more efficient than C3. There are some intrinsic costs to the C4 path, however.
Phosphoenolpyruvate is regenerated from pyruvate at the expense of 2 ATP per molecule. This is in addition to the ATP used by the Calvin cycle and is an energetic cost of C4 photosynthesis. This cost can be seen by measuring the number of absorbed photons required by C3 and C4 plants in the laboratory under conditions of elevated CO 2, where oxygenation does not occur.
C3, 1% CO2:
10 photons absorbed / CO2 fixed
2 NADPH & 3ATP / CO2 fixed
C4, 1% CO2:
14 photons absorbed / CO2 fixed
2 NADPH & 5ATP / CO2 fixed
In nature, however, C4 plants are actually more efficient than C3 because they avoid the wasteful process of photorespiration. In air, the following results occur:
C3, air :
18 photons absorbed / CO2 fixed
C4, air :
14 photons absorbed / CO2 fixed
The advantage of being C4 is affected by temperature and water availability. At higher temperatures, oxygenation reactions in C3 plants increase but C4 plants still do no oxygenation. This increases the advantage of being C4. C4 plants are also more efficient at photosynthesizing without losing water. Because PEPCase has a low Km for CO2, C4 plants can keep their stomata a little more closed than C3 plants without slowing carboxylation.
The increased water use efficiency of C4 photosynthesis can be seen by examining an equation for water use efficiency:
Water use efficiency = A/E = Ca - Ci / (ei - ea) 1.6
A = CO2 uptake by the leaf.
E = Evaporation of water from the leaf.
Ca = [CO2] outside the leaf.
Ci = [CO 2] inside the leaf.
ea = [water vapor] outside the leaf.
ei = [water vapor] inside the leaf.
1.6 = correction factor for slower diffusion of CO2 than water vapor. CO2 is a larger molecule than water and diffuses more slowly.
Ca and ea tend to be constant for all plants in a given setting. C4 plants are more water use efficient than C3 because they can maintain a lower Ci than C3 plants without slowing carboxylation. They can do this because of the low Km for CO2 of PEPCase.
The increased water use efficiency of C4 plants gives them a potential advantage in hot, dry environments where lack of water limits plant growth.