Also, activities of fructose 1,6 bis are dependent on pH and Mg2+
2-
phosphoribulose kinase, but PG3- does not. Alkal fa 3-
There is also evidence that the genes of chloroplasts are regulated at the transcriptional level and are repressed by sucrose and glucose.
Figure 21: Light dependent regulation of Calvin cycle enzymes: Glyceraldehyde-3- phosphate dehydrogenase, fructose 1, 6-bisphosphatase, sedoheptulose 1, 7-bisphosphatase
and phosphoribulokinase
Efficiency of photosynthesis
During the Linear electron flow or Z scheme of photosynthesis, PSII cycles four times i.e.
absorbs 4 photons to form one molecule of O2 and release 4 electrons which go to PSI. PSI absorbs another 4 photons to reduce NADP+. A total of 8 photons are thus required to synthesize 2 molecules of NADP and 3 of ATP per O2 evolved. This agrees with the observed experimental value of 0.12 O2 release per absorbed photon, when both PSI and PSII are operating at optimal efficiency. Thus, 48 photons must be absorbed for 12 NADPH and 18 ATP required for the synthesis of a molecule of hexose in the dark reactions. If the cyclic photophosphorylation also operates then the number of photons needed will be larger.
Forming a mole of hexose from CO2 and H2O requires 2870 kJ of energy. As the energy input of photons is dependent upon the wavelength of light used, assuming that light of 650nm wavelength is absorbed, then 48 photons of such light will correspond to about 8200 kJ of energy. Therefore, the efficiency of photosynthesis is around 35%. This is in agreement with direct experimental results under optimal conditions for photosynthesis.
Photo respiration
ubisco is a bifunctional e associated oxygenase activity It catalyzes the oxygenation of Ribulose 1,5 bisphosphate (RuBP) at catalytic site on the L subunit as
e classical competitive inhibitors. Both the enzyme activities are inhib bisphosphate, CABP. The Km for CO2 is 12µM as against 250 ch higher than that for CO2.
When O2 c 2 with Ribulose 1,5 bis
phosphate to form ycerate in the chloroplast (Figure 22)
Phosphoglycolate is toxic to plants being a powerful inhibitor of Triosephosphate Isomerase (an
important enzyme ed to glycolate by a
phosphoglycolate phosphatase. The oxygenase activity and the glycolate salvage pathway consuming O2 and releasing CO 2 cycle and occurs in three cellular compartments-chloroplasts
evolution when the hotosynthesis evolved, CO2 content was high and O2 was neglible in the atmosphere and the effect of O2 on Rubisco activity was not perceptible. However, as the O2 content in the
R nzyme and has an
the same the carboxylation where carbondioxide and oxygen are th
ited by 2-carboxyarabinitol 1,5, µM for O2 i.e. Km for O2 is mu
oncentration is high, Rubisco catalyzes the reaction of O phosphogycolate and 3-phosphogl
of glycolysis and Calvin cycle). It is dephosphorylat
2 is called Photorespiration or C , mitochondria and peroxisomes.
Figure 22: Oxygenase activity of Rubisco
Glycolate passes on to peroxisomes where it is oxidized to give hydrogen peroxide and glyoxylate. Hydrogen peroxide being toxic is acted upon by catalase. Glyoxylate is converted to glycine which then enters the mitochondria where two molecules of glycine form one molecule each of serine, CO2 and NH3. The latter two products are released and serine is returned to the peroxisome where it is converted to glycerate. Glycerate then enters the chloroplast, where it is converted by ATP dependent glycerokinase to 3-phosphoglycerate (3PG) which enters the
igure 23).
Calvin cycle (F
Thus, photorespiration is a light dependent reversal of the CO2 fixation which oxidizes RuBP, uses ATP, and is a wasteful process. About 25% of the (RuBP) is lost and 75% is available as 3PG. Infact, it decreases the efficiency of photosynthesis. Photorespiration occurs at all times to some extent in C3 plants but is very active during such conditions as bright sunlight, high
emperatures, low CO t
p 2
and high O2 concentrations. Early during
Figure 23: The photorespiration pathway for salvage of glycolate produced by oxidation of ribulose 1,5 bisphosphate (RuBP) by Rubisco
(Source : R a w n , J . D. 1 9 8 9 . Bi o ch e m i s t r y N eil Pa tt e r s o n P u b l i sh e r s. N . C a r o l en a , US A )
atmo
No Rubisco has been found that ha ation suggests that when CO2
concentratio sual to form
ATP and NADPH which accum actions due to
low availability of in the reduced electron carriers, proton
gradient across the thylakoid m s,
ing
ATP and NADP reaction to continue i.e.
help in dissipating th ts damage to the photosynthetic
components.
nation of leaves or chloroplasts
in the absence of bot versible loss of their
photosynthetic ability.
Some
PATHWAY which conserve CO late etc.) This is different
from
The C4 and CAM pathways are tw intaining photosynthesis on
hot, dry days.
C4 Pathway
ists who elucidated this millet - the agricultu
high temp
Mesophyll cells
As discussed in earlier section, CO esophyll cells.
lized bundle sheath cells
lying below the me atmospheric
oxygen (Figure 24).
The bundle sheath cells contain
carboxylase enzyme ts than those of C3
I which carries out cyclic electron flow, and therefore no O
he meseophyll cells are exposed to the outside, do not contain Rubisco but have
2
s
sphere increased, photorespiration assumed significance. Inspite of its deleterious effects, why has the oxygenase activity of Rubisco been retained over the years during evolution?
s no oxygenase activity. One explan
n is low, but the sunlight is sufficient, the light reaction proceeds as u ulate, as they are not utilized in the Calvin cycle re CO2. This, in turn, leads to an increase
embrane and photochemical excited states of the phostosystem which have deleterious effects on the Photosynthetic assemblies. Photorespiration by consum
H, provides ADP and NADP+ for the activity of the light e photochemical energy and thus preven
This hypothesis is supported by the observation that bright illumi
h CO2 and O2 results in the rapid and irre
plants have bypassed the harmful effects of photorespiration by using the C4 and CAM
2 as C4 intermediates (oxaloacetate, ma the Calvin cycle or the C3 pathway which uses C3 intermediates.
o evolutionary solutions for ma
C4 pathway is also known as Hatch-Slack pathway after the scient
pathway in 1960s. The C4 cycle operates in several thousand species such as sugarcane, corn, rally important plants and in tropical plants exposed to intense sunlight and eratures and dryness.
In C4 plants, there are two distinct types of photosynthetic cells (1) Bundle Sheath cells and (2)
2 fixation in the C3 plants occurs in the m However in C4 plants, the Calvin cycle photosynthesis occurs in specia
sophyll cells in the interior of the leaf and are protected from
chloroplasts and Rubisco but no phosphoenol pyruvate (PEP) . It has less grana and more starch in their chloroplas
plants and are unusual as they lack PSII and contain only PS
2 is evolved.
T
phosphoenolpyruvate (PEP) carboxylase which captures CO2. The C4 pathway physically separates the CO capture from the Calvin cycle reactions in C4 plants. The C4 pathway is
hown in Figure 25.
Figure 24: Diagram of the structure of leaves of C4 and C3 plants
o l e c u l a r c e l l B i o l o g y, 3r d E d i ti o n , S c i en ti f i c A m e r i c a n B o o k s W . H.
r e e m a n a n d co m p a n y , U S A )
(Source : R a w n , J . D. 1 9 8 9 . Bi o ch e m i s t r y N eil Pa tt e r s o n P u b l i sh e r s. N . C a r o l en a , US A )
Figure 25: The C4 pathway involving the capture of CO2 in the mesophyll cells and its transfer to the bundle sheath cells for fixation by Calvin cycle
(Source : L o d i s h , H ., B a l t i mo r e, D . , B e r k, A . , Z i p u r s k y , S . L . , M e t a s u d a i r a , P . a n d D a r n el l , J . 1 9 9 5 . M
f
Carbon dioxide is fixed in the mesophyll cells by PEP carboxylase catalyzed reaction between HCO-3 (which is in equilibrium with CO2) and phosphoenol pyruvate (PEP) to form oxaloacetate. This enzyme has a very low Km for CO2 and has no oxygenase activity, so that even at low CO2 concentration oxaloacetate is formed. Oxaloacetate can be converted to any of the other C4 acids. Malic acid is formed by reduction of oxaloacetate with NADPH specific malic dchydrogenase, and transported to the bundle sheath cells through plasmodesmata where malic enzyme catalyzes its oxidative decarboxylation to from pyruvate and CO2. The CO2
released enters the Calvin eycle and the pyruvate is transported to the mesophyll cells where it is converted to PEP by pyruvatephosphate dikinase using ATP to form AMP and pyrophosphate.
The pyrophosphate is further hydrolysed to give two Pi. The AMP conversion to ADP requires another molecule of ATP. So that two molecules of ATP are required in the conversion of pyruvate to PEP. The species of C4 plants which have PEPcarboxy kinase use only one ATP. The CO2 fixation, therefore, consumes 4-5 ATP in C4 plants instead of 3ATP in Calvin cycle per CO2
molecule fixed. Thus, the C4 pathway is energetically more expensive and the overall efficiency of photosynthetic production of sugars is lower than that in C3 plants. However, the net rate of photosynthesis in C4 plants can be two to three times of that in C3 plants. It is observed that the
4 plants flourish in hot, dry, tropical climate whereas C plants predominate in temperate climate (low t
C 3
emperature).
Crassulacean acid metabolism (CAM)
The CAM pathway is evolved in sacculent (water storing) plants e.g. cacti, pineapple and members of the Crassulacean family. In these species, the stomata close during the day and open during the night, as the temperature is lower at night and also humidity is usually higher. This helps the dessert plant to conserve water and reduce transpiration (prevent water loss during the day). These plants take up CO2 at night, convert it into malic acid or isocitric acid (C4 compounds). During the day, when stomata are closed, CO2 is released from these C4 organic acids and is used immediately in the Calvin cycle reaction. This process is known as Crassulacean acid metabolism (CAM) and the plants that employ it are known as CAM plants. It is analogous to the C4 pathway but in this case there is no physical separation of CO2 capture and Calvin cycle. The two processes occur at different times (night/day).
The C4 and CAM plants eventually use Calvin Cycle, like the C3 plants to make sugar from CO2.
Suggested Readings
1. Barber. J., and Anderson, B., 1994. Revealing the blue print of photosynthesis. Nature 370, 31-34.
2. Bassham, J. A., 1962. The path of carbon in photosynthesis Sci. Am. 206, 88 – 100.
3. Buchanan, B.B., 1991. Regulation of CO2 assimilation in oxygenic photosynthesis: the ferridoxin/thioredoxin system. Perspective on its discovery, present status and future development. Arch. Biochem. Biophy., 288, 1- 9.
4. Green, B.R., Pichersky, E, and Kloppstech, K, 1991. Chlorophyll a/b binding proteins: an extended family.
Trends Biochem. Sci. 16, 181-186.
5. Govindjee and W.J. Coleman, 1990. How plants mak oxygen. Sci. Am., e 262, 50-58.