Page 172 - Algae Anatomy, Biochemistry, and Biotechnology

Photosynthesis 155

Regeneration

The final stage in the Calvin cycle is the regeneration of the CO 2 acceptor RuBP. This involves a
series of reactions that convert triose phosphate first to the 5-carbon intermediate Ru5P (ribulose 5-
phosphate), then phosphorylate Ru5P to regenerate RuBP (ribulose-bisphosphate). This final step
requires ATP formed in the light reactions.
Overall, for every three turns of the cycle one molecule of product (triose phosphate) is formed
(3CO 2 :1G3P). The remaining 15 carbon atoms (5 G3P) re-enter the cycle to produce three mol-
ecules of RuBP.
The triose phosphate formed in the Calvin cycle can remain in the chloroplast where it is con-
verted to starch. This is why chloroplasts form starch grains. Alternatively, triose phosphate can be
exported from the chloroplast where it is converted to carbohydrates in the cytoplasm. Both reac-
tions involve the release of phosphate. In the case of carbohydrates, the phosphate must be returned
to the chloroplast to support continued photophosphorylation (ATP formation).
The net energy balance of six rounds of the Calvin cycle to produce one mole of hexose is thus:

6CO 2 þ 18ATP þ 12NADPH þ 12H 2 O ! C 6 H 12 O 6 þ 18ADP
þ
þ 18P i þ 12NADP þ 6H þ (3:4)

Photorespiration

Photosynthetic organisms must cope with a competing reaction that inhibits photosynthesis known
as photorespiration. Unlike photosynthesis, this process involves the uptake of oxygen and the
release of carbon dioxide.
Recall that mitochondrial respiration involves the uptake of O 2 and the evolution of CO 2 and is
associated with the burning of cellular fuel to obtain energy in the form of ATP. In contrast, photo-
respiration starts in the chloroplast and wastes energy.
Photorespiration can be defined as the light-dependent uptake of O 2 in the chloroplast. It is
caused by a fundamental “inefficiency” of RuBisCO.
During photosynthesis RuBisCO catalyzes the carboxylation of RuBP to give two molecules of
PGA. However, it can also catalyze the oxygenation of RuBP to give one molecule of PGA and one
molecule of a 2-carbon compound called phosphoglycolate. This reaction occurs because O 2 can
compete with CO 2 at the active site of RuBisCO. As oxygenation of RuBP competes with carbox-
ylation, it lowers the efficiency of photosynthesis. A significant portion (25%) of the carbon in
phosphoglycolate is lost as CO 2 . Algae must use energy to “recover” the remaining 75% of this
carbon, which further limits the efficiency of photosynthesis.
If photorespiration lowers the yield of photosynthesis, why has such a process been maintained
throughout the course of evolution? The answer to this intriguing question has to do with the origin
of RuBisCO and the CBB cycle. RuBisCO is an ancient enzyme, having evolved over 2.5 billion
years ago in cyanobacteria. During this period in Earth’s history, the atmosphere contained high
levels of CO 2 and very little oxygen. Thus, photorespiration did not present a problem for early
photosynthetic organisms. By the time oxygen levels accumulated to significant levels in the atmos-
phere (ironically, by the process of photosynthesis!), the catalytic mechanism of RuBisCO was
apparently “fixed.” In other words, because both O 2 and CO 2 compete for the same active site of
the enzyme, algae could not decrease the efficiency of oxygenation without also decreasing the
efficiency of carboxylation. To compensate, algae evolved an elaborate pathway, known as the
photorespiratory pathway, to recover at least some of the carbon that would otherwise be lost.
This pathway involved biochemical reactions in the chloroplast, mitochondria, and peroxisome.
The importance of photorespiration is easily demonstrated by the fact that nearly all plants grow
better under high CO 2 versus low CO 2 . Conditions that favor carboxylation (photosynthesis)
over oxygenation (photorespiration) include high CO 2 , moderate light intensities, and moderate