Photorespiration:
* Rubisco catalyze its oxygenation ability
omnipresent, even in anaerobic, autotrophic bacteria when exposed to
oxygen
* Loss of CO2 from cells
* Competition: decrease the efficiency of photosynthesis
* Interconnection:
determined by the kinetic properties of rubisco, the concentration
of substrates, and temperature
* C2 oxidative photosynthetic carbon cycle:
act as a scavenger operation to recover fixed carbon lost during
photorespiration
Three organelles
Carbon flow
22C13C+CO2
75%
Nitrogen flow
no changed
Oxygen flow
3 O2/2 RuBP
Malate-OAA
shuttle supply
NADH
Web Topics 8.6
[gas] µM = Pgas    106/ V0
In vitro vs. In vivo
Pgas: partial pressure; : absorption coefficient
Solubility of CO2 and O2 as a function of temperature
T  tilt toward the C2 oxidative photosynthetic cycle
ROS
Photorespiration depends on the photosynthetic electron transport system
The biological function of photorespiration
is under investigation
＊ a protective, to dissipate excess ATP and reducing
power, especially under high light intensity and low
[CO2]inter (e.g., water stress)
＊ mutants
lack glycerate kinase, not viable in normal air
＊ linked photorespiration to nitrate assimilation
 a full understanding is still not at hand
CO2-concentrating mechanisms:
0.03% CO2 / 21% O2
A. C4 photosynthetic carbon fixation (C4), in hot environment;
B. Crassulacean acid metabolism (CAM), in desert environment;
C. CO2 pumps at the plasma membrane.
In aquatic plants, such as unicellular cyanobacteria and algae.
In aquatic environment, [CO2] low  rubisco specificity activity low
CO2-HCO3- pumps at the plasma membrane are induced,
to accumulate inorganic carbon
light energy provide ATP to uptake CO2 and HCO3carbonic anhydrase:
HCO3- + H+ → H2O + CO2 → Calvin cycle
[CO2] ↑ suppress photorespiration
Cyanobacterial CO2 concentrating mechanism
— high homologous to the Rheus, a protein in erythrocytes
The C4 carbon cycle
spatial
Kranz (wreath) cells: present two distinct chloroplast-containing cells,
mesophyll and bundle sheath cells
OAA
Calvin
cycle
sugarcane
Flaveria australasica
Poa sp
The C4 photosynthetic pathway: Hatch and Slack
Gramineae (corn, millet, sorghum, sugarcane);
Chenopodiaceae (Atriplex); Cyperaceae (sedges).
external
plasmodesmata
specific
translocators
vascular
NADP-ME: in chloroplast
NAD-ME: in mitochondria
PEP carboxykinase: in cytosol
Web Topic 8.7 (?)
Three variations of C4 metabolism
The form of transportation
The manner of decarboxylation
(1) maize, crab grass,
sugarcane, sorghum;
Aspartate aminotransferase
(2) pigweed, millet;
(3) guinea grass.
PEP carboxykinase
Alanine aminotransferase
Kranz anatomy: mesophyll and bundle-sheath cells
carbon concentrating mechanism / suppressed photorespiration
Photosynthetic Carbon assimilation
reduction
plasmodesmata
Borszczowia
aralocaspica
Bienertia cycloptera
Chloroplasts containing rubisco are near mitochondria with NAD-ME