Lecture 3: CO2 acquisition by plants and photorespiration Flashcards
who were Calvin + Benson?
they illuminated micro algae (Chlorella, Scenedesmus) with 14CO2 for short periods and allayed the first products of CO2 fixation by paper chromatography and autoradiography
rubisco =
Ribulose
1,5‐bisphosphate
carboxylase‐oxygenase
what is the worlds most abundant protein and what is it the first step of
RUBISCO
first step of CO2 assimilation
The Benson-Calvin Cycle =
–> 3RuBP (3X5C)
–> [carboxylation by rubisco]
–> + 3CO2
–> 6 glycerine 3-P (6X3C)
–> + 6ATP + 6NADPH
–> [Reduction]
–> 6 triode-P (6X3C)
=1 x 3C product
–> [regeneration] (5X3C)
–> + 2ATP = REPEAT
Triode phosphate is used to make
carbohydrates
- left over triose‐P is either used to make starch inside the chloroplast or is exported from the chloroplast via the phosphate translocator , to make sucrose
Atmospheric CO2 over the last 3500 years
dropped from 50,000 ppm to today, v low.
across rubisco time, O2-evolving photosynthesis + land plants
Atmospheric O2 over the last 3500 years
Increased over time slowly
across rubisco time, O2-evolving photosynthesis + land plants
RuBP + CO2 + H2O (5C+C) —CARBOXYLASE–>
2 ( glycerine 3-P)
2X3C
RuBP + O2 (5C) —OXYGENASE–>
glycerine 3-P + glycol late 2-P
3C + 2C
peroxisome
a small organelle present in the cytoplasm of many cells, containing the reducing enzyme catalase and usually some oxidases
photorespiratory pathway equation =
2 X 2C –> 1 X 3C + CO2
photorespiratory pathway
In chloroplast: Rubisco --> 2 glycollate-2-P --> In cytoplasm (peroxisome): 2 glycollate --> 2 glycine (2C) --> In mitochondrion: [glycin carboxylase] - loss of CO2, NH3, NADH --> In cytoplasm (peroxisome): Serine (3C) --> In chloroplast glycerate-3-P
photorespiration explanation =
a respiratory process in many higher plant where they take up oxygen in the light and release CO2, contrary to the general process of photosynthesis
photorespiration definition =
the light‐ dependent loss of CO2 from leaves
why does photorespiration occur?
- Glycollate cannot be further metabolised by the Benson-Calvin cycle
- Glycollate metabolism results in the loss of CO2 & NH3 and the return of only 3/4 of the carbon originally in glycol late to the chloroplast
- photorespiration salvages glycollate
increasing CO2 ____ photorespiration
decreases
increasing temperature ___ photorespiration
increases
photorespiration is a major drain on a plants carbon how can this be improved?
- improve Rubisco
- evolution
- genetic engineering
- Evolved CO2-concentrating mechanisms
- -C4 photosynthesis
- -Active uptake of inorganic carbon by aquatic organisms (algae, cyanobacteria)
- Improve CO2 fixation by genetic engineering
In C4 plants what is the first product of CO2 fixation
a C4 acid compared to glycerine 3-P (3C) in C3 plants
what is carbon fixation
the conversion process of inorganic carbon (carbon dioxide) to organic compounds by living organisms e.g. photosynthesis
example of types of C4 plants
many are tropical grasses, including the some of world’s most important crop species (maize, sugar cane, millet)
% of C4 plants in flowering plant species and grasses
about 3% of total flowering plant species, about 50% of all grasses are C4
majority of grasses in savannas (about 15% of Earth’s vegetated surface) are C3/C4
C4
C4 plants are responsible for %% of global photosynthesis
about 20%
C4 plants are responsible for __% of the worlds grain harvest
40%
4 examples of C4 environments
- savanna grasslands
- tropical wetlands
- salt marshes
- hot deserts and semi-deserts
typical C3 crop plants
- potatoes
- rice
- wheat
typical C4 crop plants
- sugar cane
- maize
C4 anatomy in a maize leaf
- wreath (Kranz) of bundle-sheath cells
- mesophyll cells surrounding
- CO2 is concentrated in the bundle-sheath & photorespiration suppressed
- PEP carboxylase & Rubisco are compartmented between the mesophyll + bundle-sheath
C4 photosynthesis
• CO2 enters from atmosphere via stomata
MESOPHYLL:
• HCO3- is fixed by PEP carboxylase into C4 acids (malate and aspartate)
• C4 acids are transported to bundle-sheath
BUNDLE-SHEATH
-C4 acids are decarboxylated
-CO2 is fixed by Rubisco & the Benson-Calvin cycle
• C3 portion returns to mesophyll
