biochemistry of photosynthesis 2 Flashcards
RuBP
5C compound ribulose biphosphate
first step of the calvin cycle
carbon dioxide from the air combined with RuBP in the chloroplasts
carbon fixation
when carbon is attaches to RuBP it is fixed
RUBISCO
ribulose biphosphate carboxylase oxygenase
-aids carbon dioxide from air combining with 5C RuBP
-RUBISCO is rate limiting enzyme in process of photosynthesis
result of reaction between RuBP and CO2
in theory 6C compound
why is the 6C compound produced in theory only
because it is highly unstable and no one has been able to isolate it
what happens to the 6C compound produced in the reaction between RuBP and CO2
-it splits to give 2 molecules of GP 3C
what happens to GP after being produced by splitting of 6C
it is reduced by addition of hydrogen to form glyceraldehyde 3-phosphate 3C
glyceraldehyde 3-phosphate
GALP 3C sugar
where does the hydrogen that reduced GP come from
from reduced NADP and the energy required comes from ATP made in light stage
2 pathways for GALP
-much of it passes through series of steps to replace RuBP needed in first step of cycle
-some synthesised into 6C glucose or passed directly into glycolysis pathway where its used for synthesis of other molecules needed by plant
features of reactions that take place in calvin cycle
take place in both dark and light
-reactions only stop in dark when products of light reaction run out leaving no reduced NADP or ATP available in the chloroplasts
how is RUBISCO inefficient
the active site cannot distinguish between the carbon oxygen double bonds of CO2 molecules and the oxygen oxygen double bonds of 02
-as a result there is competitive inhibition between the 2
RUBISCO as carboxylase
in high levels of CO2 low O2 RUBISCO binds to oxygen and combined it with RuBP giving two GP which feeds into calvin cycle
RUBISCO as oxygenase
in low CO2 high O2 RUBISCO binds to oxygen and combined RuBP to form one molecule of GP and one glycolate 2-phosphate
-this is converted into GP in photorespiration
photorespiration
-process by which glycolate 2-phosphate is converted to GP
-reaction that used products of calvin cycle and ATP and releases CO2
-photorespiration name because it uses CO2 and O2
affinity of RUBISCO
affinity for CO2 80% higher than affinity for oxygen
inefficiency of photorespiration
needs to convert GP2 to 3C GP so wastes carbon and energy
-thats why plants 25& less efficient than they should be
why is RUBISCO so inefficient
RUBISCO evolved when atmosphere was high in co2 with little o2 so photorespiration never occurred and no selection pressure against i t
-even today high oxygen low co2 atmosphere photorespiration not problem for plants
-no selection pressure for enzyme to evolve to become more specific to CO2
-however more efficient RYBISCO useful for people because crop plants would be 25% more productive
whats the end product of photosynthesis
GALP and key molecule for synthesis of everything else needed in plant
different uses of GALP in plant
some GALP directly used in glycolysis so fed into krebs cycle
-some of GALP produced in calvin cycle used to produce glucose by gluconeogenesis
gluconeogenesis
process by which some of GALP produced in calvin cycle if used to produce glucose
what happens to glucose produced in gluconeogenesis
-glucose may be converted into disaccharides such as sucrose for transport round the plant and into polysaccharides such as starch for energy storage and cellulose for structural support
function of GALP that feeds into glycolysis and krebs cycle
used as fuel to provide energy in form of ATP for functions of the cell
