Exam 1: Photorespiration

Question 1

photorespiration

Answer 1

a costly side reaction of Rubisco

ribulose 1,5 bisP + O2 (NOT CO2) —> 3-P-G + 2-phosphoglyCOLATE

Question 2

in photorespiration there is no incorporation of …

Answer 2

CO2

costly to regenerate ribulose 1,5 bisP to continue the Calvin cycle

Question 3

2-phosphoglycolate is converted to…

Answer 3

3-phosphoglycerate and CO2 in several steps

requires ATP

net result is a loss of CO2 (purpose of Rubisco is to take up CO2!!!)

Question 4

why does Rubisco have oxygenase activity?

Answer 4

enzyme evolved before O2 was a large % of atmosphere (O2 produced from photophosphorylation built up over the years)

Rubisco is intolerant to mutation–hard to remove oxygenase activity without removing carboxylase activity

Question 5

most organisms now use ___ as the terminal electron acceptor of respiration… what did they use before?

Answer 5

O2

sulfur i think

Question 6

which plants evolved a defense against photorespiration?

Answer 6

C4–use a 4 carbon intermediate

Question 7

what is the defense of C4 plants

Answer 7

they compartmentalize CO2 uptake and CO2 assimilation

therefore O2 is partially excluded from Rubisco

Question 8

in C4 where is CO2 fixed

Answer 8

in the mesophyll cell (uptake)

CO2, now attached to a molecule is transported to a bundle sheath cell where it is released and can react with Rubisco (assimilation)

Question 9

in C4 plants all the cells have the gene for Rubisco, but it is only expressed in…

Answer 9

bundle sheath cells

therefore O2 is excluded (only makes it to mesophyll cell)

Question 10

does C4 mechanism cost energy?

Answer 10

yes, but far less than if Rubisco wasn’t compartmentalized

Question 11

why aren’t all plants C4?

Answer 11

photorespiration increases with Temp

at higher temp Rubisco more likely to use O2 than CO2 as a substrate

therefore, C4 plants found in climates with high temps

ex. tropics

not Maine b/c no evolutionary advantage