5B: Rubisco in C3, C4 and CAM photosynthesis

Question 1

Rubisco

Answer 1

a pivotal enzyme involved in initial carbon fixation during the Calvin Cycle/light-independent stage of photosynthesis

- Stands for Ribulose 1,5 Bisphosphate Carboxylase/Oxygenase

Question 2

Role of Rubisco in Photosynthesis

Answer 2

Catalyses the following reactions

Carbon fixation: turns 3 CO2  and 5 RuBP molecules into 6x 3-PGA molecules. 

Reduction: energy and hydrogen is donated from ATP and NADPH to convert 3-PGA into G3P.  One G3P molecule is removed to form glucose. 

Regeneration: ATP is used to convert 5G3P molecules into 3 RuBP molecules to restart the cycle.

Question 3

Photorespiration

Answer 3

a wasteful process in plants initiated when Rubisco uses O2 as a substrate rather than CO2, limiting photosynthesis

- Photosynthesis is disrupted as CO2 loses an opportunity to bind with Rubisco.

- Less photosynthesis = less glucose produced + wasted energy used = negative impacts on a plants ability to grow

Question 4

Factors influencing whether Rubisco Binds to CO2 or O2

Answer 4

Temperature:
- At regular temperatures, Rubisco’s affinity for CO2 is greater than for O2

- At higher temperatures, the affinity for O2 is higher, leading to rubisco binding to oxygen more often

Substrate Concentration:
- The more substrate is present, the greater chance it will bind to Rubisco

- Plants try and expose a high concentration of CO2 and a low concentration of O2 to Rubisco 

    - The stomata of the plant regulates this by opening up to allow CO2 to enter while O2 and other water vapor simultaneously diffuse out

Question 5

C3 Plants

Answer 5

plants with no evolved adaptation to minimize photorespiration

- Considered 'normal plants'

- All of the light independent stage occurs within mesophyll cells

E.g. Trees, wheat, rice

Question 6

C4 Plants

Answer 6

plants that minimize the photorespiration by separating initial carbon fixation and the remainder of the Calvin Cycle over space

• The Carbon Fixation stage occurs in the mesophyll cell whereas the rest of the Calvin cycle occurs in the bundle sheath cell

Question 7

Light Independent Reaction Process in C4 plants

Answer 7

• CO2 enters the mesophyll cell and is fixed by PEP carboxylase to joins PEP(3 Carbon molecule) to form oxaloacetate(4 carbon molecule)
  • Oxaloacetate is then converted and rearranged into malate(4 carbon molecule) which is capable of being transported into bundle-sheath cells
  • Inside the bundle-sheath cell, malate breaks down back into CO2 which then enters the Calvin Cycle leading to the production of glucose
  • When malate breaks down, pyruvate is formed which is transported back to the mesophyll cell and with the help of ATP, is rearranged and converted into PEP
    • PEP is then ready to transform new CO2 molecules and the cycle continues

Question 8

CAM plants

Answer 8

plants that minimize photorespiration by separating initial carbon fixation and the remainder of the Calvin Cycle over time

- Carbon fixation occurs at night whereas Calvin Cycle occurs during the day

- The controlled release of molecules out of vacuoles ensures a high concentration of CO2 is maintained near Rubisco, maximising photosynthesis and minimising photorespiration

Question 9

Light Independent Reaction Process in CAM plants

Answer 9

• At night, the stomata open up to bring in CO2 which is fixed and converted by PEP carboxylase into oxaloacetate(4 carbon molecule)
  • Oxaloacetate is then converted and rearranged into either malate or another organic molecule which is then stored inside vacuoles within the mesophyll cell until daytime
  • During the day, plants close their stomata to prevent water loss
  • The malate is transported out of the vacuole and broken down into CO2 which is then free to enter the Calvin Cycle and produce Glucose