5.6.4: The light-independent stage Flashcards
Where does the light-independent stage take place?
-Stroma of chloroplasts
If the plant is not illuminated, why will the light-independent stage cease?
- Doesn’t use light directly by uses products of the light-dependent stage.
- ATP and hydrogen are not available to reduce the carbon dioxide and synthesise large complex organic molecules.
How does CO2 diffuse from the atmosphere into the stroma?
- CO2 in air
- Enters leaf through stomata
- Diffuses through the spongy mesophyll layer
- Enters palisade cells through thin cellulose walls
- Through chloroplast envelope into the stroma.
What maintains the conc. gradient od CO2 and aids diffusion?
The fixation of carbon dioxide.
What is the Calvin cycle?
-The series of reactions whereby CO2 is converted to organic molecules.
What are the steps in the Calvin cycle?
Step 1: CO2 combines with CO2 acceptor, a 5C compound called ribulose biphosphate (RuBP). Reaction catalysed by enzyme RuBisCO.
- CO2 combines with CO2 acceptor, a 5C compound called ribulose biphosphate (RuBP). Reaction catalysed by enzyme RuBisCO.
What are the steps in the Calvin cycle?
Step 1:
Step 2:
- RuBp, by accepting the carboxyl (COO-) group, becomes carboxylated, forming an unstable intermediate 6C compound that immediately breaks down.
What are the steps in the Calvin cycle?
Step 2: RuBp, by accepting the carboxyl (COO-) group, becomes carboxylated, forming an unstable intermediate 6C compound that immediately breaks down.
Step 3:
- The product of this reaction is 2 molecules of a 3C compound, GP (glycerate-3-phosphate). The CO2 has now been fixed.
What are the steps in the Calvin cycle?
Step 3: The product of this reaction is 2 molecules of a 3C compound, GP (glycerate-3-phosphate). The CO2 has now been fixed.
Step 4:
- GP reduced, using hydrogens from the reduced NADP made during the light-dependent stage, to triose phosphate (TP).
- Energy from ATP, also made during the light-dependent stage, is used at this stage at the rate of 2 molecules of ATP for every molecule of CO2 fixed during stage 3.
What are the steps in the Calvin cycle?
Step 4:
GP is reduced by NADP to triose phosphate (TP).
Energy from ATP used at the rate of 2 molecules of ATP for every molecule of CO2 fixed.
Step 5:
- In 10/12 TP molecules, atoms are rearranged to regenerate 6 molecules RuBP.
- This requires phosphate groups.
- Remaining 2 molecules of TP are the product.
Why do chloroplasts contain only low levels of RuBP?
-It is being continuously converted to GP but is also being continuously regenerated.
Why does the Calvin cycle only run during
-The products of the light-dependent stage (ATP and NADPH) are continuously needed for the Calvin cycle to run.
How are the conditions in the stroma optimum for the enzyme RuBisCO?
- During the light-dependent stage, H+ pumped from stroma into the thylakoid spaces.
- Conc. of free H+ in stroma falls.
- pH raised to around 8.
- Optimum pH for enzyme RuBisCO.
What (other than pH) activates RuBisCO?
-The presence of extra ATP in the stroma.
What is the role of mg2+ in the activation of RuBisCO?
- In daylight mg2+ conc. increases in the stroma.
- These ions attach to RuBisCO, acting as cofactors to activate it.
