Photosynthesis - Dark Reactions (Lecture 5) Flashcards
What are light-independent reactions?
The set of anabolic reactions (energy-consuming) using NADPH (reductant) and ATP (energy) to synthesize carbohydrates.
Under what name are the dark reactions known?
The Calvin cycle
Where do the dark reactions take place?
In the stroma, the inner space of chloroplasts.
What are the three phases of dark reactions?
Carbon fixation;
Reduction;
Regeneration
What sustains the Calvin cycle?
The light reactions that take place prior to!
How many times does the cycle have to occur, for the synthesis of 1 G3P (sugar)?
In other words, for 1 G3P to exit the cycle and go towards glucose synthesis, ____ CO2 molecules must enter the cycle, providing ______ new atoms of fixed carbon.
3 times
Describe the first phase of the Calvin cycle, being carbon fixation.
- A CO2 molecule combines with a 5-C acceptor molecule, ribulose-1,5-biphosphate (RuBP).
- This makes a 6-C compound, splitting immediately into two molecules of a 3-C compound: 3-phosphoglyceric acid (3-PGA).
- The reaction is catalyzed by rubisco, the RuBP carboxylase/oxygenase enzyme.
What is Rubisco?
The most abundant protein in chloroplasts, and on Earth. This is partly due to its low efficiency.
It is the key enzyme for the Calvin cycle, allowing for carbon uptake.
Its structure is composed of 8 large and 8 small subunits. Chloroplasts make the large subunits, while the nucleus makes the small units.
Paradoxically, Rubisco accepts oxygen and CO2!
Describe the second phase of the Calvin cycle, being reduction.
ATP and NADPH are used to convert the 3-GPA molecules into molecules of a 3-C sugar, glyceraldehyde-3-phosphate (G3P).
- A second phosphate (P) is added using ATP.
- NADPH reduces the COOH to CHO.
- This results in G3P.
How does the second stage get its name?
NADPH donates electrons to (or reduces) a 3-C intermediate to make G3P.
Describe the third phase of the Calvin cycle, being regeneration.
Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor.
Regeneration requires ATP and involves a complex network of reactions - the “carbohydrate scramble”.
How many G3P molecules are made when 3 CO2 molecules enter the cycle?
6 G3P molecules. One exits the cycle and is used to make glucose, while the other five must be recycled to regenerate 3 molecules of the RuBP acceptors.
How many NADPH are converted to NADP+ during the reduction step?
6
To produce 1 molecule of glucose, it takes _____ turns of the cycle: ___ CO2, ____ ATP, ____ NADPH.
It takes SIX turns of the cycle.
6 CO2, 18 ATP, 12 NADPH.
True or false: The Calvin cycle is autocatalytic.
TRUE! It keeps going, as long as CO2 is added.
