Photosynthesis: (Dark Reaction)

Question 1

Light independent reactions:

Answer 1

•AKA Calvin-Benson Cycle, Dark Reaction.
•series of reactions that “fixes” atmospheric CO2 into glucose.
•if sufficient NADPH and ATP in the stroma, the energy of these molecules can be used to synthesize glucose in the presence or absence of light (thus, “independent” reaction).

Question 2

Does the dark reaction only happen at night?

Answer 2

No, it most often occurs during the day but it can also occur during the night.

Question 3

Step 1: carbon fixation:

Answer 3

•C atom from CO2 binds to a pre-existing 5 carbon molecule ribulose biphosphate (RuBP).
-1C + 5C —> 6C molecule.
•new 6 carbon is unstable and quickly breaks down into 2 x 3 carbon molecules. (2 PGA molecules -low energy).
-6C —> 2 3C molecules.
•CO2 + RuBP —> unstable C6 —> 2C3.

Question 4

Step 2: reduction:

Answer 4

•the new 3 carbon molecules are in a low energy state.
-activated by ATP.
-then reduced by NADPH.
•this produces two molecules of PGAL (glyceraldhyde-3-phosphate).
-now a higher energy state 3 carbon molecule.
•1 PGAL leaves the cycle and can be used to make glucose.
•remaining 5 PGAL stays in cycle.

Question 5

Step 3: replacing RuBP:

Answer 5

-ATP produced from the light dependent reactions breaks and reforms the 3 carbon PGAL into 5 carbon RuBP.
-6 cycles needed to make one glucose molecule.
-12 PGAL produced:
• —> 10 stay in cycle to remake RuBP.
• —> 2 make one glucose.

Question 6

Math for step 3:

Answer 6

10 x 3C = 30C\5C = 6 —> 5C.

Question 7

Summary (dark reaction):

Answer 7

•ATP and NADPH from the light-dependent reactions provide energy and reducing power to form PGAL from the newly formed three-carbon compounds.
•six cycles produce 12 PGAL molecules, 10 of which regenerate RuBP and 2 of which are used to make glucose.

Question 8

Fixing or fixation:

Answer 8

Attaching it to something else.