Photosynthesis Day 2
Noncyclic Photophosphorylation
The light reactions elevate e-s in 2 steps:
The PS 2 creates a H+ gradient that generates ATP at the ATP Synthase.
PS I generates NADPH and the NADPH “carries” away the e-s.
This is called Noncyclic Photophosphorylation.
Cyclic photophosphorylation
However If PS I can’t pass e-s to NADP the e-s cycles back to PS II & makes more ATP, but no NADPH is produced. There are 18 ATPs and 12 NADPHs required to make one C6H12O6.
Calvin Cycle
The Calvin cycle reactions can be divided into three main stages: carbon fixation, reduction, and regeneration of the starting molecule.
Carbon fixation. A CO2 molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco.
Reduction. In the second stage, ATP and NADPH are used to convert the 3-PGA molecules into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). This stage gets its name because NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P.
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, which my college bio professor liked to call the "carbohydrate scramble."
Three turns of the Calvin cycle are needed to make one G3P molecule that can exit the cycle and go towards making glucose. Let’s summarize the quantities of key molecules that enter and exit the Calvin cycle as one net G3P is made. In three turns of the Calvin cycle:
Carbon. 3 CO2 molecules combine with 3 RuBP acceptors, making 6 molecules of glyceraldehyde-3-phosphate (G3P).
*1 G3P molecule exits the cycle and goes towards making glucose.
*5 G3P molecules are recycled, regenerating 3 RuBP acceptor molecules.
ATP. 9 ATP are converted to 9 ADP (6 during the fixation step, 3 during the regeneration step).
NADPH. 6 NADPH are converted to 6 NADP+ (during the reduction step).
Calvin (Light Independent Cycle) Steps
Step 1: RuBP (5 carbon molecule) is joined with CO2 at the enzyme rubisco to form a very unstable 6 carbon molecule which quickly breaks into two 3 carbon molecules called 3-PGA.
Step 2: Each 3-PGA gains a phosphate from ATP and H’s from NADPH, making an energized 3 carbon compound G3P.
RuBP + CO2 🡪 2 PGA
2 PGA + 2ATP + 2NADPH 🡪2 G3P
Step 3: G3P molecules begin to accumulate in the stroma (G3P buildup).
Step 4: When the cycle has turned 6 times and 12 G3P molecules have built up 2 G3Ps are joined to form one 6 carbon sugar.
Step 5: The remaining 10 G3P molecules are converted back into RuBP to continue the cycle.
Total Energy use:
12 total G3Ps made need 12 ATPs & 12 NADPHs
BUT! 10 G3Ps are converted back to 5 RuBPs this needs 6 ATPs so…
Total Energy use:
12 total G3Ps made need 12 ATPs & 12 NADPHs
BUT! 10 G3Ps are converted back to 5 RuBPs this needs 6 ATPs so…
18 ATPs and 12 NADPHs required for one molecule of Glucose
In three turns of the Calvin cycle:
A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle, or 6 CO2s, 18 ATP, and 12 NADPH, to produce one molecule of glucose.