# Amount of ATP required in photosynthesis.

1. Jan 5, 2004

### wasteofo2

Does anyone know how many ATP molecules are used in the process of photosynthesis? I think with aerobic respiration 2 are used, but I could be wrong.

2. Jan 5, 2004

### Monique

Staff Emeritus
ATP required in photosynthesis? ATP is produced during photosynthesis, so I guess you are referring to the carbon-fixation cycle?

Three molecules of ATP and two molecules of NADPH are consumed for each CO2 molecule that is fixed.

3. Jan 5, 2004

### Monique

Staff Emeritus
So how much ATP and NADPH is produced out of every quantum of light?

4. Jan 5, 2004

Staff Emeritus
I seem to recall that 11 photons are required for one sugar molecule, which is later used by the cell to make other molecules. The point of what I was reading was that 3 photons would be 100% thermodynamic efficiency, so the actual process is a little under 30% efficient. This was long ago (~20 years) so maybe understanding has changed.

5. Jan 5, 2004

### Monique

Staff Emeritus
Yes, my answer was incomplete, since the net reaction of carbon-fixation is:

$3CO_2+9ATP+6NADPH+H_2O\rightarrow$
$glyceraldehyde3phosphate+8P_i+9ADP+6NADP^+$

So this glyceraldehyde then can be the substrate for glucose prodcution.

Last edited: Jan 5, 2004
6. Jan 5, 2004

### fish

Each turn of the light independent cycle (calvin-benson) requires 1 CO2, 3 ATP, and 2 NADPH. It needs 6 turns of the cycle to make 1 molecule of glucose because glucose has a 6 carbon atom backbone.

Aerobic respiration needs 2 ATP to start reactions and when completed will have a net yield of 36 ATP

7. Jan 5, 2004

### Monique

Staff Emeritus
You are partially right, I think.

This reaction takes place in the chloroplast:
$3CO_2+9ATP+6NADPH+H_2O\rightarrow$
$glyceraldehyde3phosphate+8P_i+9ADP+6NADP^+$

After which the glyceraldehyde 3-phosphate is exported into the cytosol, where it can be converted into fructose 6-phosphate and glucose 1-phosphate. The glucose 1-phosphate is then converted to the sugar nucleotide UDP-glycose, and this combines with the fructose 6-phosphate to form sucrose phosphate, the immediate precursor of the disaccharide sucrose (which the plant uses for energy).

I am not sure how much ATP is required to go from glyceraldehyde 3-phosphate to glycose 1-phosphate.

8. Jan 5, 2004

### wasteofo2

Monique, you over complicated the question a bit. Firstly, photosynthesis doesn't make ATP, it makes $C_6H_1_2O_6$

I wanted to know how many ATP molecules are used in the overall reaction of photosynthesis.

$12H_2O+6CO_2+Light Energy\rightarrow C_6H_1_2O_6+6O_2+6H_2O$

I thought that

$C_6H_1_2O_6+6O_2\rightarrow 36ATP+6CO_2+6H_2O$

Used 2 molecules of ATP in the process...

Last edited: Jan 5, 2004
9. Jan 5, 2004

### Monique

Staff Emeritus
Ok, but photosynthesis occurs in discrete steps, I was considering those. What you are giving is a net reaction, there are intermediate steps.

6 CO2 + 18 ATP + 12 NADPH = C6H12O6 + 18 (ADP + Pi) + 12 NADP+

There you go: the dark reaction of photosynthesis :)

10. Jan 5, 2004

### Monique

Staff Emeritus
The light reaction takes 6 H2O and turns it into 6 O2.
12 NADP+ gets reduced to 12 NADPH and H+ during the reaction.

The H+ gradient is used to make 18 ATPs, which go into the dark reaction.

11. Jan 5, 2004

### wasteofo2

Alright, 18 ATP moledules per glucose molecule, thanks.