Question

In a photosynthesis experiment where DPIP was used to indicate when photosynthesis is occurring, the change in the % transmittance was measured with a spectrophotometer and results were recorded. The measurements were made after 20 min incubation of DPIP with unboiled chloroplasts in presence of a flood light at four pHs – 5.8, 6.8, 7.8 & 8.8 – and results obtained are indicated in the table below. pH % Transmittance 5.6 20 30 46 52 6.8 30 50 60 70 7.8 40 60 80 94 8.4 30 40 55 60 Using the results shown, which pH is the best for photosynthesis to occur? Knowing what you do about the effects of pH on living systems, propose a reasonable hypothesis explaining the differences in the rates of reaction in differing pH ? Remember that a hypothesis is different than a prediction.

Answer 1

DPIP is a pigment used for measuring photosynthetic activity, since it has a redox feature in which it turns from blue to clear when it is reduced, that is when photosynthesis is working. The higher the transmittance received, the more DPIP is being reduced, and the more photosynthetic activity is being done.

I understand the table would look like this:

pH	%Transmittance
5.8	20
	30
	46
	52
6.8	30
	50
	60
	70
7.8	40
	60
	80
	94
8.8	30
	40
	55
	60

There are 4 transmittance values for each pH. Let us plot this to better visualize it:

The first 4 measurements correspond to pH 5.8, the next 4 to 6.8, and so on. We can see in this graph how the pH 7.8 reaches the highest transmittance values. This means 7.8 is the best pH for photosynthesis to occur.

pH is a measure of the concentration of proton in the medium, lower pH values indicate higher concentrations of protons. pH levels can change the dynamics of redox reactions, by changing the state of protonation/deprotonation of certain molecules. Photosynthesis works with redox reactions, that's why it is affected by pH.

Hypothesis: pH levels affect photosynthetic activity by affecting the redox potential of the involved molecules.