Wednesday 25th of Mach 2009

IB Photosynthesis Experiment: Hill Reaction

Aim of the Experiment: To analyse the effects of light intensity on photosynthesis via the Hill reaction.

Research Question: How does light intensity affect the rate of photosynthesis more precisely the rate of reaction in photosystem II, at the primary acceptor location?

Hypothesis:  In the light dependant reactions, an artificial electron acceptor, DCPIP can be substituted for NADP it is very similar to this molecule but changes colour when it is reduced; it changes from blue to colourless. This is because DCPIP is a redox dye. As DCPIP becomes colourless light transmittance is therefore increased and one can use a colorimeter to measure the light acceptance. This means that it is the rate of electron flow in photosynthesis that is being measured. It is also important to note that DCPIP interrupts the passage of electrons from photosystem II to photosystem I, explaining why only photosystem II activity can and is measured.

For this experiment the variables are the following:

Independent variable: Light intensity – Produced from a lamp at various distances.

Dependant variable: Transparency- Light absorbance of the solutions

Controlled variable: Ambient Light - The shutters were closed to stop sunlight from entering the room in which the lab took place.



Material Used:

  • Chloroplast solution pre-extracted from spinach
  • Diluted DCPIP solution
  • Pipette
  • Small beaker
  • Vernier Colorimeter with 8 plastic cuvettes for the DCPIP-Chloroplast-Buffer solution
  • Phosphate reaction buffer
  • Lamp with a 40W light bulb
  • 1 meter ruler
  • A timer


  1. The colorimeter wavelength was set to 635 nm.
  2. A cuvette was filled with the 2 ml of buffer and 1ml of chloroplast solution it was then placed into the colorimeter to calibrate it (Averaging 100% absorbance).
  3. A cuvette was filled with DCPIP (1ml + 3 drops) and 2ml of buffer solution and was tested for transmittance percentage.
  4. Six other plastic cuvettes were filled with 1 ml of DCPIP solution and 2 ml of reaction buffer solution. 1ml of chloroplast solution was added to these solutions.
  5. The six cuvettes were thoroughly shaken and placed alongside a ruler at distances of 15, 30, 45, 60, 75 and 90 cm each.
  6. Each cuvette had its light absorbance tested to make sure they all contained a similar solution so as to avoid data collection errors later on in the lab.
  7. Turn on the lamp and wait for 2 minutes allowing the Hill reaction to take place.
  8. Then, once the time had elapsed, each cuvette was quickly tested by using the colorimeter for its light absorbance.
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Collected Data.

Table 1.This is the table of light transmittance results percentage collected during several tries. Their contained solution is listed on the left.

Processed Data

Table 2.Raw data with the light intensity calculated with the   proportionality with 40W as the constant of this relation. Because 6 retakes where made, an average was made with its uncertainty calculated. Even if only 6 values were tested, a correlation coefficient was calculated and was 0.9953, suggesting a high probability of correlation.

Graph 1.On the graph underneath all the sequential tries have been plotted, showing their resemblance and trend. A disagreement is ...

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