BIO Laboratory Exercises
Laboratory III: The Hill Reaction of Photosynthesis
in Isolated Chloroplasts
The goal of this session is to reproduce a classic experiment1 by Dr. Robert (Robin) Hill, which provided key evidence in describing the role of sunlight in photosynthesis. Students will be introduced to the concept of using light energy as fuel for biological reactions.
Students will explore the Hill Reaction by:
1) observing the effect of light intensity on reaction rate
2) designing a variation of their choice, which may include:
– changing the type of light source (i.e. incandescent vs. natural or other light)
– lowering or raising pH of the reaction buffer
– effects of freeze/thaw cycles on chloroplast suspension
– altering the color (wavelength) of incident light
Introduction
Photosynthesis is a process by which light energy is converted to chemical energy. The mechanisms by which light energy is harvested and converted to chemical energy starts with absorption of light photons by pigments in the chloroplasts. The reaction in green plants follows:
light
CO2 + H2O (CH2O) + O2
Shown above, CH2O stands for carbohydrate. In green plants, photosynthesis involves a series of oxidation-reduction reactions in which water is the electron donor and oxygen is released (Figure 1).
In the 1930s, Robin Hill investigated electron transfer in chloroplasts using artificial electron acceptors, such as DCIP. In his experiments, Hill treated photons as though they were a reactant (like succinate in the SDH experiment). At low light intensities, the photosynthetic rate was linear but at higher intensities the photosynthetic rate became saturated. In many ways photosynthesis behaves like an enzyme-substrate reaction except that light, rather than chemical energy, serves as the fuel.
Figure 1. Broad overview of photosystems I and II in green plants. Reproduced from Alberts et al2.
The purpose of this experiment is to reproduce…