Biology: Light-Dependant Reaction in Photosynthesis (the Hill Reaction)

Biology: Light-Dependant Reaction in Photosynthesis (the Hill Reaction)

Light-Dependant Reaction Basics

  • The light-dependent reactions are the first stage of photosynthesis, which use light energy captured by chlorophyll to generate ATP and NADPH.
  • These reactions occur in the thylakoid membranes of the chloroplasts.
  • In this process, light energy is converted into chemical energy, in the form of ATP and NADPH, which are used in the next stage of photosynthesis: the light-independent reactions (Calvin Cycle).

The Hill Reaction

  • The Hill Reaction is an example of light-dependent reactions where water (electron donor) is split to produce oxygen, ATP and NADPH.
  • This reaction was first demonstrated by Robin Hill in his experiments conducted in 1937, hence named after him.
  • The process involves two photosystems, Photosystem I and Photosystem II, working together.

Measuring Light-dependent Reactions

  • The rate of the light-dependent reactions can be estimated by measuring the amount of oxygen produced, or more commonly in labs, by measuring the change in absorbance of a dye, such as DCPIP.
  • DCPIP (dichlorophenolindophenol) turns from blue to colourless as it accepts hydrogen atoms from water molecules.

Factors Affecting Light-dependent Reactions

  • Light Intensity: As light intensity increases, the rate of the light-dependent reactions also increases, until a saturation point is reached.
  • Temperature: Up to an optimum level, a rise in temperature increases enzyme activity and thus the reaction rate. If the temperature is too high, the enzymes involved may denature, slowing or stopping the reaction.
  • Wavelength of Light: Different pigments absorb different wavelengths of light. Chlorophyll absorbs red and blue light most efficiently and reflects green light, hence why plants appear green.

Experimental Techniques in Photosynthesis

  • Take care to control all other variables when measuring one specific factor’s impact on the rate of photosynthesis.
  • Typical safety precautions for lab work apply, such as wearing gloves and safety goggles, especially when handling chemicals like DCPIP.
  • Carefully note your results, running multiple trials for accuracy. Also, calibrate all equipment accurately before use.
  • Be aware of possible sources of error, including sample contaminants or inaccurate measurements, and ensure steps are taken to reduce these effects.