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Investigation Of The Light-Dependant Electron Transport In Silver Beet Chloroplast Suspension Using DCPIP Under Different Conditions

Practical 2 Lab Report: Photosynthesis


Photosynthesis is one of the most vital process that takes place in plants that allows these producers in the food chain to incorporate  CO2 to form sugars(sucrose, glucose) and other complex compounds(cellulose) required for growth. This entire process can be summarized:

nCO2 + nH2O        CHLOROPLAsTLIGHT      (CH2O)n + nH2O[pic 1]

This process is  divided into two; the light-dependant reaction and light-independent reaction.

Due to the nature of our experiment we would only be looking at the Light-dependent reaction summarized as follows:


2H2O     LIGHTCHLOROPLAST      4H+ + 4e- + O2[pic 2]

The above reaction illustrates the process of Photolysis where water molecules are broken down by enzymes present only in the PS680 photosystem and this happens only in non-cyclic photophosphorylation. The H+ protons are used for the formation of ATP(energy carrier) via chemiosmosis and the e- diffuse down the ETC  to mediate this process. NADP then acts as the final e- acceptor forming NADPH.

The aim of our experiment is to investigate this electron transport that takes place by introducing a dye(DCPIP-2,6 dichloropenolindolphenol), a blue colored dye that turns colorless when it is reduced by acting as the final e- acceptor instead of NADPH.

This allows us to estimate the rate of electron transport by measuring the rate at which DCPIP loses color(using a spectrophotometer) and in turn an estimate of the rate of photosynthesis-faster the rate, faster the dye loses color and the more rapid the decrease in absorbance. This can be summarized as follows:

2H2O     LIGHTCHLOROPLAST      4H+ + 4e- + O2[pic 3]

Oxidized dye(DCPIP-blue)                                 Reduced dye(DCPIP-colorless)[pic 4]

The rate was not measured under one condition but many: 7 test tubes under 7 different conditions were measured for absorbance:

TUBE 1.) Acts as the blank to which all the other tube's absorbance are measured to. This is to make sure that only the color change of DCPIP affects the absorbance and not any other factor such as chloroplast.

TUBE 2.) Dark reaction: placing the reaction in the dark. Since the formation of e-  happens only in the light-dependent reaction, whose enzymes require light to function. Without light no e-  are hypothesized  to form as rate of photolysis will decrease, rate of e-  formation will decrease, and hence the rate of color loss will also decrease



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