The Effect Of The Light Intensity On Photosynthesis

Photosynthesis is a chemical reaction that takes place inside a plant, it has two stages known as the light reactions and the Calvin Cycle which then divides into photosystem I and photosystem II. It is known as the process that feeds the biosphere which involves both dark and light reactions. The purpose of the light reaction is to produce different energy molecules such as ATP and NADPH and is also known as the first stage of photosynthesis, the light reaction occurs in the thylakoids. The dark reaction then uses these energy molecules to produce glucose and is known as the second stage of photosynthesis. The light and dark reactions contribute to photosynthesis by converting the carbon dioxide and water into glucose. The products of the light reaction, photosystem II, gives off oxygen as a by-product and ATP and NADPH energy as a product whereas in photosystem II, the NADP+ accepts the electrons and picks up the H+ ions to form NADPH + H+ DCPIP substitutes the NADP+ part of the system during photosynthesis.

It can be hypothesized that as the DCPIP becomes colorless, the photochemical reactions of photosynthesis increase in rate as the light intensity increases.

Firstly, a lamp and lump of Blu tack was placed on the bench with 26cm between them. Three tubes were labelled separately with C, DC and E which stand for control, dark control and experimental. The control tube had 980µL of buffer solution added to it, the dark control and the experimental tubes both had 950µL added to them. Both the dark control and experimental tubes then received 30µL of DCPIP and the control tube received none. Each tube then had 10µL of chloroplasts added to them and shaken. The three tubes were lined up on the Blu tack next to each other with the dark control being covered by a beaker wrapped in tin foil. Each tube was placed separately into the spectrophotometer to read the absorbance at 590nm. The lamp was then turned on and every 2 minutes the three tubes were placed in the spectrophotometer, one at a time and the absorbance was noted down. The purpose of the control and dark control was to see if they had any similarities to the experimental one in the results even though they had different factors. The expected change in measurement was for the experimental tube to eventually reach the same amount of absorbance as the control tube.

The graph was achieved by noting down the absorbance reading of the three different tubes every 2 minutes. The control, dark control and experimental all contained a certain number of chloroplasts and buffer solution and only the dark control and experimental had DCPIP added, the three tubes were then placed in front of a light source, 26cm between the light and tubes with the dark control covered. The data from the absorbance readings was then placed into a computer program to produce the line graph displayed above. It is visible that the graph was achieved by absorbance against time with absorbance being read at 590µL and time being in minutes with the absorbance being read up until 26 minutes had elapsed.

The trend observed in the graph can be seen to be quite consistent within the control and dark control tubes as their values only changed slightly every two minutes when the absorbance was recorded. Whereas, even though the experimental tube’s results were also quite consistent, there was a sudden decrease in absorbance around the 22-minute mark and then back up with an increase in absorbance around the 24-minute mark as seen in the graph. The overall trend in the graph shows a consistent result throughout the control and dark control tubes without any sudden increase or decrease within the absorbance readings and a steady decrease with the experimental tube even with the sudden drop.

Overall it can be concluded that the amount of light absorbed within the three separate tubes varies based on the different conditions. The purpose of the light reaction is to generate energy molecules which are required for the dark reaction. The consistent trend observed shows that the more light absorbed then the rate of light absorption decreases and then the rate of photochemical reactions decrease. The amount of light received affects the light reactions of photosynthesis because the higher the light intensity means more photons are hitting the leaves of the plant therefore the rate of photosynthesis will increase because there is more light available to help the reactions of photosynthesis.

If a compound was added to the experiment where the conversion of water to oxygen is blocked then there will be no energy obtained as there will be no extra electrons with a H+. Therefore, the absorbance would decrease as there is no electrons to be elevated.   

16 December 2021
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