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Biology: Form and Function

  1. What is the chemical species that is transferred through the light reactions? (Box A)

Answer: electron (e-)

  1. What energy-rich compound is produced by the actions of the cytochrome complex? (Box B)

Answer: Energy for the synthesis of ATP

  1. What is the gas (Box C) produced when electrons are stripped from a water molecule and fed into photosystem II?

Answer: The enzyme system that catalyzes water splitting contains magnesium and calcium and situated in photosystem II of the chloroplast.

  1. What energy is used to energise the light reactions? (Box D)

Answer: Light energy

  1. What is the substrate of the reaction that is occurring at NADP+ reductase? (Box E)

Answer: NADPH

  1. Provide names for structures I and ii? (0.5 mark for each correct answer)

Answer: i= Photosystem II

Ii = Photosystem I

  1. The electron-accepting dye, DCPIP, is the substrate of which part of the light reactions?

Answer: DCPIP substitute NADP+ in experiment 4-part B.

Introduction to Photosynthesis Concept Map

 In one paragraph (250 words or less) provide a brief introduction to the experiment described in Prac 4 Part II investigation of Photochemical Reactions of Photosynthesis (7.5marks). Use the following description and questions and the concept map on the previous page to guide you. Provide an overview of photosynthesis by describing, in brief, the light and dark reactions. That is: what is the general process of light reactions? Describe the products and by-products of the light reactions and general concept of the system. For which part of the system is DCPIP a substitute? What is synthesised by dark reactions (Calvin cycle)? What products produced by the light reaction are required by dark reactions?

Answer: Photosynthesis is the process known to produce starch and molecular oxygen from atmospheric carbon dioxide and water using light and chlorophyll pigments, but the chemical reactions during photosynthesis are not so simple. Photosynthesis reactions consist of light reaction and dark reaction. In the light reaction, light propels electrons and produces ATP, NADPH and molecular oxygen. The energy ATP and NADPH is used in the dark reaction to produce glucose from carbon dioxide through the reaction of the Calvin cycle Lambers et al., 2008.

The light reaction is completed in two photosystems: photosystems I and II. Light absorption by chlorophyll and other photosynthetic pigments is the first step of photosynthesis. This light absorption initiates the electron flow through various electron acceptors and ends in NADP +. For this reason, the chlorophyll molecules become electron-deficient and this electron deficiency is caused by the photolysis of water molecules on the photosystem II. These electrons are eventually accepted by NADP + and H + to produce NADPH. This NADPH is used in the last step of photosynthesis, carbon fixation. Carbon fixation is a process of glucose production from carbon dioxide in the dark state Lambers et al., 2008.

DCPIP stands for 2,6-dichlorophenol and is a redox dye. DCPIP remains blue in an oxidized state and absorbs a maximum wavelength of 600nm. In a diminished state, DCPIP becomes colourless. Robert Hill used DCPIP in the 1930s as an alternative electron acceptor for measuring photosynthesis rate. DCPIP replaced the electron acceptor NADP +. In this experiment, we will also use DCPIP as an alternative electron acceptor for measuring photosynthesis rate Bauer & Bauer, 1980.

Hypothesis: In 50 words or less, state the hypothesis of the experiment (2.5 marks)

Answer: The hypothesis is that "DCPIP, a blue dye, acts as an electron acceptor and becomes colourless when reduced." This reduction allows us to demonstrate the production of reducing agents by chloroplast.

Results: Using the data provided on the SLE132 site, produce a graph using Excel, of absorbance of the experimental dark controls and control samples against distance from the lamp. Include a suitable title, labelling of the x and y axes with appropriate units and a figure legend. The scale on both the x and y axes should allow the reader to estimate value s of the samples. Provide a figure legend (in the box, below your graph) that concisely (maximum 80 words) describes the graph that is shown and the experiment that underlies the measurements. A figure legend does not reproduce the entire experimental procedures in detail, but rather puts the shown result in an experimental context. (12 marks for graph, 5 marks for legend).

Absorbance

Time

Experimental solution

Dark Control

Control

0

1.87

1.870

1.585

1

1.656

1.926

1.53

2

1.578

1.840

1.499

3

1.60

1.84

1.480

4

1.53

1.785

1.495

5

1.514

1.779

1.528

Table: Representation of the absorption of different reaction mixtures at different time intervals.

Results 2:

In 250 words or less, describe the trends for the experimental, control and dark control groups observed in your results (6 marks).

Answer: The absorption of the control solution showed a decrease from zero to three minutes and then it started to increase. The absorption of the dark control showed an increase in the absorption from 0 to one minute, then a gradual decrease from one minute to five minutes. The absorption of the experimental solution also showed a gradual decrease from zero to two minutes and then a sudden increase in the absorption from two to three minutes and then again a decrease from three to five minutes. These results suggest that there is an error in the experiment with dark control. Since there was no light exposure, there should be no reduction of the DCPIP dye, but it was observed that the foil shell may not cover the cuvette properly so that the colour changes can be observed in the dark reaction solution as well. Also, the control solution shows a decrease in absorption, suggesting that there is an error or other source of electron dispensers in the system.

Conclusion on Photosynthesis Concept Map

In one paragraph (300 words or less) provide a conclusion linking your observed trend of the experimental group with the functioning of the light reactions in plants. Explain how these results support your hypothesis. How does the amount of light receive affect the light reactions of photosynthesis? Why does the absorbance in the experimental samples decrease with time?

Answer: As light intensity increases, the rate of photosynthesis increases, leading to more ATP and NADPH production, but at a very high light intensity, the chloroplast damages, leading to a decreased rate of photosynthesis. We observed the same in our experiment with an experimental solution. First, the increase in light intensity leads to an increase in photosynthesis and a high number of generated electrons, which gradually decreases DCPIP. Then, the high light intensity begins to destroy chlorophyll, resulting in a lower generation of ATP, NADP + and electrons. A smaller number of electrons cannot reduce the desired amount of DCPIP dye, which increases the absorption capacity. In the control experiment, the same thing was observed. Therefore, I can conclude that during the experiment some errors may occur, which lead to a deviation of the results from the desired ones.

An inhibitor of the photolysis of water would stop the formation of electrons, which would lead to an electron deficiency in the chloroplast. This electron deficiency leads to less or no production of ATP and NADP +, which eventually leads to less or no carbon fixation. If we had added a photolysis inhibitor in this experiment, a reduction in absorption due to a colour change of the DCPIP dye would not have been observed.

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