Chapter 20, Problem 35DAP

1.

Summary Introduction

To determine: The way that how the given information support the Emerson model, in which light energy is used directly by chlorophyll to make ATP, and the ATP then provides the energy to drive ${\text{CO}}_{\text{2}}$ fixation and reduction.

Introduction:

Chloroplast is a double membrane bound organelle present in green plants and algae. Chloroplast contains thylakoid membrane in which two photosystem units PSI and PSII are present. These photosystem units absorb sunlight and pass it to an antenna molecule to drive the photosynthesis process.

Explanation of Solution

Explanation:

The photosynthesis is a process in which the green plants and algae obtains their nutrients. This process takes place in chloroplast where chlorophyll molecules are present. The chloroplast contains thylakoid molecules where light reaction takes place. The end product of the light reaction is the formation of ATP and NADPH which is an energy storing molecule which together helps in driving the Calvin cycle in stroma of chloroplast.

Chlorophyll is a light absorbing pigment that absorbs light for photosynthesis process. Chlorophyll contains polycyclic planar structure. In center of chlorophyll ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ is present which is surrounded by polyenes structures. These polyenes structures absorb light for driving the photosynthesis.

The presence of ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ion supports the Emerson model hypothesis that chlorophyll pigment is directly involved in photophosphorylation process.

Conclusion

Conclusion:

The presence of ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ support the hypothesis and determines that chlorophyll pigment is involved in photophosphorylation process.

2.

Summary Introduction

To determine: The way Rabinowitch explain the given information based on the model in which light energy is used directly by chlorophyll to make reducing compounds.

Introduction:

Chloroplast is a double membrane bound organelle present in green plants and algae. Chloroplast contains thylakoid membrane in which two photosystem units PSI and PSII are present. These photosystem units absorb sunlight and pass it to an antenna molecule to drive the photosynthesis process.

Explanation of Solution

Explanation:

Chlorophyll is a light absorbing pigment that absorbs light for photosynthesis process. Chlorophyll contains polycyclic planar structure and ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ions. Scientist Emerson determines that the presence of ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ions in chlorophyll shows that chlorophylls directly involves in photophosphorylation process.

Further, Rabinowitch proves that many enzymes also contain ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ions, but they are not involved in phosphorylating process. So, he concluded that the presence of ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ions in center of chlorophyll does not prove that chlorophyll pigment is involved in photophosphorylation process.

Conclusion

Conclusion:

Rabinowitch shows that many enzymes also contain ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ions which are not phosphorylating. Thus presence of ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ions chlorophyll molecule does not prove chlorophyll involvement in photophosphorylation.

3.

(a)

Summary Introduction

To determine: The way in which the given information explained modern understanding of photosynthesis “chlorophyll contains a ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ions which is known to be an essential cofactor for many enzymes that catalyze phosphorylation and dephosphorylation reactions.

Introduction:

Chloroplast is a double membrane bound organelle present in green plants and algae. Chloroplast contains thylakoid membrane in which two photosystem units PSI and PSII are present. These photosystem units absorb sunlight and pass it to an antenna molecule to drive the photosynthesis process.

Explanation of Solution

Explanation:

Chlorophyll pigment contain ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ion which is an essential cofactor for enzymes that involve in dephosphorylation and phosphorylation reaction. Thus, the presence of ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ion in chlorophyll show that chlorophyll involve in ATP synthesis by photophosphorylation process. The presence of ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ in chlorophyll is also essential in absorption of light and transport of an electron in chlorophyll.

Conclusion

Conclusion:

The presence of ${\text{Mg}}^{{\text{2}}^{\text{+}}}$ ion in center of chlorophyll is important in light absorption and electron transport in chlorophyll.

(b)

Summary Introduction

To determine: The way in which the given information explained modern understanding of photosynthesis “a crude chlorophyll protein isolated from photosynthetic cells showed phosphorylating activity”.

Introduction:

Chloroplast is a double membrane bound organelle present in green plants and algae. Chloroplast contains thylakoid membrane in which two photosystem units PSI and PSII are present. These photosystem units absorb sunlight and pass it to an antenna molecule to drive the photosynthesis process.

Explanation of Solution

Explanation:

The phosphorylating activity of chlorophyll can be shown by a crude preparation of chlorophyll pigmented protein. In laboratory conditions, the extraction of chlorophyll protein involve in the formation of ATP by adding one molecule of inorganic phosphate into ADP.

Thus, there are two possible explanations that either chlorophyll may contain phosphatase enzyme protein which involves in catalyzing the reaction or the crude preparation of chlorophyll gets contaminated with phosphatase enzyme which catalyzes the reaction.

Conclusion

Conclusion:

The crude preparation of chlorophyll protein either contain phosphatase enzyme or extraction get contaminated with phosphatase enzyme that catalyzes the phosphorylating reaction.

(c)

Summary Introduction

To determine: The way in which the given information explained modern understanding of photosynthesis “the phosphorylating activity of chlorophyll protein was inhibited by light”.

Introduction:

Chloroplast is a double membrane bound organelle present in green plants and algae. Chloroplast contains thylakoid membrane in which two photosystem units PSI and PSII are present. These photosystem units absorb sunlight and pass it to an antenna molecule to drive the photosynthesis process.

Explanation of Solution

Explanation:

The presence of light can inhibit the phosphorylation activity of chlorophyll that can be seen in normal photosynthesis process. In dark conditions, ATP does not produced and dephosphorylation reaction occurs. Thus, the inhibition of phosphorylation activity in presence of light may due to the presence of artifacts. Due to this, chlorophyll does not absorb light and photosynthesis would not occur.

Conclusion

Conclusion:

The inhibition of light due to the presence of artifacts results that chlorophyll does not absorb light to drive photosynthesis process.

(d)

Summary Introduction

To determine: The way in which the given information explained modern understanding of photosynthesis “the level of several different phosphorylated compounds in photosynthetic cells changed dramatically in response to light exposure”.

Introduction:

Chloroplast is a double membrane bound organelle present in green plants and algae. Chloroplast contains thylakoid membrane in which two photosystem units PSI and PSII are present. These photosystem units absorb sunlight and pass it to an antenna molecule to drive the photosynthesis process.

Explanation of Solution

Explanation:

The changes occur in photosynthetic cells due to the exposure of light. These differences are:

(i) In present of light, ATP molecule would synthesize and other phosphorylated intermediates would be used during ATP synthesis.

(ii) The end product of light reaction drives the synthesis of glucose molecules. The cell then consumes glucose molecule for ATP generation.

(iii) Then under presence of light, ATP will produced by consuming the phosphorylated intermediate.

Conclusion

Conclusion:

In the presence of light ATP molecule will produce that drive the synthesis of glucose. This glucose further metabolized during cellular respiration.

(e)

Summary Introduction

To explain: The way these two models relate to current model of photosynthesis.

Introduction:

The photosynthesis is a process in which the green plants and algae obtains their nutrients. This process takes place in chloroplast where chlorophyll molecules are present. The chloroplast contains thylakoid molecules where light reaction takes place. The end product of the light reaction is the formation of ATP and NADPH which is an energy storing molecule which together helps in driving the Calvin cycle in stroma of chloroplast.

Explanation of Solution

Explanation:

The Emerson model explains that light is used for the production of ATP. This model is correct, light drive the formation of ATP. The Rabinowitch model explains that light is used for the formation of reducing power (NADPH). This model is also correct.

Conclusion

Conclusion:

The Emerson model state that light is used for synthesis of ATP and Rabinowitch model state that light is used to drive the synthesis of NADPH.

(f)

Summary Introduction

To determine: The way Rabinowitch value 8 to 10 photons per molecule of ${\text{CO}}_{\text{2}}$ reduced compare with the value accepted today.

Introduction:

Chloroplast is a double membrane bound organelle present in green plants and algae. Chloroplast contains thylakoid membrane in which two photosystem units PSI and PSII are present. These photosystem units absorb sunlight and pass it to an antenna molecule to drive the photosynthesis process.

Explanation of Solution

Explanation:

According to current model of photosynthesis, 8 photons are used to produce 3 molecules of ATP and 2 molecules of NADPH to reduce 1 molecule of carbon dioxide. Thus, Rabinowitch 8 to 10 photons are used for reduction of 1 molecule of carbon dioxide is accepted.

Conclusion

Conclusion:

The Rabinowitch values of 8 to 10 photons are accepted.

(g)

Summary Introduction

To determine: The way a scientist rebut Rabinowitch argument based on current knowledge of photosynthesis.

Introduction:

The photosynthesis is a process in which the green plants and algae obtains their nutrients. This process takes place in chloroplast where chlorophyll molecules are present. The chloroplast contains thylakoid molecules where light reaction takes place. The end product of the light reaction is the formation of ATP and NADPH which is an energy storing molecule which together helps in driving the Calvin cycle in stroma of chloroplast.

Explanation of Solution

Explanation:

The solar energy is used for driving the photosynthesis process. The end result of photosynthesis is the formation of ATP and NADPH. This capture of light energy by chlorophyll is more efficient than Rabinowitch supposed. Thus, plenty of light energy is available for the synthesis of more than one ATP molecule.

Conclusion

Conclusion:

Chlorophyll molecule is involved in capturing the light energy. This absorption of light energy further drives the production of ATP and NADPH. Thus, the capturing of light by chlorophyll is much more efficient than Rabinowitch supposed.