In discussion, we analyzed the initial and final states of a toy model of photosynthesis: the reaction CO2 + H2O ↔ H2CO + O2.
The binding of the carbon in the organic molecule (formaldehyde) has effectively traded a strong C=O double bond for a weaker O=O double bond, so the reaction is endothermic from left to right. (The right side has a higher chemical energy so you have to put energy in to get there.)
Although that tells the overall result, the actual mechanism of photosynthesis is more complicated. Discovering how this works is one of the great stories of figuring out what is happening in a complex biological process.
Our simple equation of the initial and final state of the transformed molecules makes it look as if the O2 is peeled off the CO2 and the C is attached to the H2O. This isn't how it happens -- the reaction is better described as involving a second water molecule, like this: CO2 + 2H2O ↔ H2CO + H2O + O2.
The second water molecule acts like a catalyst, helping the reaction take place and changing during the reaction, but being restored in the end. The actual process is quite complex and involves many helper molecules and sub-reactions. In order to get an idea of what the mechanism is and what the energy flow would be without the helper molecules, consider the following "improved toy model". This kind of analysis helps develop insight into why sunlight doesn't produce formaldehyde (or glucose) from carbon dioxide and water vapor in the air without a plant -- and what the plant needs to do energetically to make it actually happen.
A. THE LIGHT REACTION
In the reaction that takes place in the presence of sunlight, the water molecules are effectively disassembled and the oxygen molecule built. The reaction looks something like this:
Does this reaction need an energy input to go from left to right, or does it release energy? ---Select--- Energy input is required Energy is released
Calculate the magnitude (absolute value) of the energy input or released.
Energy = eV
B. THE DARK REACTION
In the reaction that takes place in the absence of sunlight, the hydrogens are attached to the carbon and one of the oxygens pulled off the CO2 to make a water molecule. It looks something like this:Does this reaction need an energy input to go from left to right, or does it release energy? _____ Energy input is required Energy is released
Calculate the magnitude (absolute value) of the energy input or released.
Energy = eV
C. FULL PROCESS
The process we're studying here makes formaldehyde in two steps. Putting in all the molecules, the overall reaction with intermediate steps looks like this:
If we only consider the initial and final states (1 and 3), and skip the intermediate (2) state, how much energy must be provided by sunlight for the reaction to occur? (This should match the result you found in discussion for the toy model of photosynthesis.)
Energy input = _ eV
When we consider all the steps (1, 2, and 3), how much energy must be provided by sunlight for the reaction to occur?
Energy input =_ eV
The latter result, the energy requirement for our "improved toy model" is larger. The larger energy requirement helps to explain why photosynthesis does not occur spontaneously in the atmosphere.
D. ADVANCED TOY MODEL
In reality, the energy requirement for photosynthesis is even higher. This is because the hydrogen atoms are actually disassembled into their component parts -- H+ (a proton) and e- (an electron) -- to get all the hydrogens into the right places during the reaction. Just as atoms in molecules have a binding energy that must be supplied to break the bonds, the protons and electrons within atoms have binding energies that must be provided to separate the electrons from the atom (this is sometimes called ionization energy). The energy required to unbind a single hydrogen atom is relatively large: 13.6 eV.
In step 2 of the full process above, the four hydrogens should be replaced by four H+ and four e-. In this "advanced toy model", how much energy must be provided by sunlight for the light reaction (1 to 2) to occur?
Energy input = ____ eV
This is way too much energy to be provided by sunlight -- there is a large energy barrier that stops the formation of formaldehyde (or glucose) in the atmosphere. The role of the plant in making photosynthesis happen is to provide biological molecules that dramatically reduce this energy barrier. That's a story you will learn in your biology classes.

Question

Chanselor5321 Chanselor5321

16-12-2022
Chemistry

Answered

In discussion, we analyzed the initial and final states of a toy model of photosynthesis: the reaction CO2 + H2O ↔ H2CO + O2.
The binding of the carbon in the organic molecule (formaldehyde) has effectively traded a strong C=O double bond for a weaker O=O double bond, so the reaction is endothermic from left to right. (The right side has a higher chemical energy so you have to put energy in to get there.)
Although that tells the overall result, the actual mechanism of photosynthesis is more complicated. Discovering how this works is one of the great stories of figuring out what is happening in a complex biological process.
Our simple equation of the initial and final state of the transformed molecules makes it look as if the O2 is peeled off the CO2 and the C is attached to the H2O. This isn't how it happens -- the reaction is better described as involving a second water molecule, like this: CO2 + 2H2O ↔ H2CO + H2O + O2.
The second water molecule acts like a catalyst, helping the reaction take place and changing during the reaction, but being restored in the end. The actual process is quite complex and involves many helper molecules and sub-reactions. In order to get an idea of what the mechanism is and what the energy flow would be without the helper molecules, consider the following "improved toy model". This kind of analysis helps develop insight into why sunlight doesn't produce formaldehyde (or glucose) from carbon dioxide and water vapor in the air without a plant -- and what the plant needs to do energetically to make it actually happen.
A. THE LIGHT REACTION
In the reaction that takes place in the presence of sunlight, the water molecules are effectively disassembled and the oxygen molecule built. The reaction looks something like this:
Does this reaction need an energy input to go from left to right, or does it release energy? ---Select--- Energy input is required Energy is released
Calculate the magnitude (absolute value) of the energy input or released.
Energy = eV
B. THE DARK REACTION
In the reaction that takes place in the absence of sunlight, the hydrogens are attached to the carbon and one of the oxygens pulled off the CO2 to make a water molecule. It looks something like this:Does this reaction need an energy input to go from left to right, or does it release energy? _____ Energy input is required Energy is released
Calculate the magnitude (absolute value) of the energy input or released.
Energy = eV
C. FULL PROCESS
The process we're studying here makes formaldehyde in two steps. Putting in all the molecules, the overall reaction with intermediate steps looks like this:
If we only consider the initial and final states (1 and 3), and skip the intermediate (2) state, how much energy must be provided by sunlight for the reaction to occur? (This should match the result you found in discussion for the toy model of photosynthesis.)
Energy input = _ eV
When we consider all the steps (1, 2, and 3), how much energy must be provided by sunlight for the reaction to occur?
Energy input =_ eV
The latter result, the energy requirement for our "improved toy model" is larger. The larger energy requirement helps to explain why photosynthesis does not occur spontaneously in the atmosphere.
D. ADVANCED TOY MODEL
In reality, the energy requirement for photosynthesis is even higher. This is because the hydrogen atoms are actually disassembled into their component parts -- H+ (a proton) and e- (an electron) -- to get all the hydrogens into the right places during the reaction. Just as atoms in molecules have a binding energy that must be supplied to break the bonds, the protons and electrons within atoms have binding energies that must be provided to separate the electrons from the atom (this is sometimes called ionization energy). The energy required to unbind a single hydrogen atom is relatively large: 13.6 eV.
In step 2 of the full process above, the four hydrogens should be replaced by four H+ and four e-. In this "advanced toy model", how much energy must be provided by sunlight for the light reaction (1 to 2) to occur?
Energy input = ____ eV
This is way too much energy to be provided by sunlight -- there is a large energy barrier that stops the formation of formaldehyde (or glucose) in the atmosphere. The role of the plant in making photosynthesis happen is to provide biological molecules that dramatically reduce this energy barrier. That's a story you will learn in your biology classes.

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