The following diagram summarizes the light reactions of...

The following diagram summarizes the light reactions of photosynthesis. The diagram shows light-dependent reactions of photosynthesis, including the reaction centers, electron transport chains, and the overall reactions within each of these. The free energy per electron is shown for the oxidation-reduction reactions. The free change of the captured radiant energy is shown. 2NADP+ + 2H + + 2H2 O + 3ADP + 3Pi ? O2 + 4H + + 2NADPH + 3ATP A. In the overall mass balance equation for the light reactions shown above, identify the source of electrons for the synthesis of NADPH. B. Calculate the number of electrons transferred in this reaction. C. Using the free energies per electron displayed, calculate the free energy change of the lightdependent reactions. D. Given that the free energy change for the hydrolysis of ATP is -31.5 kJ/mole and the free energy change for the formation of NADPH from NADP+ is 18 kJ/mole, calculate the total production of free energy for the light reactions. E. Using this definition of energy efficiency, calculate the efficiency of the light reaction of photosynthesis: energy efficiency = free energy produced/energy input

The following diagram summarizes the light reactions of photosynthesis.
The diagram shows light-dependent reactions of photosynthesis, including the reaction centers, electron transport chains, and the overall reactions within each of these. The free energy per electron is shown for the oxidation-reduction reactions. The free change of the captured radiant energy is shown.
2NADP+ + 2H
+ + 2H2 O + 3ADP + 3Pi ?
O2 + 4H
+ + 2NADPH + 3ATP
A. In the overall mass balance equation for the light reactions shown above, identify the source of electrons for the synthesis of NADPH.
B. Calculate the number of electrons transferred in this reaction.
C. Using the free energies per electron displayed, calculate the free energy change of the lightdependent reactions.
D. Given that the free energy change for the hydrolysis of ATP is -31.5 kJ/mole and the free
energy change for the formation of NADPH from NADP+ is 18 kJ/mole, calculate the total
production of free energy for the light reactions.
E. Using this definition of energy efficiency, calculate the efficiency of the light reaction of
photosynthesis: energy efficiency = free energy produced/energy input

Key Concepts

ATP Synthesis

ATP synthesis in the light reactions occurs via photophosphorylation, which is driven by the proton gradient established by the electron transport chain. The flow of protons back across the thylakoid membrane energizes ATP synthase to convert ADP and inorganic phosphate into ATP, coupling energy conversion with chemical bond formation.

Energy Efficiency Calculation

Energy efficiency in the context of photosynthetic light reactions refers to the ratio of useful chemical energy produced (in the form of ATP and NADPH) to the initial energy input from absorbed light. By comparing the free energy produced to the free energy of the electron transfers and photon absorption, one can assess how effectively the light energy is converted into stored chemical energy.

NADPH Formation

NADPH formation involves the transfer of electrons from the electron transport chain to NADP+, reducing it to NADPH. This molecule serves as a reducing agent in the subsequent Calvin cycle, providing the necessary electrons for carbon fixation and biosynthetic reactions in photosynthesis.

Stoichiometry and Electron Balance

Stoichiometry in the light reactions ensures that the number of electrons lost from water oxidation matches those gained during the reduction of NADP+ to NADPH. Maintaining electron balance is essential for the proper functioning of the entire photosynthetic process, and calculations based on this balance allow determination of the number of electrons transferred in the overall reaction.

Reaction Centers

Reaction centers are specialized complexes in the photosystems that facilitate the primary photochemical events of photosynthesis. They contain chlorophyll molecules that absorb light and undergo excitation, thereby initiating the transfer of electrons through the electron transport chain.

Electron Transport Chain

The electron transport chain in the light reactions is a series of protein complexes and mobile carriers that transfer electrons from water to NADP+. As electrons move through the chain, the energy released is used to pump protons across the thylakoid membrane, creating a proton gradient essential for ATP synthesis.

Light-Dependent Reactions

These are the initial steps in photosynthesis where light energy is absorbed by pigments and used to drive electron transfer processes. The reactions occur within the thylakoid membranes and involve photosystems that capture photons, generating electron flow that leads to the production of energy-rich compounds like ATP and NADPH.

Redox Reactions and Free Energy

Redox reactions involve the transfer of electrons between molecules, resulting in changes in oxidation states accompanied by energy transformations. The overall free energy change in these reactions is crucial for driving endergonic processes such as ATP and NADPH formation, and it is quantified in terms of energy per electron transferred.

Transcript

00:01 Okay, so problem 48 here has five parts, so it feels pretty daunting, but it starts with this equation, right? and this is an equation for the free change of energy and so the reactions that are happening here during photosynthesis, right? so i'm just going to write it out as quickly as possible.

00:28 And some of this is showing sort of the synthesis of nadp into nadph and the adp into adt.

00:42 Okay, so we start with this and those are our reactants.

00:48 Okay, and then the products are going to be some oxygen, right? photosynthesis makes oxygen.

00:55 We get some hydrogen ions and then of course, we get our n -a -d -p -h and as well our atp.

01:09 Okay, so this is a photosynthesis, right? and so here, this first section, what we start with are our reactants.

01:24 Let's try to squeeze that in there.

01:27 And what we end up with, these are our products.

01:32 Okay, so when we look at part a, it is asking us, to identify the source of electrons to synthesize the nad ph, right? so we need those extra electrons.

01:54 And what's happening is in photosystem 1, all right, i apologize, in photosystem 2, it splits water, right? so we'll see that we're starting with water here.

02:08 Okay, and this water, if we look at what's happening in in photosystem 2, the water split is going to result in the oxygen and the four hydrogen ions, right? and so we started with something that was neutrally charged, and now we end up with something that is positively charged with these ions here, right? and so you can think of when we have a o2 molecule, each one of these is going to have the six electrons, and they can bond here so that they each have the eight in their outer shell.

02:58 And with the four hydrogens, though, each hydrogen, you know, has just the one.

03:05 Okay, so these four are going to find a way to share two electrons, okay, so that they are, their outer shell is full, and then we get two electrons.

03:27 Sort of free.

03:30 Okay.

03:35 And so this sort of answers both a and b.

03:39 Okay, so where is the source of the electrons? well, it's from the breaking down of the water into oxygen and hydrogen ions, which leaves us with two extra electrons.

03:52 And in fact, you'll often see this equation.

03:55 You'll see this equation written a lot of the times as if it was one water molecule.

04:06 Let's see the textbooks a lot.

04:11 With just the two hydrogen ions, okay, so with the two hydrogen ions, one electron, and then they'll say half, half of an oxygen, which seems a little counterintuitive, but then that way working forward in the textbook you're sort of working with a with just the one electron, and that can simplify later discussions.

04:43 Okay, so part a is it comes from the water, part b, b is that it's going to create two electrons.

04:54 Okay, and now it wants us to do a little bit of math, and it looks like i'll need a new screen here.

05:01 And so we're given that, so for part c now, that the starting free energy for an electron is 77 kilojoules per electron.

05:18 So this is for the reactant...

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