Practice: Photophosphorylation 1 - Online Tutor, Practice Problems & Exam Prep

The light reactions of photosynthesis involve the absorption of photons by chlorophyll in photosystems, primarily Photosystem II, which splits water to release oxygen. The ultimate electron acceptor is NADP⁺, forming NADPH for the Calvin cycle. Cyclic electron flow generates ATP without producing NADPH or oxygen. The energy of a photon can be calculated using Planck's equation, where the energy per mole of photons is approximately 176 kJ/mol. Understanding these processes is crucial for grasping the fundamentals of photosynthesis and energy transfer in plants.

$E^{\mathrm{photon}}=\frac{hc}{\lambda }$

Practice

Which of the following about photosynthetic light reactions in plants is false? And the answer is that the ultimate electron acceptor is O₂, right? No. I mean, first of all, we can have cyclic electron flow like the next question is going to deal with. Additionally, the ultimate electron acceptor, if anything, would be NADP⁺ because it's going to get reduced to NADPH and carry those electrons to the Calvin cycle. Now, cyclic electron flow produces ATP but it does not produce oxygen and NADPH. Right? The ferredoxin is not taking the electrons to produce NADPH. It's bringing it back to the proton pump and no oxygen is produced because cyclic electron flow involves photosystem 1, not photosystem 2. And it's photosystem 2 where water is split and oxygen is released.

Now, archaea have redoxins which use light to generate a proton motive force, but rhodopsins are not limited to this function. I mean, we have rhodopsins in our eyes that have been modified for visual sensation. We also have seen rhodopsins that can be used as chloride pumps.

Now, in the chloroplast membrane, a photosystem contains many chlorophyll molecules which mainly transfer excited states to the reaction center chlorophyll. That is the main job. It’s like an antenna in the light-harvesting complex, focusing all that excitation on the reaction center to use an equation here that involves Planck’s constant (h) and c, which is the speed of light. So this is the Planck-Einstein equation. And to plug in our values:

E = h · c λ

We're going to multiply \( h = 6.62607015 \times 10^{-34} \) joules·seconds by \( c = 3 \times 10^{8} \) meters/second, all over \( \lambda = 680 \) nanometers. Be careful here. Don't forget to convert your units when you solve this through. And if you solve this through, what you should wind up with is \( E = 2.925 \times 10^{-19} \) joules. This equation is just giving us the energy of one photon. We are asked for a mole, right? So we need to take that number \( 2.925 \times 10^{-19} \) and multiply it by Avogadro's number \( 6.022 \times 10^{23} \) to get the energy for a mole of photons. So that is going to give us about \( 176 \) kilojoules. More precisely, it's \( 176,144 \) joules per mole. So that would be equivalent to 176 kilojoules per mole.

Alright. Let's flip the page and finish this up.

Here’s what students ask on this topic:

What is the ultimate electron acceptor in the light reactions of photosynthesis?

The ultimate electron acceptor in the light reactions of photosynthesis is NADP⁺. During these reactions, electrons are transferred through a series of proteins in the thylakoid membrane, ultimately reducing NADP⁺ to NADPH. This NADPH is then used in the Calvin cycle to help convert carbon dioxide into glucose.

How does cyclic electron flow differ from non-cyclic electron flow in photosynthesis?

Cyclic electron flow involves only Photosystem I and results in the production of ATP without the generation of NADPH or oxygen. In contrast, non-cyclic electron flow involves both Photosystem I and Photosystem II, leading to the production of ATP, NADPH, and oxygen. Cyclic electron flow is used to balance the ATP/NADPH ratio required for the Calvin cycle.

What is the role of chlorophyll in the light reactions of photosynthesis?

Chlorophyll plays a crucial role in the light reactions of photosynthesis by absorbing light energy and converting it into chemical energy. It is located in the photosystems within the thylakoid membrane. When chlorophyll absorbs photons, it becomes excited and transfers this energy to the reaction center chlorophyll, initiating the electron transport chain.

How is the energy of a photon calculated in the context of photosynthesis?

The energy of a photon is calculated using Planck's equation: $E_{\mathrm{photon}}=\frac{hc}{\lambda }$, where $h$ is Planck's constant (6.626 x 10^-34 J·s), $c$ is the speed of light (3 x 10⁸ m/s), and $\lambda$ is the wavelength of light. For example, for a wavelength of 680 nm, the energy per mole of photons is approximately 176 kJ/mol.

What is the significance of Photosystem II in the light reactions of photosynthesis?

Photosystem II is significant in the light reactions of photosynthesis because it is responsible for the splitting of water molecules, a process known as photolysis. This reaction releases oxygen as a byproduct and provides electrons to replace those lost by chlorophyll in Photosystem II. These electrons are then passed down the electron transport chain, contributing to the formation of ATP and NADPH.