Problem 5 - Biochemistry

Question

If noncyclic photosynthetic electron transport leads to the translocation of \(3 \mathrm{H}^{+} / e^{-}\) and cyclic photosynthetic electron transport leads to the translocation of \(2 \mathrm{H}^{+} / e^{-},\) what is the relative photosynthetic efficiency of ATP synthesis (expressed as the number of photons absorbed per ATP synthesized) for noncyclic versus cyclic photophosphorylation? (Assume that the \(\mathrm{CF}_{1} \mathrm{CF}_{0}-\mathrm{ATP}\) synthase yields \(3 \mathrm{ATP} / 14 \mathrm{H}^{+}\).)

Step-by-Step Solution

Answer

In noncyclic photophosphorylation, about 9.34 photons are absorbed per ATP and in cyclic photophosphorylation, 14 photons are absorbed per ATP.

Step 1: Phosphorylation Calculation

Calculate the photons absorbed per ATP in noncyclic phosphorylation. The noncyclic photosynthetic electron transport leads to the translocation of \(3 \mathrm{H}^{+} / e^{-}\), and each ATP synthase yields 3 ATP for 14 hydrogen ions translocated. So we have to find how many electrons are needed for 14 \( \mathrm{H}^{+}\)

Step 2: Hydrogen Ion Calculation

To find the number of electrons, divide 14 by the number of hydrogen ions translocated per electron in noncyclic phosphorylation. In this case, it is 3. \( \frac{14}{3} \approx 4.67 \) electrons.

Step 3: Photon Calculation for Noncyclic Photophosphorylation

Each electron flow in the photosynthetic transport requires 2 photons. So multiply the number of electrons by 2 to find the number of photons per ATP in noncyclic phosphorylation. In this case, \(2 \times 4.67 = 9.34 \) photons per ATP.

Step 4: Phosphorylation Calculation for Cyclic Photophosphorylation

Calculate the photons absorbed per ATP in cyclic phosphorylation by repeating the same steps as above. Cyclic photosynthetic electron transport leads to the translocation of \(2 \mathrm{H}^{+} / e^{-}\). So, we have, \( \frac{14}{2} \approx 7 \) electrons.

Step 5: Photon Calculation for Cyclic Photophosphorylation

Each electron flow requires 2 photons. So, multiply the number of electrons by 2 to find the number of photons per ATP in cyclic phosphorylation. In this case, \(2 \times 7 = 14 \) photons per ATP.

Key Concepts

Noncyclic Photophosphorylation

This is a process that occurs in the thylakoid membranes of chloroplasts during photosynthesis. It is called "noncyclic" because the electrons, once excited by sunlight, do not return to their original photosystem but are instead transferred to NADP+, forming NADPH.

• Electrons flow from water molecules to photosystem II, then to photosystem I, and finally to NADP+.
• This flow generates a proton gradient across the thylakoid membrane.
• The gradient powers the production of ATP and is linked to the generation of NADPH.

During this transport, 3 hydrogen ions (H+) are translocated per electron to further aid ATP synthesis.

Cyclic Photophosphorylation

In contrast, cyclic photophosphorylation involves the circular flow of electrons. Here, electrons cycle back to the same photosystem after passing through parts of the electron transport chain.

• Only photosystem I is involved in this cycle.
• This process generates a proton gradient to synthesize ATP, but not NADPH.
• In cyclic photophosphorylation, 2 hydrogen ions (H+) are translocated per electron.

It's a simpler route primarily focused on ATP generation when the NADPH to ATP ratio needs balancing.

ATP Synthesis

ATP synthesis during photosynthesis is powered by the proton gradient established by photophosphorylation.

• The enzyme responsible for ATP synthesis is CF₁CF₀-ATP synthase.
• This enzyme complex utilizes the flow of protons (H+) back into the stroma to produce ATP from ADP and inorganic phosphate.
• Typically, 14 protons passing through this enzyme result in the synthesis of 3 ATP molecules.

The efficiency of this synthesis is crucial for understanding how effectively light energy is converted into chemical energy.

Electron Transport Chain

The electron transport chain is a series of protein complexes found within the thylakoid membrane.

• These complexes facilitate the transfer of electrons derived from water molecules.
• The electrons move through photosystems and other carriers, generating an electrochemical gradient.
• This gradient is indispensable for ATP production via ATP synthase.

The chain's efficiency impacts the number of photons required for ATP production, influencing overall photosynthetic efficacy.