Problem 2 - Campbell Biology

Question

Which of the following sequences correctly represents the flow of electrons during photosynthesis? \(\begin{equation}\begin{array}{l}{\text { (A) } \mathrm{NADPH} \rightarrow \mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}} \\ {\text { (B) } \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{NADPH} \rightarrow \text { Calvin cycle }} \\\ {\text { (C) } \mathrm{H}_{2} \mathrm{O} \rightarrow \text { photosystem I } \rightarrow \text { photosystem II }} \\ {\text { (D) } \mathrm{NADPH} \rightarrow \text { electron transport chain } \rightarrow \mathrm{O}_{2}}\end{array}\end{equation}\)

Step-by-Step Solution

Answer

(B) H₂O → NADPH → Calvin cycle

Step 1: Understanding Electron Flow in Photosynthesis

In photosynthesis, electrons are transferred through a series of steps starting from water (H₂O) and eventually being used in the Calvin cycle. Identify what molecules or processes are commonly associated with each stage.

Step 2: Breakdown of Each Option

Review each provided option: (A) NADPH → O₂ → CO₂: This sequence does not fit photosynthesis as NADPH is not the initial electron donor.(B) H₂O → NADPH → Calvin cycle: This is a logical sequence as it starts with water, transfers electrons to NADPH, which then participates in the Calvin cycle.(C) H₂O → photosystem I → photosystem II: This is incorrect as photosystem II acts before photosystem I.(D) NADPH → electron transport chain → O₂: This sequence relates more to respiration, not photosynthesis.

Step 3: Correct Sequence Selection

Among the given options, option (B) H₂O → NADPH → Calvin cycle correctly represents the flow of electrons during photosynthesis. Water is the primary electron donor, NADPH is the carrier of electrons to the Calvin cycle.

Step 4: Conclusion

After analyzing all options, conclude that the correct sequence is option (B). This matches the transformation and flow of electrons found in the photosynthetic process.

Key Concepts

Electron Transport Chain

Photosynthesis starts with the splitting of water molecules (H₂O). This happens in the chloroplasts, specifically within the thylakoid membrane.
Electrons released from water pass through a series of protein complexes and mobile carriers. This pathway is called the electron transport chain.
The primary goal of this process is to pump protons across the thylakoid membrane, creating a gradient. This gradient is then used to produce ATP, a type of energy storage molecule.
As electrons move through the chain, they lose energy, which is harnessed to pump protons into the thylakoid lumen. Eventually, the electrons are transferred to NADP⁺, transforming it into NADPH.
Important takeaways:

• The electron transport chain is essential for ATP production during photosynthesis.
• Electrons originally come from water and move through different protein complexes.
• NADP⁺ is the final electron acceptor, forming NADPH.

Calvin Cycle

The Calvin cycle is the stage of photosynthesis where carbon fixation occurs. This means that carbon dioxide (CO₂) from the air is converted into glucose, a sugar.
The Calvin cycle takes place in the stroma of the chloroplast and is sometimes called the light-independent reactions. Despite this name, it indirectly depends on light as it needs ATP and NADPH produced during the light-dependent reactions.
The cycle involves several steps:
1) Carbon fixation: CO₂ is attached to a 5-carbon compound called ribulose-1,5-bisphosphate (RuBP).
2) Reduction: ATP and NADPH are used to convert the fixed carbon into a 3-carbon sugar (G3P).
3) Regeneration: Some G3P molecules leave the cycle to form glucose, while others are used to regenerate RuBP, allowing the cycle to continue.
Key points on the Calvin cycle include:

• It takes place in the chloroplast stroma.
• Utilizes ATP and NADPH from the light-dependent reactions.
• Involves carbon fixation, reduction, and regeneration steps.

NADPH

NADPH is a crucial molecule in photosynthesis, acting as a carrier of electrons and energy. Its full name is Nicotinamide Adenine Dinucleotide Phosphate.
During the light-dependent reactions of photosynthesis, NADP⁺ gains electrons (is reduced) to form NADPH. This process occurs in the electron transport chain located in the thylakoid membrane.
Once NADPH is formed, it carries those high-energy electrons to the Calvin cycle. Here, NADPH donates its electrons to support the transformation of CO₂ into glucose.
Important aspects of NADPH include:

• It acts as an electron carrier.
• Formed in the light-dependent reactions by the reduction of NADP⁺.
• Essential for the Calvin cycle to produce glucose from CO₂.