Question

Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the one that is best in each case. Answers can be found here. All of the following statements are correct regarding the light reaction in photosynthesis EXCEPT (A) The antennae pigments capture sunlight. (B) The reaction center in photosystem II is P680. (C) Light energy is converted to chemical energy. (D) The electrons are activated and passed along an electron transport chain. (E) Carbon dioxide is used to make glucose.

Short answer

The incorrect statement is (E) Carbon dioxide is used to make glucose. This statement describes a part of the light-independent reactions in photosynthesis, not the light-dependent reactions.

Step-by-step solution

- Understand the process

The light-dependent reactions of photosynthesis use light energy to make ATP and NADPH, which are used in the Calvin cycle. These reactions occur in two major stages: Photosystem II and Photosystem I. Both processes involve light-absorbing pigments, called antenna pigments, and reaction centers (P680 in photosystem II and P700 in photosystem I). Light energy activates electrons, which are then passed along an electron transport chain.

- Evaluate each statement

(A) The antennae pigments capture sunlight. This is true because the antenna pigments absorb light energy and pass it to the reaction centers. \n(B) The reaction center in photosystem II is P680. This is true because P680 refers to the specific form of chlorophyll a in the reaction center of photosystem II. \n(C) Light energy is converted to chemical energy. This is true because the light-dependent reactions convert sunlight into chemical energy in the form of ATP and NADPH. \n(D) The electrons are activated and passed along an electron transport chain. This is true because electrons energized by light absorption are indeed passed along an electron transport chain.

- Identify the incorrect statement

(E) Carbon dioxide is used to make glucose. This is the incorrect statement. While it is true that carbon dioxide is used to make glucose in photosynthesis, this occurs in the light-independent reactions (the Calvin cycle), not the light-dependent reactions.

Key concepts

Photosystem II

Photosystem II initiates the light-dependent reactions of photosynthesis and plays a crucial role in capturing light energy to generate ATP and NADPH. It is the first complex in the chain and is located within the thylakoid membranes of chloroplasts.
Within Photosystem II, light is absorbed by various pigments found in structures called antenna complexes. These pigments include chlorophyll a, chlorophyll b, and various carotenoids.

Once captured, light energy excites electrons in chlorophyll a at the reaction center, specifically at a pigment known as P680. Once these electrons are excited, they are transferred to a primary electron acceptor in a nearby protein complex.
The specific function of Photosystem II doesn't end here:

• It splits water molecules (a process called photolysis) to release oxygen as a byproduct.
• It provides electrons to the electron transport chain that will ultimately replace electrons lost by P680.

This movement and replacement of electrons is critical for maintaining the electron flow necessary to drive other photosynthetic reactions.

Photosystem I

After electrons travel through the electron transport chain from Photosystem II, they reach Photosystem I. This stage falls later in the light-dependent process but is just as vital.
Photosystem I is also located in the thylakoid membranes, and it works to further excite electrons with light energy. Here, the key pigment at the reaction center is P700. Just like in Photosystem II, light energy is absorbed by chlorophyll molecules, but now at this part of the photosynthetic pathway, these electrons have traveled through the chain gaining and releasing energy.

Photosystem I has several functions:

• It further excites electrons which are then passed onto another series of proteins, ultimately used to form NADPH, an important energy carrier.
• NADPH, along with ATP produced by chemiosmosis, will then be used in the Calvin cycle.

Photosystem I is strategically placed to ensure that light energy absorbed is efficiently converted into chemical energy, maintaining the seamless energy transfer necessary for subsequent photosynthesis phases.

Calvin Cycle

The Calvin Cycle, often referred to as the light-independent reactions, takes place in the stroma of the chloroplasts. This process does not require light directly but depends on the ATP and NADPH generated by the light-dependent reactions in Photosystems II and I. The primary objective of the Calvin cycle is to convert atmospheric carbon dioxide into glucose.
Here is the basic flow of the Calvin Cycle:

• Carbon fixation, where CO₂ is attached to a five-carbon sugar called ribulose bisphosphate (RuBP), a reaction catalyzed by an enzyme known as RuBisCO.
• The resulting six-carbon compound breaks down into two molecules of 3-phosphoglycerate (3-PGA).
• 3-PGA is then converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. This transformation is essential as G3P can be assembled into glucose and other carbohydrates.

The Calvin cycle is cyclic and ongoing, requiring the constant input of ATP and NADPH. Importantly, it represents the synthesis component of photosynthesis, turning inorganic CO₂ into energy-storing molecules that plants use to grow and thrive.