Problem 3 - Campbell Biology Concepts & Connections

Question

Which of the following are produced by reactions that take place in the thylakoids and consumed by reactions in the stroma? a. \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) b. ATP and NADPH c. ATP, NADPH, and CO \(_{2}\) d. ATP, NADPH, and \(\mathrm{O}_{2}\)

Step-by-Step Solution

Answer

b. ATP and NADPH

Step 1: Understand the Photosynthesis Process

Photosynthesis in plants occurs in two main stages: the light-dependent reactions and the Calvin cycle. The light-dependent reactions occur in the thylakoids and produce ATP and NADPH.

Step 2: Identify Products of Light-Dependent Reactions

The light-dependent reactions, which take place in the thylakoids, produce ATP, NADPH, and \(\text{O}_2\) as products.

Step 3: Identify Consumption in the Calvin Cycle

The Calvin cycle (light-independent reactions) occurs in the stroma of the chloroplast and requires ATP and NADPH produced by the light-dependent reactions.

Step 4: Determine Correct Answer

Among the given options, the molecules produced by the thylakoids and consumed by reactions in the stroma are ATP and NADPH. Therefore, the correct answer is option b.

Key Concepts

Light-Dependent Reactions

Photosynthesis begins with the light-dependent reactions. These reactions occur in the thylakoid membranes of the chloroplasts. Here, sunlight is absorbed by chlorophyll and other pigments. The absorbed light energy is used to split water molecules into oxygen, protons, and electrons. The oxygen is released as a by-product. The electrons then move through the electron transport chain, a series of proteins embedded in the thylakoid membrane. This movement creates a flow of protons across the membrane, generating a proton gradient used to produce ATP through a process called chemiosmosis. Additionally, the electrons are transferred to NADP+, forming NADPH. Both ATP and NADPH are essential for the next stage of photosynthesis.

Calvin Cycle

The Calvin cycle, also known as the light-independent reactions, takes place in the stroma of the chloroplasts. It does not directly rely on light but uses the ATP and NADPH generated by the light-dependent reactions. The cycle starts with carbon fixation, where carbon dioxide is attached to a five-carbon sugar named ribulose bisphosphate (RuBP). This process is catalyzed by the enzyme rubisco, leading to the formation of a six-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA). These molecules are then converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. Some G3P molecules are used to form glucose and other sugars, while others regenerate RuBP to continue the cycle. The Calvin cycle is crucial for synthesizing organic molecules plants need for growth and energy storage.

ATP and NADPH

ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) are crucial energy carriers in photosynthesis. In the light-dependent reactions, ATP is produced by the enzyme ATP synthase, which uses the energy from the proton gradient created by the electron transport chain. ATP provides the energy needed for various chemical reactions in the Calvin cycle.
NADPH is formed when electrons from the light-dependent reactions reduce NADP+ in the chloroplasts. It serves as a reducing agent, supplying the high-energy electrons required for converting carbon dioxide into glucose in the Calvin cycle. Both ATP and NADPH are essential for transforming energy from sunlight into chemical energy stored in sugars, enabling plants to grow and thrive.