Problem 5 - Campbell Biology Concepts & Connections

Question

The reactions of the Calvin cycle are not directly dependent on light, but they usually do not occur at night. Why? (Explain your answer.) a. It is often too cold at night for these reactions to take place. b. Carbon dioxide concentrations decrease at night. c. The Calvin cycle depends on products of the light reactions. d. Plants usually close their stomata at night.

Step-by-Step Solution

Answer

The Calvin cycle relies on ATP and NADPH from light reactions, which stop at night.

Step 1 - Understanding the Calvin Cycle

The Calvin cycle, also known as the light-independent reactions or dark reactions, involves the fixation of carbon dioxide into glucose. While these reactions do not require light, they depend on the energy carriers ATP and NADPH which are produced during the light-dependent reactions.

Step 2 - Dependencies on Light Reactions

Even though the Calvin cycle itself does not use light, it needs ATP and NADPH generated by the light reactions that occur in the chloroplasts during daylight. Without these molecules, the Calvin cycle cannot proceed.

Step 3 - Examining Night Conditions

At night, the light-dependent reactions stop because there is no light to drive them. Therefore, ATP and NADPH are not produced in sufficient quantities, causing the Calvin cycle to halt as it doesn't get the necessary reactants.

Step 4 - Eliminate Incorrect Options

Option a is incorrect because temperature changes are not the primary reason for the inactivity of the Calvin cycle at night. Option b is incorrect as carbon dioxide concentration does not notably decrease at night. Option d is incorrect because while plants may close their stomata at night, it doesn't directly stop the Calvin cycle.

Step 5 - Identify the Correct Answer

The correct answer is c. The Calvin cycle depends on products of the light reactions, which are ATP and NADPH, and these are not produced in the absence of light.

Key Concepts

Light-dependent reactions

Light-dependent reactions are the first stage in the process of photosynthesis. These reactions take place within the thylakoid membranes of the chloroplasts and are primarily driven by sunlight. When chlorophyll and other pigments absorb light, they convert it into chemical energy in the form of ATP and NADPH.
This energy production is crucial, as it provides the necessary inputs for the Calvin cycle. Here's a quick breakdown:

• Solar energy is captured by chlorophyll.
• Water molecules are split, releasing oxygen as a byproduct.
• The energy from the light is used to produce ATP and NADPH.

Even though the Calvin cycle doesn't require light directly, it relies heavily on these energy carriers.

ATP and NADPH

ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) are energy carriers produced during the light-dependent reactions of photosynthesis. These molecules store the energy absorbed from light and transport it to the Calvin cycle. Without them, the Calvin cycle cannot fix carbon dioxide into glucose.

Let's break down the roles of each:

• ATP provides the energy needed for the chemical reactions to occur in the Calvin cycle.
• NADPH donates electrons, which are necessary for reducing carbon dioxide molecules into sugar.

When it's dark, the light-dependent reactions halt, leading to a shortage of ATP and NADPH, causing the Calvin cycle to stop too.

Photosynthesis

Photosynthesis is the process by which plants convert light energy into chemical energy to fuel their activities. It can be divided into two main stages:
1. Light-dependent reactions (which we discussed previously)
2. The Calvin cycle, or light-independent reactions that take place in the stroma of the chloroplasts.
The Calvin cycle indeed depends on ATP and NADPH produced by the light-dependent reactions. Here’s a simplified flow of how both stages work together:

• Light-dependent reactions produce ATP and NADPH during the day.
• The Calvin cycle uses ATP and NADPH to convert carbon dioxide into glucose.
• At night, without light, ATP and NADPH are not available in sufficient quantities.

Therefore, even though light is not directly needed for the Calvin cycle, its dependency on ATP and NADPH ties it closely to the presence of light.