Question

In the light-independent reactions, ____ a. carbon is fixed b. electrons flow through electron transfer chains c. ATP forms

Short answer

In light-independent reactions, carbon is fixed (option a).

Step-by-step solution

Understand Light-Independent Reactions

Light-independent reactions, also known as the Calvin Cycle, occur in the stroma of the chloroplasts. These reactions do not require light and are responsible for converting carbon dioxide and other compounds into glucose.

Identify Key Processes

During the light-independent reactions, carbon dioxide is incorporated into organic molecules. This process is known as carbon fixation. ATP and NADPH, produced during the light-dependent reactions, are used here to fuel the conversion of these molecules into glucose.

Examine the Options

a. Carbon is fixed. This accurately describes a primary function of the light-independent reactions as carbon dioxide is fixed to produce sugar. b. Electrons flowing through electron transfer chains occurs mainly during the light-dependent reactions in the thylakoid membranes. c. ATP forms primarily during the light-dependent reactions.

Choose the Correct Answer

Given that carbon fixation is a central activity of the light-independent reactions, option a is correct: carbon is fixed.

Key concepts

Carbon Fixation

Carbon fixation is the initial step in the Calvin Cycle, which is the set of light-independent reactions in photosynthesis. It refers to the process where carbon dioxide is taken from the atmosphere and incorporated into an organic molecule, typically in the form of a three-carbon compound. This process is crucial because it transforms inorganic carbon into organic matter, which plants can then use to store energy in the form of glucose.

• This step starts with the enzyme RuBisCO, which is central to carbon fixation.
• RuBisCO adds carbon dioxide to a five-carbon sugar named ribulose bisphosphate (RuBP).
• The resulting six-carbon compound is unstable and quickly splits into two molecules of 3-phosphoglycerate, a three-carbon compound.

By converting carbon dioxide into a form that can be used in cellular respiration, carbon fixation lays the foundation for most life forms on Earth. Without this critical process, plants would be unable to create the sugars necessary for their growth and energy needs.

Light-Independent Reactions

The light-independent reactions, or Calvin Cycle, occur in the stroma of the chloroplast. Unlike light-dependent reactions, they do not require light directly to proceed. This doesn't mean they can happen in complete darkness; they still rely on the products of light-dependent reactions: ATP and NADPH.

Here's how the process works:

• The Calvin Cycle uses ATP and NADPH to convert carbon dioxide and water into glucose.
• These reactions incorporate molecules of carbon dioxide into a stable intermediate and then reduce this into sugars.
• ATP is used as an energy source, while NADPH provides the necessary reducing power.
• Throughout this cycle, a series of transformations occur to stabilize and reduce the carbon molecules, ultimately producing glucose as a by-product.

Because these reactions form the basis of a plant's anabolic pathways, they are essential for the synthesis of carbohydrates, which serve as energy storage molecules.

Photosynthesis

Photosynthesis is the overarching process by which plants, algae, and certain bacteria convert light energy into chemical energy stored in glucose molecules. It occurs primarily in the chloroplasts of plant cells. Photosynthesis is divided into two main phases: the light-dependent reactions and the light-independent reactions.

In photosynthesis:

• Light-dependent reactions convert solar energy into chemical energy in the form of ATP and NADPH. This phase takes place in the thylakoid membranes where chlorophyll captures sunlight.
• Light-independent reactions synthesize glucose from carbon dioxide using the products of the light-dependent reactions.
• The entire process not only provides the energy necessary for plant survival but also generates oxygen as a by-product.

Photosynthesis is crucial for life on Earth because it is the primary source of organic matter for nearly all organisms, and it is directly responsible for the oxygen in our atmosphere. Without photosynthesis, life as we know it would not be possible.