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Chapter 8: Problem 33

Why does it take three turns of the Calvin cycle to produce G3P, the initial product of photosynthesis? a. To fix enough carbon to export one G3P molecule. b. To fix enough oxygen to export one G3P molecule. c. To produce RuBisCO as an end product. d. To produce ATP and NADPH for fixation of G3P.

Short Answer

Expert verified
It takes three turns to fix enough carbon to export one G3P molecule. The correct answer is (a).

Step by step solution

01

Understand the Calvin Cycle

The Calvin cycle is a series of biochemical redox reactions that occur in the stroma of chloroplasts in photosynthetic organisms. It is divided into three main stages: carbon fixation, reduction phase, and regeneration of RuBP (Ribulose bisphosphate). The purpose of the Calvin cycle is to convert carbon dioxide and other compounds into glucose.
02

Analyze the Cycle's Requirement

To produce one molecule of G3P (glyceraldehyde-3-phosphate), the Calvin cycle must fix three molecules of carbon dioxide. This is because each turn of the Calvin cycle fixes one molecule of carbon dioxide.
03

Calculate Carbon Fixation

Each molecule of CO2 contributes one carbon atom. To form one G3P (which contains three carbons), three molecules of CO2 are required. This means three turns of the Calvin cycle are necessary to accumulate enough carbon atoms to form one G3P.
04

Evaluate the Answer Choices

a. Is correct because it explains that three turns are necessary to fix enough carbon for one G3P.b. Is incorrect because the Calvin cycle fixes carbon, not oxygen.c. Is incorrect because RuBisCO is an enzyme, not a product of the cycle.d. Is partially correct but not the primary reason.
05

Conclusion

The Calvin cycle turns three times to fix enough carbon to export one G3P molecule. Therefore, the correct answer is (a).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Photosynthesis
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy, mostly from the sun, into chemical energy stored in glucose. This process occurs predominantly in the chloroplasts of plant cells. Photosynthesis involves two major stages: the light-dependent reactions and the Calvin cycle. The light-dependent reactions capture energy from sunlight to produce ATP and NADPH. These molecules are then utilized in the Calvin cycle to convert carbon dioxide into glucose. During photosynthesis, oxygen is also released as a by-product, contributing to the life-sustaining oxygen in our atmosphere.
Carbon Fixation
Carbon fixation is the initial step of the Calvin cycle where carbon dioxide is 'fixed' into an organic molecule. This step takes place in the stroma of chloroplasts. The enzyme RuBisCO facilitates the attachment of carbon dioxide to a five-carbon sugar called ribulose bisphosphate (RuBP). This reaction results in a six-carbon compound that immediately splits into two three-carbon molecules called 3-phosphoglycerate (3-PGA). This step is crucial because it allows the inorganic carbon to be incorporated into an organic form that can be further processed by the plant.
G3P Production
G3P, or glyceraldehyde-3-phosphate, is the carbohydrate produced in the Calvin cycle. After carbon fixation, the 3-PGA molecules are converted into G3P through a series of reductions and energy transfers involving ATP and NADPH. For every three molecules of carbon dioxide fixed, six G3P molecules are formed. However, only one G3P molecule exits the cycle to be used by the plant in biosynthesizing glucose and other carbohydrates. The remaining G3P molecules are recycled to regenerate RuBP, enabling the cycle to continue.
RuBisCO Enzyme
RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is one of the most important enzymes in the Calvin cycle. It catalyzes the first major step of carbon fixation. RuBisCO is also one of the most abundant proteins on Earth, given its crucial role in photosynthesis. While it is highly efficient at fixing carbon dioxide, it can also bind oxygen, leading to a wasteful process called photorespiration. Despite this flaw, RuBisCO remains essential for the Calvin cycle and overall plant growth.
Redox Reactions
Redox reactions, or reduction-oxidation reactions, involve the transfer of electrons between molecules. In the context of the Calvin cycle, these reactions are essential for converting 3-PGA into G3P. Specifically, NADPH donates electrons (gets oxidized) to 3-PGA, reducing it to G3P. This process not only converts the inorganic carbon into an organic form but also stores energy in a stable form. The ATP generated during the light-dependent reactions provides the energy needed for these conversions, linking the light reactions to the Calvin cycle.
Chloroplast Stroma
The chloroplast stroma is the fluid-filled space surrounding the thylakoid membranes where the Calvin cycle takes place. Loaded with enzymes necessary for carbon fixation and sugar synthesis, the stroma provides a suitable environment for the biochemical reactions of the Calvin cycle. Besides housing the Calvin cycle enzymes, the stroma contains molecules like chloroplast DNA, ribosomes, and other compounds that support the synthetic and regulatory processes of photosynthesis.

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Most popular questions from this chapter

What is a part of grana? a. the Calvin cycle b. the inner membrane c. stroma d. thylakoids

Metabolic pathways both produce and use energy to perform their reactions. How does the Calvin cycle help to harness, store, and use energy in its pathway? a. The Calvin cycle harnesses energy in the form of 6 ATP and 6 NADPH that are used to produce Fructose- 3 - phosphate (F3P) molecules. These store the energy captured from photosynthesis. The cycle uses this energy to regenerate RuBP. b. The Calvin cycle harnesses energy in the form of 6 ATP and 6 NADPH that are used to produce Glyceraldehyde-3- phosphate (GA3P) molecules. These store the energy captured from photosynthesis. The cycle uses this energy to regenerate RuBP. c. The Calvin cycle harnesses energy in the form of 3 ATP and 3 NADPH that are used to produce Glyceraldehyde-3- phosphate (GA3P) molecules. These store the energy captured from photosynthesis. The cycle uses this energy to regenerate the RuBP. d. The Calvin cycle harnesses energy in the form of 6 ATP and 3 NADPH that are used to produce Glyceraldehyde-3- phosphate (GA3P) molecules. These store energy captured from photosynthesis. The cycle uses this energy to regenerate RuBP.

The classical theory of evolution is based on a gradual transformation, the accumulation of many random mutations that are selected. The biological evidence for evolution is overwhelming, particularly when one considers what has not changed: core conserved characteristics. A. Describe three conserved characteristics common to both chloroplasts and mitochondria Some hypotheses that have been proposed to account for biological diversity are saltatory, involving sudden changes, rather than gradualist. In defense of the classical gradualist theory of evolution, nearly all biologists in the late 1960s rejected the theory of endosymbiosis as presented by Lynn Margulis in 1967. B. Suppose that you want to disprove the theory of endosymbiosis. Explain how the following evidence could disprove the theory: i. a 鈥渢ransitional species鈥� with cellular features that are intermediate cells with and without mitochondria ii. a 鈥渢ransitional organelle鈥� with some features, such as compartmentalized metabolic processes, but not other features, such as DNA Explain how the following evidence supports the theory of endosymbiosis: iii. bacteria live within your intestines, but you still have a separate identity iv. no one has directly observed the fusion of two organisms in which a single organism results.

3\. In which compartment of the plant cell do the light-independent reactions of photosynthesis take place? a. mesophyll b. outer membrane c. stroma d. thylakoid

Why are chemoautotrophs not considered the same as photoautotrophs if they both extract energy and make sugars? a. Chemoautotrophs use wavelengths of light not available to photoautotrophs. b. Chemoautotrophs extract energy from inorganic chemical compounds. c. Photoautotrophs prefer the blue side of the d. Photoautotrophs make glucose, while chemoautophs make galactose.
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