Problem 6 - Campbell Biology Concepts & Connections

Question

Which of the following does not occur during the Calvin cycle? a. carbon fixation b. oxidation of NADPH c. consumption of ATP d. release of oxygen

Step-by-Step Solution

Answer

d. release of oxygen

Step 1: Understand the Calvin Cycle

The Calvin Cycle is a process in plants that converts carbon dioxide into glucose using the energy from ATP and NADPH, produced during the light-dependent reactions of photosynthesis.

Step 2: Identify Processes in the Calvin Cycle

The main processes in the Calvin Cycle are carbon fixation, reduction phase, and regeneration of the ribulose bisphosphate (RuBP).

Step 3: Carbon Fixation

Carbon fixation occurs when carbon dioxide is attached to a five-carbon sugar, ribulose bisphosphate (RuBP), forming a six-carbon compound that splits into two molecules of 3-phosphoglycerate (3-PGA).

Step 4: Reduction Phase

During this phase, ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).

Step 5: Regeneration of RuBP

Next, the molecule G3P is used to regenerate RuBP, allowing the cycle to continue.

Step 6: Analyze Options

Review the options: a. Carbon fixation - This does occur during the Calvin Cycle. b. Oxidation of NADPH - This occurs as NADPH is converted into NADP+. c. Consumption of ATP - This also occurs to provide energy for the cycle. d. Release of oxygen - This is not part of the Calvin Cycle but occurs during the light reactions.

Key Concepts

Photosynthesis

Photosynthesis is the process through which plants use sunlight to convert carbon dioxide and water into glucose and oxygen.
The whole process occurs in two main stages: the light-dependent reactions and the Calvin Cycle.
During the first stage, light-dependent reactions, sunlight is captured by chlorophyll, and its energy is used to produce ATP and NADPH.
These molecules (ATP and NADPH) store energy that will be used in the Calvin Cycle.

The Calvin Cycle, also known as the light-independent reactions or dark reactions, takes place in the stroma of the chloroplasts.
This cycle does not directly require light, but it uses the products of the light-dependent reactions (ATP and NADPH).
Thus, photosynthesis effectively converts the light energy into chemical energy stored in glucose.

Carbon Fixation

Carbon fixation is the first major step of the Calvin Cycle.
In this step, carbon dioxide from the atmosphere is attached to a five-carbon molecule called ribulose bisphosphate (RuBP).
When the CO2 is fixed to RuBP, it forms a six-carbon compound which is unstable.
This six-carbon compound quickly breaks down into two three-carbon molecules, 3-phosphoglycerate (3-PGA).

The enzyme that catalyzes this fixation step is called RuBisCO, which is the most abundant enzyme on Earth.
This step is crucial as it captures atmospheric CO2 and incorporates it into organic molecules, making it available for the plant to use.
Hence, carbon fixation is a critical process for plant growth and the basis for the production of glucose.

ATP Consumption

The energy currency of cells, ATP (adenosine triphosphate), is consumed during the Calvin Cycle in the reduction phase.
In this phase, ATP is used to convert 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P).
For every molecule of CO2 fixed, two molecules of ATP are consumed in converting 3-PGA to G3P.

ATP provides the necessary energy to drive these endergonic (energy-requiring) reactions.
The consumption of ATP during the Calvin Cycle highlights its role in converting energy from sunlight into chemical bonds within glucose.
This use of ATP is an example of energy coupling, where the energy from ATP hydrolysis powers biological processes.

NADPH Oxidation

NADPH serves as a reducing agent in the Calvin Cycle, providing the necessary electrons to reduce 3-PGA to G3P.
Oxidation of NADPH to NADP+ occurs during the reduction phase.
As NADPH donates electrons, it gets oxidized to NADP+.

For each molecule of CO2 fixed, NADPH provides the reducing power necessary to convert the fixed carbon into G3P.
Thus, one of the key roles of NADPH in the Calvin Cycle is to facilitate the conversion of 3-PGA to G3P, which involves the transfer of electrons.
This step is vital for forming the sugar molecules that the plant will eventually use for energy and growth.