Question

How many number of \(\mathrm{CO}_{2}\) molecules are required: synthesize one molecule of glucose during \(C_{3}\) cycle? (a) One (b) Three (c) Six (d) Five

Short answer

Six CO2 molecules are required to synthesize one molecule of glucose during the C3 cycle.

Step-by-step solution

Understand the Process of Photosynthesis in the C3 Cycle

Recognize that in the C3 cycle, also known as the Calvin cycle, photosynthesis involves fixing carbon dioxide (CO2) into a carbohydrate. Glucose (C6H12O6) is a six-carbon sugar synthesized via this process.

Determine the Number of Carbon Atoms in Glucose

Consider the molecular formula of glucose, which is C6H12O6. Since each molecule of glucose contains six carbon atoms, and each CO2 molecule contributes one carbon atom, you need six CO2 molecules to synthesize one molecule of glucose.

Count the Required CO2 Molecules

Since one CO2 molecule is fixed during each turn of the cycle and glucose has six carbon atoms, it takes six turns of the Calvin cycle to fix the six carbon atoms needed to form one glucose molecule. Therefore, six molecules of CO2 are required.

Key concepts

Photosynthesis

Photosynthesis is the remarkable biological process that allows plants to convert light energy, usually from the sun, into chemical energy stored in sugars. The process primarily takes place within the chloroplasts of plant cells, where chlorophyll, the green pigment, captures light energy. This energy drives a series of reactions that ultimately convert carbon dioxide (CO2) from the air and water (H2O) from the soil into glucose (C6H12O6), a type of sugar, and oxygen (O2) as a byproduct.

There are two main stages in photosynthesis: the light-dependent reactions and the light-independent reactions, sometimes known as the Calvin Cycle. During the light-dependent phase, the plant uses sunlight to make ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), both of which are energy-carrying molecules. These molecules are then used in the Calvin Cycle to fix CO2 into organic compounds such as glucose. Understanding these processes is not only pivotal in biology but also for our existence, as it is the primary source of oxygen and organic compounds for life on Earth.

Glucose Synthesis

Glucose synthesis, also known as gluconeogenesis, is a critical process in photosynthesis where plants, algae, and certain bacteria produce glucose using energy from light. But when it comes to plants, this synthesis occurs in the Calvin Cycle, a phase where the energy in ATP and NADPH, formed during the light-dependent reactions, is used to convert CO2 into glucose.

The molecular formula for glucose is C6H12O6, which indicates that it contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. In plants, the pathway of glucose synthesis begins with CO2 absorption. The C3 cycle, or Calvin Cycle, involves a series of enzyme-catalyzed reactions that result in the formation of a three-carbon compound, which is further processed and eventually gives rise to a single glucose molecule after several cycles.

CO2 Fixation

CO2 fixation is an essential process, being one of the core components of the Calvin Cycle. During CO2 fixation, CO2 molecules are attached to a five-carbon sugar called ribulose bisphosphate (RuBP) by the enzyme RuBisCO. This step is crucial because it converts inorganic carbon from CO2 into an organic form that can be used in the synthesis of glucose.

Each CO2 molecule is incorporated one at a time, resulting in the creation of two molecules of a three-carbon compound in the Calvin Cycle. This compound is then modified through a series of reactions, using energy from ATP and electrons from NADPH, before finally being assembled into a glucose molecule after multiple rounds of the cycle. Since each molecule of glucose has six carbon atoms, and each CO2 contributes one carbon, it takes six CO2 molecules to produce one molecule of glucose. This knowledge is fundamental to the understanding of how energy and carbon flow through ecosystems, with every organic compound in the biosphere deriving, at some point, from CO2 fixation by plants and other photosynthetic organisms.