Problem 98 - Objective Biology for NEET

Question

How many Calvin cycle forms one hexose molecule? (a) 2 (b) 6 (c) 4 (d) 8

Step-by-Step Solution

Answer

(a) 2

Step 1: Understand the Calvin Cycle

In the Calvin cycle, CO2 in the atmosphere is 'fixed:' it's converted into carbohydrates using energy from ATP and NADPH. In each Calvin cycle, one molecule of CO2 is consumed and the net output is one molecule of a three-carbon sugar molecule GDP (glyceraldehyde 3-phosphate).

Step 2: Determine the number of cycles to form one hexose molecule

Because a hexose molecule, such as glucose, is a six-carbon molecule and the product of one Calvin cycle is a three-carbon molecule GDP, it would take two Calvin cycles to form one hexose molecule. This is because two three-carbon molecules (from two Calvin cycles) will combine to form one six-carbon hexose molecule.

Step 3: Select the correct answer

With the understanding of step 2, it can be concluded that it takes two Calvin cycles to form one hexose molecule. Among the given options, the correct answer would be (a) 2.

Key Concepts

photosynthesis

Photosynthesis is the remarkable process through which plants, algae, and some bacteria convert light energy into chemical energy. This transformation occurs in the chloroplasts of plant cells, most notably in the form of glucose, a type of sugar. The whole process can be split into two main stages: the light-dependent reactions and the Calvin cycle.

The light-dependent reactions capture energy from sunlight, producing ATP and NADPH. These molecules are vital as they provide the energy required for the second stage.

• ATP stands for Adenosine Triphosphate, a high-energy molecule that stores energy.
• NADPH is a carrier molecule that transfers high-energy electrons.

Once the light energy is captured, the second stage, known as the Calvin Cycle (or light-independent reactions), kicks in. This cycle does not require light and takes place in the chloroplast's stroma. It utilizes the ATP and NADPH produced earlier to convert carbon dioxide into organic compounds, forming the basis of the plant's food. In essence, photosynthesis is nature's way to fuel life on Earth, forming the backbone of the food web for nearly all living organisms.

hexose sugars

Hexose sugars, like glucose and fructose, are six-carbon sugars that play a critical role in biology as a primary source of energy. These sugars are the end product of the Calvin cycle and other biochemical pathways. One of the best examples is glucose, a simple sugar that acts as a vital energy substrate for cells.

When plants perform the Calvin Cycle, they eventually produce hexose sugars, like glucose. These sugars don't only provide energy; they also serve structural roles and act as precursors for other important molecular pathways.

• They form the building blocks of carbohydrates, which were long considered the body's favorite energy source.
• Hexose sugars are involved in the synthesis of starch and cellulose, essential components in plants.
• They are part of glycolysis, a process that breaks down sugars to create energy in animal cells.

The production of hexose sugars highlights plants' incredible ability to harness energy from the sun to create essential nutrients that sustain life.

G3P (glyceraldehyde 3-phosphate)

G3P, or glyceraldehyde 3-phosphate, is a three-carbon sugar that is essential in the Calvin Cycle. It is the first stable product formed during the Calvin cycle, representing a key intermediate in the transformation of inorganic carbon dioxide into organic molecules.

Here's how it works inside the cycle:

• Carbon dioxide enters the cycle and attaches to a five-carbon sugar called ribulose bisphosphate (RuBP).
• ATP and NADPH produced in the light-dependent reactions help reduce and eventually form G3P.
• G3P can exit the Calvin cycle to be used directly or further combined to form hexose sugars like glucose.

Through the combination of two G3P molecules, a six-carbon sugar is eventually formed, critical to both plant metabolism and the foundational energy cycle of life. The production and utilization of G3P underscore the efficiency and self-sustainability of the plant's photosynthetic process, immensely contributing to the Earth's oxygen and food resources.