**Calvin Cycle and Glucose Production**

The Calvin cycle, also known as the Calvin-Benson cycle or the C3 cycle, is a series of chemical reactions that occur in the chloroplasts of plants during photosynthesis. It is responsible for converting carbon dioxide (CO2) into glucose, which is an essential carbohydrate for plant growth and development.

To understand the number of turns of the Calvin cycle required to make one molecule of glucose, let's break down the process step by step:

**1. Carbon Fixation:**
- The first step of the Calvin cycle is carbon fixation, where atmospheric CO2 is incorporated into an organic molecule. In this case, CO2 is combined with a five-carbon sugar called ribulose bisphosphate (RuBP) to form an unstable six-carbon compound.
- This reaction is catalyzed by the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).

**2. Formation of PGA:**
- The unstable six-carbon compound immediately splits into two molecules of 3-phosphoglycerate (PGA), which is a three-carbon compound.
- This reaction does not require any ATP or NADPH and is therefore considered an energy-neutral step.

**3. Reduction of PGA:**
- In the next step, ATP and NADPH, which are products of the light-dependent reactions of photosynthesis, are utilized.
- PGA is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate (G3P), another three-carbon compound.
- For every three molecules of CO2 fixed, six molecules of G3P are produced. However, only one molecule of G3P is used for glucose synthesis, while the remaining five molecules are used to regenerate RuBP.

**4. Glucose Synthesis:**
- Finally, the one molecule of G3P that is used for glucose synthesis undergoes a series of enzymatic reactions to form glucose.
- These reactions involve the rearrangement and rearrangement of carbon skeletons, as well as the utilization of additional ATP and NADPH.

**Number of Turns of Calvin Cycle:**
- Since one molecule of glucose is synthesized from one molecule of G3P, and for every three molecules of CO2 fixed, six molecules of G3P are produced, it follows that six turns of the Calvin cycle are required to produce one molecule of glucose.

Therefore, the correct answer is '6'.