Which one is not a hexose?

Options

A. Glucose
B. Ribose
C. Fructose
D. Galactose

Answer

To determine which of the given sugars is not a hexose, we need to analyze the number of carbon atoms in each sugar. Hexoses are monosaccharides that contain six carbon atoms.

### Step-by-Step Solution:

1. **Understand the Definition of Hexose:**
- Hexoses are sugars that contain six carbon atoms. The prefix "hex" indicates six.

2. **Identify the Sugars Given in the Options:**
- The options typically include glucose, ribose, fructose, and galactose.

3. **Analyze Each Sugar:**
- **Glucose:**
- Glucose is an aldohexose (contains an aldehyde group).
- Structure: C6H12O6 (6 carbon atoms).
- Conclusion: Glucose is a hexose.

- **Ribose:**
- Ribose is an aldopentose (contains an aldehyde group).
- Structure: C5H10O5 (5 carbon atoms).
- Conclusion: Ribose is **not** a hexose; it is a pentose.

- **Fructose:**
- Fructose is a ketohexose (contains a ketone group).
- Structure: C6H12O6 (6 carbon atoms).
- Conclusion: Fructose is a hexose.

- **Galactose:**
- Galactose is an aldohexose (contains an aldehyde group).
- Structure: C6H12O6 (6 carbon atoms).
- Conclusion: Galactose is a hexose.

4. **Final Conclusion:**
- Among the options, ribose is the only sugar that does not have six carbon atoms. Therefore, ribose is the correct answer to the question "Which one is not a hexose?"

### Answer:
**Ribose is not a hexose.**

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