To determine which of the given monosaccharides are present in a five-membered cyclic structure (furanose structure), we will analyze each option step by step.
### Step 1: Analyze Ribose
- Ribose is a pentose sugar, meaning it has five carbon atoms.
- In its cyclic form, ribose can form a five-membered ring by reacting with its own aldehyde group.
- The structure of ribose in its furanose form consists of four carbon atoms and one oxygen atom in the ring.
- Therefore, ribose does indeed exist in a furanose structure.
### Step 2: Analyze Glucose
- Glucose is an aldohexose, meaning it has six carbon atoms.
- In its cyclic form, glucose forms a six-membered ring (pyranose structure) by reacting with its aldehyde group.
- The structure of glucose in the cyclic form consists of five carbon atoms and one oxygen atom, making it a six-membered ring.
- Thus, glucose does not exist in a furanose structure.
### Step 3: Analyze Fructose
- Fructose is a ketohexose, which means it has six carbon atoms but contains a ketone group.
- In its cyclic form, fructose can form a five-membered ring (furanose structure) by reacting with its ketone group.
- The structure of fructose in its furanose form consists of four carbon atoms and one oxygen atom in the ring.
- Therefore, fructose does exist in a furanose structure.
### Step 4: Analyze Galactose
- Galactose is an aldohexose, similar to glucose, meaning it has six carbon atoms.
- In its cyclic form, galactose also forms a six-membered ring (pyranose structure) by reacting with its aldehyde group.
- The structure of galactose in the cyclic form consists of five carbon atoms and one oxygen atom, making it a six-membered ring.
- Thus, galactose does not exist in a furanose structure.
### Conclusion
Based on the analysis:
- Ribose (a) and Fructose (c) are the monosaccharides that exist in a five-membered cyclic structure (furanose structure).
- Therefore, the correct answer is (a) Ribose and (c) Fructose.
### Summary of Answers:
- (a) Ribose - Yes, furanose structure
- (b) Glucose - No, pyranose structure
- (c) Fructose - Yes, furanose structure
- (d) Galactose - No, pyranose structure
### Final Answer:
The monosaccharides that are present as five-membered cyclic structures (furanose structures) are:
- (a) Ribose
- (c) Fructose
