In cellulose, D-glycosidic units are joined by

Options

A. `alpha`-1,4 glycosidic linkage
B. `beta`-1,6 glycosidic linkage
C. `beta`-1,4 glycosidic linkage
D. peptide linkage

Explanation

To answer the question regarding how D-glycosidic units are joined in cellulose, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Monomer**:
- Cellulose is a polysaccharide made up of D-glucose units, specifically in the beta form, known as beta-D-glucose.

2. **Understand the Structure of Beta-D-Glucose**:
- Beta-D-glucose has a specific structure where the hydroxyl (OH) group on the first carbon is in the same plane as the CH2OH group on the sixth carbon.

3. **Condensation Reaction**:
- When two beta-D-glucose molecules come together, they undergo a condensation reaction. This reaction involves the removal of a water molecule (H2O).

4. **Linkage Formation**:
- The condensation occurs between the hydroxyl group on the first carbon (C1) of one glucose molecule and the hydroxyl group on the fourth carbon (C4) of another glucose molecule.

5. **Naming the Linkage**:
- The bond formed between these two glucose units is specifically known as a beta 1,4-glycosidic linkage. This is because the linkage is formed between the first carbon of one glucose and the fourth carbon of the adjacent glucose.

6. **Conclusion**:
- Therefore, in cellulose, D-glycosidic units are joined by beta 1,4-glycosidic linkages.

### Final Answer:
In cellulose, D-glycosidic units are joined by **beta 1,4-glycosidic linkages**.

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