Which of the following associations is not correct?

Options

A. Desmosomes-hold cells together to form a flexible sheet of cells in an organ
B. Tight junctions between cells serving as barriers
C. Plasmodesmata-between adjacent plant cells
D. Gap junctions between differing types of plasma membranes

Explanation

To determine which association is not correct among the given options, we will analyze each statement regarding cell junctions.

### Step-by-Step Solution:

1. **Analyze the first statement about desmosomes**:
- Desmosomes are specialized structures that connect adjacent cells, providing mechanical strength and flexibility to tissues. They are indeed responsible for holding cells together in organs.
- **Conclusion**: This statement is correct.

2. **Analyze the second statement about tight junctions**:
- Tight junctions are connections between cells that create a barrier to prevent the leakage of substances between the cells. They are crucial in maintaining the integrity of epithelial layers.
- **Conclusion**: This statement is also correct.

3. **Analyze the third statement about plasmodesmata**:
- Plasmodesmata are channels that connect adjacent plant cells, allowing for the transport of nutrients and signaling molecules. This is a well-established fact in plant cell biology.
- **Conclusion**: This statement is correct.

4. **Analyze the fourth statement about gap junctions**:
- Gap junctions are intercellular connections that allow for the direct transfer of ions and small molecules between the cytoplasm of adjacent cells. However, they are formed between similar types of cells, not between different types of plasma membranes.
- **Conclusion**: This statement is incorrect.

### Final Answer:
The association that is not correct is: **Gap junction between different types of plasma membranes**.

---