Some hormones need the secondary messenger because

Options

A. they need activator
B. They can't cross cell membrane
C. They can cross cell membrane
D. They need a prosthetic group

Answer

### Step-by-Step Solution:

1. **Understanding Hormones and Their Action**:
- Hormones are signaling molecules that regulate various physiological processes in the body. They can be classified based on their ability to cross the cell membrane.

2. **Identifying the Role of Secondary Messengers**:
- Some hormones, particularly peptide hormones and catecholamines, are too large or polar to cross the cell membrane. Therefore, they cannot directly enter the target cells.

3. **First Messenger Concept**:
- The hormone that binds to the receptor on the cell surface is referred to as the "first messenger." This binding initiates a signal transduction pathway.

4. **Activation of Secondary Messengers**:
- Upon binding of the first messenger to its receptor, it triggers the production of secondary messengers within the cell. These secondary messengers are small molecules that relay the signal from the receptor to various effector proteins inside the cell.

5. **Function of Secondary Messengers**:
- Secondary messengers amplify the signal and help in carrying out the cellular response. Examples include cyclic AMP (cAMP), inositol trisphosphate (IP3), and calcium ions.

6. **Conclusion**:
- The reason some hormones need secondary messengers is that they cannot cross the cell membrane due to their size or polarity, hence they rely on secondary messengers to transmit their signals inside the cell.

### Final Answer:
Some hormones need secondary messengers because they cannot cross the cell membrane due to their large size or polarity.

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