Characteristic of an ion channel includes which of the following

Options

A. They are transmembrane proteins
B. They are gated channels
C. They act as selective pores. Transport through the channel is always
D. All

Answer

### Step-by-Step Solution:

1. **Understanding Ion Channels**: Ion channels are specialized proteins embedded in the cell membrane that allow ions to pass in and out of the cell. They play a crucial role in various physiological processes.

2. **Characteristics of Ion Channels**:
- **Transmembrane Proteins**: Ion channels are composed of transmembrane proteins that span across the cell membrane. This structure allows them to facilitate the movement of ions from one side of the membrane to the other.
- **Gated Channels**: Many ion channels are gated, meaning they can open or close in response to specific signals (like voltage changes or ligand binding). This gating mechanism allows the cell to control the flow of ions.
- **Selective Pores**: Ion channels are selective, meaning they allow only certain types of ions to pass through. This selectivity is crucial for maintaining the cell's electrochemical gradient and overall homeostasis.

3. **Passive Transport**: The transport of ions through these channels is typically passive, meaning it does not require energy (ATP). Instead, ions move along their concentration gradient, from areas of higher concentration to areas of lower concentration.

4. **Conclusion**: Since all the characteristics mentioned (transmembrane proteins, gated channels, and selective pores) are true for ion channels, the correct answer to the question is that all of the options provided are characteristics of ion channels.

### Final Answer:
The characteristic of an ion channel includes all of the following: they are transmembrane proteins, they are gated channels, and they act as selective pores. Therefore, the correct answer is option 4: all.