Substance 'X' is the charged molecule of small molecular weight . It is found at higher concentrations outside of cells than inside . How can substance 'X' entre the cells ?

Options

A. Active transport
B. Diffusion through a channel
C. Diffusion through lipid bilayer
D. None of the above

Answer

To solve the question regarding how substance 'X', a charged molecule of small molecular weight, can enter the cells, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Nature of Substance 'X'**:
- Substance 'X' is described as a charged molecule. Charged molecules (ions) typically cannot pass through the lipid bilayer of the cell membrane directly due to their polarity and charge.

**Hint**: Consider the properties of charged molecules and how they interact with cell membranes.

2. **Concentration Gradient**:
- The question states that substance 'X' is found at higher concentrations outside the cell than inside. This indicates a concentration gradient, which can drive the movement of substances across the membrane.

**Hint**: Think about how concentration gradients influence the movement of substances in biological systems.

3. **Transport Mechanisms**:
- Charged molecules often enter cells through specific transport mechanisms. The two primary mechanisms are:
- **Facilitated Diffusion**: This process involves the use of transport proteins (such as ion channels) that allow charged molecules to pass through the membrane without the expenditure of energy.
- **Active Transport**: This process requires energy (usually from ATP) to move substances against their concentration gradient through specific transport proteins.

**Hint**: Recall the difference between passive and active transport mechanisms.

4. **Ion Channels**:
- If substance 'X' is an ion, it can enter the cell through specialized protein channels known as ion channels. These channels are selective for specific ions and allow them to move down their concentration gradient into the cell.

**Hint**: Consider the role of ion channels in cellular transport.

5. **Endocytosis**:
- In some cases, charged molecules may also enter cells through endocytosis, where the cell membrane engulfs the substance, forming a vesicle that brings it into the cell. However, this is less common for small charged molecules compared to facilitated diffusion or active transport.

**Hint**: Think about how cells can engulf substances that cannot pass through the membrane directly.

### Conclusion:
Substance 'X' can enter the cells primarily through facilitated diffusion via ion channels or through active transport mechanisms, depending on the specific nature of the molecule and the cell's requirements.