Which of the following statements is correct regarding cell membrane? I. The membrane is selectively permeable to some molecules present on either side of it. II. Water may also move across this membrane from lower to higher concentration.
Only I
Understanding the Cell Membrane Properties
The cell membrane, also known as the plasma membrane, is a vital boundary for all cells, separating the internal environment from the external world. It plays a crucial role in controlling what enters and leaves the cell. Let's carefully examine the two statements provided about its characteristics.
Analyzing Statement I: Selective Permeability of the Cell Membrane
Statement I asserts: "The membrane is selectively permeable to some molecules present on either side of it."
The term 'selectively permeable' is a key property of the cell membrane. It means that the membrane does not allow all substances to pass through indiscriminately. Instead, it regulates the passage of molecules and ions, allowing some to cross easily, others with assistance, and preventing others entirely. This selective nature is fundamental to maintaining the cell's internal balance (homeostasis) and function. For example, small, nonpolar molecules like oxygen and carbon dioxide can often diffuse directly through the lipid bilayer, while ions and larger polar molecules like glucose require specific protein channels or carriers to cross.
Therefore, Statement I accurately describes a well-established characteristic of the cell membrane.
Analyzing Statement II: Water Movement Across the Membrane
Statement II claims: "Water may also move across this membrane from lower to higher concentration."
The movement of water across a selectively permeable membrane is a special type of diffusion called osmosis. Osmosis occurs when there is a difference in solute concentration on either side of the membrane. Water moves from a region of higher water potential (which is typically a region with a lower concentration of solutes) to a region of lower water potential (a region with a higher concentration of solutes).
Statement II says water moves "from lower to higher concentration." This phrasing is potentially misleading. If it refers to water concentration, water moves from higher water concentration to lower water concentration. If it refers to solute concentration, water moves from lower solute concentration to higher solute concentration.
The standard understanding of osmosis is that water moves towards the area with a higher concentration of solutes (i.e., lower water potential). Thus, water moves from a region of lower *solute* concentration to a region of higher *solute* concentration. The statement "from lower to higher concentration" without specifying what concentration (solute or water) is ambiguous but does not correctly describe the net movement of water via osmosis as moving from lower to higher *water* concentration, nor does it clearly capture the movement relative to *solute* concentration in a way that aligns with the direction "from lower to higher concentration" without further context. Water moves towards the area where solutes are more concentrated.
Based on the principles of osmosis, Statement II, as phrased, is incorrect regarding the typical direction of net water movement.
Conclusion on Cell Membrane Statements
Based on our analysis:
- Statement I: The membrane is selectively permeable. (Correct)
- Statement II: Water may also move across this membrane from lower to higher concentration. (Incorrect - water moves from higher water potential/lower solute concentration to lower water potential/higher solute concentration via osmosis).
Therefore, only Statement I is a correct statement regarding the cell membrane.
Revision Table: Key Cell Membrane Features
| Feature | Description | Significance |
|---|---|---|
| Lipid Bilayer | Forms the basic structure, semi-fluid | Barrier function, allows passage of small nonpolar molecules |
| Proteins | Embedded or associated, various types (channels, carriers, enzymes, receptors) | Selective transport, signaling, cell recognition |
| Carbohydrates | Attached to lipids (glycolipids) or proteins (glycoproteins) | Cell recognition, adhesion |
| Cholesterol (in animal cells) | Affects membrane fluidity | Maintains membrane stability across temperature ranges |
| Selectively Permeable | Controls passage of substances | Essential for homeostasis and cell function |
Additional Information: Osmosis and Cell Tonicity
Osmosis is crucial for cell survival. When a cell is placed in a solution, water moves across the membrane depending on the relative solute concentrations:
- Isotonic Solution: Solute concentration is the same inside and outside the cell. No net water movement.
- Hypotonic Solution: Solute concentration is lower outside the cell than inside. Water moves into the cell, potentially causing it to swell or burst (lysis in animal cells).
- Hypertonic Solution: Solute concentration is higher outside the cell than inside. Water moves out of the cell, causing it to shrink (crenation in animal cells, plasmolysis in plant cells).
This demonstrates that water movement is fundamentally driven by concentration gradients (specifically, water potential gradients), moving towards areas of higher solute concentration, contrary to the phrasing in Statement II.
Important Questions from Cell
In a plant cell, which one of the following contains their own DNA?
Which one of the following structures or components is not always present in living cells?
Which one of the following statements about animal cells and plant cells is correct?
- Which one of the following statements about vacuoles' is not correct?
- A cell is unable to synthesize lipids. Which of its cell organelles might be defective?
