Question: Why is the plasma membrane called a selectively permeable membrane?
- Because it allows all substances to freely pass through.
- Because it only allows certain substances to pass through while blocking others.
- Because it is impermeable and does not allow any substances to pass through.
- Because it is selectively permeable only to water molecules.
Answer: B) Because it only allows certain substances to pass through while blocking others.
Why is the Plasma Membrane Called a Selectively Permeable Membrane Solution:
The plasma membrane, often referred to as the cell membrane, is called a selectively permeable membrane because it exhibits a unique property in regulating the movement of substances in and out of the cell.
Unlike option A, which suggests that it allows all substances to freely pass through, the correct answer, option B, explains that the plasma membrane selectively permits certain molecules and ions to cross while impeding the passage of others. This selectivity is vital for maintaining a stable internal environment within the cell.
This structure creates a barrier that prevents water and other polar molecules from passing through the plasma membrane freely. However, some substances can pass through the plasma membrane, such as:
- Small nonpolar molecules, such as oxygen, carbon dioxide, and nitrogen
- Water molecules, which can pass through the plasma membrane by osmosis
- Ions, which can pass through the plasma membrane through channels or pumps
The selective permeability of the plasma membrane is essential for many cellular processes, such as nutrient uptake, waste removal, and cell signaling. However, small nonpolar molecules, like oxygen and carbon dioxide, can diffuse freely through the lipid bilayer.
Moreover, the membrane proteins, as described in option D, also play a crucial role in this selectivity. They act as gatekeepers, facilitating the transport of specific molecules into or out of the cell. For instance, channels and transporters allow ions and nutrients to cross the membrane selectively, controlling the cellular environment.
In summary, the plasma membrane’s ability to be selectively permeable, as explained in option B, is essential for maintaining cellular integrity and function. This unique feature ensures that only necessary substances enter or exit the cell, contributing to the overall health and survival of the organism.
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