It allows the entry and exit of some materials while preventing others.
Explanation
The plasma membrane (or cell membrane) is the outermost boundary of the cell that separates the internal cellular environment from the external surroundings. Structurally, it is composed of a phospholipid bilayer embedded with proteins and carbohydrates. Its primary function is to regulate the transport of substances in and out of the cell to maintain homeostasis.
Option Analysis
- It is made of selected proteins only. is incorrect: The cell membrane is not made of proteins only. It is a fluid mosaic of lipids (phospholipids and cholesterol), proteins (integral and peripheral), and carbohydrates attached to them.
- It allows the entry and exit of some materials while preventing others. is correct: The term "selectively permeable" refers to the membrane's ability to control the passage of substances. It allows specific molecules (such as oxygen, water, and glucose) to enter or leave the cell while preventing the passage of others (such as toxins or large charged ions) without assistance.
- It is permeable only to water. is incorrect: While the membrane allows water to pass through (a process called osmosis), it is not permeable only to water. It also facilitates the movement of ions, gases, and organic molecules.
- It selects which organelles to keep inside. is incorrect: The membrane encloses the cytoplasm and organelles, but the concept of permeability relates to the transport of molecular substances, not the selection of organelles.
Key Takeaway: The plasma membrane is selectively permeable, meaning it acts as a gatekeeper that regulates the entry and exit of specific materials to ensure the cell's survival and proper functioning.