Which of the following is NOT a function of the cell (plasma) membrane surrounding a typical animal cell?

Introduction / Context:
The cell membrane, also called the plasma membrane, is a selectively permeable barrier that surrounds the cell. It has several important roles in protecting the cell, controlling what enters and leaves, and allowing communication. However, some functions belong instead to internal membranes like the nuclear envelope. This question asks you to identify which listed function does not correctly belong to the cell membrane.

Given Data / Assumptions:

• The options describe various possible roles of the cell membrane.

• Some roles are accurate, such as separation from the environment and control of substance movement.

• One role involves movement between cytoplasm and nucleus, which is actually mediated by the nuclear envelope.

• You must select the statement that is not a function of the plasma membrane.

Concept / Approach:
The cell membrane forms the outer boundary of the cell. It maintains internal conditions by separating the cytoplasm from the extracellular fluid, and it controls the passage of substances via protein channels, carriers, and pumps. The membrane is flexible, allowing shape changes and cell movement. It also carries receptors, recognition proteins, and signaling molecules involved in cell communication and tissue organization. On the other hand, passage of substances between the cytoplasm and the nucleus is regulated by the nuclear envelope, which has nuclear pores. Therefore, any statement that assigns this nuclear function to the plasma membrane is incorrect.

Step-by-Step Solution:
Step 1: Evaluate option B. The statement that the membrane keeps internal materials separated from the external environment is accurate and a fundamental function of the plasma membrane. Step 2: Look at option C. Helping control the movement of materials into and out of the cell describes selective permeability, which is another core role of the cell membrane. Step 3: Consider option D. The membrane is a flexible bilayer that allows cells to change shape slightly, especially in animal cells that lack rigid cell walls, so this is also a valid function. Step 4: Check option E. Plasma membranes contain receptor proteins and glycoproteins that are involved in cell recognition and communication, which is important in immune responses and tissue organization. Step 5: Now examine option A. It claims that the cell membrane helps substances pass directly from the cytoplasm into the nucleus, but this transport is specifically handled by nuclear pores in the nuclear envelope, not by the plasma membrane at the cell surface. Step 6: Since the question asks which statement is not a function of the cell membrane, option A is the correct choice.

Verification / Alternative check:
Cell biology diagrams show a clear distinction between the plasma membrane and the nuclear envelope. The plasma membrane surrounds the entire cell, while the nuclear envelope surrounds the nucleus and contains nuclear pores that regulate traffic of RNA and proteins. Text descriptions list the roles of the plasma membrane as selective barrier, communication interface, and structural boundary. They do not include regulation of nuclear entry as a function of the outer cell membrane. This confirms that option A misassigns a job that belongs to the nuclear envelope, not the plasma membrane.

Why Other Options Are Wrong:

Keeping materials separated from the environment: This is a core role of the cell membrane in maintaining internal conditions.

Controlling movement of materials in and out: Describes selective permeability, another essential function.

Allowing shape change under pressure: Reflects membrane flexibility, especially in animal cells.

Cell recognition and communication: Involves membrane proteins and glycoproteins and is a valid function.

Common Pitfalls:
Students may confuse the plasma membrane with internal membranes because all are lipid bilayers with embedded proteins. Another pitfall is not distinguishing clearly between the cell boundary and the nuclear boundary. To avoid these errors, remember that the plasma membrane controls exchange between the cell and its external environment, while the nuclear envelope controls exchange between the nucleus and the cytoplasm.

Final Answer:
The function that does NOT belong to the cell membrane is helping substances pass directly from the cytoplasm into the nucleus, which is the role of the nuclear envelope.