Question

The most important function of a cell membrane is that it
A. Controls the entry and exit of materials from the cells
B. Controls only the entry of materials into the cells
C. Controls only the exit of materials from the cells
D. Allows entry and exit of materials without any control

Answer 1

Hint: Cell membrane, which is the boundary of a cell, protects the cell from the outer environment and it works like a doorway for the cell. In our homes, the door is used for entry and exit purposes, similar function is also seen in case of cell membrane.

Complete answer:
The cell membrane is selectively permeable – that is it allows some selected molecules to pass from it. Such molecules are gases, waters etc. Sometimes it allows glucose also to pass. Proteins, amino acids, nucleic acids, ions, fats etc. are impermeable to cell membranes. Thus, we can say large-sized materials and lipids are not passed through the cell membrane.
So, it is clear that cell membrane helps in the entry and exit of materials.
The entry and exit is done by some channels and carrier proteins that are present on the cell membrane by diffusion, osmosis etc. Those channels are made up of amino acids.
Sometimes transport of molecules is done against the concentration gradient using pump proteins which are activated by energy of ATP molecule.

So, the correct answer is option A. Controls the entry and exit of materials from the cells.

Note: Without any control, the cell does not even allow a small particle to pass on. Ligand gated, ion gated, voltage gated channels are present that only open when a ligand or an ion or a voltage difference across a membrane is noticed across the membrane respectively. Thus, it is clear that a fine control mechanism is present on the cell membrane for passing any materials across the cell membrane.