KS3 Science Cells Practice Test 2025 – Complete Exam Prep

The correct answer indicates that a semi-permeable membrane allows only water and small molecules to pass through. This characteristic is central to the function of semi-permeable membranes, especially in biological systems.

In a biological context, semi-permeable membranes, such as the cell membrane, are selectively permeable, meaning they control what substances can enter and exit the cell. Small molecules, like water, oxygen, and carbon dioxide, can easily pass through the membrane due to their size and the nature of the lipid bilayer that makes up the membrane. This selective permeability is crucial for maintaining homeostasis within the cell, allowing it to regulate its internal environment by controlling the concentrations of various substances.

Larger molecules, like proteins, are too big to pass freely through these membranes and typically require specific transport mechanisms, such as active transport or facilitated diffusion, to move in and out of cells. This regulation allows cells to maintain their necessary conditions for metabolic processes without being overwhelmed by larger or unwanted substances.