Which type of molecules cannot pass through cell walls without being broken down?

Big molecules cannot pass through cell walls without being broken down due to their size and structure. Cell walls, particularly in plants, are semi-permeable membranes that allow the passage of certain substances while blocking larger, more complex molecules. This is essential for maintaining the integrity of the cell and regulating what enters and exits.

Small molecules, such as minerals or simple sugars, and water molecules can typically pass through the cell wall more easily due to their small size. Gaseous molecules, like oxygen and carbon dioxide, can also diffuse through cellular structures without the need for breakdown. However, larger molecules, such as proteins or polysaccharides, must be broken down into smaller units—like amino acids or simple sugars—by enzymes or other processes before they can be absorbed by the cell, leading to the conclusion that they cannot pass through cell walls unaltered.