What characterizes a semi-permeable membrane?

A semi-permeable membrane is characterized by its ability to allow certain substances to pass through while blocking others. The correct choice highlights that this type of membrane is made from proteins and contains small pores. These pores are vital because they enable the selective transport of molecules based on factors such as size, charge, and solubility.

In biological systems, semi-permeable membranes play a crucial role in processes like osmosis and diffusion, where they regulate the movement of water and solutes necessary for cellular function. The structural characteristics of proteins in these membranes contribute to their selective permeability.

Other definitions do not correctly define semi-permeable membranes. A membrane with no pores would not allow any movement of substances, effectively acting as a solid barrier, which contradicts the semi-permeable nature. Similarly, a solid barrier does not embody the intended functionality of a semi-permeable membrane. Lastly, a membrane made only from synthetic materials does not adequately capture the essence of semi-permeability, as these membranes can also be found in biological contexts, not just synthetic ones. Thus, the presence of small pores in a protein-based composition is crucial for the membrane's selective permeability.