Diffusion is not limited to gases; it can also occur in liquids and solids, albeit at different rates. In liquids, diffusion happens as molecules move and spread out within the liquid medium. For example, when a drop of dye is added to water, the dye molecules gradually spread throughout the water, resulting in a uniform colour over time. In solids, diffusion occurs much more slowly due to the tightly packed arrangement of particles. The movement of atoms or molecules within solid structures is a key mechanism in processes like the hardening of concrete and the ageing of materials. The rate of diffusion in these states is influenced by factors such as temperature, particle size, and the medium's density.

Membrane proteins play a crucial role in facilitating the diffusion of substances that cannot easily pass through the lipid bilayer of the cell membrane. These substances include polar molecules and ions that are not lipid-soluble. There are two main types of membrane proteins involved in this process: channel proteins and carrier proteins. Channel proteins form pores in the membrane, allowing specific molecules or ions to pass through by diffusion. Carrier proteins, on the other hand, bind to specific molecules on one side of the membrane, change shape, and then release the molecule on the other side. These proteins are crucial for the selective permeability of the cell membrane, allowing the cell to control the movement of different substances in and out of the cell.

The polarity of a molecule significantly affects its ability to diffuse through the cell membrane. Nonpolar molecules, which do not have a charge, can easily dissolve in the lipid bilayer of the cell membrane and pass through by simple diffusion. Examples include oxygen and carbon dioxide. On the other hand, polar molecules, which have a positive or negative charge, cannot easily pass through the hydrophobic (water-repelling) core of the lipid bilayer. These molecules, such as water, ions, and glucose, typically require specific transport proteins, like channel or carrier proteins, to facilitate their movement across the membrane. This selective permeability is essential for the cell to maintain its internal environment and regulate the movement of substances in and out of the cell.

Cells with a larger surface area to volume ratio have a more efficient diffusion process because a larger surface area allows for more molecules to cross the cell membrane at once. As cells grow larger, their volume increases more rapidly than their surface area, reducing the efficiency of diffusion. This is because the increased volume means that molecules have a longer distance to travel inside the cell, slowing down the distribution of materials. Small cells, with their relatively larger surface area compared to their volume, facilitate quicker and more efficient exchange of substances through the cell membrane, thus making diffusion more effective.

The size of a molecule significantly influences its rate of diffusion through the cell membrane. Smaller molecules can move more easily and quickly across the membrane due to less resistance encountered in the lipid bilayer. Larger molecules face greater resistance and thus diffuse more slowly. For instance, small nonpolar molecules like oxygen and carbon dioxide diffuse rapidly because they are small enough to pass through the spaces between the lipid molecules in the membrane. In contrast, larger polar molecules and ions cannot easily penetrate the hydrophobic core of the lipid bilayer and often require specific transport mechanisms such as carrier proteins to facilitate their movement.