In the small intestine, diffusion plays a crucial role in the absorption of nutrients. After the process of digestion, nutrients like glucose, amino acids, and fatty acids are present in high concentrations in the lumen of the small intestine. The cells lining the intestine, the enterocytes, have a lower concentration of these nutrients. Therefore, a concentration gradient is established. Glucose and amino acids diffuse passively from the lumen into the enterocytes. This process is facilitated by specific transport proteins that help in the movement of these nutrients across the cell membrane. Fatty acids, being non-polar molecules, can diffuse directly through the lipid bilayer of the enterocytes. Once inside the cells, these nutrients are either used for energy, stored, or transported into the bloodstream for distribution to other parts of the body. This diffusion-driven absorption is critical for the efficient uptake of essential nutrients from our diet.

The size and type of molecules significantly influence their rate of diffusion. Smaller molecules tend to diffuse faster than larger ones because they move more quickly and require less energy to pass through membranes or between molecules in a medium. Additionally, the type of molecule - whether it is polar or non-polar - also affects diffusion. Non-polar molecules, such as oxygen and carbon dioxide, diffuse more readily through the lipid bilayer of cell membranes because they are hydrophobic, like the membrane itself. Polar molecules and ions, however, find it difficult to pass through the non-polar lipid bilayer due to their charge or polarity. They often require the assistance of membrane proteins, such as channel or carrier proteins, to diffuse into or out of a cell. This is why the diffusion rates of different substances can vary greatly depending on their size and chemical nature.

While diffusion is most commonly associated with liquids and gases, it can also occur in solids, albeit at a much slower rate. In solids, the particles are more tightly packed and have less freedom to move compared to those in liquids and gases. However, diffusion in solids does happen, especially at the atomic or molecular level. An example of diffusion in solids is the process by which impurities or alloying elements spread through a metal. This is crucial in processes like hardening steel, where carbon atoms diffuse into iron. The rate of diffusion in solids is influenced by factors such as temperature and the size of the diffusing particles. At higher temperatures, the particles have more kinetic energy, which increases the rate of diffusion. However, due to the restricted movement and close packing of particles in solids, diffusion is significantly slower compared to that in liquids and gases.

Diffusion is classified as a passive process because it does not require the expenditure of energy (ATP) by the cell. In diffusion, particles move from an area of higher concentration to an area of lower concentration, driven by their intrinsic kinetic energy. This movement is dictated by the natural tendency of particles to spread out evenly in a given space, following the laws of entropy. Since the cell does not have to expend energy to fuel this process, diffusion is an efficient way of transporting substances across cell membranes, especially for small molecules like oxygen, carbon dioxide, and water. This passive nature of diffusion is crucial for cells, as it allows them to conserve energy for other vital processes that require active transport, which does use ATP. For instance, in active transport, cells actively pump ions against their concentration gradient, a process essential for functions like nerve transmission and muscle contraction, which are energy-intensive.

Cell membranes are selectively permeable structures composed mainly of a phospholipid bilayer with embedded proteins. This unique composition facilitates diffusion in several ways. Firstly, the hydrophobic lipid bilayer forms a barrier to water-soluble substances, allowing only small, non-polar molecules like oxygen and carbon dioxide to diffuse directly through it. Secondly, specific proteins embedded in the membrane function as channels or carriers for different substances. These proteins can facilitate the diffusion of ions and larger, water-soluble molecules. Channel proteins provide a passageway for ions and small molecules to diffuse down their concentration gradient. Carrier proteins, on the other hand, bind to specific molecules, change shape, and transport them across the membrane. This selective permeability of the cell membrane ensures that essential substances like nutrients and gases can enter the cell, and waste products can leave efficiently through diffusion, maintaining cellular homeostasis.