Summary
Highlights
Diffusion is defined as the net movement of molecules or ions from an area of high concentration to an area of low concentration. It's a passive process, meaning it doesn't require energy, and continues until equilibrium is reached. In biology, diffusion often occurs across a semi-permeable membrane, though it can happen in any fluid or gas.
Several factors influence the rate of diffusion. A higher concentration gradient increases the rate, and maintaining this gradient, often through good blood or air supply, is crucial. A larger surface area, such as that achieved by folding membranes, also increases the rate because more space is available for molecules to cross. Temperature is another factor; higher temperatures lead to faster diffusion due to increased kinetic energy of particles, as demonstrated by the rapid spread of potassium manganate in hot water compared to cold water. Finally, a thinner membrane allows for faster diffusion as there is less distance for the particles to travel.
Facilitated diffusion is a type of diffusion that allows large, charged, or water-soluble molecules, which can't easily pass through a membrane via simple diffusion, to cross. These molecules move down their concentration gradient with the help of specific proteins in the membrane. This process is still passive and does not require energy.
Channel proteins form pores, tunnels, or channels through the membrane, primarily for charged particles. Each channel protein is specific to the molecule it transports, such as aquaporins for water. While most channel proteins are open constantly, some are 'gated' and require a specific trigger (like molecule binding, a certain charge across the membrane, or pressure) to open or close.
Carrier proteins differ from channel proteins as they change shape to physically move molecules across the membrane. They typically transport larger molecules and are also specific to the molecules they bind. The binding of a molecule triggers the conformational change and transport. Facilitated diffusion, especially via channel proteins, is generally much faster than simple diffusion.
The concentration gradient still affects facilitated diffusion; a greater difference leads to a faster rate. However, this is only true up to a point. Once all the transport proteins are saturated (in use), the rate will plateau, and increasing the concentration gradient further will have no additional effect. Similarly, the number of channel or carrier proteins in the membrane is a limiting factor; more proteins generally lead to a faster rate, but once all are engaged, the rate cannot increase further.