Describe the similarities and differences between simple diffusion, facilitated diffusion and active transport.

Simple diffusion, facilitated diffusion and active transport are three methods to transport various molecules across a membrane. Whilst similar in the overall effect, there are some major differences between these methods.

1. Energy requirement: simple diffusion and facilitated diffusion are similar in the fact that both are passive processes that require no energy (ATP) since both transport molecules down a concentration gradient. Active transport on the other hand is an active process, thus requiring energy provided through ATP hydrolysis and transports substances against the concentration gradient.

2. Machinery requirements: As simple diffusion is passive, it requires no extra machinery. For both facilitated diffusion and active transport, transmembrane proteins are required. For facilitated diffusion, carrier proteins or channel proteins are used. For active transport, carrier proteins that also act as ATPases are required.

3. Cargo and selectivity: Simple diffusion across a biological membrane is difficult, as the membrane acts as a barrier and prevents anything large or polar to diffuse through, although small, non-polar molecules such as oxygen, carbon dioxide and small hormones like oestrogen can diffuse through non-selectively. Facilitated diffusion and active transport are a lot more selective, requiring very specific transmembrane proteins which in most cases will only be able to transport one specific molecule across the membrane. However, many different types of these proteins exist, allowing a wide array of molecules of varying charges and sizes to be transported. For the transport methods that require carrier proteins (some facilitated diffusion and all active transport), it is even possible to have various activation methods to open the carrier proteins. E.g. voltage gated protein channels involved in action potential generation.