Final answer:
Proteins diffuse slowly through the cytosol and require transport proteins to pass through the cell membrane. Channel proteins facilitate rapid diffusion, while carrier proteins are more selective and slower. Facilitated diffusion is essential for transferring substances that cannot pass freely through the membrane.
Step-by-step explanation:
Proteins diffuse through the cytosol much more slowly than small molecules due to their large size and complex structures. Small molecules, such as oxygen and carbon dioxide, can pass freely through the cell membrane due to its composition. However, polar molecules, ions, and large molecules like proteins cannot, and require assistance to cross the membrane. This assistance is offered by transport proteins, which include channel proteins and carrier proteins.
Channel proteins facilitate diffusion at a high rate, allowing tens of millions of molecules to pass per second. In contrast, carrier proteins operate at a slower pace, with a thousand to a million molecules per second. It's important to note that channel proteins are generally less selective than carrier proteins and discriminate mainly on the basis of size and charge.
For substances unable to pass the membrane through simple diffusion, there is a process known as facilitated diffusion. This method uses integral proteins to create channels or alter their shape to allow specific molecules to pass. It is a key process that ensures substances can move quickly enough to meet cellular needs without expending energy as compared to active transport.