Explain how facilitated diffusion occurs across the cell membrane.

Facilitated diffusion occurs when specific molecules move across the cell membrane via protein channels or carrier proteins.

Facilitated diffusion is a type of passive transport that allows substances to cross cell membranes with the help of special transport proteins. This process does not require energy, as it relies on the concentration gradient, moving substances from an area of high concentration to an area of low concentration.

There are two types of proteins involved in facilitated diffusion: channel proteins and carrier proteins. Channel proteins form pores or channels in the cell membrane, allowing specific molecules or ions to pass through. These proteins are selective, meaning they only allow certain types of molecules to pass. For example, aquaporins are channel proteins that facilitate the transport of water molecules across the cell membrane.

Carrier proteins, on the other hand, bind to the specific molecule that needs to be transported. When the molecule binds to the carrier protein, it causes a change in the protein's shape, which allows the molecule to be released on the other side of the membrane. This is how glucose and amino acids, for instance, are transported into cells.

It's important to note that facilitated diffusion is a regulated process. The cell can increase or decrease the number of transport proteins in the membrane to control the rate of diffusion. This allows the cell to respond to changes in the environment and maintain homeostasis.

In summary, facilitated diffusion is a crucial process that allows cells to transport specific molecules across the cell membrane. It involves the use of protein channels or carrier proteins, and is driven by the concentration gradient, moving substances from areas of high concentration to areas of low concentration. This process is essential for the survival and function of cells, enabling them to obtain necessary nutrients and expel waste products.