Actually, you could probably just ask google, but here you go, maybe this will help:
Explanation:
The simplest forms of transport across a membrane are passive. Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane. A concentration gradient is a just a region of space over which the concentration of a substance changes, and substances will naturally move down their gradients, from an area of higher to an area of lower concentration.
In cells, some molecules can move down their concentration gradients by crossing the lipid portion of the membrane directly, while others must pass through membrane proteins in a process called facilitated diffusion. Here, we’ll look in more detail at membrane permeability and different modes of passive transport.Some molecules, such as carbon dioxide and oxygen, can diffuse across the plasma membrane directly, but others need help to cross its hydrophobic core. In facilitated diffusion, molecules diffuse across the plasma membrane with assistance from membrane proteins, such as channels and carriers.
A concentration gradient exists for these molecules, so they have the potential to diffuse into (or out of) the cell by moving down it. However, because they are charged or polar, they can't cross the phospholipid part of the membrane without help. Facilitated transport proteins shield these molecules from the hydrophobic core of the membrane, providing a route by which they can cross. Two major classes of facilitated transport proteins are channels and carrier proteins.
Channel proteins span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion. Channels are very selective and will accept only one type of molecule (or a few closely related molecules) for transport. Passage through a channel protein allows polar and charged compounds to avoid the hydrophobic core of the plasma membrane, which would otherwise slow or block their entry into the cell.
Aquaporins are channel proteins that allow water to cross the membrane very quickly, and they play important roles in plant cells, red blood cells, and certain parts of the kidney (where they minimize the amount of water lost as urine).
Some channel proteins are open all the time, but others are “gated,” meaning that the channel can open or close in response to a particular signal (like an electrical signal or the binding of a molecule). Cells involved in the transmission of electrical signals, such as nerve and muscle cells, have gated ion channels for sodium, potassium, and calcium ions in their membranes. The opening and closing of these channels, and the resulting shifts in ion levels inside the cell, play an important role in electrical transmission along membranes (in nerve cells) and in muscle contraction (in muscle cells).
Another class of transmembrane proteins involved in facilitated transport consists of the carrier proteins. Carrier proteins can change their shape to move a target molecule from one side of the membrane to the other.
Diagram showing how a carrier protein can bind a target molecule on one side of the membrane, undergo a shape change, and release the target molecule on the other side of the membrane.
Diagram showing how a carrier protein can bind a target molecule on one side of the membrane, undergo a shape change, and release the target molecule on the other side of the membrane.
Like channel proteins, carrier proteins are typically selective for one or a few substances. Often, they will change shape in response to binding of their target molecule, with the shape change moving the molecule to the opposite side of the membrane. The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient (rather than acting as pumps).
Channel and carrier proteins transport material at different rates. In general, channel proteins transport molecules much more quickly than do carrier proteins. This is because channel proteins are simple tunnels; unlike carrier proteins, they don’t need to change shape and “reset” each time they move a molecule. A typical channel protein might facilitate diffusion at a rate of tens of millions of molecules per second, whereas a carrier protein might work at a rate of a thousand or so molecules per second^1
https://www.khanacademy.org/science/biology/membranes-and-transport/passive-transport/a/diffusion-and-passive-transport
