The cell membrane is often thought of as this solid, protective border surrounding the cell, but really it is designed to be less like a wall and more like a gatekeeper. It is best described as being selectively permeable. So this means it is “picky” or selective about what leaves and what enters the cell. Much like this bouncer in this photo that is checking the woman’s ID to verify whether or not she should be permitted to enter the club. So too do membranes have methods for checking and controlling what and how things can enter or leave a cell.

So there are two main ways that things can move across a cell membrane in the first place. Passive transport is the movement of materials without utilizing any of the cell’s energy (like ATP). So as you may know, ALL matter is in motion. In fact that’s what helps differentiate between a solid, liquid and a gas is how fast the particles are moving and how close they are to one another. So since matter is already in motion….This is said to be  its kinetic energy, we can then understand that matter has a tendency to move to areas where it is not…. this whole 2nd law of thermodynamics that says things are becoming more unruly, or in disorder is being kept her what matter spreads out. So if there is a lot if “stuff’ on one side a membrane than on the other side of the membrane and said stuff can pass through somehow… it indeed will. That is what we call passive transport. Matter moving DOWN ITS CONCENTRATION GRADIENT. Much like a kid sliding down a playground slide doesn’t take energy on the kids part to do down, neither does it take NRG to move matter down or across its gradient. So a nice way to recall this is to say “matter moves from where there’s a lot to where it is not”……aka as “Down its concentration gradient”.  

Secondly matter can move from low to high (small amounts to where it already had more) or can even be moved in mass quantities/bulk, this is going to require cellular energy (aka ATP) and this process is known as Active transport. Moving up its concentration gradient (or the kid climbing up the slide) is going to require NRG!!!!!

So to delve into passive transport a bit more… you’ll need to understand that diffusion is matter spreading from areas of high to low concentration. The purple dots moving in this image are showing matter in motion and the gradient here is showing that matter spreading from where there was a lot if purple molecules to where there  weren’t any. This is diffusion. Usually its lipid-based or non-polar, small molecules that diffuse well across the phospholipid bilayer…. like oxygen and carbon dioxide. 

Diffusion can take place of larger or polar molecules but they need a little help or ‘facilitation’ to getting across. This is called Facilitated diffusion. Matter moves across the membrane from areas of high concentration to areas of lower concentration but they need a protein channel to get across the membrane. In this way a protein is facilitating the diffusion of the molecule.  So a perfect example of this is osmosis, which is the diffusion if water. Water can move from high concentrations to low through protein channels called aquaporins. And just a reminder this is still WITHOUT the use of the cell’s ENERGY, it’s just using a special channel to get across the membrane.