1) Think about what a membrane is. Isn’t it kind of like a wall? Well a cell membrane functions as sort of a wall. It keeps the structure of the cell, much like how without your skin all your bones and organs would come spilling out. It acts as a defense against pathogens by keeping harmful organisms (bacteria, viruses, etc.) out of the cell. It also regulates the movement of things in and out of the cell. Inside the cell there is water, ions, nutrients, etc., as well as in the “fluid” outside of the cell. The balance of these “fluids” is very precise and the cell membrane is a major part of keeping that balance, letting just the right amount of one thing (for example, nutrients) in or keeping just the right amount of one thing (for example, water) out. Make sense to you?

2) think about the words “selective” and “permeable”. Selective means it’s picky or choosey and permeable means to penetrate, or let through. So selectively permeable means things can go through it, but not everything. Because of this the cell membrane is able to regulate what enters and exits the cell.

3) Now that you understand selectively permeable, we can talk about concentration gradient. You know how I said things like ions can travel through the cell membrane, going in and out of a cell? Well the concentration gradient is how this is done. Think about an hour glass for a second. Now when it has been sitting for a long time there is all the sand in the bottom and nothing in the top, but when you tip it over, all the sand is in the top with nothing in the bottom. And when this happens all the sand starts falling into the bottom. The concentration gradient works sort of like that. Inside the cell, aside from the cell structures, there is also water and ions floating around (such as sodium). Outside the cell there is also water and ions floating around. When there are tons of ions outside the cell and little inside the cell, the ions flow from outside to inside, like when you tip the hour glass over and the sand starts falling. Outside the cell was a high concentration (lots of ions in a set amount of water) and inside was a lower concentration (less ions in the same set amount of water). So the ions started to flow from outside to inside, or from high concentration to low concentration. The concentration gradient is the concentration across the cell and it’s surrounding area. It’s a way of looking at it to see which way the ions are going to go. If the inside of the cell is full of ions and there is very little outside, the flow will go from inside the cell to outside it. Make sense?

I kind of condensed it all down for you, trying to explain as I went along instead of just giving you the answers. Hope this helps clarify things. If you’re still confused or need some additional help, feel free to message me. Good luck on your test.