The Cell Membrane- Gateway to the cell

Cell Membrane The cell membrane is responsible for controlling what enters and leaves the cell. It does this by being selectively permeable. Selectively Permeable- allows only certain substances to go through. If something is too big, or the wrong substance it cannot pass through. What is it called when only certain things are allowed to pass through?

Selectively Permeable Objects

There are 3 ways things enter/leave a cell: 1. Diffusion- molecules of a substance moving from high concentration to low concentration. (Takes NO energy)

CONCENTRATION Have you ever mixed a powder drink and put too much powder in it? Let’s try it! Two volunteers to taste two different versions of the same powdered drink. The ratio of powder to water is the drink’s concentration- the more powder, the greater the concentration! The water is called a solvent. The powder is called a solute.

Let’s try moving around to show this Everyone stand up! If you are on the side of the room on my right, move to the other side and crowd in with your classmates. Now, which way do you want to move to be less crowded? Why? Molecules of water or other substances want to do the same thing!

Osmosis 2. Osmosis- The diffusion of WATER molecules. It is still from high to low concentration.

In cells what does osmosis do? Exp. Finding Nemo Exp. Hospital rehydration Exp. Snails -gaining water can cause a cell to _____? -losing water can cause a cell to ______?

3. Active Transport- Molecules of a substance moving from low to high concentration. THIS TAKES ENERGY!!!

Practice problems with Osmosis and diffusion You need to keep osmosis and diffusion separate in your mind. Diffusion is the movement of some substance from high to low concentration. Osmosis is a special kind of diffusion it deals with water only.

Sample diffusion problem: Do this problem on your notes sheet: A cell is sitting in a solution of Iodine. Inside of the cell, there is 50% iodine. Outside the cell, the solution or liquid is 70% iodine. Which way will the iodine move across the membrane ( into the cell or out of the cell)-remember-high to low concentration

Another diffusion practice problem A cell is in a sodium solution. Inside the cell there is already 20 % sodium. Outside the cell, the solution is 10% sodium. Which way will sodium move-into our out of the cell?

Osmosis Practice Problems Remember, now we are talking about moving water from high concentration to low concentration. Water will move from where there is a lot of water (high concentration of water) to where there is not as much water (Lower concentration of water). So while other substances are moving, water is also moving into or out of the cell depending on concentration of water.

Osmosis Practice Problems A cell that is 20% water is in a solution that is 80% water. Will water flow into the cell or out of the cell? Remember: High to low concentration A cell that is 70% water is in a solution that is 10% water. Will water flow into or out of the cell?