 The Gatekeeper

 Have you ever flown to another country or driven into the USA?  What couldn’t you take with you across the border?  What items had to be “checked” before boarding the plane?

 The cell membrane acts like a customs officer. It allows some material to enter or leave and it stops other substances.  Because it allows only certain material to cross it, the cell membrane is said to be selectively permeable or semipermeable.

 A membrane that lets all materials cross it is called a permeable membrane.

 A membrane that does not let anything cross is called an impermeable membrane.

 In a moment we will watch tea being made in order to understand how a cell membrane works.  As the tea bag is added to the water think about how it happens. Can you explain it using your understanding of membranes?  In your exercise book record the results of what happened.  Sketch a diagram(s) of your observations.

 Procedure: 1. In your groups you will have a beaker with room temperature water. 2. Make a prediction about what will happen if you add in a drop of food colouring. Write down your prediction. 3. Add one drop of food colouring. 4. Record your results for: Immediately, 10 s, 30 s, 2 min, 10 min. Sketch your observations into your notebook.

 The process of particles moving from an area where there are more of them to an area where there is less of them is called diffusion.  Another way to say this is: the movement of particles from an area of high concentration to an area of less concentration is called diffusion.

 Diffusion can be seen as the “balancing out” of particles.  Particles are the tiny pieces of matter that make up everything.

 Can you think of any real life examples of diffusion?

 In a moment you will be given an opportunity to smell two balloons. One of these balloons will be filled with air and the other will be filled with air and vanilla. 1. Can you tell which is which? 2. Why do you think you can smell the vanilla? 3. Explain this observation in your notebook using your scientific terms and the knowledge you have gained so far.

 The most common substance found inside and around cells is water.  About 70% of a cell’s content is water, and cells will die quickly without a supply of this liquid.  Water particles are small enough to pass through a cell’s membrane through diffusion.

 Diffusion of water through a selectively permeable membrane is called osmosis.

 See page 44 in the text book.

 Active transport describes what happens when a cell uses energy to transport something.  We're talking about the movement of individual molecules across the cell membrane.cell membrane  The liquids inside and outside of cells have different substances.  Sometimes a cell has to work and use some energy to maintain a proper balance of molecules.