The Cell and it’s Environment Section 4 of chapter 1-”C” pages 32-36 Cell membrane compared to a sieve/colander. When would you use a colander? What does a cell membrane and a colander have in common?

All cells are surrounded by a cell membrane Cell membranes keep things out and keep things in. This is called selectively permeable-this is to say that some substances can pass through the membrane and others cannot. Some things must come in, like oxygen and food molecules but other things must come out— i.e. waste materials.

Diffusion Diffusion is the main method by which small molecules move across the cell membrane. With diffusion molecules move from an area of higher concentration to an area of lower concentration. Concentration means how much of a substance is in a given volume.

Confused? Look at this example: The room is full– so, you want to go where there is more room and less people. So you leave the crowded room and go into the other less crowded room.

Still confused?

Let’s see how this diffusion works-- Air freshener--- The particles that came out were concentrated and then spread out to get away from the concentration. The particles actually hit into each other which causes them to spread. That is how all of you eventually were able to smell the molecules of the freshener.

What causes diffusion? Molecules are always moving. They bang into each other as they move. More molecules will mean more collisions as they hit into each other. As they hit, the molecules move farther apart. As time passes, the molecules will all move as far apart as possible. Diffusion of Oxygen. Remember our pond water critters? They use diffusion to bring oxygen across their cell membranes to live. Because there is more oxygen in the surrounding water than in the cell of the critter, oxygen will pass from the water into the cell.

What is Osmosis? Osmosis is the diffusion of water molecules through a selectively permeable membrane. Cells cannot function without water so cellular processes depend on osmosis. Water moves in the same manner as diffusion. It will move from an area of high concentration to an area of lower concentration. This movement is done through the cell membrane.

What are the effects of osmosis?

Question time-- 1. What is osmosis? The diffusion of water molecules through a selectively permeable membrane. 2. Why is it important? Cells cannot function properly without adequate water. 3. I put a cell into a glass with a very high salt solution, what will happen to the cell? It will shrink as the water in the cell goes out to the area with less water concentration. The cell will shrink, and shrivel-called crenation—and eventually it will die.

Passive Transport The movement of dissolved materials through the cell membrane without using cellular energy is called passive transport. What if the cell brings something into the cell that is larger or is against the normal flow of things? This is called active transport-requires the cell to use its own energy to transport the materials into the cell. Many times proteins are brought into the cell this way.

Active Transport

What types of things are brought into the cell with active transport Things like: Calcium Potassium Sodium

Transport by engulfing This is active transport and requires energy. The cell membrane surrounds and engulfs or encloses the particle. Once the particle is engulfed, the cell membrane wraps around the particle and forms a vacuole within the cell.

One last puzzeling question? Why are cells so small? One reason is how materials move into and out of a cell. As a cell’s size increases, more of its cytoplasm is located farther from the cell membrane. Since the streaming cytoplasm helps to move things in the cell, those things must move faster and it would take so much longer if cells were larger-now you know!!

All done with Section 4 of Chapter 1 Now you should be able to complete the concept map. It is due on Wed. Remember that your animal and plant cell needs to be complete on Wed. also. It should be labeled and colored. For Thursday---have your cell organelle parts and functions memorized and ready to do something fun.