Week 10 - Cells

Lesson 6

Science in Action 8
Pages 115-119

ScienceFocus 8
Pages 128-131, 134-137

 

Background Information

Diffusion
Definition of diffusion : Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.

Molecules in a solid, liquid, and gas are in constant motion. When molecules move from an area where there are more of them, into areas where there are few of them, it is called diffusion . In the food colouring activity that follows, there are more molecules of food colouring in a drop of food colouring than in the water you add the food colouring to. You will investigate to see if the food colouring particles move from areas of high concentration to areas of low concentration.
Cell Transport
Most of you have made macaroni and cheese or some type of boiled pasta for lunch. You cook the pasta in boiling water. When the pasta is cooked, you pour it into a strainer in your sink. The water goes through the strainer but the pasta (at least most of it) stays in the strainer or a sieve. Particles smaller than the openings in the grid pass through. In this example, the water particles are small enough to pass through the openings. Particles that are larger than the openings in the grid stay in the strainer. Obviously, the macaroni is too big to pass through the openings. Cells are surrounded by a cell membrane . Cells must obtain nutrients and oxygen form their environment. They must also release waste products into their environment. How can materials move into and out of cells? How does a cell control what enters and leaves?

A cell membrane performs in a similar way for the cell as the strainer does for the pasta. It allows some things to enter or leave the cell while keeping other things outside or inside the cell. When membranes function this way, they are called selectively permeable. The strainer (sieve) is selectively permeable based on the size of the openings. The pot, on the other hand, does not let anything pass through its sides or bottom. The pot is impermeable because it does not allow anything to pass through it.

Osmosis
Water makes up a large part of a living cell. The diffusion of water through the cell membrane is called osmosis . Water moves from an area where there is more water to an area where there is less water. In a later activity you will see how that works in the carrot cells.

Definition of osmosis : Osmosis is the diffusion of water across a cell membrane from an area where there are more water molecules to an area where there are fewer water molecules.
Exercise 2.2: Osmosis