Movement across the plasma membrane

1. Movement across the plasma membrane Osmosis and Diffusion

2. Particle Movement • The molecules that make up gases and liquids are always in motion.

3. Diffusion • In gases and liquids, substances want to move from where they are very crowded to where it is less crowded. • They want to spread themselves out

4. Diffusion • When substances move from a crowded area (we call this high concentration) to a less crowded area (we call this low concentration), it is called diffusion. • Definition: diffusion is the movement of particles from an area of high concentration to an area of low concentration.

5. Draw this Area of high concentration Barrier

6. Draw this Area of low concentration Area of high concentration

7. Draw this Completely diffused

8. Concentration Gradient • Molecules move from an area of high concentration to an area of low concentration • The difference between the two areas is called the Concentration Gradient • Molecules are said to move withthe concentration gradient.

9. Osmosis • Osmosis is a special kind of diffusion • It is only about the movement of water • It is movement across a membrane • Definition: the movement of water across a membrane from an area of higher concentration to an area of lower concentration

10. Osmosis

11. How does osmosis work? • Let’s take the example of a cell • It has water inside it and water around it • It has a membrane

12. Let’s suppose that a cell is placed in a beaker of salt water. • It is much more salty outside the cell that inside the cell • This means the concentration of water is less outside the cell than inside the cell Cell in saltwater

13. The water wants to balance things out • Water will move from the inside of the cell to the outside to try and make the concentrations the same • This will cause the cell to lose water, shrinking it Cell in saltwater

14. Now let us suppose that a cell is placed in a beaker with pure water • There is a little salt in the water inside the cell, but none in the water outside the cell • Again, the water wants to balance things out • Water will move into the cell to try and make the concentrations the same. Cell in less salty water

15. What if the saltiness of the water inside the cell is the same as the water outside the cell? • Water will still move across the membrane, but there will be no change in concentration • We are at dynamic equilibrium Cell in water of same saltiness as cell

16. Cells in Salty Water • If a cell is in a salty solution and water moves out of the cell to try and balance it out, it is called a hypertonic solution = high salt • Water moving out of the cell can cause the cell to shrink

18. Cell in non-salty water • If a cell is in a pure water solution and water wants to move into the cell to balance things out, we call that a hypotonic solution= low salt • Water moving into the cell can cause the cell to swell and burst like a balloon.

20. Cells in water that is just as salty as they are • If the concentration of water is the same on both sides of the membrane, it is called anisotonic solution • Hypo = under • Hyper = over • Iso = equal

23. How can I remember which direction the water will move? • A simple rule to remember is: • Salt is a solute. When it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty.

25. What does this have to do with cells • Cells take advantage of osmosis and diffusion to help move molecules into and out of the cell • They purposely set up concentration gradients so that substances can pass through the plasma membrane from an area of high concentration to an area of low concentration