1. Cell Membranes and Osmosis

2. - Cell membrane - Controls what enters and leaves cell What separates a cell from its surroundings?

3. Cell membrane is made out of two layers of fats. These fats are called “phospholipids” Phospholipid Since your cell membrane is made out of two layers of fats, the cell membrane is often called a “phospholipid bilayer” Lipid Bilayer

4. The outside of the cell membrane likes to be near water Hydrophilic – loves water The inside of the cell membrane hates to be near water Hydrophobic – hates or fears water

5. In addition to phospholipids, the cell membrane also has proteins in it. These proteins help a cell to control what comes in and out of the cell. Fluid mosaic model These proteins can float around on the cell membrane like icebergs in the ocean and decorate the cell like a mosaic.

6. Cell membranes are “selectively permeable” Permeable – EVERYTHING can pass through membrane Non-Permeable – NOTHING can pass through membrane Selectively-Permeable – SOME THINGS can pass through membrane As long as it is small enough, it will DIFFUSE across cell membrane from high to low concentration

7. Not all molecules can diffuse through a cell membrane. However, one molecule can always diffuse - WATER! Osmosis (diffusion of water) – water flows from an area of high concentration to an area of low concentration

8. Try osmosis with an animal cell. Assume starch – a really BIG sugar molecule – cannot move across an animal cell membrane. Start by comparing the percent water inside and outside the cell. Then, draw an arrow pointing in the direction that water will flow (high to low). Finally, draw the result. (NOTE: animal cells can swell and shrink like a balloon – to a certain extent. Your own red blood cells can explode if they swell too much). 100% water 80% water 20% starch Cell will lose water and shrivel

9. Try osmosis with an animal cell. Assume starch – a really BIG sugar molecule – cannot move across an animal cell membrane. Start by comparing the percent water inside and outside the cell. Then, draw an arrow pointing in the direction that water will flow (high to low). Finally, draw the result. (NOTE: animal cells can swell and shrink like a balloon – to a certain extent. Your own red blood cells can explode if they swell too much). 80% water 20% starch 100% water Cell will expand or even burst!

10. Use mouse to click here for an Osmosis Video Turn on speakers

11. PLASMOLYSIS Plasmolysis – a special case involving osmosis and plants Plasmolysis – water moving from high to low concentrations in plant cells Why would osmosis be different in plant cells? (Think: what makes plant cells different from animal cells?) Plant cells have a cell membrane AND a cell wall.

12. 100% water 80% water 20% starch WHAT DO YOU THINK THIS CELL WILL LOOK LIKE?

13. PLASMOLYSIS 80% water 20% starch 100% water

14. Use mouse to click here for plasmolysis Video Which one is in salt water?

15. Red blood cells in pure water Red blood cells in salt water

16. PASSIVE AND ACTIVE TRANSPORT How can cells transport nutrients and molecules that are too big to pass through their membranes? Proteins – 1. Channels open up and allow certain molecules into the cell. 2. Receptors “grab” molecules and bring them into the cell.

17. Proteins can work by “passive” or “active” transport Passive transport (also known as “facilitated diffusion”) – proteins help molecules move across cell membrane from high to low; NO ENERGY is required PASSIVE TRANSPORT Animation Link Animation Link 2

18. Active transport – proteins help move molecules across cell membrane from LOW to HIGH; ENERGY IS required ACTIVE TRANSPORT Animation Link

19. Not all molecules can move through cell membrane. Some are just too big! Different methods are needed. ENDOCYTOSIS AND EXOCYTOSIS Endocytosis – cell wraps membrane around object and pulls it in

20. There are three specific types of endocytosis ENDOCYTOSIS 1. Phagocytosis – white blood cells “eat” bacteria and digest them Animation Link

21. There are three specific types of endocytosis ENDOCYTOSIS 2. Pinocytosis – cell is “drinking” in liquids

22. There are three specific types of endocytosis ENDOCYTOSIS 3. Receptor-aided endocytosis– protein receptors work to pull molecules into cell

23. There are three specific types of endocytosis EXOCYTOSIS Exocytosis – molecules are released from cell