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Cell Membranes and Osmosis

Explore the structure and function of cell membranes, including the phospholipid bilayer, proteins, and selective permeability. Learn about osmosis, passive and active transport, and endocytosis and exocytosis.

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Cell Membranes and Osmosis

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  1. Cell Membranes and Osmosis

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

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

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

  5. Fluid mosaic model 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. 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. 80% water 20% starch 100% water 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). Cell will lose water and shrivel

  9. 80% water 20% starch 100% water 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). Cell will expand or even burst!

  10. Use mouse to clickherefor an Osmosis VideoTurn 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. 80% water 20% starch 100% water WHAT DO YOU THINK THIS CELL WILL LOOK LIKE?

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

  14. Use mouse to clickherefor plasmolysis Video Which one is in salt water?

  15. Red blood cells in salt water Red blood cells in pure 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. PASSIVE TRANSPORT 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 Animation Link Animation Link 2

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

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

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

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

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

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

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