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  1. 1. Cell Boundaries What are the main functions of the cell membrane and the cell wall?
  2. 2. <ul><li>When you study a country, you often begin by examining a map of its borders – where does it begin and where does it end? </li></ul><ul><li>This same principle applies to cells…the border of a cell is very important to understand. </li></ul>
  3. 3. The Cell Wall <ul><li>Cell walls lie outside the cell membrane in plants, algae, fungi, and many prokaryotes. </li></ul><ul><li>Most are porous enough to allow H 2 O, CO 2 , and O 2 to pass through easily. </li></ul><ul><li>Most are made from carbohydrate or protein fibers </li></ul><ul><li>The main function of the cell wall is to provide support and protection for the cell. </li></ul>
  4. 4. The cell membrane regulates what enters and leaves the cell – also provides protection and support.
  5. 5. <ul><li>The composition of nearly all cell membranes is a double-layered sheet called a lipid bilayer . </li></ul><ul><li>Embedded into the bilayer are proteins, some of which are attached to a carbohydrate. </li></ul><ul><li>Carbohydrates are identification markers. </li></ul><ul><li>Proteins without carbohydrates often serve as channels and pumps that help move substances across the cell membrane. </li></ul>
  6. 6. <ul><li>Hydrophilic region – water ‘loving’ – mix easily with water </li></ul><ul><li>Hydrophobic region – water ‘hating’ – does not mix well with water </li></ul>
  7. 7. <ul><li>[What moves into and out of cells?] </li></ul><ul><li>In  food, carbohydrates (sugars), proteins, lipids, O 2 , H 2 O </li></ul><ul><li>Out  wastes (ammonia), CO 2 , H 2 O… </li></ul><ul><li>[What determines the rate and the direction of this movement across the membrane?] </li></ul><ul><li>Concentration gradient – the difference in concentration across a space… </li></ul><ul><li>( solvent / solute ) </li></ul><ul><li>Often the movement will continue until equilibrium is reached – equal concentrations on both sides of the membrane (equal movement in both directions) </li></ul>
  8. 8. There are two main types of transport <ul><li>Passive Transport – movement that does not require energy. The cell does not have to generate any energy to accomplish this type of movement. </li></ul><ul><li>The most well known example of passive transport is… </li></ul><ul><li>DIFFUSION – the movement of a substance from an area of high(er) concentration to an area of low(er) concentration. </li></ul><ul><li>When water moves, we call it… </li></ul><ul><li>OSMOSIS – the diffusion of water through a semi-permeable membrane </li></ul>
  9. 9. Diffusion
  10. 10. Osmosis
  11. 11. How does OSMOSIS work? <ul><li>Water will move into the cell . [When the surrounding fluid contains a lower concentration of dissolved materials than the cell does…] </li></ul><ul><li>… Water moves into the cell…Why?... </li></ul><ul><li>… The solution outside of the cell has a lower solute concentration than the cell itself. </li></ul><ul><li>The cell is in a hypotonic solution. </li></ul>
  12. 12. <ul><li>Water will move out of the cell . [When the surrounding fluid has a concentration of dissolved substances greater than the concentration within the cell… </li></ul><ul><li>Water moves out of the cell – Why? </li></ul><ul><li>The solution outside the cell has a higher solute concentration than the cell itself. </li></ul><ul><li>This cell is in a hypertonic environment. </li></ul>
  13. 13. <ul><li>Water moves both into and out of the cell equally . Water-to-solute concentrations are equal on both sides of the membrane. </li></ul><ul><li>This is an isotonic environment. </li></ul>
  14. 14. The Effects of Osmosis on Cells
  15. 15. Does my life depend on diffusion?
  16. 16. Another example of passive transport is… <ul><li>… Facilitated Diffusion . This is when membrane proteins chemically bind to a particle on one side of the membrane, carry it across the membrane, and release is on the other side moving WITH the concentration gradient. </li></ul><ul><li>No cellular energy is required since its moving ‘with the flow.’ </li></ul>
  17. 17. <ul><li>New research has shown H2O enters the cell by facilitated diffusion. (Osmosis) </li></ul><ul><li>Why does water have a tough time passing through the cell membrane? </li></ul><ul><li>Many cells contain water channel proteins called aquaporins that allow water to pass right through them… </li></ul>
  18. 18. Another application… <ul><li>What disease does these people suffer from? </li></ul>
  19. 19. Cystic Fibrosis <ul><li>CF is an incurable genetic disease that affects 1 in every 3,000 Caucasian births. </li></ul><ul><li>It involves a chronic cough, lung infections, and digestion problems. </li></ul><ul><li>25% of patients survive in their 30’s. </li></ul><ul><li>What causes it? </li></ul><ul><li>CF is caused by a mutation in the gene of a membrane protein (CFTR protein) – this protein is a part of the cell membrane and helps regulate the movement of chlorine ions (Cl - ) across the cell membrane. </li></ul>
  20. 21. <ul><li>The inability to regulate the movement of Cl - ions results in an imbalance of water – a thick mucus builds up… </li></ul><ul><li>… on the cells of the lungs (this makes one vulnerable to lung infections) </li></ul><ul><li>… on cells in the digestive system (this blocks the release of digestive enzymes) </li></ul><ul><li>… on reproductive organs (infertility may be a problem – sperm can’t get to egg) </li></ul><ul><li>The POINT  Serious issues may arise when your cell losses its ability to regulate what substances can and can’t move across its membrane… </li></ul>
  21. 22. <ul><li>Active Transport - movement that does require energy. The cell plays an active role in accomplishing these particle movements. </li></ul><ul><li>Examples – All animal cells use the sodium-potassium pump to move Na + and K + across a membrane. [see next slide] </li></ul>
  22. 24. <ul><li>Another example – proton pumps – hydrogen ions (protons) need to get moved across a membrane </li></ul>
  23. 25. The movement of large particles into or out of cells… <ul><li>Endocytosis (when materials are taken into the cell) </li></ul><ul><li>Phagocytosis – “cell eating” – the cell ingests large solid particles such as bacteria and food </li></ul><ul><li>Pinocytosis – “cell drinking” – the cell takes in dissolved materials </li></ul><ul><li>Receptor-mediated – specific molecules combine with receptor proteins in the membrane (this is the main way eukaryotic cells take in macromolecules) </li></ul><ul><li>Exocytosis (when materials are removed from the cell </li></ul>
  24. 28. Transport Summary…
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