CVA Biology I - B10vrv3073

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CVA Biology I - B10vrv3073

  1. 1. Lesson OverviewLesson Overview7.3 Cell Transport7.3 Cell Transport
  2. 2. Lesson OverviewLesson Overview Cell TransportCell TransportTHINK ABOUT ITWhen thinking about how cells move materials in and out, it can be helpfulto think of a cell as a nation. The boundaries of a nation are its borders, andnearly every country tries to regulate and control the goods the move acrossthose borders.Cells have their own borders, which separate the cell from its surroundingsand also determine what comes in and what goes out.How can a cell separate itself from its environment and still allow material toenter and leave?
  3. 3. Lesson OverviewLesson Overview Cell TransportCell TransportPassive TransportWhat is passive transport?
  4. 4. Lesson OverviewLesson Overview Cell TransportCell TransportPassive TransportWhat is passive transport?The movement of materials across the cell membrane without usingcellular energy is called passive transport.
  5. 5. Lesson OverviewLesson Overview Cell TransportCell TransportPassive TransportEvery living cell exists in a liquid environment.One of the most important functions of the cell membrane is to keep thecell’s internal conditions relatively constant. It does this by regulating themovement of molecules from one side of the membrane to the other side.
  6. 6. Lesson OverviewLesson Overview Cell TransportCell TransportDiffusionThe cytoplasm of a cell is a solution of many different substances dissolvedin water.In any solution, solute particles tend to move from an area where they aremore concentrated to an area where they are less concentrated.The process by which particles move from an area of high concentration toan area of lower concentration is known as diffusion.Diffusion is the driving force behind the movement of many substancesacross the cell membrane.
  7. 7. Lesson OverviewLesson Overview Cell TransportCell TransportDiffusionSuppose a substance is present in unequal concentrations on either side ofa cell membrane.
  8. 8. Lesson OverviewLesson Overview Cell TransportCell TransportDiffusionIf the substance can cross the cell membrane, its particles will tend to movetoward the area where it is less concentrated until it is evenly distributed.
  9. 9. Lesson OverviewLesson Overview Cell TransportCell TransportDiffusionAt that point, the concentration of the substance on both sides of thecell membrane is the same, and equilibrium is reached.
  10. 10. Lesson OverviewLesson Overview Cell TransportCell TransportDiffusionEven when equilibrium is reached, particles of a solution will continue tomove across the membrane in both directions.Because almost equal numbers of particles move in each direction, there isno net change in the concentration on either side.
  11. 11. Lesson OverviewLesson Overview Cell TransportCell TransportDiffusionDiffusion depends upon random particle movements. Substances diffuseacross membranes without requiring the cell to use additional energy.The movement of materials across the cell membrane without using cellularenergy is called passive transport.
  12. 12. Lesson OverviewLesson Overview Cell TransportCell TransportFacilitated DiffusionCell membranes have proteins that act as carriers, or channels, making iteasy for certain molecules to cross.Molecules that cannot directly diffuse across the membrane pass throughspecial protein channels in a process known as facilitated diffusion.Hundreds of different proteins have been found that allow particularsubstances to cross cell membranes.The movement of molecules by facilitated diffusion does not require anyadditional use of the cell’s energy.
  13. 13. Lesson OverviewLesson Overview Cell TransportCell TransportOsmosis: An Example of FacilitatedDiffusionThe inside of a cell’s lipid bilayer ishydrophobic—or “water-hating.” Becauseof this, water molecules have a toughtime passing through the cell membrane.Many cells contain water channelproteins, known as aquaporins, thatallow water to pass right through them.Without aquaporins, water would diffusein and out of cells very slowly.The movement of water through cellmembranes by facilitated diffusion is anextremely important biological process—the process of osmosis.
  14. 14. Lesson OverviewLesson Overview Cell TransportCell TransportOsmosis: An Example of FacilitatedDiffusionOsmosis is the diffusion of water through a selectively permeablemembrane.Osmosis involves the movement of water molecules from an area of higherconcentration to an area of lower concentration.
  15. 15. Lesson OverviewLesson Overview Cell TransportCell TransportHow Osmosis WorksIn the experimental setup below, the barrier is permeable to water but not tosugar. This means that water molecules can pass through the barrier, but thesolute, sugar, cannot.
  16. 16. Lesson OverviewLesson Overview Cell TransportCell TransportThere are more sugar molecules on the right side of the barrier than on theleft side. Therefore, the concentration of water is lower on the right, wheremore of the solution is made of sugar.How Osmosis Works
  17. 17. Lesson OverviewLesson Overview Cell TransportCell TransportHow Osmosis WorksThere is a net movement of water into the compartment containing theconcentrated sugar solution.Water will tend to move across the barrier until equilibrium is reached. At thatpoint, the concentrations of water and sugar will be the same on both sides.
  18. 18. Lesson OverviewLesson Overview Cell TransportCell TransportHow Osmosis WorksWhen the concentration is the same on both sides of the membrane, the twosolutions will be isotonic, which means “same strength.”
  19. 19. Lesson OverviewLesson Overview Cell TransportCell TransportHow Osmosis WorksThe more concentrated sugar solution at the start of the experiment washypertonic, or “above strength,” compared to the dilute sugar solution.The dilute sugar solution was hypotonic, or “below strength.”
  20. 20. Lesson OverviewLesson Overview Cell TransportCell TransportOsmotic PressureFor organisms to survive, they must have a way to balance the intake andloss of water.The net movement of water out of or into a cell exerts a force known asosmotic pressure.
  21. 21. Lesson OverviewLesson Overview Cell TransportCell TransportOsmotic PressureBecause the cell is filled with salts, sugars, proteins, and other molecules, itis almost always hypertonic to fresh water.As a result, water tends to move quickly into a cell surrounded by freshwater, causing it to swell. Eventually, the cell may burst.
  22. 22. Lesson OverviewLesson Overview Cell TransportCell TransportOsmotic PressureIn plants, the movement of water into the cell causes the central vacuole toswell, pushing cell contents out against the cell wall.Since most cells in large organisms do not come in contact with fresh water,they are not in danger of bursting.
  23. 23. Lesson OverviewLesson Overview Cell TransportCell TransportOsmotic PressureInstead, the cells are bathed in fluids, such as blood, that are isotonic andhave concentrations of dissolved materials roughly equal to those in thecells.Cells placed in an isotonic solution neither gain nor lose water.
  24. 24. Lesson OverviewLesson Overview Cell TransportCell TransportIn a hypertonic solution, water rushes out of the cell, causing animal cells toshrink and plant cell vacuoles to collapse.Osmotic Pressure
  25. 25. Lesson OverviewLesson Overview Cell TransportCell TransportOsmotic PressureSome cells, such as the eggs laid by fish and frogs, must come into contactwith fresh water. These types of cells tend to lack water channels.As a result, water moves into them so slowly that osmotic pressure does notbecome a problem.
  26. 26. Lesson OverviewLesson Overview Cell TransportCell TransportOsmotic PressureOther cells, including those of plants and bacteria, that come into contactwith fresh water are surrounded by tough cell walls that prevent the cellsfrom expanding, even under tremendous osmotic pressure.
  27. 27. Lesson OverviewLesson Overview Cell TransportCell TransportOsmotic PressureNotice how the plant cell holds its shape in hypotonic solution, while theanimal red blood cell does not.However, the increased osmotic pressure makes such cells extremelyvulnerable to injuries to their cell walls.
  28. 28. Lesson OverviewLesson Overview Cell TransportCell TransportActive TransportWhat is active transport?
  29. 29. Lesson OverviewLesson Overview Cell TransportCell TransportActive TransportWhat is active transport?The movement of materials against a concentration difference is known asactive transport. Active transport requires energy.
  30. 30. Lesson OverviewLesson Overview Cell TransportCell TransportActive TransportCells sometimes must move materials against a concentration difference.The movement of material against a concentration difference is known asactive transport. Active transport requires energy.
  31. 31. Lesson OverviewLesson Overview Cell TransportCell TransportActive TransportThe active transport of smallmolecules or ions across a cellmembrane is generally carriedout by transport proteins, orprotein “pumps,” that are found inthe membrane itself.
  32. 32. Lesson OverviewLesson Overview Cell TransportCell TransportActive TransportLarger molecules and clumps ofmaterial can also be activelytransported across the cellmembrane by processes knownas endocytosis and exocytosis.The transport of these largermaterials sometimes involveschanges in the shape of the cellmembrane.
  33. 33. Lesson OverviewLesson Overview Cell TransportCell TransportMolecular TransportSmall molecules and ions arecarried across membranes byproteins in the membrane that actlike pumps.Many cells use such proteins tomove calcium, potassium, andsodium ions across cellmembranes.Changes in protein shape seem toplay an important role in thepumping process.
  34. 34. Lesson OverviewLesson Overview Cell TransportCell TransportMolecular TransportA considerable portion of the energy used by cells in their daily activities isdevoted to providing the energy to keep this form of active transport working.The use of energy in these systems enables cells to concentrate substancesin a particular location, even when the forces of diffusion might tend to movethese substances in the opposite direction.
  35. 35. Lesson OverviewLesson Overview Cell TransportCell TransportBulk TransportLarger molecules and even solidclumps of material may betransported by movements of thecell membrane known as bulktransport.Bulk transport can take severalforms, depending on the size andshape of the material moved intoor out of the cell.
  36. 36. Lesson OverviewLesson Overview Cell TransportCell TransportEndocytosisEndocytosis is the process oftaking material into the cell bymeans of infoldings, or pockets,of the cell membrane.The pocket that results breaksloose from the outer portion ofthe cell membrane and forms avesicle or vacuole within thecytoplasm.
  37. 37. Lesson OverviewLesson Overview Cell TransportCell TransportEndocytosisLarge molecules, clumps of food,and even whole cells can be takenup by endocytosis.Two examples of endocytosis arephagocytosis and pinocytosis.
  38. 38. Lesson OverviewLesson Overview Cell TransportCell TransportEndocytosisIn phagocytosis, extensions of cytoplasm surround a particle and package itwithin a food vacuole. The cell then engulfs it.Amoebas use this method for taking in food.Engulfing material in this way requires a considerable amount of energy and,therefore, is a form of active transport.
  39. 39. Lesson OverviewLesson Overview Cell TransportCell TransportEndocytosisIn pinocytosis, cells take up liquid from the surrounding environment byforming tiny pockets along the cell membrane.The pockets fill with liquid and pinch off to form vacuoles within the cell.
  40. 40. Lesson OverviewLesson Overview Cell TransportCell TransportExocytosisMany cells also release largeamounts of material from thecell, a process known asexocytosis.During exocytosis, themembrane of the vacuolesurrounding the material fuseswith the cell membrane, forcingthe contents out of the cell.

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