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Passive Transport and Water Balance in Cells

This chapter explores the processes of passive transport, facilitated diffusion, osmosis, and active transport in cells. It also discusses the importance of water balance and the role of membranes in maintaining it. Additionally, it highlights the impact of faulty membranes on blood cholesterol levels.

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Passive Transport and Water Balance in Cells

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  1. Chapter 5 The Working Cell

  2. 5.14 Passive transport is diffusion across a membrane • Diffusion is the tendency for particles to spread out evenly in an available space • From an area of high concentration to an area of low concentration • Passive transport across membranes occurs when a molecule diffuses down a concentration gradient • Small nonpolar molecules such as O2 and CO2 diffuse easily across the phospholipid bilayer of a membrane

  3. LE 5-14a Molecules of dye Membrane Equilibrium

  4. LE 5-14b Equilibrium

  5. Animation: Diffusion

  6. 5.15 Transport proteins may facilitate diffusion across membranes • In facilitated diffusion • Transport proteins that span the membrane bilayer help substances diffuse down a concentration gradient • To transport the substance, a transport protein may • Provide a pore for passage • Bind the substance, change shape, and then release the substance

  7. LE 5-15 Solute molecule Transport protein

  8. 5.16 Osmosis is the diffusion of water across a membrane • In osmosis water, molecules diffuse across a selectively permeable membrane • From an area of low solute concentration • To an area of high solute concentration • Until the solution is equally concentrated on both sides of the membrane • The direction of movement is determined by the difference in total solute concentration • Not by the nature of the solutes Animation: Osmosis

  9. LE 5-16 Lower concentration of solute Higher concentration of solute Equal concentration of solute H2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water

  10. 5.17 Water balance between cells and their surroundings is crucial to organisms • Osmoregulation is the control of water balance • Tonicity is the tendency of a cell to lose or gain water in solution • Isotonic solution: solute concentration is the same in the cell and in the solution • No osmosis occurs • Animal cell volume remains constant; plant cell becomes flaccid

  11. Hypotonic solution: solute concentration is greater in the cell than in the solution • Cell gains water through osmosis • Animal cell lyses; plant cell becomes turgid • Hypertonic solution: solute concentration is lower in the cell than in the solution • Cell loses water through osmosis • Animal cell shrivels; plant cell plasmolyzes

  12. LE 5-17 Isotonic solution Hypertonic solution Hypotonic solution H2O H2O H2O H2O Animal cell (2) Lysed (3) Shriveled (1) Normal Plasma membrane H2O H2O H2O H2O Plant cell (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed)

  13. Video: Plasmolysis Video: Turgid Elodea

  14. 5.18 Cells expend energy for active transport • Active transport requires energy to move solutes against a concentration gradient • ATP supplies the energy • Transport proteins move solute molecules across the membrane Animation: Active Transport

  15. LE 5-18 Transport protein P P Protein changes shape Phosphate detaches P ATP ADP Solute Transport Solute binding Phosphorylation Protein reversion

  16. 5.19 Exocytosis and endocytosis transport large molecules • To move large molecules or particles through a cell membrane • A vesicle may fuse with the membrane and expel its contents outside the cell (exocytosis) • Membranes may fold inward, enclosing material from the outside (endocytosis)

  17. 5.19 Exocytosis and endocytosis transport large molecules • To move large molecules or particles through a cell membrane • A vesicle may fuse with the membrane and expel its contents outside the cell (exocytosis) • Membranes may fold inward, enclosing material from the outside (endocytosis)

  18. LE 5-19a Fluid outside cell Vesicle Protein Cytoplasm

  19. LE 5-19b Vesicle forming

  20. Endocytosis can occur in three ways • Phagocytosis ("cell eating") • Pinocytosis ("cell drinking") • Receptor-mediated endocytosis

  21. LE 5-19c Pseudopodium of amoeba Food being ingested LM 230 Phagocytosis Plasma membrane Material bound to receptor proteins PIT TEM 54,000 TEM 96,500 Cytoplasm Pinocytosis Receptor-mediated endocytosis

  22. Animation: Receptor-Mediated Endocytosis Animation: Exocytosis and Endocytosis Introduction Animation: Exocytosis Animation: Pinocytosis Animation: Phagocytosis

  23. CONNECTION • 5.20 Faulty membranes can overload the blood with cholesterol • Cholesterol is carried in the blood by low-density lipoprotein (LDL) particles • Normally, body cells take up LDLs by receptor-mediated endocytosis • Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors • People with hypercholesterolemia have more than twice the normal level of blood cholesterol

  24. LE 5-20 Phospholipid outer layer LDL particle Vesicle Cholesterol Protein Plasma membrane Receptor protein Cytoplasm

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