Homeostasis & Transport

1. Homeostasis & Transport

2. Cell Membrane • Helps organisms maintain homeostasis by controlling what substances may enter or leave cells—selectively permeable

3. Passive Transport Movement of substances across the cell membrane that does NOT require energy Active Transport Movement of substances across the cell membrane that DOES require energy Types of Transport

4. Types of Passive Transport • Diffusion • Osmosis • Facilitated Diffusion • Diffusion through Ion Channels

5. Diffusion • Simplest type of passive transport • Movement of molecules from area of higher concentration to area of lower concentration • Movement created by concentration gradient • Driven by molecule’s kinetic energy • Molecules are in constant motion • Continues until equilibrium is reached • Concentration of molecules is same throughout

6. Diffusion of Molecules Through Cell Membrane • Move from area of high concentration to area of low concentration • Factors that determine ability of molecule to diffuse • Size • Type • Chemical nature of membrane • Molecules that can dissolve in lipids • Oxygen & carbon dioxide • Pores formed from proteins allow molecules to pass

7. Osmosis • Solution has 2 parts: • Solute • Solvent • In cells, solutes are usually organic & inorganic compounds & solvent is water • Osmosis is process by which water molecules diffuse across cell membrane from area of higher concentration to area of lower concentration • Does NOT require energy

8. Direction of Osmosis • Net direction of osmosis depends on relative concentration of solutes on the two sides of the membrane • 3 types of solutions: • Hypotonic • Hypertonic • Isotonic

9. Hypotonic Solutions • “Hypo-” means less • Occurs when concentration of solute molecules outside cell is LOWER than concentration in the cytosol (inside cell) • Solution outside is hypotonic to the cytosol • Water moves into the cell • Cell will gain water until equilibrium is reached • Cell gets larger

10. Hypertonic Solution • “Hyper-” means more • Occurs when concentration of solute molecules outside cell is HIGHER than concentration in the cytosol (inside cell) • Solution outside is hypertonic to cytosol • Water diffuses out of the cell until equilibrium is reached • Cell shrinks

11. Isotonic Solution • “Iso-”means same • Occurs when concentration of solutes outside cell is equal to concentration of solutes inside the cell • Outside solution is isotonic to cytosol • Water diffuses into & out of cell at equal rates—no net movement of water

12. Osmosis Animation • http://glencoe.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_osmosis.html

13. How Cells Deal with Osmosis • Cells exposed to isotonic external environments usually have no difficulty keeping homeostasis • Cells of vertebrate land animals & organisms living in sea

14. How Cells Deal with Osmosis • Cells exposed to hypotonic require a means to rid themselves of the excess water • Case for unicellular freshwater organisms • Paramecia rid excess water using contractile vacuole • Contractile vacuole organelle collect excess water & contract—pumping water out of cell • Requires energy • Case for some multicellular organisms • Solute Pumps: pump solutes out of cytosol which lowers concentration of solutes in cell, bringing it closer to concentration of environment

15. Paramecia • http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/articles/param1.html

16. How Plant Cells Deal with Osmosis • Most plant cells live in hypotonic environments • Water moves into plant cells by osmosis causing plant cell to swell • Swells until cell membrane is pressed against cell wall • Pressure exerted on cell wall by water molecules is called turgor pressure • Cell wall strong • Plants in hypertonic environments shrink as water leaves & turgor pressure is lost • Referred to as plasmolysis • Reason why plants wilt

17. How Some Animal Cells Deal with Osmosis • Some cells cannot compensate for changes in solute concentrations of their environments • Causes them to lose their normal shape • Hypertonic environment causes them to shrink & shrivel • Hypotonic environment causes them to swell & eventually burst • Cytolysis refers to bursting of a cell

18. BLOOD CELLS Hypertonic Environment Isotonic Environment Hypotonic Environment

19. Facilitated Diffusion • Used for molecules that cannot diffuse fast through cell membrane if they are… • Not soluble in lipids • Too large to pass through pores • Examples: Glucose & amino acids • Movement is assisted by proteins called carrier proteins • Movement occurs from area of high to low concentration • No energy expended • 2 Important properties of facilitated diffusion • Help substances move in or out of cell depending on concentration gradient • Carrier proteins are specific for one type of molecule

20. How Facilitated Diffusion Works • Carrier protein bind to molecule on one side of cell membrane • Carrier protein changes shape, shielding molecule from interior of membrane • Molecule is released on other side of membrane http://www.bact.wisc.edu/MicrotextBook/BacterialStructure/FacDiff.html

21. Diffusion Through Ion Channels • Passive transport that involves membrane proteins known as ion channels • Ions such as Na+, K+, Ca+, Cl- are important for cell function, but they are not lipid soluble • Ion channels provide small passageways for ions to diffuse • Each ion channel is specific for type of ion • Some ion channels are always open • Some have “gates” that open or close in response to 3 stimuli: • Stretching of cell membrane • Electrical signas in cytosol or environment • Chemical signals in cytosol or environment

22. Active Transport • Movement of materials from area of LOW concentration to an area of HIGH concentration • Requires energy

23. Types of Active Transport • Cell Membrane Pumps • Endocytosis • Exocytosis

24. Cell Membrane Pumps • Carrier proteins involved with active transport are called cell membrane pumps • Move substances against their concentration gradient • Both passive & active carrier proteins behave in similar ways

25. Sodium-Potassium Pump • Carrier protein that transports Na+ & K+ ions against their concentration gradient • For animal cells to function normally, need higher concentration of Na+ ions outside cell and have higher concentration of K+ ions inside cell • Na+- K+ pump helps maintain these concentrations

26. 6-Step Process of Sodium-Potassium Pump • 1. Three Na+ ions located in cytosol bind to carrier protein • 2. At same time, carrier protein removes a phosphate group from ATP. Phosphate group binds to carrier protein • 3. Binding of phosphate changes shape of carrier protein, allowing 3 Na+ ions to be released into cell’s environment

27. 6-Step Process of Sodium-Potassium Pump • 4. Two K+ ions located outside cell bind to carrier protein • 5. Phosphate group is released from carrier protein, causing it to change shape • 6. Two K+ ions are then released into cytosol

28. Sodium-Potassium Pump Animation

29. http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.htmlhttp://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.html

30. Electrical Gradient • Exchange of 3 Na+ ions for 2 K+ ions creates an electrical gradient across cell membrane • Outside has positive charge • Inside has negative charge • Difference in charge is important for conducting electrical impulses along nerve cells

31. Endocytosis • Process by which cells ingest external fluid, macromolecules, & large particles, including other cells • Cell membrane folds in & forms small pouch • Pouch pinches off from cell membrane to become a vesicle • Some vesicles fuse with lysosomes

32. Types of Endocytosis • 1. Pinocytosis • Transport of solutes or fluids • 2. Phagocytosis • Transport of large particles or whole cells • Many unicellular organisms feed by phagocytosis • Animal cells may have phagocytes which fuse with lysosomes • Phagocytes & lysosomes ingest bacteria or viruses that invade the body

33. Pinocytosis Animation

34. Phagocytosis Animation

35. Exocytosis • Reverse of endocytosis • Vesicles fuse with cell membrane, releasing contents to cell’s external environment • Used to release large molecules like proteins • http://www.biology.washington.edu/bsa/IonTransport/images/exocytosis.mov