Passive Transport

1. Passive Transport Section 4-1

2. Alabama State Course Objectives to Be Covered • AL COS 2 Describe cell processes necessary for achieving homeostasis, including active and passive transport, osmosis, diffusion, exocytosis, and endocytosis. • AL COS 2.2 Comparing the reaction of plant and animal cells in isotonic, hypotonic, and hypertonic solutions.

3. Diffusion is Caused by the Random Movement of Particles • RECALL… When an organism adjusts internally because the external environment is changing, this is HOMEOSTASIS. • One way cells maintain homeostasis is by controlling the movement of substances across the cell membrane.

4. Random Motion and Concentration • Passive Transport – movement across the cell membrane that DOES NOT require energy. • Gradient – a difference in the concentration of a substance across a space • Equilibrium – a condition in which the concentration of a substance is equal throughout a space

5. Movement of Substances • If there is a concentration gradient in the solution, the substance will move from an area of high concentration to an area of lower concentration. • Diffusion – movement of a substance from an area of high concentration to low concentration caused by the random motion of particles

6. Movement of Substances (con’t) • Many substances (molecules and ions) dissolved in cytoplasm and in fluid outside the cell enter/leave cells by diffusion 1. Inside cell – concentration of most of these substances are different from concentration outside cell. This forms a concentration gradient across the cell membrane. 2. To diffuse “down” the concentration gradient, the molecule/ion goes from an area of high concentration to an area of low concentration across the cell membrane.

7. Movement of Substances (con’t) • Cell Membrane is selectively permeable. 1. Nonpolar interior of lipid bilayer repels ions and most polar molecules. This prevents diffusion across the cell membrane of these substances. 2. Very small molecules or nonpolar molecules can diffuse across cell membrane down their concentration gradient. • Diffusion of small or nonpolar molecules is the simplest type of passive transport.

8. Water Diffuses into and out of Cells by Osmosis • Osmosis – diffusion of water through a selectively permeable membrane • Osmosis involves the movement of water down its concentration gradient. • The direction of water movement across cell membrane depends on relative concentrations of free water molecules in cytoplasm and in fluid outside of cell.

9. Water Diffuses into and out of Cells by Osmosis (con’t) • In the solutions on either side of the cell membrane, many ions and polar molecules are dissolved in water. When these substances dissolve in water, water molecules are attracted to them and so are no longer free to move around. If solutions on either side of cell membrane have different concentrations of dissolved particles, they will also have different concentrations of “free” water molecules. Osmosis will occur as free water molecules move into the solution with the lower concentration of free water molecules.

10. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

11. Three Possibilities for Direction of Water Movement • Water Moves Out • When water diffuses out of the cell, CELL SHRINKS • Hypertonic – solution that causes a cell to shrink because of osmosis. • If fluid outside cell has a higher concentration of dissolved particles than cytoplasm, outside fluid has lower concentration of free water molecules than cytoplasm.

12. Three Possibilities for Direction of Water Movement (con’t) • Water Moves In • When water diffuses into cell, CELL SWELLS • Hypotonic – a solution that causes a cell to swell because of osmosis • If fluid outside the cell has a lower concentration of dissolved particles than cytoplasm has, then the outside fluid also has a higher concentration of free water molecules than cytoplasm

13. Three Possibilities for Direction of Water Movement (con’t) • No Net Water Movement • Cytoplasm and fluid outside cell have same concentration of free water molecules - isotonic, water diffuses into and out of the cell at equal rates. • Results in no net movement of water across cell membrane • Cell stays the same size http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

14. Animal Cells and Plant Cells • If left unchecked, a hypotonic solution can cause a cell to burst. Different cells have evolved different methods to deal with this. • Plants and Fungi – rigid cell walls to prevent cell from expanding too much • Unicellular Eukaryotes – contractile vacuoles – collect excess water and force it out • Animal Cells – avoid swelling by osmosis by removing dissolved particles from cytoplasm. This increases concentration of free water molecules inside cell.

15. Proteins Help Some Substances Cross the Cell Membrane • Ions and polar molecules cross the cell membrane with the help of transport proteins • Transport Proteins form channels or polar passageways • Each channel allows only specific substances to pass through cell membrane • This selectivity enables a cell to control what enters and leaves.

16. Diffusion Through Ion Channels • Ions such as Na+ (sodium), K+ (potassium), Ca2+ (calcium), and Cl- (chloride) are involved in many important cell functions. • These ions diffuse across the cell membrane through ion channels. • Ion Channel – doughnut-shaped transport protein with a polar pore through which ions can pass.

17. Diffusion Through Ion Channels (con’t) • Pores of some ion channels always open. Others, the pores can be closed by ion channels gates • Ion channel gates may open or close in response to several stimuli (ex. Stretching of the cell membrane, change in electrical charge, or the binding of specific molecules to the ion channel). • Stimuli able to control ability of particular ions to cross cell membrane

18. Diffusion Through Ion Channels (con’t) • Passive transport because no use of energy because ions move down their concentration gradients • The effect of electrical charge on ion transport • Rate of movement of a substance across cell membrane is generally determined by the concentration gradient of the substance. • Movement of a charged particle across cell membrane influenced by particle’s positive or negative charge.

19. Diffusion Through Ion Channels (con’t) • Inside Cell – negative • Outside Cell – positive • Opposite charges attract, so positive ions typically diffuse into cell and negative ions typically diffuse out of cell. • Direction of movement caused by an ion’s concentration gradient may oppose the direction of movement caused by ion’s electrical charge. • http://www.cellsalive.com/channels.htm • http://www.bioanim.com/CellTissueHumanBody6/index.html

20. Facilitated Diffusion • Carrier Proteins – bind to specific substances on one side of the cell membrane and carry the substance to the other side. It then releases the substance. • Facilitated Diffusion – type of passive transport because it moves substances down their concentration gradient without using the cell’s energy.

21. Facilitated Diffusion (con’t) • Figure 4-4 p. 80 • Carrier protein binds substance and changes shape • Change in shape of carrier protein exposes substance to other side of cell membrane. Molecule is transported across cell membrane. • Carrier protein shields molecule from interior of cell membrane. Molecule released from carrier protein. Carrier protein back to original shape.

22. Facilitated Diffusion

23. Review • Passive transport is the movement of substances across the cell membrane without the use of energy by the cell. • Diffusion is the movement of a substance from an area of high concentration to an area of lower concentration. • Osmosis is the diffusion of free water molecules across a selectively permeable membrane. • Ion channels are proteins that have a pore through which ions can cross the cell membrane.

24. Review • In facilitated diffusion, a carrier protein transports a substance across the cell membrane down the concentration gradient of the substance.

25. HOMEWORK Section 4-1 Review Questions Page 80 Questions 1-5