Summary Section 2 – pages 152-156

1. Summary Section 2 – pages 152-156 The process of diffusion • Diffusion – net movement of particles from high concentration to low concentration. • Diffusion results because of the random movement of particles (Brownian motion). • Rate of diffusion is affected by temperature, concentration.

2. Summary Section 2 – pages 152-156 Diffusion in living systems • Concentration gradient – difference in concentration of a substance across space. • Ions and molecules diffuse from an area of higher concentration to an area of lower concentration.

3. Summary Section 2 – pages 152-156 The results of diffusion • Dynamic equilibrium occurs when there is no longer a concentration gradient. • In a cell, water always moves to reach an equal concentration on both sides of the membrane. Material moving out of cell equals material moving into cell

4. Section 8.1 Summary – pages 195 - 200 Osmosis: Diffusion of Water • The diffusion of water across a selectively permeable membrane is called osmosis. • Regulating the water flow through the plasma membrane is an important factor in maintaining homeostasis within a cell.

5. Section 8.1 Summary – pages 195 - 200 What controls osmosis? • Unequal distribution of particles, called a concentration gradient, is one factor that controls osmosis. Before Osmosis After Osmosis Water molecule Sugar molecule Selectively permeable membrane

6. The -tonics • Isotonic refers to two solutions with the same concentration of dissolved substances. • If two solutions have different concentrations of dissolved substances, one is hypertonic and the other hypotonic. • Hypertonic = higher concentration • Hypotonic = lower concentration

7. Section 8.1 Summary – pages 195 - 200 Cells in an isotonic solution • Water molecules move into and out of the cell at the same rate • In an isotonc solution, cells retain their normal shape. H2O H2O Water Molecule Dissolved Molecule

8. Section 8.1 Summary – pages 195 - 200 Cells in a hypotonic solution • More water enters a cell by osmosis. • In a hypotonic solution, a cell will swell. H2O H2O Water Molecule Dissolved Molecule

9. Section 8.1 Summary – pages 195 - 200 Cells in a hypertonic solution • More water leaves a cell by osmosis. • In a hypertonic solution, cells shrink. H2O H2O Water Molecule Dissolved Molecule

10. Isotonic solution Hypertonic solution Hypotonic solution

11. Figure 4-18a-cPage 85 250 µm 250 µm 250 µm Water out Cell wall Plasma membrane Water in Vacuole Plasma membrane Nucleus Hypertonic solution Isotonic solution Hypotonic solution

12. Section 8.1 Summary – pages 195 - 200 Passive Transport • Passive transport occurs when a cell uses no energy to move particles across a membrane. Concentration gradient Plasma membrane

13. Section 8.1 Summary – pages 195 - 200 Passive Transport by proteins • Passive transport of materials across the membrane using transport proteins is called facilitated diffusion. • Facilitated diffusion is accomplished by two types of transport proteins: carriers and channels.

14. Section 8.1 Summary – pages 195 - 200 Transport Proteins Click image to view movie.

15. Section 8.1 Summary – pages 195 - 200 Passive Transport by proteins • Channel proteins form holes that allow specific molecules to flow through the membrane. • The movement is with the concentration gradient, and requires no energy input from the cell. Channel proteins Concentration gradient Plasma membrane

16. Section 8.1 Summary – pages 195 - 200 Passive transport by proteins • Carrier proteins change shape to allow substances to pass through the plasma membrane. • Again, the movement is with the concentration gradient and requires no energy input from the cell. Carrier proteins Concentration gradient Plasma membrane Step 1 Step 2

17. Section 8.1 Summary – pages 195 - 200 Active Transport • Active Transport - movement of materials through a membrane against a concentration gradient. • Requires energy from the cell. Carrier proteins Concentration gradient Plasma membrane Cellular energy Step 1 Step 2

18. Section 8.1 Summary – pages 195 - 200 How active transport occurs • Each carrier has a shape that fits a specific molecule or ion. Once a molecule binds to the carrier, chemical energy changes its shape moving the molecule from one side of the membrane to the other. Carrier proteins Concentration gradient Plasma membrane Cellular energy Step 1 Step 2

19. Section 8.1 Summary – pages 195 - 200 How active transport occurs Click image to view movie.

20. Limits to Cell Size • Diffusion limits cell size. • Many nutrients and wastes, e.g. glucose, O2 and CO2, move by diffusion. • Cells need large surface area-to-volume ratios in order to regulate the internal environment.

21. Surface area-to-volume ratio Section 8.2 Summary – pages 201 - 210 4 mm 4 mm 2 mm 1 mm 2 mm 1 mm 1 mm 2 mm Surface area = 6 mm2 Volume = 1 mm3 4 mm Surface area = 24 mm2 Volume = 8 mm3 • As a cell’s size increases, its volume increases much more than its surface area.

22. Surface area-to-volume Ratios • For a giant cell 20 cm in diameter, oxygen molecules would take more than 200 days to diffuse to the center of the cell! • How does surface area-to-volume ratio relate to the shape of the inner mitochondrial membrane?