Understanding Diffusion and Membrane Crossings in Biology

1. Chapter 5.3 Diffusion, Membranes, and Metabolism AP Biology Fall 2010

2. Bell Ringer

3. Objectives • Differentiate between the factors that influence the rate and direction of diffusion • Understand how substances cross membranes • Know what a concentration gradient is • State the definition of diffusion

4. What Is A Concentration Gradient? • Concentration Gradient: difference in the number of molecules or ions of a substance in a given volume of fluid between two adjoining regions • In absence of other forces, molecules move from a region where they are more concentrated to a region where they are not as concentrated

5. What Is A Concentration Gradient? • Thermal energy keeps molecules in motion • Collide at random and bounce off each other • Happens more in regions where molecules are most concentrated  net movement is toward the region where they are not colliding

6. What Is A Concentration Gradient? • Molecules flow down their concentration gradient • High concentration  low concentration

7. What Is A Concentration Gradient? • Diffusion: net movement of like molecules or ions down a concentration gradient • From high to low concentration • Each substance diffuses independently (ex. Dye molecules in water)

8. The Rate of Diffusion • Size • Smaller molecules diffuse faster than larger ones • Smaller = faster

9. The Rate of Diffusion • Temperature • More heat energy makes molecules move faster • Higher = faster

10. The Rate of Diffusion • Steepness of the concentration gradient • Rates are high with steep gradients

11. The Rate of Diffusion • Charge • A difference in electric charge between adjoining regions • Ex. Each ion dissolved in fluids bathing a cell membrane contributes to a local electric charge • Opposite charges attract • The fluid having more negative charge overall exerts the greatest pull on positively charged substances

12. The Rate of Diffusion • Pressure • Difference in exerted force per unit areas in two adjoining areas

13. The Rate of Diffusion • Dynamic Equilibrium: when gradients no longer exist there is no net movement • Still movement, but “equal”

14. Membrane Crossing Mechanisms • Selective permeability: allow some substances but not others to enter and leave a cell • All cell membranes structured to show this • Control when and how much crosses

15. Membrane Crossing Mechanisms • Lipids and nonpolar molecules pass easily through cell membrane • Glucose and other large polar molecules cannot pass through the bilayer directly • Rely on passage through interior of transport proteins

16. Membrane Crossing Mechanisms • Passive transport- material passes through interior of transport proteins without use of energy • Known as facilitated diffusion • Active transport- proteins become activated to move a solute against its concentration gradient

17. Membrane Crossing Mechanisms • Substances move in bulk across cell membrane by exocytosis and endocytosis • Endocytosis: vesicle forms around particles when a patch of plasma membrane sinks inward and seals back on itself • Exocytosis: vesicle that formed in the cytoplasm fuses with the plasma membrane, so that its contents are released to the outside

18. Membrane Crossing Mechanisms

19. Membrane Permeability • Cells keep extracellular fluid contents separate from the contents of the cell with membranes that are selectively permeable • Raw materials enter the cell to be used in metabolism • Wastes are expelled from the cell into the extracellular fluid

20. Membrane Permeability • Cell volume is adjusted and maintained within normal ranges as the environment around the cell changes • pH is adjusted to maintain homeostasis by movement of substances into and out of the cell

21. Review • T/F With diffusion, molecules move from areas of low concentration to high concentration. • T/F Endocytosis is the process of moving large particles out of the cell. • T/F With dynamic equilibrium, there is a net movement of molecules.

22. Answers • False • False • False