A Closer Look at Cell Membranes

1. A Closer Look at Cell Membranes Chapter 5 Part 1

2. Impacts, Issues:One Bad Transporter and Cystic Fibrosis • Transporter proteins regulate the movement of substances in and out of cells; failure of one of these proteins causes cystic fibrosis

3. 5.1 Organization of Cell Membranes • The basic structure of all cell membranes is the lipid bilayer with many embedded proteins • A membrane is a continuous, selectively permeable barrier

4. Revisiting the Lipid Bilayer • Phospholipid molecules in the plasma membrane have two parts • Hydrophilic heads interact with water molecules • Hydrophobic tails interact with each other, forming a barrier to hydrophilic molecules • Study Figure 5.2a

5. The Fluid Mosaic Model • Fluid mosaic model • Describes the organization of cell membranes • The bilayer is a mosaic mixture of phospholipids, steroids, proteins, and other molecules • Predominantly proteins floating in lipids

6. 5.2 Membrane Proteins Plasma Membrane aka Cell membrane • Cell membrane function begins with the many proteins associated with the lipid bilayer • Most of the functions are carried out by proteins

7. Membrane Proteins • Each type of protein in a membrane has a special function – Know the names of each and functions • Adhesion proteins- connects cells • Recognition proteins – recognizes self from non-self • Receptor proteins – binds water soluble hormones (signaling molecules) • Enzymes – increases/speeds chemical reactions • Transport proteins (active and passive/channel allows large and small molecules to cross.)

8. Common Types of Membrane Proteins

9. Animation: Cell membranes

10. 5.3 Diffusion, Membranes, and Metabolism • Ions and molecules tend to move from one region to another, in response to gradients

11. Membrane Permeability • Selective permeability • The ability of a cell membrane to control which substances and how much of them enter or leave the cell

12. A Gases (such as oxygen and carbon dioxide), small nonpolar molecules, and water cross a bilayer freely. B Other solutes (molecules and ions) cannot cross a lipid bilayer on their own. Lipid Bilayer Fig. 5-6, p. 82

13. Animation: Selective permeability

14. Concentration Gradients • Concentration gradient • The difference in concentration between two adjacent regions • Molecules move from a region of higher concentration to one of lower concentration • Stops/ceases to exist when there is no net movement of molecules

15. Diffusion • Diffusion • The net movement of molecules down a concentration gradient • A substance diffuses in a direction set by its own concentration gradient

16. The Rate of Diffusion • Rate of diffusion depends on the following factors • Size/charge • Temperature and pressure • Steepness of the concentration gradient • The movement is away from the region of highest conc. • Movement of individual molecules is random • The movement of molecules or substances move independently of each other

17. How Substances Cross Membranes • Gases (oxygen, carbon dioxide) and nonpolar molecules diffuse freely across a lipid bilayer via diffusion • Ions and large polar molecules require other mechanisms to cross the cell membrane • Passive transport AKA Facilitated diffusion • Active transport • Endocytosis and exocytosis • Study Figure 5.8 A-E

18. Extracellular Fluid Cytoplasm B Passive Transport A solute moves across bilayer through interior of passive transporter; movement is driven by concentration gradient. STUDY THIS FIGURE C Active TransportActive transporter uses energy (often, ATP) to pump a solute through bilayer against its concentration gradient. A Diffusion A substance simply dif- fuses across lipid bilayer. Fig. 5-8 (a-c), p. 83

19. D Endocytosis Vesicle movement brings substances in bulk into cell. E Exocytosis Vesicle movement ejects substances in bulk from cell. STUDY THIS FIGURE!!!!!!! Fig. 5-8 (d-e), p. 83

20. 5.4 Passive and Active Transport • Many types of molecules and ions diffuse across a lipid bilayer only with the help of a specific transport protein

21. Passive Transport • Passive transport aka (facilitated diffusion) • Requires no energy input • A passive transport protein allows a specific solute (such as glucose) to follow its concentration gradient across a membrane • A gated passive transporter changes shape when a specific molecule binds to it

22. Active Transport • Active transport • Requires the expenditure of energy or energy input (usually ATP) • Moves a solute against its concentration gradient, to the concentrated side of the membrane • Calcium pumps (Example) • Active transporters move calcium ions across muscle cell membranes into the sarcoplasmic reticulum • Na+-K+ Pump (Example) (Co-transporter)

23. calcium Sarcoplasmic Reticulum Cytoplasm Calcium ions bind to a calcium transporter (calcium pump). Calcium moves from the intracellular fluid (inside) to the extracellular fluid (outside the cell) A Fig. 5-10a, p. 85

24. ATP energy B A phosphate group is transferred from ATP to the pump. The pump changes shape so that it ejects the calcium ions to the opposite side of the membrane, and then resumes its original shape. Fig. 5-10b, p. 85

25. Cotransport • Cotransporter • An active transport protein that moves two substances across a membrane at the same time • Example: The sodium-potassium pump

26. Cotransport: Sodium-Potassium Pump