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. and carbon dioxide), small nonpolar molecules, and
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
Figure 5.6
The selectively permeable nature of cell membranes. Small, nonpolar molecules, gases, and water molecules freely cross the lipid bilayer. Polar molecules and ions cross with the help of proteins that span the bilayer.
Fig. 5-6, p. 82

13. Animation: Selective permeability

14. Concentration Gradients
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. A Diffusion A substance simply dif- fuses across lipid bilayer.
Extracellular Fluid
Cytoplasm
Figure 5.8
Overview of membrane-crossing mechanisms.
A Diffusion A substance simply dif- fuses across lipid bilayer.
B Passive Transport
A solute moves across bilayer through interior of passive transporter; movement is driven by concentration gradient.
STUDY THIS FIGURE
C Active Transport Active transporter uses energy (often, ATP) to pump a solute through bilayer against its concentration gradient.
Fig. 5-8 (a-c), p. 83

19. D Endocytosis Vesicle movement brings substances in bulk into cell.
Figure 5.8
Overview of membrane-crossing mechanisms.
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 Calcium pumps (Example)
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. Sarcoplasmic Reticulum
calcium
Sarcoplasmic Reticulum
Cytoplasm
Figure 5.10
Active transport. This model shows a calcium transporter. After two calcium ions bind to the transporter, ATP transfers a phosphate group to it, thus providing energy that drives the movement of calcium against its concentration gradient across the cell membrane. A
Calcium ions bind to a calcium transporter (calcium pump).
Calcium moves from the intracellular fluid (inside) to the extracellular fluid (outside the cell)
Fig. 5-10a, p. 85

24. energy
ATP
Figure 5.10
Active transport. This model shows a calcium transporter. After two calcium ions bind to the transporter, ATP transfers a phosphate group to it, thus providing energy that drives the movement of calcium against its concentration gradient across the cell membrane.
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