1. Membranes and Transport Unit 4

2. What does the Membrane Do?
Support keeps cell shape
Transport moves material in and out of the cell
Recognition receives info on material around the cell
Communication send info to outside of cell
Adherence stick cell to other materials; hold it in place

3. Membrane Structure What are the parts?
1) Phospholipids hydrophilic and hydrophobic ends; 2 layers
2) Sterols non-polar rings and polar alcohol groups; cholesterol
3) Embedded proteins Transport proteins and enzymes
4) Glycolipids and glycoproteins
protective coat or communication (act like receptors)

4. Fluid Mosaic Model
Membrane and it proteins are in a semi-fluid state which allows proteins to move freely in all directions
How do cells maintain fluidity at low temps?
More unsaturated fatty acid chains
Raise cholesterol levels
How do cells maintain stability at high temps?
Cholesterol reduces fluidity of the membrane; nonpolar region keep nonpolar tails close together

5. How do we know it is fluid?
Frye and Edidin; 1970
Created antibodies for human (glow red) and mice (glow green) proteins
Mixed human and cell membranes and watched the red and green lights over time
After 40 mins the lights went from half an half to completely diffused around the membrane
Found membrane fluidity is similar to machine oil

6. Membrane Proteins 4 Types of proteins:
Transport move H2O, ions, and molecules
Recognition cell recognize each other in a system; communication
Receptor bind to signal molecules from the environment
Adhesion bind cells to each other or other materials

7. Two Structures
Integral cross through both layers of the membrane; all 4 types
Peripheral bind to one side of the membrane, mostly the inside, through non-covalent bonds; cytoskeleton parts, glycolipids, and glycoproteins

8. How do we know these types exist?
Freeze faction experiments:
Cool cell down and cut apart membrane layer
Under an electron microscope, holes and dents in the membranes match like puzzle pieces

9. Why Are Cells SOOO Small?
Cell Size Limitations:
Diffusion limits cell size too big and it will take too long for things for move around the cell
DNA limits cell size Too big and it will take too long to make the proteins needed
Surface area-to-volume ratio Too big and the ratio between the surface area and cell size takes too long to take nutrients and release waste
6:1
3:1
96:64
1.5:1

10. Transportation through the Membrane
Transport of material is the most activity role of the membrane
Always must be:
Directional in or out
Specific only one type of cargo
2 Ways to Go:
1) Passive requires no energy by the cell
2) Active requires energy for at least one step

11. Passive Transport All movement “requires” energy
Where does the energy come from for passive transport?
Kinetic energy
Diffusion the net movement of molecules from a region of high conc. to a region of low conc. down a conc. gradient
Is it directional?
Yes, though molecules move in all directions, the over movement is from high to low
What happens when the gradient levels out?
Dynamic equilibrium

12. Simple Diffusion What can diffuse through the membrane?
Non-polar molecules can pass through hydrophobic barrier in the middle
Steroid hormones
Inorganic gases O2, N2, and CO2
H2O?
If water is polar why does it diffuse?
It is small enough to pass though the phospholipids but very slowly
Water needs a little help

13. Facilitated Diffusion
1) Aquaporins water channels
Billion/sec
2) Ion channels Na+, Ca2+, and K+
2 types:
Gated Channels have to be opened by a stimulus that changes proteins shape
Carrier Proteins doors that allow one solute at a time (uniport)
What is the rate limiter for facilitated diffusion?
saturation all the doors are full; can only go so fast

14. Osmosis
The movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
Movement controlled by water potential difference
3 solution types:
Hypotonic high water potential outside the cell; water flows into the cell (turgor pressure)
Hypertonic low water potential outside the cell; water flows out of the cell (plasmolysis)
Isotonic equal water potentials; water flows in and out at same rate

15. Active Transport
The energy-consuming transport of molecules or ions across a membrane against a conc. gradient made possible by transferring energy from respiration
3 main functions:
Brining important nutrients into the cell
Removing waster from the cell
Maintaining concentrations of ions across the membrane
What ions are most important?
H+, Na+, K+, and Ca2+
Ions create membrane potential electrical potential difference between sides of membranes

16. Active Transport Na+/K+ Pumps:
Essential for all animal cells and the nerve system of complex organisms
1 ATP 3 Na+ out and 2 K+ in
Generates membrane potentials from -20mV to – 200mV
Electrochemical gradient concentration gradient produces both a movement of chemicals and an electrical charge

17. Bulk Transport Exocytosis: What limits exocytosis rates? Endocytosis:
Remove waste material and release secretions
Vesicle fuses with plasma membrane
What limits exocytosis rates?
Cell size; plasma membrane can only grow to a certain point
Endocytosis:
Bulk-phase (pinocytosis)
Cell pulls in part of ECF through the vesicle; non-specific
What might trigger this?
Cytosol is too thick; lack of general nutrition; reduce plasma membrane

18. Bulk Transport Receptor mediated Endocytosis
Receptor protein in membrane binds to specific substrate
Receptors form coated pit after activation and pull in molecule
Bind to lysosome to breakdown contents
Phagocytosis “cell eating”
Cell consumes whole other bacteria cell for energy or defense (white blood cells)
Pinocytosis “cell drinking”
Cell consumes fluid from ECF