1. Chapter 7~ Membrane Structure & Function

2. What You Must Know: Why membranes are selectively permeable.
The role of phospholipids, proteins, and carbohydrates in membranes.
How water will move if a cell is placed in an isotonic, hypertonic, or hypotonic solution.
How electrochemical gradients are formed.

3. Early membrane model (1935) Davson/Danielli – Sandwich model
phospholipid bilayer between 2 protein layers
Problems: varying chemical composition of membrane, hydrophobic protein parts

4. Membrane structure, I Selective permeability
Amphipathic~ hydrophobic & hydrophilic regions
Singer-Nicolson: fluid mosaic model

5. The freeze-fracture method: revealed the structure of membrane’s interior

6. Fluid Mosaic Model

7. Membrane structure, II Phospholipids~ membrane fluidity
Cholesterol~ membrane stabilization
“Mosaic” Structure~
Membrane carbohydrates ~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins
Integral proteins~ transmembrane proteins
Peripheral proteins~ surface of membrane
Eg. blood transfusions are type-specific

8. Synthesis and sidedness of membranes

9. Membrane Proteins Integral Proteins Peripheral Proteins
Embedded in membrane
Determined by freeze fracture
Transmembrane with hydrophilic heads/tails and hydrophobic middles
Extracellular or cytoplasmic sides of membrane
NOT embedded
Held in place by the cytoskeleton or ECM
Provides stronger framework

10. Transmembrane protein structure
Hydrophobic interior
Hydrophilic ends

11. Integral & Peripheral proteins

12. Membrane fluidity
Low temps: phospholipids w/unsaturated tails (kinks prevent close packing)
Cholesterol resists changes by:
limit fluidity at high temps
hinder close packing at low temps
Adaptations: bacteria in hot springs (unusual lipids); winter wheat ( unsaturated phospholipids)
Stays fluid even in cold temperatures.

13. Membrane structure, III
Membrane protein function: •transport •enzymatic activity •signal transduction •intercellular joining •cell-cell recognition •ECM attachment

14. Membrane traffic
Diffusion~ tendency of any molecule to spread out into available space
Concentration gradient
Passive transport~
NO ENERGY (ATP) needed!
Diffusion down concentration gradient (high  low concentration)
Osmosis~ the diffusion of water across a selectively permeable membrane

15. Water balance Isotonic~ equal concentrations of solutes
Osmoregulation~ control of water balance
Isotonic~ equal concentrations of solutes
Hypertonic~ higher concentration of solutes
Hypotonic~ lower concentration of solutes
Cells with Walls:
Turgid (very firm)
Flaccid (limp)
Plasmolysis~ plasma membrane pulls away from cell wall

16. Osmoregulation Control solute & water balance
Contractile vacuole: “bilge pump” forces out fresh water as it enters by osmosis
Eg. paramecium caudatum – freshwater protist

17. Tonicity is a relative term
Isotonic Solution - both solutions have same concentrations of solute.
10% ions
in water
If the outside solution is isotonic: Concentration of ions and water is equal on both sides of the cell membrane.
10% ions
in cell
H2O moves in both directions
K. White

18. Tonicity is a relative term
Hypertonic Solution - one solution has a higher concentration of solute than another.
20% ions
in water
If the outside solution is hypertonic: Concentration of salt is lower and water concentration is higher inside the cell than the solution outside the cell. Water will move out of the cell.
10% ions
in cell
H2O
H2O moves out of the cell
K.White

19. Tonicity is a relative term
Hypotonic Solution - One solution has a lower concentration of solute than another.
10% ions
in water
If the outside solution is hypotonic: concentration of salt is higher and water concentration is lower inside the cell than outside. Water will move into the cell.
H2O
H2O
H2O
H2O
20% ions
in cell
H2O moves into the cell
K. White

20. Specialized Transport
Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient
Transport proteins (channel or carrier proteins) help hydrophilic substances cross
Two ways:
Provide hydrophilic channel
Loosely bind/carry molecule across
Eg. ions, polar molecules (H2O, glucose)
Active transport~ movement of a substance against its concentration gradient with the help of cellular energy

21. Aquaporin: channel protein that allows passage of H2O

22. Glucose Transport Protein (carrier protein)

23. Specialized Transport
Active transport~
Proteins transport substances against concentration gradient (low  high conc)
Requires ENERGY (ATP)
Eg. Na+/K+ pump, proton pump

24. Membrane potential cytoplasm is negative compared to ECM
Unequal distribution of cations and anions
-50 to -200 mV
Membrane potential acts like a battery
Helps determine movement of cations/anions in and out of cell
electrochemical gradient – movement of cations/anions affected by their concentration as well as charge

25. Electrogenic Pumps: generate voltage across membrane
Na+/K+ Pump
Proton Pump
Pump Na+ out, K+ into cell
Nerve transmission
Push protons (H+) across membrane
Eg. mitochondria (ATP production)

26. Eg. sucrose-H+ cotransporter (sugar-loading in plants)
Cotransport: membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other
Eg. sucrose-H+ cotransporter (sugar-loading in plants)

27. Bulk Transport Transport of proteins, polysaccharides, large molecules
Endocytosis: take in macromolecules, form new vesicles
Exocytosis: vesicles fuse with cell membrane, expel contents

28. BulkTransport Endocytosis~ take in macromolecules, form new vesicles
Bulk transport -Transport of proteins, polysaccharides, large molecules
Endocytosis~ take in macromolecules, form new vesicles
Exocytosis~ vesicles fuse with cell membrane, expel contents

29. Types of Endocytosis Phagocytosis: “cellular eating” - solids
Pinocytosis:
“cellular drinking” - fluids
Receptor-Mediated Endocytosis:
Ligands bind to specific receptors on cell surface

30. Passive vs. Active Transport
Little or no Energy
High  low concentrations
DOWN the concentration gradient
eg. diffusion, osmosis, facilitated diffusion (w/transport protein)
Requires Energy (ATP)
Low  high concentrations
AGAINST the concentration gradient
eg. pumps, exo/endocytosis