1. Membranes

2. Outline
Phospholipid Bilayer
Fluid Mosaic Model
Membrane Proteins
Diffusion
Facilitated Diffusion
Osmosis
Osmotic Balance
Bulk Transport
Active Transport

3. Nonpolar (hydrophobic) region
Phospholipid Bilayer
Phospholipid has two fatty-acid chains attached to its backbone.
One end is strongly nonpolar while the other end is strongly polar.
polar head oriented toward water and nonpolar tails oriented away from water
bilayer is stable because water’s affinity for hydrogen bonding never stops
Fatty acid
Phosphorylated
alcohol
Polar
(hydrophilic) region
Nonpolar (hydrophobic) region G L Y C E R O

4. Fluid Mosaic Model
Plasma membrane is composed of both lipids and globular proteins.
Membrane proteins are not very soluble in water.

5. Cell Membrane
Cell membrane components
phospholipid bilayer
transmembrane proteins
interior protein network
cell surface markers
extra cellular matrix (ECM)

6. Membrane Proteins transporters enzymes cell surface identity markers
cell adhesion proteins
attachments to cytoskeleton

7. Plasma Membrane Proteins

8. Membrane Protein Structure
Anchoring proteins in the bilayer
Transmembrane proteins
Cytoplasmic side
of cell membrane
Cytoskeletal
proteins
Junctional
complex
100 nm
Actin
Spectrin
Linker
protein

9. Membrane Protein Structure
Transmembrane proteins
single-pass anchors
multiple-pass channels and carriers
pores
Phospholipids
Polar areas
of protein
Cholesterol
Nonpolar areas of
protein
NH2 H+
COOH
Cytoplasm
Retinal
chromophore
Nonpolar
(hydrophobic)
a-helices in the
cell membrane

10. Solutions Intra cellular fluid (ICF) - within cells
Extra cellular Fluid (ECF) - outside cells
Inter cellular = tissue fluid = interstitial fluid
Plasma = fluid portion of blood
Composition of fluids change as substances move between compartments
nutrients, oxygen, ions and wastes move in both directions across capillary walls and cell membranes

11. Selective Permeability of Membrane
Lipid bilayer
permeable to nonpolar, uncharged molecules -- oxygen, CO2, steroids
Transmembrane proteins act as specific channels
small and medium polar & charged particles
Macromolecules unable to pass through the membrane
vesicular transport

12. Passive Transport
Diffusion
Random motion causes a net movement of substances from regions of high concentration to regions of low concentration – down their concentration gradient. Animation

13. Diffusion Rates
Factors affecting diffusion rate through a membrane
temperature -  temp.,  motion of particles
molecular weight - larger molecules move slower
steepness of concentrated gradient - difference,  rate
membrane surface area -  area,  rate
membrane permeability -  permeability,  rate

14. Selective Membrane Transport
Cell transport channels are selectively permeable, as only certain molecules are allowed to enter.
Ions cannot move between the cytoplasm of a cell and the extracellular fluid without the assistance of membrane transport proteins.
ion channels

15. Facilitated Diffusion
Carriers transport ions and other solutes across the plasma membrane.
Facilitate movement by physically binding molecules on one side of the membrane, and releasing them on the other down their concentration gradient. animation
essential characteristics
specific
passive (no ATP)
transport saturation
Outside of cell
Inside of cell

16. Osmosis
In an aqueous solution, both water and solutes diffuse down concentration gradients.
Net movement of water through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration
Only occurs if membrane is permeable to water but not to certain solutes
Aquaporins = channel proteins specialized for osmosis

17. Osmotic Pressure
Amount of hydrostatic pressure required to stop osmosis
Osmosis slows due to filtration of water back across membrane due to increased hydrostatic pressure
Animation

18. Tonicity
Tonicity - ability of a solution to affect fluid volume and pressure within a cell
depends on concentration and permeability of solute
Hypotonic (hypoosmotic) solution
low concentration of nonpermeating solutes (high water concentration)
cells absorb water, swell and may burst (lyse)
Hypertonic solution (hyperosmotic)
has high concentration of nonpermeating solutes (low water concentration)
cells lose water + shrink (crenate)
Isotonic (isoosmotic) solution = normal saline

19. Maintaining Osmotic Balance

20. Osmotic Pressure
Hydrostatic pressure - pressure of the cytoplasm pushing out against the cell membrane
Osmotic pressure - pressure that must be applied to stop the osmotic movement of water across a membrane

21. Bulk Transport Endocytosis - enveloping food
phagocytosis - material taken in is in particulate form
pinocytosis - material taken in is in liquid form
receptor-mediated - transport of specific molecules
Exocytosis - discharge of material from vesicles at the cell surface

23. Active Transport
Active transport involves the expenditure of energy to move substance against their concentration gradient.
involves highly selective protein carriers within the membrane
sodium-potassium pump
Secondary active transport
coupled transport - using energy stored in a gradient of a different molecule

25. Secondary Active Transport
Uniporter
carries only one solute at a time
Symporter
carries 2 or more solutes simultaneously in same direction (cotransport)
Antiporter
carries 2 or more solutes in opposite directions (countertransport) (animation)
sodium-potassium pump brings in K+ and removes Na+ from cell
Any carrier type can use either facilitated diffusion or active transport (primary or secondary)

27. Digitalis
Slows the sodium pump, which lets more Na+ accumulate in the heart muscle cells.
Less Na+ concentration gradient across the membrane
Na+/Ca+2 antiporters slow down so more Ca+2 remains inside the cardiac cells
Strengthening the force of contraction
Balance between concentration of Na+ and Ca+2 in cytosol & extracellular fluid is important

28. Summary
Phospholipid Bilayer
Fluid Mosaic Model
Membrane Proteins
Diffusion
Facilitated Diffusion
Osmosis
Osmotic Balance
Bulk Transport
Active Transport