1. MembraneStructure & Function
Fluid mosaic model

2. Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Transmembrane
proteins
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.
Phospholipids
Filaments of cytoskeleton
Cholesterol
Peripheral
protein
Cytoplasm

3. Phospholipids Phosphate head Fatty acid tails Arranged as a bilayer
“attracted to water”
Phosphate head
hydrophilic
Fatty acid tails
hydrophobic
Arranged as a bilayer
Fatty acid
“repelled by water”

4. Arranged as a Phospholipid bilayer
Serves as a cellular barrier / border
sugar
H2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
lipids

5. Permeability to polar molecules?
Membrane becomes semi-permeable via protein channels
specific channels allow specific material across cell membrane
inside cell
H2O aa
sugar
salt
outside cell
NH3

6. Proteins domains anchor molecule
Within membrane
nonpolar amino acids
hydrophobic
On outer surfaces of membrane in fluid
polar amino acids
hydrophilic
Polar areas
of protein
Nonpolar areas of protein

7. H+
NH2 H+
COOH
Cytoplasm
Retinal
chromophore
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
Examples
aquaporin = water channel in bacteria
Porin monomer
b-pleated sheets
Bacterial
outer
membrane
H2O H+
proton pump channel in photosynthetic bacteria
function through conformational change = protein changes shape
H2O

8. Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme activity
Cell surface receptor
“Antigen”
Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.
Cell surface identity marker
Cell adhesion
Attachment to the cytoskeleton

9. Membrane Proteins Classes of membrane proteins: peripheral proteins
loosely bound to surface of membrane
ex: cell surface identity marker (antigens)
integral proteins
penetrate lipid bilayer, usually across membrane
transmembrane protein
ex: transport proteins
channels, permeases (pumps)

10. Membrane carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish one cell from another
antigens
basis for rejection of foreign cells by immune system
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

11. Movement across the Cell Membrane

12. Simple Diffusion Move from HIGH to LOW concentration movement of water
“passive transport”
no energy needed
Small, nonpolar molecules
movement of water
diffusion
osmosis

13. Facilitated Diffusion
Diffusion through protein channels
Large and/or polar molecules (like water)
no energy needed
facilitated = with help
open channel = fast transport
HIGH
LOW
Donuts!
Each transport protein is specific as to the substances that it will translocate (move).
For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.
Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.
These channel proteins allow fast transport.
For example, water channel proteins, aquaporins, facilitate massive amounts of diffusion.
“The Bouncer”

14. Active Transport
Cells may need to move molecules against concentration gradient (low to high)
protein “pump”
“costs” energy = ATP
conformational change
LOW
HIGH
Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport.
ATP
“The Doorman”

15. Transport summary simple diffusion facilitated diffusion
ATP
active transport

16. How about large molecules?
Moving large molecules into & out of cell
through vesicles & vacuoles
Endocytosis~ in
phagocytosis = “cellular eating”
Exocytosis ~out
exocytosis

17. The Special Case of Water Movement of water across the cell membrane

18. Concentration of water
Direction of osmosis is determined by comparing total solute concentrations
Hypertonic - more solute, less water
Hypotonic - less solute, more water
Isotonic - equal solute, equal water
hypotonic
hypertonic
water
net movement of water

19. Managing water balance
Cell survival depends on balancing water uptake & loss
freshwater
balanced
saltwater

20. Pumping water out
Contractile vacuole in Paramecium
ATP