2. Phospholipids
ī§ Phosphate head
īĩ
âattracted to waterâ
hydrophilic
ī§ Fatty acid tails
īĩ
Phosphate
hydrophobic
ī§ Arranged as a bilayer
Fatty acid
ârepelled by waterâ
Aaaah,
one of those
structureâfunction
examples
AP Biology
3. Arranged as a Phospholipid bilayer
ī§ Serves as a cellular barrier / border
sugar
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
H2O
salt
impermeable to polar molecules
polar
hydrophilic
heads
waste
AP Biology
lipids
4. Cell membrane defines cell
ī§ Cell membrane separates living cell from
aqueous environment
īĩ
thin barrier = 8nm thick
ī§ Controls traffic in & out of the cell
īĩ
allows some substances to cross more
easily than others
ī§ hydrophobic (nonpolar) vs. hydrophilic (polar)
AP Biology
5. Permeability to polar molecules?
ī§ Membrane becomes semi-permeable via
protein channels
īĩ
specific channels allow specific material
across cell membrane
inside cell
NH3
AP Biology
salt
H2O
aa
sugar
outside cell
6. Cell membrane is more than lipidsâĻ
ī§ Transmembrane proteins embedded in
phospholipid bilayer
īĩ
create semi-permeabe channels
lipid bilayer
membrane
AP Biology
protein channels
in lipid bilyer membrane
7. Why are
proteins the perfect
molecule to build structures
in the cell membrane?
AP Biology
2007-2008
8. Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
AP Biology
9. Classes of amino acids
What do these amino acids have in common?
I like the
polar ones
the best!
AP Biology
polar & hydrophilic
10. Proteins domains anchor molecule
ī§ Within membrane
īĩ
Polar areas
of protein
nonpolar amino acids
ī§ hydrophobic
ī§ anchors protein
into membrane
ī§ On outer surfaces of
membrane in fluid
īĩ
polar amino acids
ī§ hydrophilic
ī§ extend into
AP Biology
extracellular fluid &
into cytosol
Nonpolar areas of protein
11. H+
H
+
Examples
Retinal
chromophore
NH2
aquaporin =
water channel in bacteria
Porin monomer
H2O
β-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
Îą-helices in the
cell membrane
COOH
H
H+
+
Cytoplasm
proton pump channel
in photosynthetic bacteria
HO
AP Biology 2
function through
conformational change =
protein changes shape
12. Many Functions of Membrane Proteins
âChannelâ
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell adhesion
Attachment to the
cytoskeleton
âAntigenâ
AP Biology
Cell surface
identity marker
13. Membrane Proteins
ī§ Proteins determine membraneâs specific functions
īĩ
cell membrane & organelle membranes each have
unique collections of proteins
ī§ Classes of membrane proteins:
īĩ
peripheral proteins
ī§ loosely bound to surface of membrane
ī§ ex: cell surface identity marker (antigens)
īĩ
integral proteins
ī§ penetrate lipid bilayer, usually across whole membrane
ī§ transmembrane protein
AP Biology
ī§ ex: transport proteins
īˇ channels, permeases (pumps)
14. Cell membrane must be more than lipidsâĻ
ī§ In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
Itâs like a fluidâĻ
Itâs like a mosaicâĻ
Itâs the
Fluid Mosaic Model!
AP Biology
15. Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
AP Biology
Cytoplasm
Transmembrane
proteins
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
16. Membrane carbohydrates
ī§ Play a key role in cell-cell recognition
īĩ
ability of a cell to distinguish one cell
from another
ī§ antigens
important in organ &
tissue development
īĩ basis for rejection of
foreign cells by
immune system
īĩ
AP Biology
19. Diffusion
ī§ 2nd Law of Thermodynamics
governs biological systems
īĩ
universe tends towards disorder (entropy)
ī§ Diffusion
īĩ
AP Biology
movement from HIGH â LOW concentration
20. Simple Diffusion
ī§ Move from HIGH to LOW concentration
âpassive transportâ
īĩ no energy needed
īĩ
AP Biology
diffusion
movement of water
osmosis
21. Facilitated Diffusion
ī§ Diffusion through protein channels
īĩ
īĩ
channels move specific molecules across
cell membrane
facilitated = with help
no energy needed
open channel = fast transport
HIGH
LOW
AP Biology
âThe Bouncerâ
22. Active Transport
ī§ Cells may need to move molecules against
concentration gradient
īĩ
īĩ
īĩ
conformational shape change transports solute
from one side of membrane to other
protein âpumpâ
âcostsâ energy = ATP LOW conformational change
ATP
HIGH
AP Biology
âThe Doormanâ
24. Getting through cell membrane
ī§ Passive Transport
īĩ
Simple diffusion
ī§ diffusion of nonpolar, hydrophobic molecules
īˇ lipids
īˇ HIGH â LOW concentration gradient
īĩ
Facilitated transport
ī§ diffusion of polar, hydrophilic molecules
ī§ through a protein channel
īˇ HIGH â LOW concentration gradient
ī§ Active transport
īĩ
diffusion against concentration gradient
ī§ LOW â HIGH
īĩ
īĩ
AP Biology
uses a protein pump
requires ATP
ATP
26. How about large molecules?
ī§ Moving large molecules into & out of cell
through vesicles & vacuoles
īĩ endocytosis
īĩ
ī§ phagocytosis = âcellular eatingâ
ī§ pinocytosis = âcellular drinkingâ
īĩ
AP Biology
exocytosis
exocytosis
28. The Special Case of Water
Movement of water across
the cell membrane
AP Biology
2007-2008
29. Osmosis is just diffusion of water
ī§ Water is very important to life,
ī§
so we talk about water separately
Diffusion of water from
HIGH concentration of water to
LOW concentration of water
īĩ
AP Biology
across a
semi-permeable
membrane
30. Concentration of water
ī§ Direction of osmosis is determined by
comparing total solute concentrations
īĩ
Hypertonic - more solute, less water
īĩ
Hypotonic - less solute, more water
īĩ
Isotonic - equal solute, equal water
water
AP Biology
hypotonic
hypertonic
net movement of water
31. Managing water balance
ī§ Cell survival depends on balancing
water uptake & loss
AP Biology
freshwater
balanced
saltwater
32. 1
Managing water balance
ī§ Hypotonic
īĩ
a cell in fresh water
īĩ
high concentration of water around cell
ī§ problem: cell gains water,
swells & can burst
ī§ example: Paramecium
īˇ ex: water continually enters
KABOOM!
Paramecium cell
ī§ solution: contractile vacuole
īˇ pumps water out of cell
ATP
īˇ ATP
īĩ
AP Biology
plant cells
No problem,
here
ī§ turgid = full
ī§ cell wall protects from bursting
freshwater
34. 2
Managing water balance
ī§ Hypertonic
Iâm shrinking,
a cell in salt water Iâm shrinking!
īĩ low concentration of water
around cell
īĩ
ī§ problem: cell loses water &
can die
ī§ example: shellfish
ī§ solution: take up water or
pump out salt
I
īĩ
AP Biology
plant cells
ī§ plasmolysis = wilt
ī§ can recover
will
survive!
saltwater
35. 3
Managing water balance
ī§ Isotonic
Thatâs
perfect!
īĩ
animal cell immersed in
mild salt solution
īĩ
no difference in concentration of
water between cell & environment
ī§ problem: none
īˇ no net movement of water
flows across membrane equally, in
both directions
I could
īˇ cell in equilibrium
be betterâĻ
īˇ
īŽ
volume of cell is stable
ī§ example:
blood cells in blood plasma
īˇ slightly salty IV solution in hospital
AP Biology
balanced
36. 1991 | 2003
Aquaporins
ī§ Water moves rapidly into & out of cells
īĩ
evidence that there were water channels
ī§ protein channels allowing flow of water
across cell membrane
AP Biology
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
37. Do you understand OsmosisâĻ
.05 M
.03 M
Cell (compared to beaker) â hypertonic or hypotonic
Beaker (compared to cell) â hypertonic or hypotonic
AP Biology
Which way does the water flow? â in or out of cell
40. Diffusion through phospholipid bilayer
ī§ What molecules can get through directly?
īĩ
fats & other lipids
inside cell
NH3
ī§ What molecules can
lipid
salt
NOT get through
directly?
īĩ
polar molecules
ī§ H2O
īĩ
outside cell
sugar aa
H2O
ions (charged)
ī§ salts, ammonia
īĩ
large molecules
ī§ starches, proteins
AP Biology
41. Membrane fat composition varies
ī§ Fat composition affects flexibility
īĩ
membrane must be fluid & flexible
ī§ about as fluid as thick salad oil
īĩ
% unsaturated fatty acids in phospholipids
ī§ keep membrane less viscous
ī§ cold-adapted organisms, like winter wheat
īˇ increase % in autumn
īĩ
AP Biology
cholesterol in membrane
42. Diffusion across cell membrane
ī§ Cell membrane is the boundary between
inside & outsideâĻ
īĩ
separates cell from its environment
Can it be an impenetrable boundary?
NO!
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
AP Biology
OUT
IN
waste
ammonia
salts
CO2
H2O
products
cell needs materials in & products or waste out
Editor's Notes
{"16":"The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.\n","22":"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.\n","23":"Plants: nitrate & phosphate pumps in roots.\nWhy?\nNitrate for amino acids\nPhosphate for DNA & membranes\nNot coincidentally these are the main constituents of fertilizer\nSupplying these nutrients to plants\nReplenishing the soil since plants are depleting it\n","12":"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.\n","19":"Movement from high concentration of that substance to low concentration of that substance.\n","8":"<number>\n","9":"<number>\n","15":"The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.\n","21":"Donuts!\nEach transport protein is specific as to the substances that it will translocate (move).\nFor example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.\nSome 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.\nThese channel proteins allow fast transport.\nFor example, water channel proteins, aquaporins, facilitate massive amounts of diffusion.\n"}