2. Phospholipids
Phosphate head
“attracted to water”
hydrophilic
Fatty acid tails
Phosphate
hydrophobic
Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah,
one of those
structure–function
examples
AP Biology
3. Arranged as a Phospholipid bilayer
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
Cell membrane separates living cell from
aqueous environment
thin barrier = 8nm thick
Controls traffic in & out of the cell
allows some substances to cross more
easily than others
hydrophobic (nonpolar) vs. hydrophilic (polar)
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5. Permeability to polar molecules?
Membrane becomes semi-permeable via
protein channels
specific channels allow specific material
across cell membrane
inside cell
NH3
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salt
H2O
aa
sugar
outside cell
6. Cell membrane is more than lipids…
Transmembrane proteins embedded in
phospholipid bilayer
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
Within membrane
Polar areas
of protein
nonpolar amino acids
hydrophobic
anchors protein
into membrane
On outer surfaces of
membrane in fluid
polar amino acids
hydrophilic
extend into
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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)
α-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
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell adhesion
Attachment to the
cytoskeleton
“Antigen”
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Cell surface
identity marker
13. Membrane Proteins
Proteins determine membrane’s specific functions
cell membrane & organelle membranes each have
unique collections of 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 whole membrane
transmembrane protein
AP Biology
ex: transport proteins
channels, permeases (pumps)
14. Cell membrane must be more than lipids…
In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
It’s like a fluid…
It’s like a mosaic…
It’s 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
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Cytoplasm
Transmembrane
proteins
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
16. Membrane carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish one cell
from another
antigens
important in organ &
tissue development
basis for rejection of
foreign cells by
immune system
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19. Diffusion
2nd Law of Thermodynamics
governs biological systems
universe tends towards disorder (entropy)
Diffusion
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movement from HIGH → LOW concentration
20. Simple Diffusion
Move from HIGH to LOW concentration
“passive transport”
no energy needed
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diffusion
movement of water
osmosis
21. Facilitated Diffusion
Diffusion through protein channels
channels move specific molecules across
cell membrane
facilitated = with help
no energy needed
open channel = fast transport
HIGH
LOW
AP Biology
“The Bouncer”
22. Active Transport
Cells may need to move molecules against
concentration gradient
conformational shape change transports solute
from one side of membrane to other
protein “pump”
“costs” energy = ATP LOW conformational change
ATP
HIGH
AP Biology
“The Doorman”
24. Getting through cell membrane
Passive Transport
Simple diffusion
diffusion of nonpolar, hydrophobic molecules
lipids
HIGH → LOW concentration gradient
Facilitated transport
diffusion of polar, hydrophilic molecules
through a protein channel
HIGH → LOW concentration gradient
Active transport
diffusion against concentration gradient
LOW → HIGH
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uses a protein pump
requires ATP
ATP
26. How about large molecules?
Moving large molecules into & out of cell
through vesicles & vacuoles
endocytosis
phagocytosis = “cellular eating”
pinocytosis = “cellular drinking”
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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
Water is very important to life,
so we talk about water separately
Diffusion of water from
HIGH concentration of water to
LOW concentration of water
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across a
semi-permeable
membrane
30. 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
water
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hypotonic
hypertonic
net movement of water
31. Managing water balance
Cell survival depends on balancing
water uptake & loss
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freshwater
balanced
saltwater
32. 1
Managing water balance
Hypotonic
a cell in fresh water
high concentration of water around cell
problem: cell gains water,
swells & can burst
example: Paramecium
ex: water continually enters
KABOOM!
Paramecium cell
solution: contractile vacuole
pumps water out of cell
ATP
ATP
AP Biology
plant cells
No problem,
here
turgid = full
cell wall protects from bursting
freshwater
34. 2
Managing water balance
Hypertonic
I’m shrinking,
a cell in salt water I’m shrinking!
low concentration of water
around cell
problem: cell loses water &
can die
example: shellfish
solution: take up water or
pump out salt
I
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plant cells
plasmolysis = wilt
can recover
will
survive!
saltwater
35. 3
Managing water balance
Isotonic
That’s
perfect!
animal cell immersed in
mild salt solution
no difference in concentration of
water between cell & environment
problem: none
no net movement of water
flows across membrane equally, in
both directions
I could
cell in equilibrium
be better…
volume of cell is stable
example:
blood cells in blood plasma
slightly salty IV solution in hospital
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balanced
36. 1991 | 2003
Aquaporins
Water moves rapidly into & out of cells
evidence that there were water channels
protein channels allowing flow of water
across cell membrane
AP Biology
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
37. Do you understand Osmosis…
.05 M
.03 M
Cell (compared to beaker) → hypertonic or hypotonic
Beaker (compared to cell) → hypertonic or hypotonic
AP Biology
Which way does the water flow? → in or out of cell
40. Diffusion through phospholipid bilayer
What molecules can get through directly?
fats & other lipids
inside cell
NH3
What molecules can
lipid
salt
NOT get through
directly?
polar molecules
H2O
outside cell
sugar aa
H2O
ions (charged)
salts, ammonia
large molecules
starches, proteins
AP Biology
41. Membrane fat composition varies
Fat composition affects flexibility
membrane must be fluid & flexible
about as fluid as thick salad oil
% unsaturated fatty acids in phospholipids
keep membrane less viscous
cold-adapted organisms, like winter wheat
increase % in autumn
AP Biology
cholesterol in membrane
42. Diffusion across cell membrane
Cell membrane is the boundary between
inside & outside…
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
Hinweis der Redaktion
{"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"}