Download Cell Membrane

The Cell Membrane
The Fluid Mosaic Model
AP Biology
Cell membrane must be more than lipids…
 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
Phosphate
1. Phospholipids
 Phosphate head + Choline

hydrophilic
 Fatty acid tails

“attracted to water”
hydrophobic
 Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah,
one of those
structure–function
examples
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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

1b. cholesterol in membrane
 Maintains the fluidity
of the cell
 especially in cold conditions
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Arranged as a Phospholipid bilayer
 Serves as a cellular barrier / border
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
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lipids
2. Proteins
 Determines what goes in and out
specific channels allow specific material
across cell membrane
 W/O water would never diffuse through

inside cell
NH
AP
Biology
3
salt
H 2O
aa
sugar
outside cell
Proteins domains anchor molecule
Polar areas
(EXTRA)
 Within membrane
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
+
H
H+
Examples
Retinal
chromophore
NH2
aquaporin =
water channel in bacteria
Porin monomer
H 2O
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
H++
H
Cytoplasm
proton pump channel
in photosynthetic bacteria
H O
AP Biology 2
function through
conformational change =
protein changes shape
Membrane Proteins
A. integral proteins
penetrate lipid bilayer, usually across whole
membrane
 transmembrane protein


1.
ex: transport proteins
Channel – have a tube that allows materials to go
from one side of the membrane to the other
 Aquaporin – no energy required
2.
Carrier channel is specific, changes its shape for
the substance, no E required
 Glucose transport protein
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Membrane Proteins
B. peripheral proteins

Embedded either on the ECM (outside) or
cytoplasmic (inside) side

ECM side – links to ECM

Cytoplasmic – links to
microfilaments or tethers
organelles
AP Biology
Fig. 6-30a
Collagen
Proteoglycan
complex
EXTRACELLULAR FLUID
Fibronectin
Integrins
Plasma
membrane
Microfilaments
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CYTOPLASM
Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Signal
Transduction
Cell adhesion
Attachment to the
cytoskeleton
“Antigen”
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Cell surface
identity marker
Functions of Proteins in Cell Membrane
1. Transfer
1.
Sodium-potassium pump (low high
con) Requires Energy, ATP is constant
2. Enzymatic Activity
1.
2.
Perform as enzymes
Carbohydrate metabolism
3. Signal Transduction
1.
2.
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Receiving signals  effect
Stimulus  response!
4. Intercellular Joining
4.
5.
Plasmodesmata
Gap/tight junctions
5. Cell-cell recognition
6. Attachment
4.
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To ECM or filament
3. Oligarsaccharides (carb chains)
 Glycolipids
A,B,O, Rh+, Rh Play a key role in cell-cell recognition

 Glycoporteins
Ligans (anything that fits
into carbon chains)
 The carb chains vary from
person to person

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Traffic Across the Membrane
 Cross easily (no assitance)
Lipid soluble molecules (ex: sex
hormones)
 Gases (oxygen or CO2)

 Cross w/ assistance
Water (and water soluble materials
 Large molecules
 Ions (sodium, potassium)
 Proteins, glucose

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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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Transmembrane
proteins
Cytoplasm
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
Any Questions??
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PASSIVE TRANSPORT
AP Biology
2007-
Diffusion
 The tendency of a substance to spread
out evenly inro space to reach dynamic
equilibrium
 Diffusion

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movement from HIGH  LOW concentration
 Down concentration gradient
 DOES NOT REQUIRE ENERGY
 Due to the kinetic energy in the
molecules
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Passive Transport
 Diffusion across a membrane
movement of water
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diffusion
osmosis
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
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“The Bouncer”
The Special Case of Water
Movement of water across
the cell membrane
AP Biology
2007-
Osmosis is just diffusion of water
 Diffusion of water from
HIGH concentration of water to
LOW concentration of water

across a
semi-permeable
membrane
 Tonicity - comparing the
solute concentrations on
opposite sides of a membrane,
and the direction water moves
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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
 Hyperosmotic – area w/more water
(hypotonic solution)
 Hypoosmatic – area w/ less water
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Managing water balance
 Cell survival depends on balancing
water uptake & loss
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freshwater
balanced
saltwater
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
Do you understand Osmosis…
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
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SEE WATER POTENTIAL
POWERPOINT
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Active Transport
 Cells may need to move molecules against
concentration gradient – requires ATP Hydrolysis


conformational shape change transports solute from
one side of membrane to other (caused by ATP)
Low High concentration
conformational change
Use carrier proteins
High

ATP
Low
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Active transport
 Many models & mechanisms
ATP
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ATP
antiport
symport
1. Ion pumps
1.
Sodium – Potassium pump (Na+/K-)
2. Endocytosis
AP Biology
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
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 Moving large molecules into & out of cell
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating” Engulfing
 Intake of solid particles
 Nonspecific
 Creates a phagosome to be digested w/ a lysosome
 pinocytosis = “cellular drinking”
 Intake of large fluid
 nonspecific
 Receptor mediated endocytosis
 Intake of specific molecules (due to receptors)
 Creation of clathrin-enclosed coated pit (vesicle)
with specific molecules inside
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3. Exocytosis
the release of substances from the
cell
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Transport summary
simple
diffusion
facilitated
diffusion
active
transport
AP Biology
ATP
Any Questions??
AP Biology
Ghosts of Lectures Past
(storage)
AP Biology
2007-
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
 H 2O

outside cell
sugar aa
H 2O
ions (charged)
 salts, ammonia

large molecules
 starches, proteins
AP Biology
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
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