Download The Cell Membrane

AP Biology
Cell and Cell Parts Reivew videos
 Inner Life of a Cell (with dramatic music)
 Inner Life of a Cell (technically narrated)
 Human skin cells, up close and personal

Dust mites (just for fun)
 Through the virtual cell (in cell submarine
with all of the key cellular processes)
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What is the purpose of all this stuff?
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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
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Arranged as a Phospholipid bilayer
 Serves as a 5-10 nm wide cellular border
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
waste
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lipids
Permeability to polar molecules?
 Membrane becomes semi-permeable via
protein channels

specific channels allow specific material
across cell membrane
inside cell
NH
AP
Biology
3
salt
H 2O
aa
sugar
outside cell
Cell membrane is more than lipids…
 Transmembrane proteins embedded in
phospholipid bilayer

create semi-permeabe channels
lipid bilayer
membrane
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protein channels
in lipid bilyer membrane
Why are
proteins the perfect
molecule to build structures
in the cell membrane?
AP Biology
2007-2008
Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
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Classes of amino acids
What do these amino acids have in common?
I like the
polar ones
the best!
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polar & hydrophilic
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
+
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
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
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
 ex: transport proteins
 channels, permeases (pumps)
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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
Membrane carbohydrates
 Play a key role in cell-cell recognition

ability of a cell to distinguish one cell
from another
 antigens

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basis for rejection of
foreign cells by
immune system
Any Questions??
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Movement across the
Cell Membrane
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2007-2008
Diffusion
 2nd Law of Thermodynamics
governs biological systems

universe tends towards disorder (entropy)
 Diffusion

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movement from HIGH  LOW concentration
Simple Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

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diffusion
movement of water
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”
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
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“The Doorman”
Active transport
 Many models & mechanisms
ATP
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ATP
antiport
symport
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 proton pump
requires ATP
ATP
How Ion Pumps Maintain Membrane
Potential
 Two combined forces, collectively called
the electrochemical gradient, drive the
diffusion of ions across a membrane


A chemical force (the ion’s concentration
gradient)
An electrical force (the effect of the
membrane potential on the ion’s
movement)
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© 2011 Pearson Education, Inc.
• An electrogenic pump is a transport protein
that generates voltage across a membrane
•
The sodium-potassium pump is the major
electrogenic pump of animal cells
•
The main electrogenic pump of plants, fungi,
and bacteria is a proton pump
 Electrogenic pumps help store energy that can be
used for cellular work
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© 2011 Pearson Education, Inc.
Figure 7.20
ATP



Proton pump
H

CYTOPLASM
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

EXTRACELLULAR
FLUID

H
H
H


H
H
Figure 7.21
Cotransport: Coupled Transport by a
Membrane
Protein
ATP

H
H

H
Proton pump
H

H


H
H

H
Sucrose-H
cotransporter
Sucrose
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
Diffusion of H

Sucrose
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
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ATP
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
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
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The Special Case of Water
post lab
AP Biology
2007-2008
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
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
Managing water balance
 Cell survival depends on balancing
water uptake & loss
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freshwater
balanced
saltwater
1
Managing water balance
 Hypotonic

a cell in fresh water

high concentration of water around cell
 problem: cell gains water,
swells & can burst
KABOOM!
 example: Paramecium
 ex: water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
ATP
 ATP

plant cells
No problem,
here
 turgid = full
 cell wall protects from bursting
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freshwater
Pumping water out
 Contractile vacuole in Paramecium
ATP
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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

plant cells
will
survive!
 plasmolysis = wilt
 can recover
AP Biology
saltwater
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
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
Plasmolysis
of onion
cells video
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
AP Biology
Any Questions??
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Review vids
 Crash Course Biology
 Another including transport
AP Biology