Download Chapter 8. Movement across the Membrane

Chapter 8.
Movement across the Cell
Membrane
AP Biology
2005-2006
Diffusion
 2nd Law of Thermodynamics
governs biological systems

Universe tends towards disorder
 Diffusion

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movement from high  low concentration
Diffusion of 2 solutes
 Each substance diffuses down its own
concentration gradient, independent of
concentration gradients of other
substances
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Diffusion
 Move for HIGH to LOW concentration
“passive transport”
 no energy needed

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diffusion
osmosis
Lipids of cell membrane
 Membrane is made of phospholipids

phospholipid bilayer
inside cell
phosphate
hydrophilic
lipid
hydrophobic
outside cell
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Phospholipids
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Fluid Mosaic Model
 In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
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Figure 7.6
Lateral movement occurs
107 times per second.
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Flip-flopping across the membrane
is rare ( once per month).
Figure 7.8
Fluid
Viscous
Unsaturated hydrocarbonSaturated hydrocarbon tails
tails
(a) Unsaturated versus saturated hydrocarbon tails
(b) Cholesterol within the animal
cell membrane
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Cholesterol
Phospholipid bilayer
 What molecules can get through directly?
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Permeable cell membrane
 Need to allow more material through

membrane needs to be permeable to…
 all materials a cell needs to bring in
 all waste a cell needs excrete out
 all products a cell needs to export out
inside cell
Haa
sugar
2O
“holes”, or
channels, in cell
membrane allow
material in & out
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outside cell
NH
salt3
lipid
Semi-permeable cell membrane
 But the cell still needs control

membrane needs to be semi-permeable
 specific channels allow
specific material in & out
inside cell
salt
outside
NH3 cell
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H 2O
aa
sugar
Why proteins?
 Proteins are mixed molecules

hydrophobic amino acids
 stick in the lipid membrane
 anchors the protein in membrane

hydrophilic amino acids
 stick out in the watery
fluid in & around cell
 specialized “receptor”
for specific molecules
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Membrane Proteins
 Proteins determine most of membrane’s
specific functions

cell membrane & organelle membranes each
have unique collections of proteins
 Membrane proteins:


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peripheral proteins = loosely
bound to surface of membrane
integral proteins = penetrate into
lipid bilayer, often completely
spanning the membrane =
transmembrane protein
2005-2006
Figure 7.10
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
Glycoprotein
(d) Cell-cell recognition (e) Intercellular joining
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(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Figure 7.11
HIV
Receptor
(CD4)
Co-receptor
(CCR5)
HIV can infect a cell that
has CCR5 on its surface,
as in most people.
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Receptor (CD4)
but no CCR5
Plasma
membrane
HIV cannot infect a cell lacking
CCR5 on its surface, as in
resistant individuals.
A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer
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Membrane Carbohydrates
 Play a key role in cell-cell recognition
ability of a cell to distinguish
neighboring cells from another
 important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system

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2005-2006
Figure 7.12
Transmembrane
glycoproteins
Secretory
protein
Golgi
apparatus
Vesicle
ER
ER lumen
Glycolipid
Plasma membrane:
Cytoplasmic face
Extracellular face
Transmembrane
glycoprotein
Secreted
protein
Membrane
glycolipid
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Getting through cell membrane
 Passive transport

diffusion of hydrophobic (lipids) molecules
 high  low concentration gradient
 Facilitated transport


diffusion of 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
Simple diffusion across membrane
Which way will
lipid move?
lipid
inside cell
low
lipid
lipid
lipid
lipid
lipid

high
outside cell
lipid
lipid
lipid
lipid
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lipid
lipid
lipid
lipid
Facilitated Diffusion
 Globular proteins act as doors in membrane

channels to move specific molecules through
cell membrane
open channel = fast transport
high
low
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“The Bouncer”
Facilitated diffusion
 Move from HIGH to LOW concentration
through a protein channel
passive transport
 no energy needed
 facilitated = with help

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2005-2006
Gated channels
 Some channel proteins open only in presence of
stimulus (signal)

Stimulus often different than transport molecule
 ex: ion-gated channels
when neurotransmitters bind to a specific gated
channels on a neuron, these channels open = allows
Na+ ions to enter nerve cell
 ex: voltage-gated channels
change in electrical charge across nerve cell membrane
opens Na+ & K+ channels
see next slide
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2005-2006
Active Transport
 Globular proteins act as ferry for specific molecules


shape change transports solute from one side of
membrane to other  protein “pump”
“costs” energy
low
high
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conformational change
“The Doorman”
Active transport
 Cells may need molecules to move
against concentration situation
need to pump against concentration
 protein pump
 requires energy
 ATP

Na+/K+ pump
in nerve cell
membranes
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2005-2006
Active transport
 Many models & mechanisms
using ATP
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using ATP
Sodium Potassium Pump Video
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2005-2006
Transport summary - Video
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2005-2006
Figure 7.19
Diffusion
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Passive transport
Facilitated diffusion
Active transport
ATP
Cotransport: Coupled Transport by a
Membrane Protein
 Cotransport occurs when active transport

of a solute indirectly drives transport of
other solutes
Plants commonly use the gradient of
hydrogen ions generated by proton pumps
to drive active transport of nutrients into
the cell
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© 2011 Pearson Education, Inc.
Figure 7.21
ATP

H
H

H
Proton pump
H

H


H
H

H
Sucrose-H
cotransporter
Sucrose
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
Diffusion of H

Sucrose
How about large molecules?
 Moving large molecules into & out of cell
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”
 receptor-mediated
endocytosis

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exocytosis
exocytosis
2005-2006
Figure 7.22
Phagocytosis
Pinocytosis
Receptor-Mediated Endocytosis
EXTRACELLULAR
FLUID
Solutes
Pseudopodium
Receptor
Ligand
Plasma
membrane
Coat proteins
Coated
pit
“Food” or
other particle
Coated
vesicle
Vesicle
Food
vacuole
CYTOPLASM
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Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
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fuse with
lysosome for
digestion
non-specific
process
triggered by
ligand signal
The Special Case of Water
Movement of water across
the cell membrane
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2005-2006
Osmosis is diffusion of water
 Water is very important, 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
2005-2006
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
hypotonic
hypertonic
net movement of water
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Managing water balance
 Cell survival depends on balancing
water uptake & loss
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freshwater
balanced
saltwater
Managing water balance
 Isotonic

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animal cell immersed in
isotonic solution
 blood cells in blood
 no net movement of water
across plasma membrane
 water flows across
membrane, at same rate in
both directions
 volume of cell is stable
Managing water balance
 Hypotonic

animal cell in hypotonic solution
will gain water, swell & burst
 Paramecium vs. pond water
 Paramecium is hypertonic
 H2O continually enters cell
 to solve problem, specialized
organelle, contractile vacuole
 pumps H2O out of cell = ATP

plant cell
 turgid
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Water regulation
 Contractile vacuole in Paramecium
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Managing water balance
 Hypertonic

animal cell in hypertonic
solution will loose water, shrivel
& probably die
 salt water organisms are
hypotonic compared to their
environment
 they have to take up water &
pump out salt

plant cells
 plasmolysis = wilt
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Figure 7.15
(a) Animal cell
Hypotonic
solution
H2O
(b) Plant cell
Isotonic
solution
H2O
Lysed
Normal
H2OCell wall H2O
H2O
Turgid (normal)
Flaccid
Osmosis
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H2O
Hypertonic
solution
H2O
Shriveled
H2O
Plasmolyzed
Water Balance of Cells Without Walls
 Tonicity is the ability of a surrounding



solution to cause a cell to gain or lose water
Isotonic solution: Solute concentration is the
same as that inside the cell; no net water
movement across the plasma membrane
Hypertonic solution: Solute concentration is
greater than that inside the cell; cell loses
water
Hypotonic solution: Solute concentration is
less than that inside the cell; cell gains water
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© 2011 Pearson Education, Inc.
1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells

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evidence that there were water channels
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Figure 7.UN03
“Cell”
0.03 M sucrose
0.02 M glucose
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“Environment”
0.01 M sucrose
0.01 M glucose
0.01 M fructose
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 of2005-2006
cell
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