Transcript Traffic across Membranes-2008

Ch. 5
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Cytoplasm
Figure 5.10
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
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Membranes are selectively permeable
– They control the flow of substances into and out of a cell
Membranes can hold teams of enzymes that
function in metabolism
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Phospholipid Bilayer
 2 layers of
phospholipids
 Proteins
 Transport
 Receptors
 Enzymes
 Cholesterol
 Maintains fluidity
Figure 5.11
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
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In water, phospholipids form a stable bilayer
– The heads face outward and the tails face inward
Water
Hydrophilic
heads
Hydrophobic
tails
Water
Figure 5.11B
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
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Membrane composed of different molecules
 Phospholipids
 Cholesterol
 Proteins
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Phospholipid molecules form a flexible bilayer
 Cholesterol and protein molecules are embedded
in it
 Carbohydrates act as cell identification tags
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Molecules move laterally
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Because of polar and nonpolar regions of the
phospholipid bilayer, the membrane allows certain
materials in and certain materials out of the cell
Permits exchange of nutrients, waste products,
oxygen, and inorganic ions.
Allows some substances to cross more easily than
others:
 Hydrophobic molecules—hydrocarbons, CO2,
and O2 dissolve in and cross membrane
 Very small polar molecules, including H2O can
cross easily
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Diffusion caused by the random
movement of particles across a
membrane
Movement due to concentration
gradient
 Moving from a higher concentration to
a lower concentration
 No energy used
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Movement continues until
equilibrium reached
 Concentration is the same on both
sides of the membrane
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A substance will diffuse from where it is more
concentrated to where it is less concentrated
Only small, uncharged particles diffuse without
assistance (CO2, and O2)
http://sussexhigh.nbed.nb.ca/swift/biology11/Cell%20Boundari
es%20ONLINE.ppt#276,25,Section G
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Diffusion of water across a selectively
permeable membrane
Water diffuses down its own concentration
gradient (from hypotonic solution to
hypertonic solution)
Does not use energy
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Hypertonic solution
 Solution contains more solute than the solution it
is compared with
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Hypotonic solution
 Solution contains less solute than the solution it is
compared with
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Isotonic solution
 Solution contains same amount of solute as the
solution it is compared with
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What kind of solution
is inside the cell?
 Isotonic
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Outside the cell?
 Isotonic
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Which direction will
water move?
 In and out at the same
rate
10 % NaCl
10 % NaCl
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What kind of solution
is inside the cell?
 Hypertonic
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Outside the cell?
 Hypotonic
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Which direction will
water move?
 Into the cell
10 % NaCl
20 % NaCl
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What kind of solution
is inside the cell?
 Hypotonic
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Outside the cell?
 Hypertonic
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Which direction will
water move?
 Out of the cell
30 % NaCl
10 % NaCl
Copyright © 2001
Pearson Education, Inc.
publishing Benjamin
Cummings
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Water balance between cells and their
surroundings is crucial to organisms
The control of water balance is
osmoregulation
Passive transport—no energy used
Uses transport proteins embedded in the plasma
membrane (ion channels)
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Aides transport of many
polar molecules and
ions that are inhibited
by phospholipid bilayer
(sugar, some hormones,
ions such as K+ and
Na+)
Pores—always open
Gated channels—
open/close in response
to stimuli
Solute
molecule
Transport
protein
http://sussexhigh.nbed.nb.ca/swift/biology11/Cell%20Boundari
es%20ONLINE.ppt#276,25,Section G
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Transport proteins share similar properties
with enzymes:
They are specific for the solutes they
transport
They can be saturated with solute—
maximum rate occurs when all binding
sites are occupied
They can be inhibited by molecules that
resemble the solute (similar to competitive
inhibition)
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Substances are transported across a membrane,
against the concentration gradient
Use carrier proteins embedded in the membrane
Use energy (ATP)
http://student.ccbcmd.edu/~gkaiser/biotutorials/eustru
ct/images/sppump.gif
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Active
transport in
two solutes
across a
membrane
FLUID
OUTSIDE
CELL
Phosphorylated
transport protein
Transport
protein
First
solute
1
First solute,
inside cell,
binds to protein
2
ATP transfers
phosphate to
protein
3
Protein releases
solute outside
cell
5
Phosphate
detaches from
protein
6
Protein releases
second solute
into cell
Second
solute
Figure 5.18
4
Second solute
binds to protein
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
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3 Na+ ions inside the cell bind to the pump. ATP donates
a phosphate to the pump
The pump changes shape, transporting 3 Na+ across the
membrane, and are released outside of the cell
2 K+ ions outside the cell
bind to the pump
The phosphate group is
released and
2 K+ ions are transported
across the membrane
2 K+ are released inside
the cell
http://gotoknow.org/file/somluckv/Cell_membrane06.jpg
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Substances that are too large for carrier proteins
(proteins and polysaccharides)
Endocytosis—movement of substances into the
cell (a.k.a. phagocytosis)
Exocytosis—movement of substance out of the
cell
http://www.stanford.edu/group/Urchin/GIFS/exocyt.gif
Exocytosis
Process of exporting
macromolecules from a cell by
fusion of vesicles with the cell
membrane
Endocytosis
Process of importing
macromolecules into a cell by
forming vesicles derived from
the cell membrane
Vesicle usually budded from the
ER or Golgi and migrates to cell
membrane
Vesicle forms from a localized
region of cell membrane that
sinks inward; pinches off into
cytoplasm
Used by secretory cells to export Used by cells to incorporate
products (insulin in pancreas;
extracellular substances
neurotransmitter from neuron)
Figure 5.19A
Figure 5.19B
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
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Phagocytosis—
endocytosis of solid
particles
 Forms food vacuoles that
fuse with lysosome to be
digested
http://student.ccbcmd.edu/~gkaiser/biotutoria
ls/eustruct/images/phagocyt.gif
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Pinocytosis—
endocytosis of fluid
droplets
 Takes in solutes dissolved
in the droplet
http://student.ccbcmd.edu/~gkaiser/biotutoria
ls/eustruct/images/pinocyt.gif
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Imports specific macromolecules into the cell by
inward budding of vesicles formed from coated pits
 Occurs in response to binding specific ligands to receptors on
cell’s surface
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Harmful levels of cholesterol can accumulate in the blood
if membranes lack cholesterol receptors
Figure 5.19
Three kinds of endocytosis
Pseudopod of
amoeba
Food being
ingested
Plasma
membrane
Material bound to
receptor proteins
PIT
Cytoplasm
Figure 5.19C
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Protein that binds a specific signal molecule, allowing the cell
to respond to the signal molecule
Some receptor proteins are attached to ion channels
 Changes permeability to a specific ion
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Some may cause the formation of
a second messenger, which acts
as a signal molecule in the
cytoplasm
Figure 5.20
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Essential Biology with Physiology, 2nd ed., by Campbell, Reece, and
Simon, ©2007. These images have been produced from the originals by
permission of the publisher. These illustrations may not be reproduced in
any format for any purpose without express written permission from the
publisher.
BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell,
Reece, Mitchell, and Taylor, ©2003. These images have been produced
from the originals by permission of the publisher. These illustrations may
not be reproduced in any format for any purpose without express written
permission from the publisher.
BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell,
Reece, Mitchell, and Taylor, ©2001. These images have been produced
from the originals by permission of the publisher. These illustrations may
not be reproduced in any format for any purpose without express written
permission from the publisher.