Download Traffic across Membranes

Ch. 7
Biology
Cell Boundaries
All cells are surrounded by a thin,
flexible barrier known as the cell
membrane.
 Many cells also produce a strong
supporting layer around the membrane
known as a cell wall.

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Cell Membranes are
Selectively Permeable

The cell membrane regulates what enters
and leaves the cell and also provides
protection and support.
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Cell Membrane

The lipid bilayer gives cell membranes a
flexible structure that forms a barrier
between the cell and its surroundings.
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Cell Membrane

The composition of
nearly all cell
membranes is a
double-layered
sheet called a
phospholipid
bilayer.
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Fluid-Mosaic Model

Membrane composed of different
molecules
 Phospholipids
 Cholesterol
 Proteins

Phospholipid molecules form a flexible
bilayer
 Cholesterol and protein molecules are
embedded in it
 Carbohydrates act as cell identification tags

Molecules move laterally
Fluid Mosaic Model
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Selectively Permeable
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

Cell Walls




The main function of the cell wall is to provide support and
protection for the cell.
Cell walls are found in plants, algae, fungi, and many
prokaryotes.
The cell wall lies outside the cell membrane.
Most cell walls are porous enough to allow water, oxygen,
carbon dioxide, and certain other substances to pass through
easily.
Diffusion Through Cell
Boundaries

Diffusion Through Cell Boundaries
○ Every living cell exists in a liquid environment.
○ The cell membrane regulates movement of
dissolved molecules from the liquid on one
side of the membrane to the liquid on the
other side.
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Diffusion of Liquids
11
Passive Transport
A substance will diffuse from where it is more
concentrated to where it is less concentrated
 Only small, uncharged particles diffuse without
assistance (H2O, CO2, and O2)

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es%20ONLINE.ppt#276,25,Section G
Diffusion Through Cell Boundaries
 Diffusion
○ Particles in a solution tend to move from an area
where they are more concentrated to an area
where they are less concentrated.
○ This process is called diffusion.
○ When the concentration of the solute is the
same throughout a system, the system has
reached equilibrium.
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Diffusion Through Cell
Boundaries
There is a higher concentration of
solute on one side of the membrane
as compared to the other side of the
membrane.
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Diffusion Through Cell
Boundaries
Solute particles move from the side of
the membrane with a higher
concentration of solute to the side of
the membrane with a lower
concentration of solute. The solute
particles will continue to diffuse across
the membrane until equilibrium is
reached.
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Diffusion Through Cell
Boundaries
When equilibrium is reached, solute
particles continue to diffuse across
the membrane in both directions.
Diffusion depends upon
random particle movements.
Therefore, substances diffuse
across membranes without
requiring the cell to use
energy.
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Osmosis

Osmosis
 Osmosis is the diffusion of water through a
selectively permeable membrane.
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Osmosis
Water tends to diffuse
from a highly
concentrated region to
a less concentrated
region.
 If you compare two
solutions, the more
concentrated solution
is hypertonic.
 The more dilute
solution is hypotonic.

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Osmosis
When concentrations of solutions are
the same on both sides of a membrane,
the solutions are isotonic.
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http://www.uic.edu/classes/bios/bios100/summer2003/isotonic.jpg


What kind of
solution is inside
the cell?
 Isotonic

Outside the cell?
 Isotonic

10 % NaCl
Which direction will
water move?
 In and out at the
same rate
10 % NaCl

What kind of
solution is inside
the cell?
 Hypertonic

Outside the cell?
 Hypotonic

10 % NaCl
Which direction will
water move?
 Into the cell
20 % NaCl

What kind of
solution is inside
the cell?
 Hypotonic

Outside the cell?
 Hypertonic

30 % NaCl
Which direction will
water move?
 Out of the cell
10 % NaCl
Osmosis

Osmotic Pressure
 Osmosis exerts a pressure known as
osmotic pressure on the hypertonic side of a
selectively permeable membrane.
 Because the cell is filled with salts, sugars,
proteins, and other molecules, it will almost
always be hypertonic to fresh water.
 If so, the osmotic pressure should produce a
net movement of water into the cell. As a
result, the volume of the cell will increase
until the cell becomes swollen or bursts.
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Osmoregulation
Water balance between cells and their
surroundings is crucial to organisms
 The control of water balance is
osmoregulation
 Cells in large organisms are not in danger
of bursting because they are bathed in
fluids, such as blood, that are isotonic.
 Other cells are surrounded by tough cell
walls that prevent the cells from expanding
even under tremendous osmotic pressure.

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http://ilovebacteria.com/Images/blood.jpg
Osmosis
Animal
cell
Plant
cell
Plasmolysis
Flaccid
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Turgid
Facilitated Diffusion

The movement of specific molecules across cell
membranes through protein channels is known as
facilitated diffusion.
Hundreds of different protein channels have been
found that allow particular substances to cross
different membranes.
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http://www.dwm.ks.edu.tw/bio/activelearner/05/images
/ch05c3.gif

Facilitated Diffusion



Although facilitated diffusion is fast and specific, it
is still diffusion.
Therefore, facilitated diffusion will only occur if
there is a higher concentration of the particular
molecules on one side of a cell membrane as
Solute
compared to the other side.
molecule
No energy is used.
Transport
protein
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es%20ONLINE.ppt#276,25,Section G
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Facilitated Diffusion
Molecules will randomly move through the
pores in Channel Proteins.
https://mattwells.wikispaces.com/file/view/PLASMA~1.PPT
28
Facilitated Diffusion
Some carrier proteins
do not extend
through the
membrane.
 They bond and drag
molecules through
the lipid bilayer and
release them on the
opposite side.

https://mattwells.wikispaces.com/file
/view/PLASMA~1.PPT
https://mattwells.wikispaces.com/file/view/PLASMA~1.PPT
29
Active Transport
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

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
Active Transport:
Sodium Potassium Pump
Sodium-Potassium Pump
1.
2.
3.
4.
5.
6.
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
Active Transport
 Endocytosis and Exocytosis
○ Large molecules and even solid clumps of material
may undergo active transport by means of the cell
membrane.
○ Endocytosis is the process of taking material into the
cell by means of infoldings, or pockets, of the cell
membrane.
○ The pocket breaks loose from the outer portion of the
cell membrane and forms a vacuole within the
cytoplasm.
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Endocytosis

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

Pinocytosis —
endocytosis of fluid
droplets
 Takes in solutes
dissolved in the droplet
http://student.ccbcmd.edu/~gkaiser/biotutoria
ls/eustruct/images/pinocyt.gif
Active Transport
 Exocytosis
○ Many cells also release large amounts of
material from the cell, in a process called
exocytosis.
○ During exocytosis, the membrane of the
vacuole surrounding the material fuses with
the cell membrane, forcing the contents out of
the cell.
http://www.stanford.edu/group/Urchin/GIFS/exocyt.gif
Three kinds of endocytosis
Pseudopod of
amoeba
Food being
ingested
Plasma
membrane
Material bound to
receptor proteins
PIT
Cytoplasm
Figure 5.19C
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 5.19B
Figure 5.19A
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Receptor-mediated Endocytosis

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

Harmful levels of cholesterol can accumulate in the
blood if membranes lack cholesterol receptors
Figure 5.19
Cell Boundaries and
Transport
Unlike a cell wall, a cell membrane
a) is composed of a lipid bilayer.
b) provides rigid support for the surrounding
cell.
c) allows most small molecules and ions to
pass through easily.
d) is found only in plants, fungi, algae, and
many prokaryotes.
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If a substance is more highly concentrated
outside the cell than inside the cell and the
substance can move through the cell membrane,
the substance will
a) move by diffusion from inside the cell to
outside.
b) remain in high concentration outside the cell.
c) move by diffusion from outside to inside the
cell.
d) cause water to enter the cell by osmosis.
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The movement of materials in a cell against a
concentration difference is called
a) facilitated diffusion.
b) active transport.
c) osmosis.
d) diffusion.
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The process by which molecules diffuse across a
membrane through protein channels is called
a) active transport.
b) endocytosis.
c) facilitated diffusion.
d) osmosis.
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The Diversity of Cellular
Life

The differences among living things
arise from the ways in which cells are
specialized to perform certain tasks and
the ways in which cells associate with
one another to form multicellular
organisms.
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Unicellular Organisms

Unicellular Organisms
○ Unicellular organisms are made up of only
one cell.
○ Unicellular organisms dominate life on Earth.
Paramecium
Euglena
Lactobacillus acidophilis
http://www1.broward.edu/~ssimpson/imgev5.jpg
http://media-2.web.britannica.com/ebmedia/15/12015-004-F85E856C.jpg
http://bioinfo.bact.wisc.edu/themicrobialworld/Lac
tobacillus.jpg
Multicellular Organisms


Organisms that are
made up of many
cells are called
multicellular.
There is a great
variety among
multicellular
organisms.
http://api.ning.com/files/ORxcmZcFiP6I7PYb0qOoWVxTv5ycZwBGGYXUO8Q5Vt4Jt
2C1cvgUvrvIFwf4erW0i31kd7vYS-0skTwphV6eIL69sBS254X/Animal_diversity_October_2007.jpg
Multicellular Organisms


Cells throughout an
organism can develop
in different ways to
perform different
tasks.
This process is called
cell specialization.
http://219.221.200.61/ywwy/zbsw(E)/pic/ech12-2.jpg
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Animal cells are specialized in
many ways.
Red blood cells transport
oxygen.
Cells in the pancreas
produce proteins.
Animal cells are specialized in
many ways.
Muscle cells allow
movement.
Nerve cells coordinate
functions throughout the
body.
Multicellular Organisms
 Specialized Plant Cells
○ Plants exchange carbon dioxide, oxygen, water
vapor, and other gases through tiny openings
called stomata on the undersides of leaves.
○ Highly specialized cells, known as guard cells,
regulate this exchange.
Stomata enclosed by
guard cells.
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Levels of Organization

Levels of Organization
 The levels of organization in a multicellular
organism are:
○ individual cells
○ tissues
○ organs
○ organ systems
http://www.learner.org/courses/essential/life/images/show1.levels.jpg
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Levels of Organization
In multicellular organisms, cells are the first
level of organization.
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Levels of Organization
 Tissues
○ Similar cells are grouped into units called
tissues.
○ A tissue is a group of similar cells that
perform a particular function.
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Levels of Organization

Most animals have four main types of
tissue:
○ muscle
○ epithelial
○ nervous
○ connective
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Levels of Organization
 Organs
○ Organs are groups of tissues that work
together to perform a specific function.
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Levels of Organization

Organ Systems
 In most cases, an organ
completes a series of
specialized tasks.
 A group of organs that
work together to perform a
specific function is called
an organ system.
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Cell Diversity
Cell specialization is characteristic of
a) bacteria.
b) all unicellular organisms.
c) yeasts.
d) multicellular organisms.
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Which of the following cells is specialized for
contraction?
a) muscle cell
b) red blood cell
c) pancreatic cell
d) nerve cell
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The stomach is an example of a(an)
tissue.
b) organ.
c) organ system.
d) organism.
a)
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Which of the following shows the levels of
organization in an organism from the simplest to
the most complex?
a) organ system, organ, cell, tissue
b) tissue, cell, organ, organ system
c) cell, tissue, organ, organ system
d) cell, organ, tissue, organ system
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Which of the following would probably contain the
greatest variety of specialized cells?
a) an organ system
b) a tissue
c) an organ
d) a multicellular organism
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Acknowledgements
Biology, by Miller and Levine. 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
 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.
