Cell Transport
Ch. 9: Cell Transport
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Passive Transport
Targets
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Distinguish between diffusion & osmosis
Define equilibrium & explain how it is established
Explain what is meant by a concentration gradient
Explain how substances cross the cell membrane
through facilitated diffusion
Explain how ion channels assist the diffusion of ions
across the cell membrane
Concentration

how much of a given substance there is mixed
with another substance
Kinetic Motion
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All particles of matter are in
constant (random) motion
Molecules travels in a straight line
until they hit something, bounce
off, and travel in a new direction
Molecules move down the
concentration gradient from an
area more concentrated to an
area less concentrated
Concentration Gradient

Difference in concentration of a substance
across a space
Types of Cellular Transport

2.
3.

Weeee!!
!
Passive Transport
cell doesn’t use energy
1.
Diffusion
Facilitated Diffusion
Osmosis
•Animations of Active
Transport & Passive
Transport
high
low
Active Transport
cell does use energy
1.
2.
3.
Protein Pumps
Endocytosis
Exocytosis
This is
gonna
be hard
work!!
high
low
Passive Transport
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Diffusion
-
-
-
Movement of molecules from an area of
higher concentration to an area of lower
concentration
Driven entirely by the kinetic motion of
molecules
Continues until the concentration of molecules
is the same throughout the space they occupy
Factors Affecting Diffusion Rate
Equilibrium


Exists when the concentration of the molecules of a
substance is the same throughout a space
Random motion of molecules continues so equilibrium
is maintained (no concentrated gradient)
Diffusion Across Membranes
right guard commercial

Cell membranes are selectively permeable

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Phospholipid bilayer (non-polar)
Diffusion depends on size, type of molecule, &
chemical nature of molecule
About Cell Membranes (continued)
3.
Structure of cell membrane
Lipid Bilayer -2 layers of
phospholipids
Phosphate head is polar
(water loving)
b. Fatty acid tails non-polar
(water fearing)
c. Proteins embedded in
membrane
a.
Phospholipid
Lipid Bilayer
Diffusion Across Membranes
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Non-polar molecules (O2
& CO2) can dissolve in
lipid bilayer and thus
pass through by diffusion
Very small insoluble
molecules may pass
through membrane
pores by diffusion
Large or Polar
substances move
through membrane with
help from transport
proteins as do ions
Osmosis

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Osmosis
Diffusion of water molecules
through a cell membrane from an
area of higher water molecule
concentration to an area of lower
concentration
Quick Fact!

Process of osmosis was first described
in 1748 by French scientist Abbé Jean
Antoine Nollet. He observed that
water spontaneously diffused through
a pig bladder membrane into alcohol!
Direction of Osmosis

Depends upon the relative concentration
of solutes on the two sides of the
membrane
Osmotic Potential


The tendency of water to move across a
membrane into a solution
The lower the solute concentration, the
higher the osmotic potential
Isotonic Solution


A solution in which the concentration of dissolved
substances is the same as the concentration inside the
cell; therefore, the concentration of water is the same
on both sides of the plasma membrane
Diffusion of water in = water out
Turgor Pressure

Pressure that exists inside a cell
Hypotonic Solution

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
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A solution in which the concentration of a
dissolved substance is lower than the
concentration inside the cell; therefore,
the concentration of water is greater
outside the cell than inside the cell
Water diffuses into the cell & the cell
swells-Turgor Pressure increases
Cytolysis – cell bursts
Plant v. Animal
Osmosis (Cell in Hypotonic Solution
Hypertonic Solution

A solution in which the concentration of
dissolved substances is higher than the
concentration inside the cell; the concentration
of water is greater in the cell than outside
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Result: water moves out of the cell & the turgor
pressure decreases & the cell shrivels
Plasmolysis – loss of water pressure in a cell
Osmosis (Cell in Hypertonic Environment)
Click for animation
How Cells Deal with Osmosis
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Example: Paramecia (freshwater protozoan)
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Live in a hypotonic environment
Water constantly diffuse into them
Contractile vacuoles (organelle) removes the excess water
A common remedy for a sore throat is to gargle with
salt water. Using the concept of osmosis, explain how
this remedy might work.
Quick Fact!

Wooden drawers in cabinets absorb water from the air on
humid days causing the wood to swell & hard to open!
Osmotic Pressure

Pressure required to stop osmosis in a solution
-
-
Pressure that builds up as water diffuses into a cell
If a physical barrier prevents expansion of a
hypertonic solution as water moves in by
osmosis, a buildup of pressure is created as
water continues to flow in
-
As pressure ↑, the net flow of water molecules will slow
(stops in plants due to cell wall – turgor pressure)
Reverse Osmosis


The movement of
water particles from
an area of lower
concentration to an
area of higher
concentration with
the
Addition of pressure
Facilitated Diffusion

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Used for molecules that cannot diffuse
rapidly across the membrane (size,
polarity)
Molecules are assisted by membrane
proteins
2 Types
Carriers & Channels
Move down the concentration gradient
(High [ ] to low [ ]) no energy is used by
cell
Facilitated Diffusion
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Molecule binds to carrier protein
Protein changes shape shielding molecule from
lipids as it crosses
Molecule is released to cell interior
Carrier protein returns to original shape


Accelerates glucose movement
Carrier proteins are specific to the molecules
they carry
Facilitated Diffusion
Carriers
Channels
Diffusion through Ion Channels
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Membrane proteins
Passageway through membrane for ions to pass
through (Na+, Ca2+, Cl-)
Each ion channel is specific to one type of ion
Some channels are always open, others have gates
Gates open in response to stimuli
1.
2.
3.
Stretching of cell membrane
Electrical signals
Chemical signals
Ion Channels
Active Transport
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Differentiate between active & passive
transport
Compare & contrast endocytosis & exocytosis
Active Transport
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Movement of materials up a concentration
gradient, from an area of low
concentration to an area of higher
concentration
Requires cell to use energy
Uses carrier proteins
[ ] = Concentration
Active Transport
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Endocytosis &
Exocytosis
Transport of
macromolecules &
food particles too
large to pass to pass
through cell
membrane
Endocytosis


Process by which cells ingest large
materials by surrounding them with the
cell membrane forming a pouch
Pouch pinches off from membrane &
forms a vesicle
Endocytosis

Types:

Pinocytosis

Transports solutes or fluids
(cell drinking)
Endocytosis

Phagocytosis

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Ingestion large particles or whole cells (cell eating)
Many unicellular organisms feed this way
Used to ingest bacteria & viruses
Cells called phagocytes – allows lysosomes to fuse with vesicles
containing bacteria & viruses to destroy them
Exocytosis
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Reverse of endocytosis
Cell excretion
Vesicles in cytoplasm fuse with cell membrane to release
contents to external environment
Releases proteins packaged in Golgi apparatus
ENDOCYTOSIS/EXOCYTOSIS
Kidspiration by: Riedell