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Passive Transport
Bell Ringer
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What is osmosis?
Objectives
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Relate Concentration Gradients, diffusion, and
Equilibrium
Predict the direction of water movement into
and out of the cells
Describe the importance of ion channels in
passive transport
Identify the role of carrier proteins in facilitated
diffusion
Challenge

List at least two things that could cause the
movement of the dye in the beaker.
Challenge
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What are you observing?
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What process is at work?
Movement of particles in Cells
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Cells maintain Homeostasis by controlling the
movement of substances across the cell
membrane
Movement in Cells
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Passive Transport – movement across the cell
membrane that does not require energy
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Concentration Gradient – Difference in the
concentration of a substance across a distance.
Equilibrium
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A condition which the concentration of a
substance is equal throughout space.
Challenge

For equilibrium to occur, what has to happen to
the spheres in this picture?
Diffusion
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Diffusion – the movement of particles from
regions of high density to regions of lower
density
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Diffusion works quickly over short distances,
such as the dye moving through the beaker of
water.
Diffusion
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Many substances enter and leave the cell
through diffusion.
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When substances cross the cell membrane they
move “down” the concentration gradient. They
go from an area of high concentration to an area
of low concentration
Osmosis
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Osmosis – the diffusion of water through a
selectively permeable membrane. Movement
down the concentration gradient.
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Osmosis is a type of passive transport
Direction of Water
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3 Types
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Hypertonic
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Hypotonic
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Isotonic
Hypertonic Solution
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The fluid outside the cell has a lower free water
concentration than the cytosol, then the outside
fluid is hypertonic and water moves out of the
cell
Hypotonic

The fluid outside the cell has a higher free water
concentration than the cytosol, then the outside
fluid is hypotonic and water moves into the cell
Isotonic

The fluid outside the cell has the same free
water concentration than the cytosol, then the
outside fluid is isotonic and water moves into
and out of the cell at equal rates.
Crossing the Cell Membrane
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Transport Proteins called Channels provide
polar passageways through which ions and polar
molecules move across the cell membrane.
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Each channel only allows specific substances to
pass through the membrane
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Why is this important?
Diffusion through Ion Channels
Sodium
 Calcium
 Potassium
 Chloride
All are involved in many important cell
functions

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Ions
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Essential to the ability of nerve cells to send
electrical signals

Movement of ions allows your heart to beat
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Cross the cell membrane by diffusing through
ion channels.
Ion Channel
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Transport Protein with a polar pore through
which ions can pass.

The pore spans the thickness of the cell
membrane
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Why is this important?
Ion Channel
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Some pores are always open, others have a gate
which can be open or closed.
They can open or close in response to different
kinds of stimuli.
ex. Electrical charge or binding sites.
Facilitated Diffusion
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Facilitated Diffusion – a type of passive
transport that moves substances down their
concentration gradient without using the cells
energy.
Transport proteins transport substances across
the cell membrane. These are called carrier
proteins.
Facilitated Diffusion
Challenge

Why do drawers get sticky when there has been
a lot of rain or high humidity.
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During the winter , salt is sprinkled over icy
roads. Plants growing along side the road are
often seriously damaged or dead. Why does this
happen?