Passive Transport: Diffusion and
Osmosis
Section 3.4
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Objectives
• SWBAT describe passive transport
• SWBAT distinguish between osmosis,
diffusion, and facilitated transport.
• Main Idea 1: diffusion and osmosis are types
of passive transport.
• Main Idea 2: some molecules diffuse through
transport proteins.
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Vocabulary Section 3.4
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Osmosis (ósmosis)
Diffusion (difusión)
Passive transport (transporte pasivo)
Concentration gradient (gradiente de
concentración)
Isotonic (isotónica)
Hypertonic (hipertónica)
Hypotonic (hipotónica)
Solute (Soluto)
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Two Types of Transport
• There are two ways of transporting materials
across a cell membrane. They are:
– Passive Transport – the movement of molecules
across a cell membrane without energy input
from the cell (the diffusion of molecules across a
membrane).
– Active Transport – actively drives molecules across
the cell membrane from a region of lower
concentration to a region of higher concentration.
Requires energy input.
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Diffusion
• Diffusion - is the movement of
molecules in a fluid or gas from a
region of higher concentration to
a region of lower concentration.
• Concentration gradient – is the
difference in the concentration of
a substance from one location to
another.
Higher
concentration
Lower
concentration
The egg swelled because water
molecules moved from outside the egg,
higher water concentration, to inside the
egg where there was a lower
concentration of water molecules.
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Diffusion
• Diffusion results from the natural motion of
particles. Diffusion across a membrane from higher
to lower concentrations is part of nature – it is a
natural force.
• Concentration is
defined as the
number of
molecules of a
substance in a
given volume.
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Diffusion
• In cells, diffusion is indispensable in moving
substances across the cell membrane!
– Small lipids (remember our hormone example –
aldosterone) and other nonpolar molecules (like
carbon dioxide and oxygen) easily diffuse across
the membrane.
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Oxygen Diffusion Example
• Most cells continually consume oxygen.
• Thus, oxygen concentration is almost always higher outside
the cell than inside it.
• As a result, oxygen almost always is diffusing from outside the
cell, across the cell membrane, to the inside of the cell.
• The cell expends no energy.
Oxygen high
concentration
Oxygen
Diffuses
Lower oxygen
concentration
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Osmosis
• Water molecules also diffuse across
semipermeable (selectively permeable)
membranes from higher concentration to
lower concentration, which we call osmosis.
• Osmosis – the diffusion of water molecules
across a semipermeable membrane (a cell
membrane in our case) from higher
concentration of water molecules to lower
concentration of water molecules.
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Osmosis – Our Naked Egg Example
In our naked egg experiment,
we saw osmosis at work.
• First, we saw water move
from the vinegar (where
there was a higher
concentration of water
molecules) into the egg
(where there was a lower
concentration of water
molecules. The egg
expanded.
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Osmosis – Our Naked Egg Example
• Then, we put the egg in
corn syrup. In the corn
syrup there was a higher
concentration of water
molecules in the egg than
the corn syrup.
• Therefore, the water
flowed from the egg,
higher concentration, to
the corn syrup (lower
concentration).
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Isotonic, Hypertonic, Hypotonic
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Isotonic, Hypertonic, Hypotonic
• Isotonic – a solution is isotonic to a cell if it has the
same concentration of dissolved particles, solutes, in
the solution outside the cell as inside the cell.
– Example: water molecules move into and out of the cell at
equal rates because the concentration of dissolved
particles is the same outside the cell as inside the cell.
– This is a state of equilibrium (remember the word from
biochemistry).
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Isotonic, Hypertonic, Hypotonic
• Hypertonic – a solution is hypertonic to a cell if it has
a higher concentration of dissolved particles than
inside the cell.
– Example: if there is a higher concentration of dissolved
particles in the solution outside the cell than inside the
cell, water will move out of the cell into the solution.
Think of our egg in corn syrup (the
corn syrup was hypertonic to the
cell. Water from in the cell flowed
out of the cell into the corn syrup.
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When we say solutes,
as in “a hypertonic
solution has more
solutes than a cell,”
we are talking about
the concentration of
dissolved particles
(for example, salts,
sugars, etc.)
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Isotonic, Hypertonic, Hypotonic
• Hypotonic – the solution outside the cell has a lower
concentration of dissolved particles than that inside
the cell.
– Example: water molecules will diffuse into the cell when the inside of
the cell has a higher concentration of dissolved particles and a lower
concentration of water molecules than outside of the cell.
Water flowed from the vinegar, where there
was a lower concentration of dissolved
particles, through the cell membrane to the
interior of the egg (where there was a higher
concentration of dissolved particles).
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Facilitated diffusion
• Facilitated diffusion – is the diffusion of molecules
across a cell membrane through transport proteins.
Facilitated diffusion is for molecules
that cannot easily diffuse across a cell
membrane.
The word facilitate means to make
easier – thus, the transport proteins
facilitate (make easier) the passage of
some molecules into our out of the cell.
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Facilitated diffusion
This is still passive transport since the cell
does not need to use any of its energy.
There are many types of transport proteins
– most types only allow specific molecules
to pass through.
The transport proteins act as tunnels
through the cell membrane.
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Question
• What will happen to a houseplant if you
water it with salt water (a hypertonic
solution)?
• Why are transport proteins needed in the cell
membrane?
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