Biology: 4.1 Cells and Their Environment
Passive Transport
in Cells
Today’s Objectives: Key ideas
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All organisms and their cells respond to external conditions
to maintain a constant internal condition (homeostasis)
One way cells maintain homeostasis is by controlling the
movement of substances across their cell membrane.
Cells must use energy to transport some substances across
the cell membrane. (active transport)
Other substances move across the cell membrane without
any use of energy by the cell. (passive transport)
Passive Transport
Movement across the cell membrane that does not
require energy from the cell is called passive transport
1: Randomly moving
particles are on one side of
a barrier.
2: When openings occur,
the particles begin to
gradually enter the
adjacent space, to diffuse
to the other space.
Passive Transport
Movement across the cell that does not require energy
from the cell is called passive transport
3: When the point of
equilibrium is reached, the
diffusion of particles is equal
in both spaces. Diffusion
continues until equilibrium is
reached.
4: A difference in the
concentration of a substance
across the space is called a
concentration gradient.
Movement of Substances
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The movement of a
substance from an area
of high concentration to
an area of lower
concentration is called
diffusion.
Dyes diffusing into water
Movement of Substances
•Many substances, such as
molecules and ions dissolved
in the cytoplasm and in the fluid
outside cells, enter or leave cells
by diffusion across cell
membranes.
•The cell membrane is selectively
permeable to certain substances
•Selectively permeable means it
allows certain substances to pass
and holds other substances back.
Osmosis
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Water molecules can diffuse through
channels in the cell membrane.
The diffusion of water through a
selectively permeable membrane is
called osmosis.
Like other forms of diffusion, osmosis
involves the movement of a substance
without energy.
Osmosis is a passive transport
Osmosis
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The direction of water movement
across the cell membrane
depends on the concentrations of
free water molecules in the
cytoplasm and in the fluid outside
the cell.
There are three possibilities for
the direction of this water
movement . . . .
Osmosis
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1: Water moves
out: cell shrinks
When water diffuses out of the
cell, the cell grows smaller.
A solution that causes a cell to
grow smaller because of osmosis
is called a hypertonic solution.
This occurs because the fluid
outside the cell has less free
water molecules than the fluid
inside the cell.
Osmosis
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2: Water moves in:
Cell grows
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When water diffuses into the cell,
the cell grows larger.
A solution that causes a cell to
grow larger because of osmosis is
called a hypotonic solution.
This occurs because the fluid
outside the cell has more free
water molecules than the fluid
inside the cell.
Osmosis
3: No net water
movement: No change
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If the cytoplasm and the fluid
outside the cell have the same
concentration of free water
molecules, water diffuses in and out
of the cell at equal rates.
This results in no change and the
cell stays the same size.
A solution that produces no change
in cell size because of osmosis is
called isotonic solution.
Osmosis
Summary: 3 types of
Osmosis
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Water moves out through
membrane: cell grows smaller:
hypertonic solution
Water moves into cell: cell
grows: hypotonic solution
Water moves in/out equal
amounts: no change in size:
isotonic solution
Transport Proteins
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Recall that most ions and polar
molecules cannot pass across the
cell membrane because they
cannot pass through the nonpolar
interior of the lipid bilayer.
However, such substances can
cross the cell membrane when
they are aided by transport
proteins.
Transport proteins called channels
provide polar passageways through
which ions and polar molecules can
move across the cell membrane.
Lipid bilayer with
channel
Transport Proteins
Transport proteins called channels
provide passageways through which
ions and polar molecules (+ or –
charge) can move across the cell
membrane.
Each channel allows only a specific
substance to pass through the cell
membrane.
This selectivity is one of the most
important properties of the cell
membrane because it enables a cell
to control what enters and leaves.
Diffusion Through Ion Channels
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Ions such as sodium, potassium,
calcium, and chloride are
involved in many important cell
functions.
Although ions cannot diffuse
through the nonpolar interior
of the lipid bilayer, they can
cross the cell membrane by
diffusing through ion channels.
An ion channel is a transport
protein with a polar pore
through which ions pass.
Electrical Charge and Ion Transport
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The movement of a charged
particle, such as an ion, across a
cell membrane is also influenced by
the particle’s positive or negative
electrical charge.
The inside of a typical cell is
negatively charged with respect to
the outside of the cell (which is
positively charged).
Opposite charges attract and like
charges repel.
Electrical Charge and Ion Transport
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Thus a more positively charged ions(+)
located outside the cell are more
likely to diffuse into the cell, where
the charge is negative.
Conversely, negatively charged ions (-)
located inside the cell are more likely
to escape the cell and diffuse outward
where the charge is positive.
Thus, an ion’s electrical charge often
affects the movement of the ion
across the cell membrane.
Facilitated Diffusion
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carrier proteins are a
different kind of transport
protein that can
1. bind to a specific
substance on one side of a
cell membrane,
2. carry the substance across
the cell membrane,
3. and release it on the other
side.
They carry the substance
across the cell’s membrane
through these channels.
Facilitated Diffusion
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When transport proteins are
used to move specific
substances across the
membrane (down their
concentration gradient) the
transport is called facilitated
diffusion.
This transport occurs without
using
the cell’s energy so it is a
passive transport.