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Passive Transport
Section 4-1
Alabama State Course Objectives
to Be Covered
 AL COS 2
Describe cell processes necessary for
achieving homeostasis, including active and
passive transport, osmosis, diffusion,
exocytosis, and endocytosis.
 AL COS 2.2
Comparing the reaction of plant and animal
cells in isotonic, hypotonic, and hypertonic
solutions.
Diffusion is Caused by the
Random Movement of Particles
 RECALL… When an organism adjusts
internally because the external environment
is changing, this is HOMEOSTASIS.
 One way cells maintain homeostasis is by
controlling the movement of substances
across the cell membrane.
Random Motion and
Concentration
 Passive Transport – movement across the
cell membrane that DOES NOT require
energy.
 Gradient – a difference in the concentration
of a substance across a space
 Equilibrium – a condition in which the
concentration of a substance is equal
throughout a space
Movement of Substances
 If there is a concentration gradient in the
solution, the substance will move from an
area of high concentration to an area of
lower concentration.
 Diffusion – movement of a substance from
an area of high concentration to low
concentration caused by the random motion
of particles
Movement of Substances (con’t)
 Many substances (molecules and ions) dissolved
in cytoplasm and in fluid outside the cell
enter/leave cells by diffusion
1. Inside cell – concentration of most of these
substances are different from concentration
outside cell. This forms a concentration
gradient across the cell membrane.
2. To diffuse “down” the concentration
gradient, the molecule/ion goes from an area
of high concentration to an area of low
concentration across the cell membrane.
Movement of Substances
(con’t)
 Cell Membrane is selectively permeable.
1. Nonpolar interior of lipid bilayer repels
ions and most polar molecules. This
prevents diffusion across the cell
membrane of these substances.
2. Very small molecules or nonpolar
molecules can diffuse across cell
membrane down their concentration
gradient.
 Diffusion of small or nonpolar molecules is
the simplest type of passive transport.
Water Diffuses into and out of
Cells by Osmosis
 Osmosis – diffusion of water through a
selectively permeable membrane
 Osmosis involves the movement of water
down its concentration gradient.
 The direction of water movement across cell
membrane depends on relative
concentrations of free water molecules in
cytoplasm and in fluid outside of cell.
Water Diffuses into and out of
Cells by Osmosis (con’t)
 In the solutions on either side of the cell
membrane, many ions and polar molecules are
dissolved in water. When these substances
dissolve in water, water molecules are attracted to
them and so are no longer free to move around. If
solutions on either side of cell membrane have
different concentrations of dissolved particles, they
will also have different concentrations of “free”
water molecules. Osmosis will occur as free water
molecules move into the solution with the lower
concentration of free water molecules.
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2
/animation__how_osmosis_works.html
Three Possibilities for Direction of
Water Movement
 Water Moves Out
When water diffuses out of the cell, CELL
SHRINKS
Hypertonic – solution that causes a cell to
shrink because of osmosis.
If fluid outside cell has a higher concentration of
dissolved particles than cytoplasm, outside fluid
has lower concentration of free water molecules
than cytoplasm.
Three Possibilities for Direction of
Water Movement (con’t)
 Water Moves In
When water diffuses into cell, CELL SWELLS
Hypotonic – a solution that causes a cell to
swell because of osmosis
If fluid outside the cell has a lower concentration
of dissolved particles than cytoplasm has, then
the outside fluid also has a higher concentration
of free water molecules than cytoplasm
Three Possibilities for Direction of
Water Movement (con’t)
 No Net Water Movement
Cytoplasm and fluid outside cell have same
concentration of free water molecules isotonic, water diffuses into and out of the cell
at equal rates.
Results in no net movement of water across cell
membrane
Cell stays the same size
http://www.tvdsb.on.ca/westmin/science/sbi3a1/C
ells/Osmosis.htm
Animal Cells and Plant Cells
 If left unchecked, a hypotonic solution can cause a
cell to burst. Different cells have evolved different
methods to deal with this.
 Plants and Fungi – rigid cell walls to prevent cell
from expanding too much
 Unicellular Eukaryotes – contractile vacuoles –
collect excess water and force it out
 Animal Cells – avoid swelling by osmosis by
removing dissolved particles from cytoplasm. This
increases concentration of free water molecules
inside cell.
Proteins Help Some Substances
Cross the Cell Membrane
 Ions and polar molecules cross the cell
membrane with the help of transport
proteins
 Transport Proteins form channels or polar
passageways
 Each channel allows only specific
substances to pass through cell membrane
 This selectivity enables a cell to control what
enters and leaves.
Diffusion Through Ion Channels
 Ions such as Na+ (sodium), K+ (potassium),
Ca2+ (calcium), and Cl- (chloride) are
involved in many important cell functions.
 These ions diffuse across the cell
membrane through ion channels.
 Ion Channel – doughnut-shaped transport
protein with a polar pore through which ions
can pass.
Diffusion Through Ion Channels
(con’t)
 Pores of some ion channels always open.
Others, the pores can be closed by ion
channels gates
 Ion channel gates may open or close in
response to several stimuli (ex. Stretching of
the cell membrane, change in electrical
charge, or the binding of specific molecules
to the ion channel).
 Stimuli able to control ability of particular
ions to cross cell membrane
Diffusion Through Ion Channels
(con’t)
 Passive transport because no use of energy
because ions move down their
concentration gradients
 The effect of electrical charge on ion
transport
Rate of movement of a substance across cell
membrane is generally determined by the
concentration gradient of the substance.
Movement of a charged particle across cell
membrane influenced by particle’s positive or
negative charge.
Diffusion Through Ion Channels
(con’t)
Inside Cell – negative
Outside Cell – positive
Opposite charges attract, so positive ions
typically diffuse into cell and negative ions
typically diffuse out of cell.
Direction of movement caused by an ion’s
concentration gradient may oppose the direction
of movement caused by ion’s electrical charge.
 http://www.cellsalive.com/channels.htm
 http://www.bioanim.com/CellTissueHumanB
ody6/index.html
Facilitated Diffusion
 Carrier Proteins – bind to specific
substances on one side of the cell
membrane and carry the substance to the
other side. It then releases the substance.
 Facilitated Diffusion – type of passive
transport because it moves substances
down their concentration gradient without
using the cell’s energy.
Facilitated Diffusion (con’t)
 Figure 4-4 p. 80
 Carrier protein binds substance and
changes shape
 Change in shape of carrier protein exposes
substance to other side of cell membrane.
Molecule is transported across cell
membrane.
 Carrier protein shields molecule from interior
of cell membrane. Molecule released from
carrier protein. Carrier protein back to
original shape.
Facilitated Diffusion
Review
 Passive transport is the movement of
substances across the cell membrane
without the use of energy by the cell.
 Diffusion is the movement of a substance
from an area of high concentration to an
area of lower concentration.
 Osmosis is the diffusion of free water
molecules across a selectively permeable
membrane.
 Ion channels are proteins that have a pore
through which ions can cross the cell
membrane.
Review
 In facilitated diffusion, a carrier protein
transports a substance across the cell
membrane down the concentration gradient
of the substance.
HOMEWORK
Section 4-1 Review Questions
Page 80
Questions 1-5