Ch 4
PASSIVE AND ACTIVE
TRANSPORT IN CELLS
Passive Transport

Movement across the
cell membrane that
does not require the
cell’s energy
Concentration Gradient

A difference in concentration of a substance
throughout a space
Equilibrium

Concentration of a
substance is equal
throughout a space
Diffusion

Movement of a substance from an area of high
concentration to an area of low concentration
Selectively Permeable Membrane

Only allows some
substances to pass
through
Draw this cell membrane
oxygen, carbon
dioxide, and other
small, nonpolar
molecules; some
water molecules
glucose and other large,
polar, water-soluble
molecules; ions (e.g.,
H+, Na+, K+, Ca++,
Cl–); water molecules
Diffusion Through Ion Channels



A transport protein with a polar pore
that allows ions to diffuse into and out
of the cell
The electrical charge of an ion can
affect its ability to diffuse- The inside
of a typical cell is negative, so it is
more likely a positive ion will diffuse
into the cell and a negative ion will
diffuse out.
Opposites attract and like charges
repel
Carrier Proteins

Most cells have a different kind of protein that can
bind to a specific substance on one side of the cell
membrane and carry it to the other side
Facilitated Diffusion

Carrier proteins are
used to transport
specific substances
without using the
cell’s energy
OSMOSIS

Diffusion of water across a selectively
permeable membrane



Hypertonic- water diffuses out of cell- cell
shrinks
Hypotonic- water diffuses into cell- cell swells
Isotonic- water diffuses into and out of cells
at equal rates
water molecules
semipermeable membrane
between two compartments
protein molecules
2%
sucrose
solution
1 liter of
distilled water
1 liter of
10% sucrose
solution
Hypotonic
Conditions
Hypertonic
Conditions
1 liter of
2% sucrose
solution
Isotonic
Conditions
VESICLES

Substances that are too large to be
transported by carrier proteins, such as
proteins and polysaccharides, are moved
across the membrane by vesicles


Endocytosis- movement of a substance into a
cell by a vesicle
Exocytosis- movement of a substance out of a
cell by a vesicle
ENDOCYTOSIS
EXOCYTOSIS
Active Transport


Movement of a substance across the cell
membrane against the concentration
gradient – against diffusion i.e. from low
to high concentration
REQUIRES ENERGY from the cell supplied
by ATP
Receptor Proteins

Cells must also respond to important
information and filter out unimportant
information. To receive messages sent by
signal molecules, cell membranes contain
specialized proteins
called receptor proteins that
bind to the signal molecules
allowing the cell to respond
to the signal