Cell Transport
Passive Transport
A. Introduction
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1. When organisms adjust internally to changing
external conditions, this is called homeostasis.
2. One way cells maintain homeostasis is by
controlling the movement of substances across
their cell membrane.
3. There are two main types of cell transport:
passive transport & active transport.
B. Passive Transport
• 1. Passive transport: movement across the
cell membrane that does NOT require
energy
• 2. 3 types include: diffusion, osmosis, and
protein-aided diffusion
• Concentration gradient: A difference
in the concentration of a substance
across a space.
• a. In all forms of passive transport,
substances move from an area of high
concentration to an area of low
concentration.
• b. This is called moving DOWN the
concentration gradient.
• Draw in movement down a
concentration gradient:
I. Diffusion: NO energy required!
• a. Diffusion depends on random particle
movements
• b. Movement continues until equilibrium
occurs
• c. Equilibrium: a condition in which
the concentration of a substance is
equal throughout a space.
• i. Even at equilibrium, particles
continue to move back and forth
II. Osmosis: NO energy required!
• a. Osmosis refers to the diffusion of water
through a selectively permeable membrane.
• b. Water will always move from an area of high
concentration to an area of low concentration.
• c. There are 3 types of solutions in osmosis:
hypertonic, hypotonic, and isotonic.
1. Hypertonic solution – water moves out of
a cell, the cell shrinks
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a. The fluid outside the
cell has a higher
concentration of dissolved
particles thus it has a
lower concentration of
“free” water molecules.
b. There is a higher water
concentration inside the
cell
• c. Water will move DOWN the
concentration gradient (out of
the cell) causing the cell to
shrink.
• i. plant cells will go flaccid,
plant will wilt
• ii. animal cells will shrink and
die if too much water is lost
Elodea in normal water
Elodea after adding salt
solution
1. Hypotonic solution – water moves into of
a cell, the cell swells
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a. The fluid outside the
cell has a lower
concentration of dissolved
particles thus it has a
higher concentration of
free water molecules.
b. There is a lower water
concentration inside the
cell
• c. Water will move DOWN the
concentration gradient (into of the
cell) causing the cell to swell.
• i. plant cells will become turgid due
to water pressing outward against
cell wall, helps plant cells keep their
shape (and plant standing upright!)
• ii. animal cells will burst if they take
in too much water
• Red blood cells in a hypotonic solution
1. Isotonic solution – no NET water
movement in or out of cell
• a. The fluid outside the
cell has the same
concentration of
dissolved particles as
inside the cell.
• b. State of equilibrium
exists.
• c. Water moves in and out of
cell at equal rates, so the cell
size stays the same.
• i. Animal cells do best in
isotonic solutions
• ii. Plants cells will survive but
prefer a hypotonic solution
Hypertonic, Isotonic,
Hypotonic?
• Isotonic
• Hypotonic
• Hypertonic
III. Protein-aided diffusion across
membrane:
• a. Certain molecules (ions and polar molecules)
cannot pass through the non-polar interior of cell
membrane (i.e. Na+, K+, Ca2+, Cl-)
• b. Transport proteins can provide a passageway
for ions & polar molecules across the cell
membrane
1. Ion Channels
• -Doughnut shaped transport protein with
polar pore through which ions can pass
• a. The pores of some ion channels will
always be open Other channels may be
closed off by a gate.
• b. The opening and closing of the channel
gates may be stimulated by:
• i. Stretching of the cell membrane
• ii. Change in electrical charge
• iii. Binding of specific molecules
2. Facilitated Diffusion
• -Carrier proteins transport
substances (amino acids &
sugars) down the
concentration gradient
• a. Specific molecules outside
the cell bind to carrier
proteins
• b. Carrier protein transports
the molecule across
membrane
• c. The molecule is released
inside, and the carrier protein
reverts back to normal shape
C. Rate of Diffusion
• 1. How fast or how slow diffusion
occurs depends on the temperature,
size of molecules, concentration of
molecules, and type of molecules
(polor vs. non-polar).