Chapter 5 Homeostasis
and Cell Transport
Section 1 Passive Transport
Passive Transport
 Substances cross the cell membrane without
any input of energy
 Ex. diffusion
Red dye
diffusing into
water
Diffusion
 The movement of
molecules from an
area of high
concentration to an
area of lower
concentration
Diffusion occurs until equilibrium is
reached…
 …the
concentration
of molecules
is the same
throughout a
space
Diffusion Across Membranes
 Called simple diffusion- cell membranes only
allow certain substances to pass through
Cell membranes are
semi-permeable
Materials that enter the cell through
simple diffusion…
 1) carbon dioxide
 2) oxygen
Earthworms
“breathe” through
their skin!
Osmosis- a form of passive transport
concerning water
 The movement of water from an area of high
concentration to an area of low concentration
The direction of osmosis…
Water moves
out- cell
shrivels
Same amt. of
water moves
in and out
Water moves
in-cells may
burst
What is the fate of the cell in each
condition???
Isotonicnothing; it’s
normal
 Hypotonic- burst
Hypertonicshrivel/shrink
Hypotonic (Hippo-)
 The cell gets
bigger.
For red blood cells, this is fatal…
Hypotonic 
isotonic 
hypertonic
How Cells deal with Osmosis
 For unicellular freshwater organisms, this is a
problem. Some of them have special organs
for removing water.
Ex. Paramecia have pumps called contractile
vacuoles that constantly remove water from their
bodies
Section 1 cont.
Turgor pressure- the
pressure that water
molecules exert
against the cell wall
Do the cells of this plant
have turgor pressure?
 Plasymolysis- when a plant doesn’t receive
enough water and the cell membrane shrinks
from the cell wall; turgor pressure is not
maintained
Facilitated Diffusion
 A type of passive transport that is used for
1) molecules too large to pass through
the cell membrane
2) molecules that do not dissolve in lipids
Facilitated diffusion uses a carrier protein.
What might
this
molecule be?
Facilitated
Diffusion
1.
2.
3.
4.
Molecule
attaches to
protein.
Protein
changes
shape.
Molecule is
released to
other side.
Protein
returns to
original
shape.
Ion Channels
 Membrane proteins move Na+, K+, Ca+2, Cl-
across the cell membrane
Three kinds of stimuli determine
whether “gates” are open:
 1) stretching of the cell membrane
 2) electrical signals
 3) chemical signals