Chapter 5
Homeostasis
and
Transport
Homeostasis & Transport
BIG IDEAS:
Cell Membranes help organisms
maintain homeostasis by controlling
what substances enter and leave the
cell.
Some substances can cross the cell
membrane without any energy input
from the cell.
Some substances require a cell to
expend energy to enter or leave the
cell.
How Materials Enter and Leave Cells
How does the cell move
materials into and out of a
cell?
For cells to remain alive, a
certain combination of
chemicals must be able to be
present inside its cell
membrane.
One way molecules of a substance
get into and out of a cell is by a
process known as passive transport
The cell membrane is one of
the structures that helps
maintain this special
environment inside and
outside the cell.
Passive transport is the diffusion
or movement of molecules across a
cell membrane. It does NOT
REQUIRE any ENERGY input from
the cell.
The Cell Membrane
The cell membrane
is Selectively
Permeable.
Meaning, it allows
the passage of some
substances and
blocks other or not
required substances.
The Cell Membrane
This selectively permeable
membrane only allows certain
things in/out of the cell.
•Made of protein and lipids
•BILAYER-two layers; the
head of the molecules face
outward while the tails face
inwards.
PHOSPHOLIPID-molecule
that makes up the cell
membrane. Has two parts.
1. The phosphate head
(hydrophilic – water loving)
2. Two lipid tails
(hydrophobic – water hating)
Passive transport
Substances crossing the cell
membrane without any input of
energy by the cell – move down
their concentration gradient
Types:
• Simple Diffusion
• Osmosis
• Facilitated Diffusion
• Ion Channels
http://www.youtube.com/watch?v=JShwXBWGMy
Molecules diffuse or
move from an area of
higher concentration
to an area of lower
concentration. THEY
MOVE DOWN A
CONCENTRATION
GRADIENT!
?? Concentration Gradient
= a gradient results from an unequal
distribution of dissolved molecules
across the cell membrane. When this
happens, solutes downconcentration
gradient.
This kind of movement is called
diffusion.
DOWN A
CONCENTRATION
GRADIENT.
Diffusion
The simplest type of passive
transport is diffusion. Diffusion
is the natural movement of
molecules from an area of high
concentration to an area of low
concentration.
Molecules naturally intermingle
as a result of their kinetic
energy of random motion.
Diffusion Stops
when…Molecules reach
EQUILIBRIUM
The concentration of the
molecules is the same
throughout a space.
No more diffusion! But
molecules keep moving.
EXAMPLE: food coloring,
cookie smell in your
house, etc.
Diffusion
Diffusion
Diffusion Across a Cell
Membrane
REMENBER - The cell
membrane is Selectively
Permeable. There are tiny
pores or holes in it that
allow some molecules to
diffuse into the cell.
For example, oxygen
molecules diffuse into your
blood stream through the
membranes of cells in your
lungs.
http://videos.howstuffworks.com/hsw/
23424-cell-diffusion-across-the-cellmembrane-video.htm
Osmosis
A cell is made up mostly of water
and dissolved substances. Water
can move into or out of a cell
through the cell membrane.
The diffusion of water through a
cell membrane is called osmosis.
Osmosis takes place
through passive transport.
Osmosis
The direction that water
moves depends on the amount
of dissolved substances inside
and outside of a cell.
If there are more dissolved
substances inside the cell,
water will move into the cell
and vise-versa.
Water will move to where
there is LESS WATER.
Osmosis maintains the
balance of water inside and
outside the cell.
Question??
If there is more salt outside of
the cell, will water move into the
cell or out of the cell??
Water
goes
where it
aint !!
Example of
Osmosis
http://www.stolaf.edu/people/giannini/flashanimat/transport/osmosis.swf
3 Types of Environments
The net movement of water,
or osmosis, depends on the
type of environment the cell
is in.
Three types of environments:
1. Isotonic
2. Hypertonic
3. Hypotonic
1. Isotonic:
Concentration of water is equal inside
and outside of the cell.
•Water will diffuse into and out of
the cell at an equal rate
•No net movement of water.
•Cell stays the same.
2. Hypertonic
More water inside the cell, less
outside.
More solutes outside the cell than
INSIDE the cell.
•Water moves out of the cell until
equilibrium is reached.
•Net movement of water is out of
the cell.
•Can cause the cell to shrink or
shrivel.
•Crenation in animal cells
•Plasmolysis in plant cells
3. Hypotonic
More water outside the cell, less
inside the cell.
More solutes inside, less solutes
OUTSIDE the cell.
• Net movement of water into the
cell.
• Can cause cell to burst.
• CYTOLYSIS-cells take in so
much water that they burst.
The Effects of Osmosis on
Animal Cells
osmosis
Osmosis in Plants
FYI - Plants whose cells do not have enough
water appear dry and may droop.
Remember
Plant cells store water
in vacuoles.
http://www.kscience.co.u
k/animations/turgor.htm
However, once the plant receives
water, its cells may absorb the water through
osmosis.
Once water is absorbed into its
cells, the plant swells.
It will probably stand up
straight and appear healthy. This is referred
to as “Turgor” and is determined by osmosis.
DRAW
Osmosis in Plant Cells
Plant Cell
Cell Wall
Vacuole
The Cell absorbs water
by osmosis…
Cytoplasm
Cell Membrane
Plant cell
membranes can be
seen pulling away
from their cell
wall, in a process
called
plasmolysis, as
their cells lose
water.
…But the cell wall stops
the cell from expanding
DRAW
Cells in 3 Types of Solution!
cytolysis
crenation
plasmolysis
Remember:
SALT SUCKS!!!
Wherever there is more solute (i.e.
salt, sugar), that is where water will
be pulled to 
WATER WILL MOVE TO WHERE IT
AINT!!
Do Now
Respond to the following in
your journal:
Why do supermarkets have
sprinkler systems set up in
their produce section?
Explain.
Facilitated Diffusion
Facilitated Diffusion:
Facilitated diffusion is the form of
passive transport in which carrier
proteins embedded in the cell
membrane help or facilitate the
movement across it.
It does not require the cell to use
any energy.
Like diffusion, molecules
are still moving from an
area of higher
concentration to an area of
lower concentration.
Ion Channels
Some ions are important for cell functions
(Na+, K+, Ca2+ and Cl-)
Since they are charged and hydrophilic,
they can’t get across the lipid bilayer.
Ion channels – specific membrane
proteins that help ions get across c
ell membrane – some are always
open, some are gated and require a
stimuli to open.
• EXAMPLE: impulse from nerve travels
to muscle cell-causes some gated
channels to open.
• It does not require the cell to use any
energy.
Like diffusion, molecules
are still moving from an
area of higher
concentration to an area of
lower concentration.
Active Transport
Active Transport:
Movement through the cell membrane that
is the opposite of diffusion.
Molecules of certain substances move
through the cell membrane from an area that
is less crowded to an area that is more
crowded is called ACTIVE TRANSPORT.
• Energy is needed to move these materials
across the cell membrane - Molecules move
AGAINST the concentration gradient!!
http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related
Most plants get water
and minerals they need
from their roots.
Minerals enter the
root by active
transport.
Active transport is
also needed to move
certain waste
materials out of a cell.
Types of Active
Transport
• Cell Membrane Pumps
• Endocytosis
• Exocytosis
Cell Membrane Pumps
Carrier proteins also help
out in active transport and
work against the
concentration gradient:
Low
High
Similar to facilitated
diffusion, but a cell’s
energy is required.
SODIUM (Na+)/Potassium
(K+) Pump
Uses a special carrier
molecule.
•K+ (potassium) goes
into the cell.
•Na+ (sodium) leaves
the cell
Uses energy from the cell
Passive vs. Active
Transport
Low Concentration
High Concentration
Passive Transport
High Concentration
Active Transport
Low Concentration
Endocytosis & Exocytosis
Cells have two processes of moving large
particles across the cell membrane:
Endocytosis and Exocytosis
Endocytosis is the process by which the cell
Membrane wraps around the large particle,
forms a vesicle, and pulls it inside the cell.
Once inside, the vesicle releases the particle.
2 Ways:
1. Pinocytosis-movement of solute or fluid into the cell.
2. Phagocytosis-movement of large food particles or
entire microorganisms into the cell.
Endocytosis & Exocytosis
Exocytosis is exact opposite.
When the cell wants to get rid of a large
particle, it encloses the particle inside a
vesicle. The vesicle fuses with the cell
membrane and the cell can then release
the particle.
http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media
/interactivemedia/activities/load.html?7&G
Endocytosis & Exocytosis