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Homeostasis means maintaining a
balance internally despite what is going
on externally.
 Organisms must adjust to changes in the
environment in order to survive or they
could die!
For example, if it is below
freezing outside, you must
put on a heavy coat,
gloves, double socks, a
warm hat, ear muffs, and
lots and lots of layers to
maintain a healthy
temperature despite the
EXTERNAL temperature
What structure maintains homeostasis in
our cells? The Cell Membrane
 How does it maintain homeostasis?
By controlling the passage of molecules,
such as water, salts, food particles, etc in
and out of the cell.
(it regulates what enters and leaves)
How? Through passive transport,
active transport and cell to cell
The Cell Membrane is:
 A phospholipid bilayer
 Two layers made of proteins and lipids
Phospholipid Structure:
 Polar Head (hydrophilic—”water loving”)
 Nonpolar Tails—(hydrophobic—”water fearing”)
Polar Head
Nonpolar Tails
Bilayer arrangement
 Draw:
The proteins embedded in the cell membrane
help it to function properly
 3 Types of Membrane Proteins
Transport — helps molecules enter or leave the cell
Marker — identify the cell
Receptor — allow cells to communicate
Transport Proteins
 Function as
Allow sugars, salts, etc
to cross the membrane.
Special channel proteins:
› Gated ion channels—
gates that open/close
› Carrier proteins—change
shape to allow specific
molecule to pass
› Aquaporins - allow water
to diffuse through (called
Marker Proteins
 Cell’s “Name Tag”
 Protein sticks out of phospholipid
 Often has carbohydrates attached
to outside end
 Functions in cell identification to
identify the cell to other cells and
Marker Protein
 Important in
› immunity—so various white blood
cells in your body do not mistake
your cells for foreign cells
- blood typing – so you can’t
receive just any ol’ type of blood
Receptor Proteins
 Function as
 Receive information from
the environment
(extracellular fluid, blood,
interstitial fluid) and
transmit that info to the
inside of the cell
 Protein has specific
shape/charge to only
allow certain molecules
(like hormones) to bond
 Triggers a response in cell
Receptor Protein
 Remember
these proteins
are embedded
or floating in the
lipid bilayer.
The Cell Membrane is:
 Selectively permeable/semipermeable
 permeable means to let objects travel through
 so if it is selectively or semi permeable it only
allows SOME things through and not others
This represents a selectively permeable
It allows (selects) certain things to pass
through it.
Is the membrane permeable to
Is the membrane permeable to
Molecules pass through the cell
membrane through:
› Passive Transport
 Diffusion
 Facilitated Diffusion
 Osmosis
› Active Transport
 Vesicles
 Pumps
Molecule- the smallest unit
of a compound/substance
› Cannot be seen with the
naked eye
› 1 drop of water has
molecules of H20
› 1 grain of salt has
molecules of salt
Concentration Gradientan area of high concentration next to an
area of low concentration
of Molecules
of Molecules
Equilibrium is when there is an EQUAL amount
of molecules on each side of the membrane
There is no net movement
meaning, there is continuous
movement of molecules
back and forth through the membrane.
Passive Transport is when molecules
move from an area of HIGH
concentration to an area of LOW
concentration until equilibrium is
 Requires NO ENERGY!
 Examples:
› Diffusion
› Osmosis
› Facilitated Diffusion
Diffusion - the process by which
molecules (“stuff”) spread from
areas of high concentration, to
areas of low concentration
Molecules are said to go
“Down” or “with” the
concentration gradient.
Requires no energy
What will
is the
the gate is
of cows?
This process is called…
Facilitated Diffusion
› What does facilitate mean?
› When substances move from high to low
concentration (down the concentration
gradient) using channel/carrier proteins
located in membrane
› Does not require energy
Click, hold, and drag slider to move forward, bckword, or slow down
A special type of diffusion is called
 Osmosis- the process by which WATER
molecules move from an area with a
high concentration of water to an area
of low concentration of water.
› OR the “diffusion of water”
Requires no energy
In Osmosis water will move in where there is
a low concentration of SOLVENT, and a
high concentration of SOLUTE
›A solute = “stuff” (salt, glucose, food particles)
›A solvent = “water”
There are three ways
water can move in a
› Hypotonic- water moves
INTO a cell and the cell
swells (gets bigger)
› Hypertonic – water moves
OUT of a cell and the cell
shrinks (gets smaller)
› Isotonic- water moves
into AND out of a cell at
an equal rate
(remember- molecules
are constantly moving)
Hypotonic -Describes a
solution that is LESS
less solute, more
o When a cell is placed
in a hypotonic solution,
the cell will swell as
water moves INTO the
Hypotonic (continued) there is more
solvent/less solute on
the outside of the cell
that the inside
 The solvent (water)
moves to an area of
low solvent (water)
 The cell could burst
Hypertonic-- describes
a solution that is
more solute, less
o When a cell is placed
in a hypertonic
solution, water moves
out of a cell and the
cell shrinks (gets
Hypertonic (continued)
 There is less
solvent/more solute
on the outside on the
cell than the inside
 The solvent (water)
moves to an area of
low solvent (water)
Isotonic – describes a
solution that is of EQUAL
concentration to another
water moves into and out
of the cell at an equal
rate(remember –
molecules are constantly
No change in the size of
the cell will be observed
The concentration of
solvent and solute on the
inside of the cell is equal
to the concentration of
solvent and solute on the
outside of the cell
Draw arrows to show the flow of water
 Label at the top if the solution is isotonic,
hypotonic or hypertonic
Where is the greatest
concentration of solvent?
Outside of the bag
Which way does the
water move?
Into the Bag
What type of
solution is this?
selectively permeable membrane
Where is the greatest
concentration of
In the bag
Which way does the
water move?
Out of the bag
90% water
10% salt
What type of solution is
selectively permeable membrane
What happens to
the bag?
90 % solvent
10% salt
selectively permeable membrane
What happens to
the bag?
90 % solvent
10% salt
selectively permeable membrane
What happens to
the bag?
90 % solvent
10% salt
selectively permeable membrane
What happens to
the bag?
The bag shrinks.
90 % solvent
10% salt
selectively permeable membrane
Passive transport Recap:
› requires no energy
› moves from high concentrations to low
› Moves down the concentration gradient
› Includes diffusion, facilitated diffusion and
When molecules are forced from a low
concentration to a high concentration
they must use active transport
Cells must transport certain amino acids,
sugars, etc. into their cytoplasm from the
surrounding fluid.
 Some of these substances, however, are
already in higher concentrations inside
versus outside.
This requires energy!
Opposite of Passive Transport
 Forces molecules to go against the
concentration gradient
 Energy is provided by the ATP made in
the mitochondria
Pumps allow molecules to travel against
their concentration gradient
1. Sodium/Potassium Pump
 For example, the sodium/potassium
pump actively transports sodium
molecules and potassium molecules
through the cell membrane
 Requires energy
 Na+
pumped out of a cell
 K+ pumped into a cell
 Important
because it
prevents cells from bursting
by lowering the sodium
inside causing less water to
enter through osmosis.
Sodium Potassium Pump
Proton (H+) pump – forces protons out of a
membrane enclosed space (organelle or cell), often
to create a proton gradient down which the
protons can flow back in
Why would the cell “waste” energy on a proton
*Because the cell needs isolated areas of the cell
with different pH for particular functions; ex)
lysosomes – have proton pumps to maintain a
*Because the cell only uses one ATP to pump a
proton out, and that proton can be used in cotransport
Co-transport – process cells use to bring large
molecules, such as sugars, into a cell with a
minimum amount of energy used; usually a
proton and a sugar enter a double tunneled
protein at the same time; the tunnel only
“works” when both molecules are present
Vesicles can transport molecules across the
cell membrane through:
› Endocytosis- moving INTO the cell
 Pinocytosis – moving a liquid into a cell
 Phagocytosis – moving a solid into a cell
Pinocytosis (drinking)
Phagocytosis (eating)
› Exocytosis- moving OUT of the cell
 Excretion – moving waste out of the cell after
something is digested by a lysosome
 Secretion – moving a cell product (like glucose
or insulin) out of a cell
Opposite of Passive Transport
 Molecules go against the concentration
gradient (from low to high concentration)
 Energy is provided by the ATP made in the
 Active transport can involve pumps or
 Requires energy