CELL MEMBRANE &
CELL TRANSPORT
Homeostasis:
Maintaining a Balance
Organisms must adjust
to changes in their
environment.
If not…DEATH!
A formal definition is—
maintaining a stable
internal state despite
what is going on
externally.
What Maintains Homeostasis?
The PLASMA or CELL
MEMBRANE maintains
the proper
concentrations of
materials by controlling
the passage of molecules
in and out of the cell.
Therefore, the cell
membrane’s function is
to maintain
HOMEOSTASIS through
passive transport, active
transport and cell
communication!
This electron
micrograph of the
cell membrane
shows the
appearance of the
phospholipid
bilayer using
several staining
processes. The
magnification on
this structure is
53,260 X
Cell Membrane
X 53, 260
Characteristics of
the Cell Membrane
The cell membrane is Selectively Permeable
aka Semi-permeable
Allows some things in/out and not others…
 Oxygen, nitrogen,carbon dioxide, and
other small, nonpolar molecules can
diffuse directly
 Water was once thought to move directly
through, but it is now understood to
travel through aquaporins (a type of
transport protein).
 Ions, sugars, and larger molecules move
through transport proteins or in vesicles.
Plasma/Cell Membrane-Structure
Phospholipid Bilayer with
proteins
embedded/floating in it
Phospholipid Structure:
 Polar Head (hydrophilic—
”water loving”)
 Nonpolar Tails—
(hydrophobic—”water
fearing”)
Phospholipid
Polar Head
Nonpolar Tails
Bilayer Arrangement
Outside of cell
Inside cell
Proteins are embedded/floating in
the lipid bilayer.
Protein in membrane
Cell Membrane Structure-Proteins
These proteins are needed
for the membrane to function
properly.
3 Types of Membrane Proteins
1. Transport—regulate what enters or leaves cell
2. Marker—identify the cell
3. Receptor—allow cells to communicate
Transport Proteins—Channel Proteins
• Function as
“gates/passageways”
• Allow polar sugars, amino
acids, and ions to cross the
membrane.
• Special channel proteins:
Channel Protein
Transport
Protein
– Gated ion channels—gates
that open/close
– Carrier proteins—change
shape to allow specific
molecule to pass
– Aquaporins—allow water
to diffuse through (called
osmosis)
http://personal.tmlp.com/Jimr57/textboo
k/chapter3/cms2.htm
Carrier Protein
Marker Proteins
• Cell’s “Name Tag”
• Protein sticks out of
phospholipid layer
• Often has carbohydrates
attached to outside end
• Functions in cell
identification to identify
the cell to other cells and
molecules
• 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
http://personal.tmlp.com/Jimr57/text
book/chapter3/cms2.htm
Marker Protein
Receptor Proteins
• Function as
“messenger/receiver”
• 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
• Ex) epinephrine can bind to a
receptor protein and send a
message inside that says “break
down glycogen”
http://personal.tmlp.com/Jimr57/textboo
k/chapter3/cms2.htm
Receptor Protein
CELL TRANSPORT
• Concentration gradient - the
difference the concentration of a
particular substance across a space. .
• Equilibrium is reached when the
molecules become even throughout a
space.
Cell Transport Types
• Passive Transport—no energy required, molecules
move from high to low concentration (down or with
concentration gradient)
– Diffusion
– Osmosis
– Facilitated Diffusion
• Active Transport—energy (ATP) required,
molecules move from low to high concentration (up
or against the concentration gradient)
– Pumps
– Vesicles
Passive Transport
Diffusion, Osmosis, Facilitated Diffusion
Diffusion
 Diffusion - the
process by which
molecules spread from
areas of high
concentration,
to areas of low concentration
 Molecules are said to go “Down” or
“with” the concentration gradient.
 Passive Transport-requires no
energy
Osmosis
• Osmosis - the
diffusion of water
molecules through a
semi-permeable
membrane requires no
energy
Ex. Water will move in the
direction
where there is a higher
concentration
of solute (and hence a lower
concentration of water).
About this diagram: this represents a cell in a solution. The cell will not let
the red particles pass through the membrane. The green molecules can
pass through, as can water molecules. The arrows show the direction of
particle movement. The green particles are moving in to the cell where
their concentration is lower, and water is moving out of the cell because its
concentration is higher inside
diffusion
osmosis
Isotonic Solution
• Concentration of solute
and solvent outside the
cell is equal to/the
same as the solution you
are comparing it to
(often the cytoplasm in
cell)
• Water is moving in and
out at equal rates, no
net movement into or
out of cell since
• Cell size would stay the
same
Red blood cells in
isotonic solution
X 1000
Note that all the cells
appear normal.
Hypertonic Solution
 There are more solute
(ex. salt) molecules in
solution outside the cell
when compared to the
inside of cell
 Water will move out of
the cell
 Cell would shrink
 Plants cells shrink
(plasmolysis) because cell
membrane pulls away from
cell wall, so plant wilts
because water has left
the central vacuole.
Animal cell shrink
(crenate). In both cases,
the cell may die.
Crenated red blood
cells in hypertonic salt
solution
X 1000
Notice that the cells
have shrunk.
Hypotonic Solution
 There are less solute
(ex. salt) molecules in
the solution outside the
cell than inside the cell.
 Water will move into
the cell.
 Cell will swell
 Cell could burst (lyse)
 Plant cells have vacuoles
to collect extra water
Red blood cells in
hypotonic solution
X 1000
Note that the pinkish
cells have swollen
(the little dip in the
middle of a normal
rbc is not visible and
one side bows out).
Osmosis Links
Osmosis Examples
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosi
s.htm
Hypotonic, Hypertonic, Isotonic Interactive Quiz
http://www2.nl.edu/jste/osmosis.htm#Osmosis
Facilitated Diffusion
(uses Transport Proteins, passive)
• Moves substances
(from high to low
concentration) down
the concentration
gradient without using
cell’s energy using
channel/carrier
proteins located in
membrane
Active Transport in
Cells
Why do cells need 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.
How does active transport work?
• Some types involve carrier
proteins that function as
“pumps”.
• Other types use vesicles.
• Energy is provided by ATP.
Active Transport
Using Pumps
Sodium-Potassium Pump
Proton Pumps
Example of active transport types using
pumps
Sodium/Potassium Pump
• 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.
• Used by many cells, including nerve
cells, to send a chemo-electric message.
Sodium-Potassium Pump
Sodium/Potassium pump:
3 Na+ out of cell (yellow
diamonds)
2 K + into cell (red/purple
squares)
Here the energy of a
phosphate from ATP
(shown in pink) is used to
exchange sodium atoms
for potassium atoms.
Proton 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 pump?
*Because the cell needs isolated areas of the cell with
different pH for particular functions; ex) lysosomes –
have proton pumps to maintain a pH=5
*Because the cell only uses one ATP to pump a proton
out, and that proton can be used in co-transport
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
Active Transport
Using Vesicles
Endocytosis & Exocytosis
Endocytosis
Substances are moved into a cell
by a vesicle that pinches off
from the cell membrane
Requires energy (ATP)
Types of Endocytosis
• Pinocytosis—when the nutrient
particles are dissolved in a
liquid; “cellular drinking”
• Phagocytosis—when the
nutrient particles are solids;
“cellular eating”
Exocytosis
Exocytosis- substances inside a vesicle are
released from a cell as the vesicle fuses with
the cell membrane
Involves the cell getting rid of waste or the cell
secreting cell products (ex. Hormones, insulin)
• Requires Energy (ATP)
Animations—Exocytosis,
Endocytosis
Exocytosis using ER and Golgi:
http://www.wisc-online.com/objects/index_tj.asp?objid=AP11203
McGraw-Hill: Cotransport, Na-K
pump, Endo/Exocytosis, & Proton
Pump
http://highered.mcgrawhill.com/sites/0072437316/student_
view0/chapter6/animations.html#
Cell Membrane Physiology-Interactive
http://www.getbodysmart.com/ap/nervoussyst
em/neurophysiology/membranephys/menu/
menu.html