THE CELL MEMBRANE
1. The cell membrane is selectively permeable; it
controls which substances pass into and out of the
cell.
2. The cell membrane is composed primarily of a
phospholipid bilayer with proteins embedded
within the membrane.
SELECTIVELY PERMEABLE (SEMI PERMEABLE)
MEMBRANE
HIGH CONCENTRATION
LOW CONCENTRATION
3.FLUID MOSAIC MODEL
The cell membrane is composed of lipids, proteins &
carbohydrate tails (a “mosaic” of molecules). It is also a
dynamic structure, with the molecules constantly
shifting & moving (it is “fluid” (like buoys on the ocean) ).
Scientists therefore describe the structure of the cell
membrane as a Fluid Mosaic Model.
4. The Cell Membrane
Composed of a lipid bilayer. And two general types of proteins:
peripheral proteins – proteins located on both the interior
surface and the exterior surface of the cell membrane.
integral proteins – proteins that are embedded in the membrane
bilayer.
channel
marker
receptor
fig. 4-5 p. 74
MEMBRANE PROTEINS
Membrane proteins can further be classified by their particular
function.
1. Carrier proteins – proteins that transport specific substances
across the membrane.
2. Channel proteins – proteins that provide a passageway across
the cell membrane through which small molecules or ions can
diffuse.
3. Marker/Recognition proteins – Marker/Recognition proteins
extend across the cell membrane and serve to identify the cell.
The immune system uses these proteins to tell friendly cells
from foreign invaders. They are as unique as fingerprints. They
play an important role in organ transplants. If the marker
proteins on a transplanted organ are different from those of
the original organ the body will reject it as a foreign invader.
4. Receptor proteins – proteins within the cell membrane that are
able to receive signals from the outside of the cell which can
cause the cell to respond.
(5-1)
1. PASSIVE TRANSPORT
All molecules have movement due to vibrations caused by heat (all except
those at Absolute Zero (theoretically)). The molecules do not need to be
physically moved; they move because they have their own energy.
Therefore a cell can transport molecules using this natural movement.
It is called “Passive Transport” because the cell does not have to use
additional energy in the process.
Different forms of passive transport include:
A. Diffusion
B. Osmosis
C. Facilitated Diffusion
D. Diffusion Through Ion Channels
DIFFUSION
MEMBRANE
HIGH CONCENTRATION
LOW CONCENTRATION
A. DIFFUSION
The simplest type of passive transport is diffusion.
1. The difference in the concentration of molecules across a space is
called a concentration gradient.
2. “ Molecules tend to move "down" their concentration gradient, from
areas where they are more concentrated to areas where they are less
concentrated.”
3. When the concentration of the molecules of a substance is the
same throughout a space, a state of equilibrium exists.
4. The ability of a molecule to diffuse across a cell membrane
depends on:
1. the size of the molecule
2. the type of molecule
3. the chemical nature of the membrane
What types of molecules can diffuse through the cell membrane?
Small molecules (water) and molecules that can dissolve in lipids
(oxygen & carbon dioxide).
B. OSMOSIS
The process by which water molecules diffuse
across a cell membrane from an area of high
concentration to an area of lower
concentration is called osmosis.
Because water moves down its concentration
gradient, osmosis does not require cells to
expend energy. Osmosis, therefore, is a type
of passive transport.
WATER MOLECULES
OSMOSIS
MEMBRANE
HIGH CONCENTRATION
LOW CONCENTRATION
b.1 HYPOTONIC/HYPERTONIC/ISOTONIC
Solute – a substance dissolved in a solution.
Solution – a mixture in which one or more substances are uniformly dissolved in another
substance.
Solvent- in a solution, the substance in which a solute is dissolved.
Hypotonic environment
(“Fresh water”) a solution
whose solute
concentration is lower than
that inside the cell.
Hypertonic environment
(“salty”) a solution whose
solute concentration is
higher than that inside the
cell.
Isotonic environment
(“same”) a solution whose
solute concentration
equals that inside a cell.
B.2.Contractile Vacuole
Paramecium live in fresh water which is Hypotonic.
That means water is constantly diffusing in through
osmosis. It will pop like a water balloon if the water is
not removed by the contractile vacuole.
Click here to see a video of a Paramecium using its contractile vacuole
b.3 Turgor Pressure and Plasmolysis
In a hypertonic (“salty”)
environment the plant cells
lose water and plasmolysis
results.
In a hypotonic (“fresh”)
environment the plant
cells fill with water. This
creates pressure called
turgor pressure.
b.4. Cytolysis
In a hypotonic solution, animal cells (which do not have a
cell wall) can fill up too much and burst! (Remember the
paramecium?)
Also remember the “Hold your wee for a Wii tradgedy”?
Hitler’s doctors apparently killed people in hospitals by
giving high doses of pure water in ivs!
CHANNEL & CARRIER PROTEINS
C. FACILITATED DIFFUSION
1. This process is used for molecules that cannot diffuse
rapidly through cell membranes, even when there is a
concentration gradient across the membrane.
2. The movement of these kinds of molecules across the cell
membrane is assisted by proteins known as carrier proteins.
Molecule binds to carrier protein. Carrier protein changes shape.
Molecule can then pass through hydrophobic interior of
membrane. Carrier protein returns to original shape.
3. Glucose as an
example shows that:
>Carrier proteins are
specific to the type of
molecule.
>Facilitated diffusion
speeds up the
diffusion process.
D. DIFFUSION THROUGH ION CHANNELS
Membrane proteins that allow for passive transport of ions.
1. Ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and
chloride (Cl-) are important for a variety of cell functions. Because
they are not soluble in lipids, ions cannot diffuse across the lipid
bilayer without assistance. Ion channels provide small passageways
across the cell membrane through which ions can diffuse.
2. Stimuli that can open ion channels: Stretching of membrane,
electrical signals & chemicals in the cytosol or external environment.
(5-2) ACTIVE TRANSPORT
1,2 & 4. Phospholipids
The cell membrane
is like a oreo cookie
The inside is hydrophobic which means it doesn’t soak up
milk (water). That is because it is made of “fatty acid tails”.
The outside soaks up the milk (water) and so we call it
hydrophilic. It is not made of fat but of a water-loving
phosphate group.
3. Channel Marker & Receptor Proteins
MEMBRANE PROTEINS
Membrane proteins can further be classified by their particular function.
Carrier proteins – proteins that transport specific substances across the membrane.
Channel proteins – proteins that provide a passageway across the cell membrane through
which small molecules or ions can diffuse.
Marker/Recognition proteins – Marker/Recognition proteins extend across the cell
membrane and serve to identify the cell. The immune system uses these proteins to tell
friendly cells from foreign invaders. They are as unique as fingerprints. They play an
important role in organ transplants. If the marker proteins on a transplanted organ are
different from those of the original organ the body will reject it as a foreign invader.
Receptor proteins – proteins within the cell membrane that are able to receive signals from
the outside of the cell which can cause the cell to respond.
II. ACTIVE TRANSPORT
1. In many cases, cells must move
materials up their concentration
gradient, from an area of lower
concentration to an area of higher
concentration. Such movement of
materials is known as active
transport. Unlike the passive
transport of facilitated diffusion, Require Energy!!! (ATP)
active transport requires a cell to
expend energy.
2. Carrier proteins that go against
the concentration gradient are called
“pumps” and they are similar to
channels because they both shield
molecules from the phospholipids
inside the membrane.
B. Sodium(Na)/
Potassium(K)
Pumps:
1. Pump against the
concentration
gradient.
2. 3 Na are pumped out
3. 2 K are pumped in.
4. ATP is used as
energy.
5. This creates an
electrical potential
which means it has
the potential to send
messages (nerves).
Endocytosis & Exocytosis
Some substances such as macromolecules
and food particles, are too large to pass
through the cell membrane by the
transport processes mentioned thus far.
Cells employ two other mechanisms –
endocytosis and exocytosis – to move
such substances across their membranes.
C. ENDOCYTOSIS
1.Endocytosis is the process by which cells ingest external
fluid, macromolecules, and large particles, including other
cells.
2. External materials are enclosed by a portion of the cell,
which folds into itself and forms a pouch. The pouch then
pinches off from the cell membrane and becomes a
membrane-bound organelle called a vesicle.
3. Pinocytosis – a type of
endocytosis in which the
cell engulfs solutes or
fluids.
Phagocytosis – a type of
endocytosis in which the
cell engulfs large particles
or whole cells.
D. EXOCYTOSIS
Exocytosis is essentially the reverse of endocytosis.
During exocytosis, vesicles in the cytoplasm fuse
with the cell membrane, releasing their contents
into the cell’s external environment.
Nerves for instance send
messages from one cell
to the next by chemicals
called neurotransmitters
that are released & send
the message to the next
nerve cell.
Drugs and alcohol
interfere with this and
make it difficult to react
appropriately!