Download The Cell Membrane

The Cell Membrane
(a.k.a. Plasma Membrane)
Fluid-Mosaic Model

Membranes have certain properties.
They:
– act as a barrier between the cell and its
environment, allowing a complex organized
system to exist inside the cell.
– permit the passage of selected
substances into and out of the cell.
– flex, bend and flow to allow the cell to
change shape.
Barrier between the cell
and its environment


All cells, from all organisms, are
surrounded by a CELL MEMBRANE
The cell membrane is a thin layer of
lipid and protein that separates the
cell's interior from its surroundings.



The cell membrane is
SELECTIVELY PERMEABLE.
The cell membrane functions like a
gate, controlling what enters and
leaves the cell.
The cell membrane controls the
ease with which substances pass
into and out of the cell-some
substances easily cross the
membrane, while others cannot
cross at all.
What can pass through the
membrane is determined by
– the SIZE of the particle,
– the CHARGE of the
particle,
– whether or not it needs
the help of a CARRIER
MOLECULE
– or if it requires the
cell to spend ENERGY.




Cell membranes are made mostly of
PHOSPHOLIPID MOLECULES.
Lipid is a simple form of fat.
Phospholipids are a kind of lipid that
consists of 2 fatty acids (tails) and a
phosphate group (Heads).
A phospholipid molecule has a polar “head"
and 2 nonpolar “tails.”




The phosphate head is
hydrophilic meaning
”water loving".
Because of its hydrophilic
nature, the head of a
phospholipid will orient
itself so that it is as
close as possible to water
molecules.
The lipid tails are
hydrophobic meaning
“water fearing.”
The hydrophobic tails will
tend to orient themselves
away from water.

Proteins in membranes
• Help to move material into and out of the cell
– peripheral proteins
• are attached to the surface of the cell
membrane,
• located on both the internal and external
surface.
-integral proteins
• Embedded in the lipid bilayer
• Some extend across entire membrane, others
extend to interior or exterior surface only


Some integral proteins form channels
or pores through which certain
substances can pass.
Other proteins bind to a substance
on one side of the membrane and
carry it to the other side of the
membrane.

Integral proteins exposed to the cell's external
environment often have carbohydrates attached
to them serve as identification badges that allow
cells to recognize each other (immune function)
and may act as a site where viruses or chemical
messengers such as hormones can attach.
Glycoproteins and Glycolipids

Glycoproteins
– proteins covalently
attached to
carbohydrates (ex.
glucose, galactose,
lactose)

Glycolipids
– Carbohydrates
attached lipids. Their
role is to provide
energy and also serve
as markers for
cellular recognition.
The antigens which
determine blood types
belong to glycoproteins
and glycolipids
Fluid Mosaic
Model



Membranes are FLUID and have the
consistency of vegetable oil.
The lipids and proteins of the cell
membrane are always in motion.
Phospholipids are able to drift across
the membrane, changing places with
their neighbor.


Cells may vary the
variety and the
relative amounts of
different lipids to
maintain the
fluidity of their
membranes despite
changes in
temperature.
Cholesterol
molecules in the
bilayer assist in
regulating fluidity.
Membrane cholesterol

The following figure shows the steroid
structure of cholesterol. The non-polar and
polar regions are also illustrated.
Membrane Cholesterol



The cholesterol
molecule inserts itself
in the membrane with
the same orientation
as the phospholipid
molecules.
The figure shows
phospholipid molecules
with a cholesterol
molecule in between.
Note that the polar
head of the
cholesterol is aligned
with the polar head of
the phospholipids.



Without cholesterol
– cell membranes too fluid, not firm enough
– and too permeable to some molecules.
Cholesterol
– adds firmness and integrity to the plasma
membrane
– prevents it from becoming overly fluid
– helps maintain its fluidity.
– helps separate the phospholipids so that the
fatty acid chains can't come together and
crystallize.
Cholesterol helps prevent membrane extremes
– too fluid, or too firm
The cytoskeleton undergirds the
cell membrane and provides
anchoring points for integral
membrane proteins
Cell Junctions: connect
cells together


structures that help cells coordinate as
part of a tissue
Plant cells:
– Plasmodesmata - channels between
adjacent plant cells that form a circulatory
and communication system
Cell junctions in animal cells




Tight junctions - bind cells
forming leakproof sheet.
Ex. Lines digestive tract
Anchoring junctions - attach
adjacent cells with
cytoskeletal fibers but still
allow materials to pass along
the spaces between cells
Communicating junctions allow water and small
molecules to flow between
cells
Passive Transport



Does not require the cell to use energy
Diffusion is the movement of molecules from an
area of higher concentration to an area of lower
concentration.
This difference in the concentration of molecules
across a space is called a concentration gradient.
Diffusion




Driven by the kinetic energy of motion
Occurs when molecules move randomly
away from each other in a liquid or gas.
Rate depends on the temperature, size,
and the type of molecules that are
diffusing.
Molecules diffuse faster at higher
temperatures than at lower
temperatures.


Diffusion always occurs down a
concentration gradient (high to low).
When molecules are dispersed evenly,
there is no longer any diffusion
because there is no longer a
concentration gradient and the
molecules are evenly distributed
throughout the space the molecules
occupy.
When the concentration of the
molecules of a substance is the same
throughout a space, a state of
equilibrium exists.
Osmosis


The diffusion of water across a
semipermeable membrane is called
osmosis.
Occurs down the concentration
gradient so no energy is used.
Osmosis
Osmosis
Why does salt kill slugs?
Hypertonic
Solution


Concentration of the solute molecules
outside the cell is high than the
concentration of solutes inside the
cell.
WATER DIFFUSES OUT OF THE
CELL until equilibrium is established.
Hypotonic
Solution


Concentration of solute molecules
outside the cell is lower than the
concentration of solutes inside the
cell.
WATER DIFFUSES INTO THE
CELL until equilibrium is established.
Isotonic
Solutions


Concentration of solutes outside and
inside the cell are equal.
Under these conditions, water
diffuses into and out of the cell at
equal rates, so there is no net
movement of water.
Plants
•Plants in a hypotonic solution will
have water diffuse into the cell
until the cell membrane pushes
against the cell wall.
•The cell wall is strong enough to
resist the pressure, the pressure
that water molecules exert against
the Cell Wall is called turgor
pressure.
Plasmolysis

In a hypertonic
environment,
the cells shrink
away from the
cell wall, and
turgor pressure
is lost. This
condition is
called
plasmolysis, and
is the reason
plants wilt.
http://www.linkpublishing.com/videotransport.htm#Elodea_-_Osmosis
View: osmosis, hypertonic and hypotonic
videos (elodea, RBC, onion, paramecium)


Animal cells placed in a hypertonic
environment will have water leave the cells,
making them shrink and shrivel.
Placed in a hypotonic environment, water
diffuses into the cells, causing them to
swell and eventually burst - lyse or
cytoloysis.
Protozoans often live in a hypotonic solution.
This presents a problem! The solution…
Facilitated Diffusion


Molecules move
across a
membrane with
the help of
transport proteins
in the membrane.
This takes place
down the
concentration
gradient so it does
not require
energy.
Facilitated Diffusion



Carrier proteins are embedded in the cell
membrane.
Carrier proteins change shape when
molecules attach to them.
The change in shape of the carrier protein
enables the molecule to cross the
membrane.
Carrier Proteins

In order to transport
solutes across the
membrane, carrier
proteins alternate
between two
conformations.
– In the first conformation
the protein’s binding site
for the solute, located in
the core of the protein, is
open to one side of the
membrane,
– In the second confirmation
the binding site is open to
the other side of the
membrane.
Channel Proteins

Channel proteins form
an aqueous pore in the
membrane to allow
solutes to move across
the membrane without
coming into contact
with the hydrophobic
core of the lipid
bilayer.
A good example of facilitated diffusion
is the transport of glucose into the cell.
Many cells depend on glucose for much
of their energy needs.
Active Transport


In many cases, cells must move
materials up their concentrated
gradient, from and area of lower
concentration to an area of higher
concentration.
Unlike passive transport, active
transport requires a cell to expend
energy (ATP).
Carrier-mediated active
transport


Carrier-mediated active
transport systems use
energy and membrane
proteins to "pump"
certain substances
against a concentration
gradient. This causes the
substance to accumulate
on one side of the plasma
membrane.
Ex. Na+/K+ Pump
Sodium Potassium pump



3 Na+ ions from inside the
cell first bind to the
transport protein.
A phosphate group is
transferred from ATP to the
transport protein causing it
to change shape and release
Na+ outside the cell.
2 K+ from outside the cell
then bind to the transport
protein and as the phosphate
is removed, the protein
returns to its original shape
and releases the K+ inside
the cell.
Sodium-Potassium Pump

This animation
shows operation of
the pump. Upper
side = outside of
cell; lower side =
cytosol. The
colored ball
represents ATP;
the three yellow
diamonds Na+ and
the two red
diamonds K+.
Endocytosis (3 Types)

Solutes or fluids outside
the cell membrane can be
brought into the
cytoplasm.
– Phagocytosis - “Cell
eating”. The cell engulfs a
food particle or other cells
– Pinocytosis - "Cell
drinking”.
– Receptor-mediated
endocytosis – needs
special proteins in the
membrane
Phagocytosis


The food vesicle can then fuse with a
lysosome that contains digesive enzymes.
White Blood Cells (WBC, phagocytes)
destroy bacteria and other unwanted cells
by phagocytosis
Exocytosis



Process by which waste
and cell products leave
the cell
Products made in the
cell are packaged in
vesicles made by the
Golgi apparatus which
then fuse with the cell
membrane and secrete
material out of the cell.
Mucus and waste
products are materials
secreted by exocytosis.
Notice the merging of the
membrane…
Relationship of endo- and exo- cytosis
Summary of types of transport