Transcript The Cell Membrane

Cell Membrane Structure &
Cellular Transport
Fluid-Mosaic Model
Central Problem #1: A living system MUST be
separated from its environment if it is to maintain
complex order & homeostasis.

We know that 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.
– They permit the passage of selected
substances into and out of the cell.
– They 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
content from the world around it.

The cell membrane functions like a
gate, controlling what enters and
leaves the cell.

The cell membrane controls how
easily substances enter & leave the
cell-some substances easily cross the
membrane, while others cannot cross
at all. For this reason, the cell
membrane is said to be
SELECTIVELY PERMEABLE.
What can pass through?

What can pass through the membrane
is determined by:
– the size of the particle,
– whether or not it needs the
help of a carrier molecule
– or if it requires the
cell to spend energy.
Size, charge, and E

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.”

Because of its
hydrophilic (water
loving) 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 (water
fearing), so the
hydrophobic tails will
tend to orient
themselves away from
water.

A variety of protein molecules are
embedded in the lipid bilayer.

Some proteins are attached to the
surface of the cell membrane (peripheral
proteins) and are located on both the
internal and external surface.

Others are embedded in the lipid bilayer
(integral proteins)

They extend across the entire cell.
Others extend only to the inside or only
to the exterior surface

There are many kinds of proteins in
membranes; they help to move material
into and out of the cell.

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.
A Fluid Mosaic Model:
The basic foundation...

The “fluid mosaic model” is used to
describe the cell membrane because
the phospholipids & proteins are fluid.
Because of this fluidity, the structure
changes or is it is said to be “mosaic”.

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.
Another model….
The cytoskeleton holds together the cell
membrane and provides anchoring points
for 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
Formation of Cell Wall



Primary cell wall: Forms 1st. Made of
cellulose, stretchy so cell can grow.
Secondary cell wall: Rigid, made of
lignin, forms once cell is full grown.
Pectin is a sticky substance that holds
neighboring cell walls together. (Pectin
is used to make jelly!)
Cellular Transport
How things get in & out of the
cell…….
PASSIVE TRANSPORT
1.
2.
3.
DIFFUSION
OSMOSIS
FACILITATED DIFFUSION
((((No Energy Required)))))
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 is driven by the kinetic energy (E of motion)
the molecules possess. Diffusion occurs when
molecules move randomly away from each other in a
liquid or gas.

The rate of diffusion 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 diffusion (& not longer
a concentration gradient) because the
molecules are evenly distributed
throughout the space they occupy.
When the concentration of the molecules of a
substance is the same throughout a space, a
state of equilibrium exists.

The diffusion of water across a
semipermeable membrane is called
OSMOSIS.

Occurs down the concentration
gradient so no energy is used.
Osmosis
Hypertonic Solution


In a hypertonic solution, the
concentration of the solute molecules
outside the cell is high than the
concentration of solutes inside the cell.
In hypertonic solutions, WATER
DIFFUSES OUT OF THE CELL until
equilibrium is established. The cell
shrinks.
Hypotonic Solution


In a hypotonic solution, the
concentration of solute molecules
outside the cell is lower than the
concentration of solutes inside the cell.
In hypotonic solutions, WATER
DIFFUSES INTO THE CELL until
equilibrium is established. The cell
grows larger.
Isotonic Solutions

In an isotonic solution, the 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 in a hypotonic solution will have
water diffuse into the cell until the cell
membrane pushes against the cell wall.

The pressure that water molecules exert
against the Cell Wall is called TURGOR
PRESSURE.

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.

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.
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.
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
1.
2.
3.
Active Transport
Endocytosis
Exocytosis
(((((Energy is Required))))
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. (against the
concentration gradient)
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
Endocytosis

Pinocytosis - "Cell
drinking”.

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
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.
Comparing Plant and Animal Cells