Download Plasma Membrane Notes

Homeostasis
&
the Plasma
Membrane
Structure of the Plasma
Membrane
2 layers of lipids with proteins embedded within each
layer
The lipids are called “phospholipids”
They have a polar head and a non-polar tail
HEAD
 phosphate group which is soluble in water
(hydrophilic)
TAIL
fatty acid chain that is not soluble in water
(hydrophobic)
Structure of the Plasma
Membrane
Structure of the Plasma
Membrane
The plasma membrane is referred
to as a fluid mosaic model
The plasma membrane is made
up of many molecules which
are free to move sideways
Fatty acid chains can be saturated
or unsaturated
The more unsaturated fatty
acids a membrane has, the more
fluid it is.
Saturated or cholesterol chains
are more rigid and stable
Structure of the Plasma
Membrane
Maintaining a Balance
Living cells maintain a balance by controlling
what goes into and out of the cell
The plasma membrane is a selectively permeable
membrane – it has the ability to allow some
materials to pass through and keep others out
This also allows some cells to carry out
different jobs than other cells
Maintaining a Balance
Some molecules can
readily pass across the
membrane while others
cannot.
The size, polarity, and
nature of the molecules will
determine whether it passes
readily or not
Diffusion
Diffusion is the net movement of particles from
high concentration to low concentration
Diffusion cannot occur unless a substance is in a
higher concentration in one region than in another.
This difference is called a concentration gradient.
Dynamic equilibrium – there is continuous
movement, but no overall change
Dynamic = movement
Equilibrium = balance
Diffusion
Osmosis
Osmosis is the diffusion of water molecules
through a selectively permeable membrane from
an area of high concentration to an area of low
concentration
A cell will lose water if it is placed in an
environment in which the water
concentration is lower than that of the cell
contents
ISOTONIC SOLUTION
The concentration of a dissolved
substance is the same as the concentration
inside the cell.
Water molecules will still move, but there
is no net movement of water.
No osmosis occurs b/c the cell is in
dynamic equillibrium
HYPOTONIC SOLUTION
The concentration of dissolved
substances is lower than the concentration
within the cell
Water will rush into the cell causing an
increase in pressure
This pressure is called “turgor
pressure”
HYPERTONIC SOLUTION
The concentration of dissolved
substances is higher than the concentration
inside the cell
Water will rush out of the cell causing it
to shrink
This loss of water from the cell is
called plasmolysis
EFFECT of OSMOSIS
on RED BLOOD CELLS
EFFECTS OF OSMOSIS ON CELLS WHEN
PLACED IN DIFFERENT SOLUTIONS
SOLUTION
TYPE
ISOTONIC
HYPOTONIC
HYPERTONIC
Concentration
of dissolved
substances
(solutes)
Concentration
Gradient
Results in
Animal Cells
Results in
Plant Cells
Same as the
living cell
Zero
No change
No Change
Lower than
the living cell
Net water
movement into
the cell
** Pressure
created by
Cell swells and
excess water
bursts
stored in central
vacuole
Higher than
the living cell
Net water
movement out
of the cell
Central vacuole
collapses and
plasmolysis
occurs
Cell shrinks
* plasmolysis
Passive transport
When the cell does no work in moving particles across the
membrane…
Transport proteins allow needed substances or waste
materials to move through the plasma membrane
The passive transport of materials across the plasma
membrane by means of transport proteins is called
facilitated diffusion.
Active transport
The transport of materials
against a concentration gradient
which requires energy.
BOTH endo- and exocytosis
requires energy to move
materials through the
membrane
Endocytosis
the cell surrounds
and takes in materials
from its environment
Exocytosis
used to expel wastes