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7-3 Cell Boundaries
Annette Lualhati
Objectives
 1. Identify the main functions of the
cell membrane and the cell wall.
 2. Describe what happens during
diffusion.
 3. Explain the process of osmosis,
facilitated diffusion, and active
transport.
Vocabulary
 Cell membrane
hypertonic
 Cell wall
hypotonic
 Lipid bilayer
facilitated diffusion
 Concentration
active transport
 Diffusion
endocytosis
 Equilibrium
phagocytosis
 Osmosis
pinocytosis
 Isotonic
exocytosis
Review
Cell Membrane – flexible barrier of
cell that regulates what enters and
leaves the cell.
The cell membrane is
“semipermeable”.
The lipid bilayer and the
structure and composition of a
glycerophospholipid molecule
 Figure 1: (A) The plasma membrane of a
cell is a bilayer of glycerophospholipid
molecules. (B) A single
glycerophospholipid molecule is
composed of two major regions: a
hydrophilic head (green) and
hydrophobic tails (purple). (C) The
subregions of a glycerophospholipid
molecule; phosphatidylcholine is shown
as an example. The hydrophilic head is
composed of a choline structure (blue)
and a phosphate (orange). This head is
connected to a glycerol (green) with two
hydrophobic tails (purple) called fatty
acids. (D) This view shows the specific
atoms within the various subregions of
the phosphatidylcholine molecule. Note
that a double bond between two of the
carbon atoms in one of the hydrocarbon
(fatty acid) tails causes a slight kink on
this molecule, so it appears bent.
The Cell Membrane
 The fluid-mosaic model
describes the plasma
membrane of animal cells.
The plasma membrane that
surrounds these cells has
two layers (a bilayer) of
phospholipids (fats with
phosphorous attached),
which at body temperature
are like vegetable oil (fluid).
And the structure of the
plasma membrane supports
the old saying, “Oil and
water don’t mix.”.
 Each phospholipid
molecule has a head that
is attracted to water
(hydrophilic: hydro =
water; philic = loving)
and a tail that repels
water (hydrophobic:
hydro = water; phobic =
fearing). Both layers of
the plasma membrane
have the hydrophilic
heads pointing toward
the outside; the
hydrophobic tails form
the inside of the bilayer.
 Because cells reside in a
watery solution
(extracellular fluid), and
they contain a watery
solution inside of them
(cytoplasm), the plasma
membrane forms a circle
around each cell so that
the water-loving heads
are in contact with the
fluid, and the waterfearing tails are
protected on the inside.
 Proteins and substances
such as cholesterol
become embedded in the
bilayer, giving the
membrane the look of a
mosaic. Because the
plasma membrane has
the consistency of
vegetable oil at body
temperature, the
proteins and other
substances are able to
move across it.
 The molecules that are
embedded in the plasma
membrane also serve a
purpose. For example, the
cholesterol that is stuck in there
makes the membrane more
stable and prevents it from
solidifying when your body
temperature is low. (It keeps
you from literally freezing when
you’re “freezing.”) Carbohydrate
chains attach to the outer
surface of the plasma
membrane on each cell. These
carbohydrates are specific to
every person, and they supply
characteristics such as your
blood type.
Cell Membrane & Cell Wall
Cell Wall –
protects cell
and give cell
structure.
 Found only in
plant cells.
 Made mostly of
cellulose.
Introduction
Student Activity
Diffusion
 In a solution, particles
move constantly.
 Diffusion – tendency of
particles to move from
an area of high
concentration to an area
of low concentration.
Diffusion Through Cell Boundaries
 Every living cell exist in a liquid environment that it
needs to survive.
 The cytoplasm of a cell contains a solution of many
different substances in water.
 Concentration of a solution = mass of solute/volume of
solution.
Diffusion
When do the particles of solute stop
moving?
When the concentration of solute is
the same throughout, and the
system has reached equilibrium
(homeostasis).
Osmosis
Water diffuses across
membranes more easily than
other substances.
Osmosis – diffusion of water
across semi-permeable
membrane.
How Osmosis Works
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/ani
mation__how_osmosis_works.html
Tonicity of a Solution
Isotonic – The concentration of
solutes is the same inside and outside
of the cell.
Hypotonic – Solution has a lower
solute concentration than the cell.
Hypertonic – Solution has a higher
solute concentration than the cell.
For organisms to survive their cells
must balance the intake of water, salts,
sugars, and other molecules.
 What if a cell is placed
into a isotonic solution?
 What if a cell is placed
into a hypertonic
solution?
 What if a cell is placed
into a hypotonic
solution?
The Effect of Osmosis on Cells
Osmotic Pressure
Hypertonic
Isotonic
Hypotonic
Facilitated Diffusion: So why don’t
our cells burst open shrivel up?
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/ani
mation__how_facilitated_diffusion_works.html
Facilitated Diffusion
 A concentration gradient is when there is an uneven
distribution of a substance across a border. For example, think
of a balloon. The air inside the balloon is more concentrated
than the air outside of it. There is a concentration gradient
because of the differences in concentration. And what happens
when you release the tip of the balloon? The air inside the
balloon shoots out because things like it when the
concentration is equal everywhere. So when something moves
against its concentration gradient, it is being forced from an
area where it is less concentrated into a place where it is more
concentrated. Conversely, when something moves down its
concentration gradient, it is going from a place where it is
more concentrated to where it is less concentrated. However, it
is important to note that ENERGY MUST BE EXPENDED TO
MOVE SOMETHING AGAINST ITS CONCENTRATION GRADIENT,
but no energy must be used to move something down its
concentration gradient.

Facilitated Diffusion
Some molecules seem to pass
through the cell membrane
more easily than they should.
Example: RBC’s have a glucose
(sugar) channel that allows
glucose to pass in and out.
Only glucose can pass
through the channel.
Facilitated Diffusion
 . Facilitated Diffusion –
is the diffusion of
particles through
protein channels,
Hundreds of channels
have been found to
allow only one specific
material through.
 Carbohydrates and
sugars mostly
Hyper link
Active Transport
Active transport – transport
of materials that requires
energy.
 Uses “pumps” that are
found in the membrane.
 The most common
pump is the sodium
(Na) – potassium (K)
(salts)
Active Transport
 Uses energy to move
molecules from low to
high concentration
(against concentration
gradient).
 http://highered.mcgrawhill.com/sites/00724958
55/student_view0/chapt
er2/animation__how_the
_sodium_potassium_pu
mp_works.html
Active Transport
+
1 Cytoplasmic Na binds to
the sodium-potassium
pump.
EXTRACELLULAR+
[Na ] high
FLUID
[K+] low
Na+
Na+
[Na+] low
Na+
CYTOPLASM [K+] high
+
+
3 K is released and Na
sites are receptive again;
The cycle repeats.
2 Na+ binding stimulates
phosphorylation by ATP.
Na+
Na+
Na+
P
ADP
ATP
4 Phosphorylation causes the
protein to change its conformation,
expelling Na+ to the outside.
Na+
Na+
Na+
K+
P
K+
5 Loss of the phosphate
restores the protein’s
original conformation.
+
6 Extracellular K binds to the
protein, triggering release of the
Phosphate group.
K+
K+
K+
K+
P
Pi
To Relieve Confusion…
Passive
Transport
1)Diffusion
2)Osmosis
3)Facilitated
Diffusion
NO ENERGY
REQUIRED!!!
Active Transport
1)Active
Transport
REQUIRES
ENERGY!!!
Passive transport
Active transport
Endocytosis and Exocytosis
 Endocytosis is a process for moving items that are
outside of the cell into the cytoplasm of the cell.
Exocytosis is a process for moving items from the
cytoplasm of the cell to the outside.
 http://www.youtube.com/watch?v=4gLtk8Yc1Zc
Phagocytosis
 The process of engulfing and ingestion of particles by
the cell or a phagocyte (e.g. macrophage) to form a
phagosome (or food vacuole), which in turn fuse with
lysosome and become phagolysosome where the
engulfed material is eventually digested or degraded and
either released extracellularly via exocytosis, or released
intracellularly to undergo further processing.
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/ani
mation__phagocytosis.html