CP Biology
Name
______
____________
UNIT 3B: Cell Membrane & Transport
The Keys to Moving Materials Into and
Out of Cells
Introduction to Cell Transport
Transport is the name of the life process in which
materials are exchanged between and organism and its
environment.
Transport can be as simple as moving materials between a
one celled organism and its environment or as complex as
the transport systems in higher organisms such as humans.
Whether it is simple or complex, the final point of transfer
is at the cell membrane.
What types of materials need to be transported into or
out of living cells?
IN: oxygen, water,
glucose, amino acids,
other building blocks
and good things
OUT: carbon dioxide,
wastes, hormones
(packaged products),
water, salts, other bad
things
Example of transport in unicellular and multicellular
organisms:
CO2
unicellular
wastes
Blood Vessel O2
All cells live at least partly in touch with water-based
("aqueous") solutions. In order for substances to move into
or out of a cell, they must cross the cell membrane between
the water-based solution on the outside of the cell (its
ions
wastes
O2
CO2
"environment") and the water-based solution on the inside
Multicellular
(w/ organ systems)
of the cell (="cytosol" or "cell solution").
Notes, summary, questions:
What is a solution? A solute dissolved in a solvent.
In Biology, the universal solvent is water!! Many things dissolve in water.
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CELLULAR BOUNDARIES
The Properties of Lipids
(Textbook 7.2 pg. 204)
Phosphorus group
All cells contain some type of boundary that separates the
inside of the cell from the outside environment.
What does
hydrophilic mean?
_water “loving”__
Cell Wall
Some cells have rigid supportive structure called a cell wall
surrounding the outside of the cell.
Recall: what polysaccharide makes up cell walls?
___cellulose_____________________________
Cell walls are porous and DO NOT regulate what goes in
and out of a cell.
Cell Membrane
Fatty acids
What does
hydrophobic mean?
_water “fearing”__
Phospholipids have two main portions:
1) a hydrophilic head
Contains a polar (charged)
phosphate group
2) Two hydrophobic
fatty acid tails
Non-polar (not charged)
oily regions of the
molecule
ALL cells have a cell membrane.
 Made of a double layered sheet called a phospholipid
bilayer
Use the words attracted or not attracted to fill in the
 It is a flexible structure and forms a boundary
blanks below.
between the cell and its environment.
 The cell membrane regulates what enters and leaves The hydrophilic heads are _attracted_______ to water.
the cell and also protects and supports the cell
The hydrophobic tails are __not attracted to water.
Cell membranes have this typical layered structure because
of the chemical properties of the lipids that make up the
membrane.
When these lipids are mixed with water, their hydrophobic
fatty acid "tails" cluster together while their hydrophilic
"heads" are attracted to water.
A phospholipid bilayer is the result. (see picture on next
page)
Notes, summary, questions:
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The Fluid Mosaic Model
Protein (and carbs & other lipids) molecules are
embedded in the phospholipid bilayer of most cell
membranes.
Dynamic!
Because the protein molecules can move around and
"float" among the lipids (fluid), and because so many
different kinds of molecules make up the cell membrane
(mosaic), scientists describe the membrane as a “Fluid
Mosaic”, i.e.
What are all these different molecules doing?
Proteins
The Phospholipid Bilayer
a) Some proteins form channels and pumps to help
Which component of the phospholipids are hydrophobic?
move materials across the cell.
____fatty acid (lipid) tails__________________
b) Some proteins
How are they oriented (positioned) in the bilayer?
1) act as enzymes
(Which way are they facing?)
2) can attach to the cell’s cytoskeleton
________face in (or toward each other)__
(internal support structure of the cell)
helping cells to respond to the
Which component of the phospholipids are hydrophilic?
environment in order to move or change
____phosphorus head_____________________
shape.
Carbohydrates act like chemical identification signs
(markers, allowing individual cells to recognize
themselves (also to recognize itself from invaders).
How are they oriented (positioned) in the bilayer?
_____face outwards (away from the tails)
Circle a membrane protein in the diagram above.
Put a box around a carbohydrate attached to the bilayer in the
diagram above.
Notes, summary, questions:
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Selective Permeability
Some materials are allowed to enter and leave a cell,
and some are not.
Since a cell needs to constantly exchange materials with its
environment, many substances can cross biological
membranes.
However, some substances are
 too large or
 too strong of a charge
to pass across the phospholipid bilayer of the cell
membrane.
Most biological membranes are selectively permeable,
which means some substances can pass across the
membrane, while others cannot pass through
What does it mean if we say that a membrane is
impermeable to a substance?
The membrane
does not allow that substance to pass through.
Which of the following materials would move easily
through the cell membrane?

water __yes – because it is small (even though it is
slightly charged, polar, – we’ll talk about aquaporins
later).
 small molecules like O2 and CO2 __YES – small
and nonpolar (not charged!).____
 charged particles
NO – hydrophobic lipid tails
of the membrane are nonpolar and do not like charged
substances.
any other particles that the cell must have to
survive __DEPENDS! – may or may not, depends on
their size and charge (the cell will find a way if it is
important to survival).
Selectively permeable membranes can also be referred to as
semipermeable membranes.
selectively
permeable
Not the same as selectively permeable!
Notes, summary, questions: The cell wall is an example of a semi-permeable membrane.
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I.
PASSIVE TRANSPORT
Solute: a substance that is dissolved in a solution
Solvent: the dissolving substance in a solution
One of the most important functions of the cell membrane
is to keep the cell's internal conditions relatively stable.
Two examples of solutions (solutes in a solvent):
In order to maintain this condition of homeostasis, the
1) particles in air (faster)
cell membrane must control the transport of materials
into or out of the cell (in other words, from one side of
Examples: _food cooking, __
the membrane to the other).
_smoke, perfume, body odor,
Passive Transport: the movement of materials across a
cell membrane without using cellular energy(No added
ATP!!!).
Passive transport includes:
1) Diffusion
2) Facilitated Diffusion
3) Osmosis
_etc.______________________
2) particles in liquid (slower)
Examples: _apple juice, iced tea, Kool-Aid, lemonade,
____coffee, swimming pool, salt water___
All matter contains a certain amount of heat. This heat
causes molecules to tend to spread out into the available
space. In any solution, particles are moving constantly
because of the kinetic energy of the particles.
Every living cell exists in a liquid environment; therefore
we can look at the movement of molecules between the
solution inside the cell and the solution outside the cell.
How does heating a liquid affect the movement of solutes and
solvents? It speeds up the movement of the particles,
because you are increasing the kinetic energy.
Notes, summary, questions:
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Diffusion
As a result of molecules moving constantly, colliding with
one another and spreading out randomly the particles tend
to move from an area where they are more concentrated to
an area where they are less concentrated. (High  Low)
Concentration: the amount of particles in a given area
(solution) in relation to other particles (often expressed as
a percent). Usually the amount of solute per unit of
solvent.
Diffusion: the process by which particles move from an
area of high concentration to an area low lower
concentration.
The process of diffusion drives the movement of many
molecules which move across cell membranes.
Suppose a substance is present in unequal amounts on
either side of a cell membrane. If the substance can cross
the membrane, the particles will tend to move toward the
area where it is less concentrated until it is evenly
distributed.
Diffusion of dye (solute) in water (solvent):
Medicine dropper
High conc.
gradient
Lower conc.
gradient
No (zero)
conc.
gradient
In your own words summarize what occurred when the dye
was placed in the water.
_______________________________________________
_______________________________________________
_______________________________________________
Concentration Gradient: conditions in which the
concentration of particles in two different areas are
different.*NOTE each molecule has its own concentration
gradient in any given solution.
Notes, summary, questions:
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When a solute is first added to a solvent, the concentration What will happen to the concentration gradient over time
gradient is high. After the solute spreads out, the
as diffusion continues?_____________________________
concentration gradient is low (or non-existent). In
__high gradient  low gradient  no (zero) gradient
diffusion, molecules move "down" or "with" the
concentration gradient, from higher concentration to lower
concentration.
Once the concentration of the substance on both sides of
the cell membrane is the same, equilibrium is reached.
Particles of the solution will continue to move across the
membrane in almost equal numbers, so there is no further
net change to the concentration of the solutions inside or
outside the cell.
Dynamic Equilibrium: condition in which the
concentration of solute particles in a given area is equal
throughout the entire area.
NO CONCENTRATION GRADIENT REMAINS.
There is no “net” movement!
When dynamic equilibrium is reached, diffusion is equal in
all directions. Do the molecules in the solution stop
moving? ________________________________________
________________________________________________
________________________________________________
Notes, summary, questions:
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