Unit 3B Cell Membrane &
Transport
What is Cell Transport?
• Transport- the movement of materials
between an organism and its environment
• Cellular Transport- movement of materials
into and out of a cell
• Materials move from the water based solution
outside the cell (its environment) to the water
based solution inside the cell (the
cytoplasm/cytosol), or vice versa
• Movement occurs across the cell membrane
Solutions
• Solution- a type of mixture in which all
components are evenly distributed
• Made of a
– Solvent- the substance that does the dissolving
(ex: water is an excellent solvent)
– Solute- the substance that is dissolved
• Cells are surrounded by aqueous (water
based) solutions and contain an aqueous
solution called cytoplasm
Cell Membranes
•
•
•
•
Found surrounding all cells
Made of a lipid bilayer
Protects and supports the cell
Regulates what comes in and out of the cell
The Lipid Bilayer
• Has 2 layers of lipids (“Bi” means two)
• The layers are made up of molecules called
phospholipids
• Each phospholipid has a HYDROPHOBIC fatty acid tail
region
• and a HYDROPHILIC head region
• HYDRO = means water
• PHOBIC = means afraid
• PHILIC = means loving
Hydrophilic head region
Hydrophobic tail region
Lipid Bilayer (Continued)
Because the fatty acid tails are “afraid” of water
(hydrophobic), they turn towards each other so they won’t be
exposed to the cytoplasm, or the outside of the cell (both of
which contain water)
The Fluid Mosaic Model
• “Fluid” means moving
• “Mosaic” means many different things put
together
– The Cell membrane contains different types of
molecules that can move around through the
phospholipids
Carbohydrates
attached to proteins
and act as signals for
molecules to attach
(NOT AS MANY)
Proteins in Membraneallow for transport of
molecules across the
membrane (ABUNDANT)
The Cell Membrane is Selectively
Permeable (semipermeable)
• Some substances can pass through the
membrane and some cannot.
• The structure of the membrane and the
proteins in it decide which molecules can
enter and which can leave.
How does a cell membrane help a
cell maintain homeostasis?
–By transporting substances across
the membrane to achieve
equilibrium (the correct amount of
each material inside and outside
each cell)
Build a Cell
Membrane
Activity
2 Main Types of Transport
–passive -no energy needed from the
cell
–active -energy from cell needed
1. Passive Transport
• Diffusion- particles move from an area of high
concentration to an area of low concentration to
maintain equilibrium
– This is called movement “down the concentration
gradient”
– A concentration gradient occurs where there are two
different concentrations of a particle on either side of
the membrane
– Particles move straight through the lipid bilayer and
no energy is required. Ex: O2 and CO2
Dynamic Equilibrium
• Once diffusion has occurred and the cell has reached
equilibrium, will the solutes stop moving entirely?
• No- they still continue to move across the membrane
but with equal amounts going in and out (no “net
Movement”)
• This is called dynamic equilibrium (dynamic means
movement)
What happens if a molecule/solute is too
large and/or not dissolvable in lipids?
Facilitated Diffusion
-the molecule/solute will go through a carrier
protein specific for that molecule.
-still goes from high to low concentration;
so no energy needed.
Facilitated Diffusion
Water Can Use a Channel to get through
the Cell Membrane
• Although possible, water has
difficulty dissolving through the
cell membrane by regular
diffusion. Why do you think this
is?
• The inside of the cell membrane
is hydrophobic
• Water needs the aquaporin
protein channel to get through
quickly!
• This is an example of facilitated
diffusion
Osmosis
• The simple or facilitated diffusion of water
molecules across a cell membrane
• From where there is more water (less solute) to
where there is less water (more solute)
• Depends upon the solvent/solute ratio
Solutions can be classified by the amount of
solutes they contain compared to other
solutions:
• HYPERTONIC: A solution with a greater solute
[concentration]
– (Ex: a solution with 20% sugar and 80% water is
hypertonic to a solution of 10% sugar and 90% water)
• HYPOTONIC: a solution with a lower solute
concentration
– (Ex: a solution of 5% NaCl and 95% water is hypotonic to
a solution of 20% NaCl and 80% water)
• ISOTONIC: a solution with equal solute
concentration with another solution
– (Ex: outside of a cell has 10% NaCl and the cytoplasm
inside the cell also has 10% NaCl)
What happens to our cells if they
are placed into a/an…
HypErtonic Solution• Since the [solute] is higher
and the [water] is lower
outside the cell, the cell
attempts to correct this by
rushing water out
(osmotic pressure)
• Result= cell shrinks and dies
• E for water exiting the cell
HypOtonic Solution• Since the [solute] is
lower and [water] is
higher outside the cell,
water rushes into the
cell (osmotic pressure)
• Result= cell enlarges,
can burst and die
• O for the cell “opens
the door” to let water
in
Isotonic Solution-
 Passive transport animations
• Since the solution has a
similar [solute] and
[water] as the
cytoplasm, water
moves in/out at equal
rates
• Dynamic equilibrium
• In addition to osmosis,
solutes can also be
moving at the same
time by diffusion or
facilitated diffusion
Osmosis in Plant and Animal Cells
What is different about the plant and animal cells during the process of
Plant cells have cell walls which help them keep their shape .
osmosis? _______________________________________________________
The central vacuole in each plant cell stores the water.
Practice
• What is the solvent concentration of a solution
97%
with a 3% concentration of solute? __________
• What is the solvent concentration of a solution
with a 15% concentration of glucose?
85%
__________
• What is the solute concentration of a solution
with 98% solvent?
2%
_________________________
• What is the solute concentration of a solution
25%
with 75% water? _________________________
Practice
Predict how water will move in each of the following conditions.
Name each environment with the correct solution type.
Conditions
Solute concentration in
the environment is
equal to that in the cell
Solute concentration in
the environment is
greater than the cell
Solute concentration in
the environment is less
than the cell
Water will…
Environment is…
(move in, out, or both?)
(Hypotonic, hypertonic, or isotonic?)
Fill in…
Conditions
Water will…
Environment is...
Move in and out in
equal amounts
No net movement
Isotonic to the cell
Cell is isotonic to its
environment
Solute concentration in
the environment is
greater than the cell
Move OUT of the cell
The Cell Shrinks
Hypertonic to the cell
Cell is hypotonic to its
environment
Solute concentration in
the environment is less
than the cell
Move INTO the cell
The Cell Swells
Hypotonic to the cell
Cell is hypertonic to its
environment
Solute concentration in
the environment is equal
to that in the cell
Practice
Practice
Practice
• Draw the following cells if placed in each type
of environment:
Hypertonic
Cell shrinks
Hypotonic
Cell swells
and
possibly
bursts
Isotonic
Cell stays
the same
2. Active Transport
• Cells move substances from low
concentrations to high concentrations
• This is also called moving substances “UP
their concentration gradient”
• Energy is needed in the form of ATP
A. Molecular Active Transport
• Uses a protein in the cell membrane to carry the
substance across
• Allows for a cell to purposely concentrate a
molecule in a particular location even when the
forces of diffusion will want the molecule to
move in the other direction
Example:
Sodium-Potassium
Pump
B. Bulk Transport (for large Molecules)
1. Some substances are too large to pass
through any of the pores, channels, and
pumps in the cell membrane
2. Or large quantities need to be transported
into/out of the cell
3. Vesicles are produced by the membrane:
pouches that are extensions of the cell
membrane that pinch off and surround the
substance
Bulk Transport: a. Endocytosis
• Bring in large particles (ingestion)
• 2 Types:
Pinocytosis- transports large amounts of small solutes or
liquids
Phagocytosis- transport of large molecules or whole cells
(ex. white blood cells “eat” damaged cells)
Cell membrane
surrounds the
particles and
pinches inward to
bring the materials
into the cell using a
vesicle.
Bulk Transport:
b. Exocytosis
• Reverse of endocytosis
• Materials to be released
from the cell are taken
into a vesicle
• The vesicle fuses with the
cell membrane and
releases the materials
from the cell (ex:
releasing proteins)
• This is how wastes are
removed from many cells