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Unit 5- Cell Membrane &
Cell Transport
5b- Cellular Transport
About Cell Membranes
• All cells have a cell membrane
• Functions:
a. Maintains homeostasis
b. Provides protection and
support for the cell
TEM picture of a real
cell membrane.
•Animations of Active Transport & Passive
Transport
What is the purpose of cellular transport?
• Homeostasis depends upon appropriate movement
of materials across the cell membrane.
– Required materials must pass into the cells so they can be
used.
• Ex. Oxygen and glucose for cellular respiration
– Waste materials must pass out of the cells as they are
produced
• Ex. The CO2 produced as a waste product of cellular respiration
How?
Membrane
movement
animation
• Each individual cell exists in a fluid environment, and the
cytoplasm within the cell is a fluid environment.
– The presence of a liquid makes it possible for substances (such as
nutrients, oxygen, and waste products) to move into and out of
the cell.
• A cell membrane is semipermeable (selectively permeable).
• Materials can enter or exit through the cell membrane by
passive transport or active transport.
Passive Transport
•
•
Cell uses no energy
The movement of molecules
occurs along a concentration
gradient
–
meaning molecules move from an
area of high concentration to an
area of low concentration
Weeee!!!
• 3 types of passive transport
hig
h
low
Passive Transport:
Simple Diffusion
Animation
1. Diffusion: spreading out of molecules across a cell
membrane until they are equally concentrated
(equilibrium) *molecules never stop moving, just stay spread out*
–
–
–
results from the random motion of molecules
Occurs along a concentration gradient
Molecules can diffuse across a membrane by dissolving in the
phospholipid bilayer (ex O2, CO2) or by passing through pores
in the membrane
Passive Transport:
2. Osmosis: diffusion of water
across a membrane
• Water moves from a high
concentration of water to a low
concentration of water
• Net direction of osmosis is
Aquaporin
determined by the relative
solute concentrations on the
two sides of the membrane.
Osmosis
animation
Solutions
• Solute- substance
being dissolved
• Solvent- substance
solute dissolves in
• Solution=
solute+solvent
Passive Transport-Osmosis
Hypotonic Solution: The solute concentration outside the
cell is lower than the solute concentration inside the cell.
(Low solute; High water)
Result: Water moves from the solution to inside the cell.
Cell swells & may burst (cytolysis).Animation
Passive Transport-Osmosis
Hypertonic Solution: The solute concentration outside the
cell is higher than the solute concentration inside the cell.
(High solute; Low water)
Plasmolysis
shrinks
Result: Water moves out of the cell into the solution.
Cell shrinks! Animation
Passive Transport-Osmosis
Isotonic Solution: The solute concentrations outside the
cell & inside the cell are equal.
Result: No net movement. Water moves equally in both
directions & the cell remains same size. (Dynamic
Equilibrium)
Hypertonic, Hypotonic, Isotonic
Solutions
In what type of solution are these cells?
A
B
Hypertonic
Isotonic
C
Hypotonic
How Cells Deal with Osmosis
•
Paramecium (protist)
removing excess
water video
• Cells must compensate for the water that enters the cell in
hypotonic environments & leaves the cell in hypertonic
environments.
• Protists, like paramecia, have contractile vacuoles that collect
& pump out excess water.
• Plants & bacteria have cell walls that prevent them from
over-expanding.
– In plants the pressure exerted on the cell
wall is called turgor pressure.
– Plasmolysis- in a hypertonic environment the
cells shrink away from the cell walls &
turgor pressure is lost.
How Cells Deal with Osmosis
• Animal cells are bathed in
blood. Kidneys keep the blood
isotonic by removing excess
salt & water.
• Many multicellular organisms
have outer surfaces that are
waterproof to prevent water
loss on land or water gain in
freshwater.
Passive Transport:
3. Facilitated diffusion: diffusion of
specific particles along a
concentration gradient with the
help of transport proteins
• Transport Proteins are specific –
they “select” only certain
molecules to cross the membrane
a. Transports larger or charged
molecules that cannot pass
through the membrane on their
own
b. Glucose is an example of a
molecule that passes into the
cell through facilitated diffusion
A
B
Facilitated
diffusion
(Channel
Protein)
Diffusion
(Lipid
Bilayer)
Carrier Protein
•
http://bio.winona.edu/berg/Free.htm
Passive Transport: 3. Facilitated Diffusion
Glucose
molecules
Cellular Transport From aHigh Concentration
High
• Channel Proteins
animations
Cell Membrane
Low Concentration
Through a 
Transport
Protein
Protein
channel
Low
Active Transport
• Requires the cell to use energy
• Molecules move against the
concentration gradient –
– from an area of low concentration to an
area of high concentration
high
This is
gonna
be hard
work!!
• Active transport is used to:
– move large molecules
– concentrate molecules within the cell
– remove waste from the cell
low
Types of Active Transport
1. Protein Pumps (aka
cell membrane pumps) –
transport proteins that
require energy to do
work
• Example: Sodium /
Potassium Pumps
are used in nerve
cells to transmit
messages.
Sodium Potassium
Pumps (Active
Transport using
proteins)
Active Transport- Protein Pumps
• Sodium-Potassium Pump
– ATP supplies the energy that drives the pump
Types of Active Transport
2. Endocytosis:
• Cells take in large molecules by
folding around them & forming
a pouch
– Pouch then pinches off from
the membrane & becomes
an organelle called a vesicle
• This is how white blood cells
ingest bacteria
Types of Active Transport
3. Exocytosis:
– vesicles made by the cell fuse
with the cell membrane,
releasing their contents into
the external environment
– Used to release large
molecules, waste products, or
toxins
– Used to release proteins after
Golgi apparatus packages them
into vesicles
Endocytosis &
Exocytosis
animations
Endocytosis & Exocytosis