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REVIEW – MEMBRANE
TRANSPORT
Passive and Active transport across a cell membrane
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Thin, flexible barrier
Cell (plasma) membrane
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Membranes also organize the interior of a
cell. Cell organelles are defined by
membranes.
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Cell (plasma) Membrane
What does it do for cell?
Controls what goes in and out
Protects/isolates
Communicates
Identification
Every cell is covered by a membrane
that controls what can enter and leave
the cell.
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Homeostasis – Maintaining a Balance
The plasma membrane is selectively
permeable – it will allow some things to
pass through, while blocking other things.
Cells must keep the proper concentration
of nutrients and water and eliminate
wastes.
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Structure of the Plasma Membrane
Lipid bilayer – two sheets of lipids (phospholipids).
Embedded with proteins and strengthened with cholesterol molecules.
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Membrane Proteins
Determine what particles can pass through the membrane.
Serve as enzymes (may speed reactions).
Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell.
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Proteins
Proteins help things get across membrane
Some go all the way through like a
channel
Some use energy
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Cell Function:
transport
osmosis and diffusion
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homeostasis and
transport
• Cell membranes help organisms maintain
homeostasis by controlling what substances may
enter or leave cells
• To stay alive, a cell must exchange materials with its
environment
• These materials must cross the cell (plasma)
membrane
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Cell membranes have protein channels
Selectively permeable: Allows some
molecules in and keeps other molecules
out
Remember…
The structure helps it be selective!
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• simple diffusion
• facilitated diffusion
• osmosis
NO expenditure of Energy
uses concentration gradient
expenditure of Energy
does NOT use
concentration gradient
the cell regulates movement in and out by two main methods:
• protein pumps
• endocytosis
• exocytosis
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Biological membranes are…
SEMI-PERMEABLE -- A.K.A.
Selectively permeable
Cell membranes are picky
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What diffuses across the
membrane???
Small, uncharged
(non-polar) molecules
Examples:
Carbon dioxide
Oxygen
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Types of Cellular Transport
Passive Transport
cell does not use energy
diffusion
facilitated diffusion
osmosis
Active Transport
cell does use energy protein pumps
exocytosis
endocytosis
high
low
This is
gonna be
hard
work!!
high
low
Weeee!!!
http://www.northland.cc.mn.us/biology/biology1
111/animations/transport1.html
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Diffusion Particles constantly move
Collide randomly
Spread out randomly
Diffusion is moving from area of HIGH conc. to
area of LOW conc.
This is what we call the CONCENTRATION
GRADIENT
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diffusion
Caused by Brownian motion
(movement of particles because of
the movement of their atoms).
Continues until an equilibrium is
reached (no gradient).
Dynamic equilibrium – particles
continue to move freely and are
evenly distributed.
http://www.northland.cc.mn.us/biology/
biology1111/animations/transport1.html
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What happens when we reach
equilibrium?
Particles continue moving across membrane but
in both directions!
***No more changes in concentration
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Osmosis
Diffusion of water across a selectively permeable membrane.
Occurs until water is balanced on both sides of the membrane.
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How Osmosis works…
We have water molecules and some other molecules, let’s
say sugar
If we have more solute concentration on one side of the
membrane, then the water will diffuse across…
Membrane will let water thru but not sugar
Water can move back and forth (not sugar)
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Cell Concentrations
Hypertonic solutions – more dissolved solute.
Hypotonic solutions – less dissolved solute.
Isotonic solutions – the same dissolved solute.
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hypotonic
solutions
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hypertonic
solutions
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isotonic
solutions
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Why don’t all our cells
burst?
What are our cells (animal) contained in? Fresh water?
No…..blood or other fluids, which are ISOTONIC The conc. Of sugars, salts, proteins and molecules in these
fluids is the same as the conc. in the cell
Bacteria and plant cells They DO come in contact with fresh water…what do
they have PROTECTING them?
CELL WALL Prevent cells from expanding even when there’s a lot of
osmotic pressure
But, cell walls are prone to injuries when there is too much osmotic pressure
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Overcoming Osmosis
Contractile vacuoles – expel excess water from bacterial cells that live in water.
Turgor pressure – water pressure in a plant cell. Loss of turgor pressure causes wilting (plasmolysis).
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Osmotic Pressure
Central vacuole fills with water and exerts and
out ward pressure on cell membrane and cell
wall
Cell wall does NOT allow cell to expand past a
certain size
Osmoregulation
Osmoregulation
means by which cells keep the
concentration of cell cytoplasm or blood
at a suitable concentration.
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Problems in Plant Cell
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How do big molecules diffuse
across a membrane so
quickly if the membrane is
selectively permeable?
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Problems for diffusion…
Things too large (like Glucose!)
Charged molecules and Polar molecules
Positive/negative
Opposites attract but likes do not…
These all present serious problems for things
getting across membranes…
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Facilitated Diffusion
What does facilitate mean?
Proteins are the extra help
“escorts” across the membrane
We call these membrane proteins…
Protein channels
Carrier proteins
LIGAND receptor proteins
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Facilitated Diffusion
•takes place through proteins, or assemblies of proteins, embedded in
the plasma membrane
Protein channels
Carrier proteins
LIGAND receptor proteins
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Facilitated Diffusion
(continued)
FAST
SPECIFIC
Still diffusion
so we only see
it from high
concentration
to low
Does NOT
require energy
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What about when we want to
go against the
concentration?
(From low concentration to
high?)
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What do we need???
ENERGY!!!!
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Cellular Transport [2]
Active transport – energy is needed to move particles.
Carrier proteins – embedded proteins change shape to open and close passages across the membrane.
Endocytosis – taking something into the cell.
Exocytosis – expelling something from the cell.
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Exocytosis
Exo- means…
Cyto means…
Sis means…
When cell releases large amounts of material
Excretes stuff
Membrane of vacuole surrounding particle inside cell fuses with the cell membrane
The contents in vacuole are then forced out of the cell
We see this in removal of water by contractile vacuoles
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