CELL BOUNDARIES
The Cell Membrane and Transport
Processes
I. The Structure and Function of the
Cell Membrane
Components of the cell membrane:
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Phospholipid bilayer – nonpolar fatty
acid tails turn inside, polar phosphate
groups turn outside – forms a flexible
structure as a boundary
Integral proteins – act as ion channels or
molecular channels also help in cell
identification
Peripheral proteins – act as receptors
Carbohydrates – Identification tags
outside of the cell membrane (glycolipids
and glycoproteins)
Cholesterol – helps stabilizing the
membrane by making it more solid
The Function of the Cell Membrane
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Surrounds the cell.
Regulate the transport of molecules
in and out of the cell -semipermeable
Immune response
Attaches the cells to other cells or
surfaces.
The model that describes cell
membrane, the Fluid Mosaic Model
http://www.youtube.com/watch?v=ULR79TiUj80
http://www.youtube.com/watch?v=GW0lqf4Fqpg
Selective Permeability
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Cell membranes control what goes
in and out of the cell
It allows some substances to cross
more easily than others
Cell membrane is amazingly thin
II. Passive Transport Processes
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Transport means the movement of
molecules from one side of the cell
membrane to the other
Transport is influenced by:
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The
The
The
The
size of substances
polarity of substances
concentration of substances
permeability of the cell membrane
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Passive transport: requires no
energy from the cell, the energy
comes from the difference in
concentration not ATP.
Moves smaller substances from the
higher to the lower concentration
area.
Important in transporting O2, CO2,
water, ions, small molecules.
http://bcs.whfreeman.com/thelifewire/content/chp05/0502001.html
Types of Passive Transport
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Diffusion:The principal method of
moving substances within the cell
and small molecules across the cell
membrane.
http://www.stolaf.edu/people/giannini/flashani
mat/transport/osmosis.swf
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Osmosis: the diffusion of water
across the cell membrane.
Especially important when the
solute cannot move through the
membrane.
Tonicity:
Describes the tendency of a cell in a given
solution to lose or gain water.
 Isotonic, hypertonic, and hypotonic
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Osmosis in action
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Isotonic solutions – when cells are put
into solutions that have equivalent salt
concentration, visible osmosis does not
take place.
Hypotonic solutions – Cells in very
dilute solutions will take in water and
eventually can burst (animal cells).
Hypertonic solutions – Cells in very
concentrated solutions will lose water and
shrink.
http://bcs.whfreeman.com/thelifewire/cont
ent/chp05/0502001.html
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Facilitated Diffusion – a special
type of diffusion in which the
transported molecules are moved
by a the help of a transport protein.
III. Active Transport
http://bcs.whfreeman.com/thelifewire/content/chp05/0502002.html
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Transport processes that can move
substances from the lower concentration
area to the higher by using energy.
Energy is gained by using ATP molecules.
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A type of active transport is the NaK ion pump
3 sodium ions move out of the cell
with the help of a transport protein,
while 2 potassium ions move into
the cell.
This process requires energy in the
form of ATP.
IV. Transport of Large Particles
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Endocytosis – a process by which large
particles can move into the cell by using
membrane vesicles
Types of endocytosis:
 Phagocytosis – engulfing solid particles
 Pinocytosis – engulfing liquids, solutions
 Receptor-mediated endocytosis – moving
into the cell by first binding with receptor
molecules on the cell’s surface.
http://bcs.whfreeman.com/thelifewire8e/content/cat_0
10/0504003.html
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Exocytosis – the process by which
the cell releases large molecules
through its cell membrane