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Diffusion
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Concentration Gradient
The difference in the
concentration of
molecules across a
distance
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Diffusion
 The net movement of particles from an
area where there are many particles of a
substance to where there are few
particles (due to Brownian motion)
Until dynamic
equilibrium is reached
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Simple diffusion
 Molecules that can dissolve in lipids can
diffuse across the cell membrane (nonpolar-O2 and CO2 )
 Very small molecules that don’t dissolve
in water move through using pores
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Osmosis
 The diffusion of water through a
selectively permeable membrane is
called osmosis.
 Like other forms of diffusion, osmosis
involves the movement of a substance—
water—down its concentration gradient.
 Osmosis is a type of passive transport
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Osmosis
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Hypertonic Solutions: contain
a high concentration of solute
relative to another solution (e.g.
the cell's cytoplasm). When a
cell is placed in a hypertonic
solution, the water diffuses out
of the cell, causing the cell to
shrivel.
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plasmolysis
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 Isotonic Solutions: contain the
same concentration of solute as an
another solution (e.g. the cell's
cytoplasm). When a cell is placed
in an isotonic solution, the water
diffuses into and out of the cell at
the same rate. The fluid that
surrounds the body cells is
isotonic.
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 Hypotonic Solutions: contain a
low concentration of solute
relative to another solution (e.g.
the cell's cytoplasm). When a
cell is placed in a hypotonic
solution, the water diffuses into
the cell, causing the cell to swell
and possibly explode. (cytolysis)
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 Contractile Vacuole in a paramecium
 http://www.youtube.com/watch?v=pahUt0
RCKYc
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Diffusion into a cell
 Cells need ions & small molecules
 Facilitated diffusion – uses proteins to
move needed items across membrane
with the concentration gradient (glucose)
 Called channel proteins or carrier
proteins
 Does NOT require energy – passive
transport
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Facilitated Diffusion
Carrier proteins-bind to a
specific substance and carry it
across the membrane (change
shape)
down their concentration gradient
- facilitated diffusion.
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Chapter 4
Section 1 Passive Transport
Facilitated Diffusion
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Crossing the Cell
Membrane
Diffusion Through Ion Channels
Ion Channel proteins spans the
thickness of the cell membrane
A more positively charged ion located
outside the cell is more likely to diffuse
into the cell, where the charge is
negative
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Chapter 4
Section 1 Passive Transport
Channel Proteins
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Active Transport
transport of a substance
across the cell membrane
against its concentration
gradient-cell membrane
pumps
 uses energy
 ATP
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Sodium-Potassium Pump
1 of the most important membrane
pumps in animal cells
 In a complete cycle, the sodiumpotassium pump transports 3
sodium ions, Na+, out of a cell
and 2 potassium ions, K+, into the
cell (450Na+ and 300K+ per
second!)
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Chapter 4
Section 2 Active Transport
Sodium-Potassium Pump
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Large substances, such as proteins
and polysaccharides, are moved
across the membrane by vesicles
 The movement into a cell by a
vesicle is endocytosis
 The movement by a vesicle to the
outside is exocytosis
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2 types of endocytosis
 Pinocytosis-solutes or fluids
 Phagocytosis-large particles or whole
cells
 Amoeba feeding
 Amoeba eating paramecium
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Exocytosis in paramecium
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Brain eating amoeba
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