Transcript Transport Across Membranes

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You must have lab ready for tomorrow
Lecture #6
 Take
in nutrients
 Expel waste
 Communicate with it’s
environment
 Communicate with
neighbouring cells
The plasma membrane must be highly
selective
 It must be able to take in a very large
food molecule while preventing very
small and valuable molecules from
leaving the cell
 It must recognize and block harmful
foreign substances while expelling the
cell’s toxic waste products
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 Membranes
within the cell must
also be crossed by important
materials
 Ex. in the mitochondria and
chloroplasts reactions occur
which require reactants from
outside the organelle and produce
products that need to leave
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The movement of a substance across a
membrane without the need use energy
Diffusion is the main type of passive
transport
Diffusion is the movement of molecules from
a place of higher concentration to a place of
lower concentration
The rate of diffusion depends on the
concentration difference (aka concentration
gradient) between the two areas
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Simple Diffusion: the ability of
substances to move across a
membrane unassisted (ex. Water,
Oxygen and Carbon Dioxide)
Facilitated Diffusion: when
substances require help by transport
proteins to cross a membrane (ex.
Sugars or amino acids), though this is
still based on concentration gradient
1. Channel Proteins: a hydrophilic pathway in
a membrane that enables water and ions
(ex. sodium, potassium, calcium and
chloride) to pass through
2. Carrier Proteins: bind to a specific solute
(ex. glucose molecule or particular amino
acid) and transports it by changing shape to
move it across the lipid bilayer. *Each
protein is VERY specific (one for glucose
could not transport fructose) which means
SUPER tight control
Diffusion of water across a membrane
 In living cells this movement can
cause swelling and shrinking
depending on the cell’s surrounding
conditions
 There are three kinds of surrounding
conditions (hypotonic, hypertonic and
isotonic) and each impacts the cell in a
different way
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Hypotonic: a solution that has a lower
solute concentration than another
(water moves into the cell)
Isotonic: a solution that has the same
solute concentration than another
(the cell remains unchanged)
Hypertonic: a solution that has a
higher solute concentration than
another (water moves out of the cell)
The movement of substances across
membranes against their
concentration gradient using pumps
 Energy-dependent (about 25% of a
cell’s energy requirements are used
for active transport)
 Classified as either primary or
secondary active transport
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Use transport pumps
ONLY move specific positively charged
ions (ex. hydrogen)
A hydrogen pump (aka proton pump)in
the plasma membrane pushes hydrogen
ions form the cytosol to the cell exterior
The pump will bind to a phosphate
group from ATP to provide the energy to
move the ion
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Uses the concentration gradient of an ion as
it’s energy source
Facilitated by two mechanisms – symport
and antiport
Symport: a solute moves through the
membrane channel in the same direction as
the driving ion
Antiport: the driving ion moves through the
membrane channel in one direction,
providing energy for the transport of
another molecule in the opposite direction