Transcript Cells and Their Environment

Cells and Their
Environment
Chapter 4
4.1 Passive Transport
Part 1: Diffusion
Remember: Living things need to maintain
homeostasis (responds to external
conditions to maintain a stable internal
condition)
One way to do this is controlling the
movement of things in and out of a cell
Passive Transport



Requires no energy
Diffusion is the
movement of a
substance from an area
of high concentration to
an area of low
concentration by
random movement
A difference in
concentration of a
substance across a
space is a
“concentration gradient”

Equilibrium is where
the concentration is
equal
 Since the interior of a
cell is different from
its external
environment
substances move
back and forth
through the cell
membrane
 The membranes lipid
interior repels ions
and polar molecules.
WHY!!
 The
diffusion of
small or nonpolar
particles across the
cell membrane is
the simplest form
of passive
transport
Part 2: Osmosis

The diffusion of water
through a selectively
permeable membrane
 Water moves down the
concentration gradient
 When something is
dissolved in water its
molecules are attracted
to the water molecules
 This causes a difference
between “free water” and
“non-free water”
 This causes the
concentration gradient
 Hypertonic
Solution



Fluid outside has
higher
concentration of
dissolved
substances than
fluid inside the cell
Causes water to
diffuse out of cell
Cell shrinks
 Hypotonic

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Solution
Fluid inside has
higher
concentration of
dissolved
substances than
fluid outside cell
Causes water to
diffuse into cell
Cell swells
 Isotonic
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Solution
Fluid inside has
same concentration
of dissolved
substances as fluid
outside cell
Causes no net
water movement
Water moves in
and out at equal
rates
Equilibrium – no
change in size
How does this affect you?


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Cells have ways of
dealing with swelling
or shrinking
Cell walls keep cells
from expanding too
much
Some unicellular
eukaryotes have
contractile vacuoles
to collect excess
water and force it out
Animal cells remove
dissolved particles to
stop osmosis
Part 3: Crossing the Cell
Membrane
 Transport
Proteins called
channels help
polar substances
cross cell
membrane
 Each only allows
1 kind of
substance
through

Diffusion through ion
channels

Many ions are important
for cell functions
• Important in heartbeat
and nerve cells

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Ion channels are
transport proteins with a
polar pore so ions can
pass through
membrane
Ions pass without
touching nonpolar lipid
tails

Ion channel gates
allow some to open
and close in
response with
stimuli

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

Stretching of cell
membrane
Change in electrical
charge
Binding of specific
molecules to channel
No use of energy so
Passive Transport

Electrical Charge and
Ion Transport
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Positive or negative
charge affects movement
of charged particle across
membrane
Inside of cells are
negatively charged
compared to outside
This causes positively
charged particles to want
to move in and the
negative ones to want to
move out (trying to reach
equilibrium)
 Facilitated


Diffusion
Carrier Proteins (a kind of transport protein)
bind to a specific substance and carry it
across membrane and release it
Still moving down concentration gradient
• Carrier protein attaches to substance on one side
• Shape of protein changes bringing substance
through
• Substance protected from lipids and is released
and protein goes back to original shape

Passive Transport
4.2 Active Transport
Part 1: Movement Against a Concentration Gradient

When concentration is higher inside than
outside, cells must move things against gradient
from low to high
 Called Active Transport
 Requires energy, usually ATP

Sometimes uses carrier proteins
 Since moving against gradient, called
membrane pumps

Sodium-Potassium Pump
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3 sodium ions, Na+, out and two potassium ions, K+,
in
Active transport of both
• Sodium higher outside and potassium higher inside

Energy supplied by ATP
How Does ATP Work?

ATP is Adenosine TriPhosphate
 3 phosphate groups attached to the sugar
Ribose and the base, Adenine

Phosphates are negative so they repel each
other and are unstable
 A little energy in causes one phosphate to break
off and more energy is given off
 This causes ATP to become ADP, Adenosine
DiPhosphate
 Cell uses energy given off from this reaction
Sodium-Potassium Pump

3 Na+ inside bind to
pump and a
phosphate group
leaves ATP and binds
to pump
 Pump changes shape
and 3 Na+ are
released outside
 2 K+ bind to pump
and are transported
across
 2 K+ and P from ATP
is released inside cell

Pump prevents Na+ from building up in cell
since it constantly flows in due to diffusion


This build up would cause water to flow in due to
osmosis, causing the cell to swell or burst
Allows K+ to be brought in

Many cells use Na+ pump to bring in things they need
Part 2: Movement in Vesicles

Some things are too large to use carrier proteins
 Vesicles used instead
 Endocytosis


when moved in
Cell membrane forms a pouch around a
substance, closes up and pinches off
forming vesicle
Vesicle may fuse with lysosomes or
other organelles

Exocytosis when moved out
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Vesicles inside fuse with membrane and
release contents
Used for proteins modified by Golgi apparatus
Nerve cells and gland cells release proteins