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The process of achieving a relatively stable
internal environment
CELLS MUST
CONSTANTLY
RESPOND TO
NATURAL FORCES IN
THEIR
ENVIRONMENT IN
ORDER TO MAINTAIN
HOMEOSTASIS
Therefore, materials like _Water, O2, ions, nutrients, &
hormones are needed to come into a cell and wastes
need to be let out of a cell.
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Two thin layers of phospholipids and proteins.
▪ The membrane is not rigid but fluid.
▪ This enables it to regulate flow of substances
into and out of the cell.

SELECTIVE PERMEABILITY:
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the membrane regulates the exchange of
materials into and out of a cell. Polarity, size,
and electric charge or molecules determine
whether they can pass through a membrane.
SMALL HYDROPHOBIC molecules
pass right through (CO2, O2, and N2)
SMALL, UNCHARGED or POLAR
molecules pass through as well (H2o,
glycerol, ethanol)
LARGER, UNCHARGED or POLAR
molecules have a harder time passing
through (amino acids, glucose,
nucleotides)
IONS can’t pass through (H+, Na+,
K+,Ca+2, and Cl-), they need the help of
TRANSPORT PROTEINS.
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Cells survive only if they can maintain homeostasis.
Organelles work to maintain this state.
Homeostasis is challenged because molecules move.
In order for cells to maintain conditions, they need to
be able to control the movement of “stuff” into and
out of them.
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It encourages the movement of good stuff in
and out.
It discourages the movement of bad stuff in
and out.
How materials move into or out of the
cell
The difference in concentration of a
substance across an area.
To move “down” or “along” a
concentration gradient means to move
from HIGH to LOW concentration. To
move “up” or “against” a concentration
gradient means to move from LOW to
HIGH concentration.
Passive Transport- the movement of
molecules along a concentration
gradient (high low)…it does NOT
require energy.
Can you guess what Active transport
means?

PASSIVE TRANSPORT involves diffusion without
any input of energy. It moves substances down
their concentration gradients. Three types:
DIFFUSION-The natural movement of small molecules
from high to low concentration
 OSMOSIS- The diffusion of water
 FACILITATED DIFFUSION requires the help of transport
proteins in the membrane, but still move down their
concentration gradient.

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Diffusion
The passive movement of
molecules from regions of
[high] to [low]*
* [ ] = scientific symbol
for concentration
All molecules contain heat, causing them to
be in constant random motion, colliding
with one another. As they move, they will
naturally move (diffuse) into available
space if they can easily cross the membrane
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When a difference in concentration exists
across a cell membrane a
CONCENTRATION GRADIENT exists.
Concentration
Total % of molecules in a certain place
By diffusing, the rate of movement of molecules from [high] to [low]
exceeds the rate from [low] to [high] until the concentration gradient
no longer exists. so, rate in = rate out!
People Molecule Example: Field Trip!!!!!!!!!
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The size of the
concentration gradient

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The steeper the gradient, the
faster the molecules move
The surface area of the
membrane

A greater surface area relative
to the enclosed volume results
in a great rate of diffusion.
Requires the help of transport proteins in the
membrane, but still move down their
concentration gradient.
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The movement of water across a selectively
permeable membrane.
Down/Along the concentration gradient.

Water moves towards the hypertonic place
•Tonic
•dissolved
substances
•Hyper
• high
•Hypo
•low
Hypertonic Solution
high
concentration of
dissolved stuff, low in
water- net water loss
cell shrinks

low concentration in
dissolved stuff and high in
water, net water gain, cell
swells

equal amount of dissolved material inside and
outside of cell- no net loss

Cells don’t necessarily want to be isotonic, but
unless they fight it, they’ll go that way.
Example:
•A frog in a freshwater pond will tend
to lose salts from his/her body to the
environment by diffusion.
•Therefore, the frog has a mechanism
to take back salts from the
environment- against the [gradient]!
•This requires chemical energy which
is supplied by ATP.
This leads us to the “other” type of transport:
ACTIVE TRANSPORT
 WHEN MOLECULES NEED TO BE IMPORTED OR
EXPORTED AGAINST THEIR [GRADIENT], CELLS
EXPEND ATP TO MOVE MOLECULES FROM [LOW]
to [HIGH] USING ENERGY!

SODIUM - POTASSIUM PUMP
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One of the most widely occurring active transport
proteins in eukaryotes.
Used to transport sodium ions out of cells and
potassium ions into cells.
Example: nerve cells have 30 times more potassium
in them than extracellular fluids.
animation
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Sodium ions bind to the
protein on the inside of
the cell membrane;
ATP is hydrolyzed and
the phosphate produced
is linked to the protein
The shape of the protein
is changed in such a way
that the sodium ion can
be expelled out of the
cell
Potassium ions bind to
the protein
Phosphate group is
removed causing the
protein to snap back to
its original shape
Potassium ion moves
into the cell
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ENDOCYTOSIS

Large molecular with materials are
enclosed within invaginations of
the plasma membrane,
subsequently pinching off to form
cytoplasmic vesicles.
Phagosytosis- cell ingests large
particles such as bacteria or pieces
of debris





Entrap
Engulf
Digest
Absorb
Pinocytosis- cell ingests liquid
and/or dissolved solutes and small
suspended particles. Also known
as “receptor-mediated endocytosis”
animation

Helps cells remove larger molecular waste
materials