Cellular Transport
Section 8.1
p. 195 - 200
8.1 Osmosis worksheet
Osmosis: Diffusion of Water
• The plasma membrane controls what can enter
or leave a cell.
• The plasma membrane does not limit the
amount of water that passes or diffuses
through.
• Remember…
diffusion is the movement of particles from
an area of higher concentration to an area of
lower concentration.
Osmosis: Diffusion of Water
• In cells, the water wants to have equal
concentrations of substances on both sides of
the membrane.
• The diffusion of water across a selectively
permeable membrane is called osmosis.
• Regulating water flow through the plasma
membrane is important to maintaining
homeostasis.
What controls osmosis?
• If a strong solution and a weak solution of the
same things are put in direct contact, the
water molecules will diffuse in one direction.
• The water will go toward the stronger side
until the concentration is the same
everywhere.
• The water wants to go to the area of higher
concentration (the stronger solution)
Selective permeability =
like a screen
Some things can come in and some things are kept out
6 sugar molecules
on this side
24 sugar
molecules on
this side
24 sugar
molecules on
this side
6 sugar molecules
on this side
During osmosis, water diffuses across a selectively permeable membrane.
Notice that the number of sugar molecules did not change on each side of the
membrane, but the number of water molecules on either side of the membrane
did change.
Cells in an isotonic solution
• Same concentration on both sides
• Diffuses in and out at the same rate
• Cells retain their normal shape
Cells in an isotonic solution – neutral …same concentration on both sides
In an isotonic solution, water molecules move into and out
of the cell at the same rate, and the cells retain their
normal shape as in (A).
Notice the concave (normal) disc shape of a red blood cell
in (B).
A plant cell has its normal shape and pressure in an isotonic
solution in (C).
Put the fish in an isotonic solution
It would be like putting a freshwater fish in a
freshwater fish tank…
Put the fish in an isotonic solution
The fish would stay the same size.
Cells in a hypotonic solution
• Concentration of dissolved substance is lower
in the solution outside the cell
• There is more water outside the cell
• Cells in hypotonic solutions experience
osmosis that FLOWS IN
• The cell swell and internal pressure increases
Cells in an hypotonic solution –
concentration of dissolved molecule lower outside the cell
In a hypotonic solution, water enters a cell by osmosis, causing the cell to
swell (A).
Animal cells, like these red blood cells, may continue to swell until they
burst. (B).
Plant cells swell beyond their normal size as pressure increases (C).
Put the fish in an hypotonic solution
It would be like putting a saltwater fish in a
freshwater fish tank…
Put the fish in an hypotonic
solution
The fish would swell… it may even burst!
Cells in a hypertonic solution
• The concentration of dissolved substances is
higher outside the cell than inside the cell
• Cells in hypertonic solutions experience
osmosis that causes water to FLOW OUT
• Animal cells shrink in hypertonic solutions
because of decreased pressure
• Plants wilt under these conditions
Cells in an hypertonic solution –
concentration of dissolved molecule higher outside the cell
In a hypertonic solution, water leaves a cell by osmosis, causing the cell to
shrink (A).
Animal cells like these red blood cells shrivel up as they lose water (B).
Plant cells lose pressure as the plasma membrane shrinks away from the cell wall
(C).
Put the fish in an hypertonic solution
It would be like putting a freshwater fish in a
saltwater fish tank…
Put the fish in an hypertonic solution
The fish would shrivel like a raisin.
Salt sucks… if you are a slug… is the a hyper or hypotonic environment for the slug?
Remember…Higher concentration on the outside = ?
8.1 Transport worksheet
Concentration gradient
• This diagram shows a “concentration
gradient”.
• There is a higher concentrate of dissolved
particles on one side.
Concentration gradient
• When we move from areas of higher
concentration to lower concentration, we are
moving with the gradient.
• When we move from areas of lower
concentration to higher concentration, we are
moving against the gradient.
Passive transport
• With the concentration gradient – from
higher concentration to lower
concentration
• Requires no energy
Passive transport by proteins
• Transport proteins in plasma membrane
help or “facilitate” movement of
substances through the membrane
• This movement is with the
concentration gradient
• It requires no energy
Passive Transport
Passive transport can occur by (A) simple diffusion,
(B) facilitated diffusion by channel proteins,
and (C) facilitated diffusion by carrier proteins.
Active Transport
• Cells can move substances from areas of
lower concentration to areas of higher
concentration
• This is moving against the concentration
gradient
• This will require extra help from carrier
proteins
• Energy is needed!
How active transport occurs
• A particle binds with a carrier protein –
usually pretty specific (like a lock and key)
• When the correct ones fit, chemical energy
allows the cell to change the shape of the
carrier protein like opening a door.
• Once the particle is on the other side, the
carrier protein goes back to its normal shape.
Carrier proteins are used in active transport to pick up ions or
molecules from near the cell membrane, carry them across the
membrane, and release them on the other side.
Why does active transport require energy?
Molecules tend to move from higher to lower concentration. Active
transport reverses the trend requiring energy input.
Transport Through the
Plasma Membrane
Type of
Transport
Simple
Diffusion
Facilitated
Diffusion
Active
Transport
Transport
Protein
Used?
Direction of
Movement
Requires
Energy
Input from
Cell?
Classification
of
Transport
Transport of large particles
• Endocytosis = cells surrounds and takes in
materials from its environment.
• The material does not just pass through the
plasma membrane but instead is engulfed and
covered by a bubble of membrane that moves
inside the cell.
• Some single cell organisms like amoebas eat
this way.
• This does require energy.
Transport of large particles
• Exocytosis = the opposite of
endocytosis.
• The expulsion of waste materials from
the cell.
Some unicellular organisms ingest food by endocytosis
and release waste or cell products from a vacuole by
exocytosis.