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Standard B-2
B- 2.5 Explain how active, passive
and facilitated transport serve to
maintain the homeostasis of the cell.
Homeostasis
• The necessity of the organism to maintain
constant or stable conditions.
Examples:
#1 -Your body maintains the same body
temperature. That is homeostasis.
#2- Your body maintains a constant blood sugar
level. Normal blood sugar is about 100 mg/ cm3
Cells must maintain homeostasis
• Homeostasis of cells depends on appropriate
movement of materials across the cell
membrane.
– Materials needed for cellular processes must pass
into cells
– Waste materials from cell processes must pass
out of cells as they are produced
– The cell membrane regulates the passage of
material into and out of the cell.
Each cell exists in a fluid environment
• The cytoplasm within the cell also has a fluid
environment.
• The liquid environment allows for substances
the cell needs to move into the cell
• The liquid environment allows for waste
products to move out of cell.
The cell membrane is semipermeable
• Some things can get through
• Other substances cannot get through
• How do the substances get through?
• Passive transport – go through without energy
• Active transport- go through with energy
Materials pass through by
• Passive transport- without energy
– From high to low concentration
• Or
• Active transport
– From low to high
– Requires energy
Passive Transport
• Substances pass through membrane without
energy
• High concentration to low
• Diffusion- spreading out of molecules across a
cell membrane until they are equal on both sides
of the membrane.
• Diffusion goes with the concentration gradient
• High to low concentration
High to low requires no energy- like
going down steps
Concentration Gradient
Diffusion ( passive transport)
• Particles of solute move across the membrane
from a high concentration gradient to a low
concentration gradient.
• ( from high to low)
• No energy needed
• Goes from high to low
• Goes without energy.
Facilitative diffusion(passive)
• Particles move through proteins in cell
membranes
• No energy used
• Particles go from high concentration to low
concentration
• No energy used
Active Transport
• Particles go from low concentration to high
• Requires energy
• It is like stuffing a large sleeping bag into a
small bag.
• It requires energy
• Goes from
Low concentration
To high
Osmosis (passive diffusion of water)
• Diffusion of WATER molecules across
membrane
• From an area of greater water molecules to
lesser amount of water molecules
• Water molecules will pass through
semipermeable membranes until
concentration on both sides is the same
EQUILIBRIUM- Concentration is equal on both
sides
Osmosis- Water travels through the
membrane until concentration of
particles is even on both sides.
In Osmosis water will travel across
membrane until concentration of
solute is even (in equilibrium ) on both
sides.
Three types of solutions a cell could be
in ( Osmotic situations only)
• Hypertonic solution- More solute in outer
solution than in the cell
– Water will leave cell
– Cell will shrink
• Hypotonic solution- Less solute in outer solution
than in the cell
– Water will enter cell
– Cell will explode (lyse)
• Isotonic solution- The solute inside cell is equal
to outside cell
– Water will pass in and out at equal rates
– Cell will stay the same size