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Homeostasis
7-3
Biology
Corsicana High School
Homeostasis
• a biological balance that cells
maintain with their
environment
• by controlling what gets into
and out of the cell
• “homeo” = same
• “stasis” = staying
Diffusion
• the process by which
molecules spread from an area
of greater concentration to an
area of lesser concentration
High
concentration
Low concentration
High
concentration
Concentration
gradient
Low concentration
High
concentration
Concentration
gradient
Low concentration
UNTIL:
EQUILIBRIUM
Concentration Gradient
• the differences in
concentrations of a substance
across space
• Molecules will tend to move
down the gradient (from
higher to lower concentration)
Equilibrium
• is reached when the
concentration of molecules is
the same throughout a space
• “equi” = equal
• “librium” = balance
Movement Across
Membranes
• molecules move across
membranes by passing between
the molecules of the membrane
• permeable---if a substance can
move through a particular
membrane, the membrane is said
to be permeable to that
substance
Permeability Depends
On:
• the size of the molecule
• the type of molecule
• the molecular structure of
the membrane
Selectively Permeable
• a membrane which allows some
substances to pass through
but not others
• The cell membrane is
selectively permeable
Osmosis
• the process by which water
molecules diffuse through a
membrane from an area of higher
water concentration to an area of
lower water concentration
• The direction of osmosis depends
on the relative concentrations of
water and solutes inside and
outside the cell
Hypotonic Solution
• concentration of solute
molecules is less outside the
cell than inside
• “hypo” = below
• Water molecules will move
into the cell, since the water
concentration is greater
outside
10%
solutes
30 %
solutes
90%
water
70%
water
10%
solutes
30 %
solutes
90%
water
70%
water
Hypertonic Solution
• concentration of solute
molecules is greater outside
the cell than inside
• “hyper” = over
• Water molecules will move out
of the cell, since the water
concentration is less outside
30 %
solutes
50%
solutes
70%
water
50%
water
30 %
solutes
50%
solutes
70%
water
50%
water
Isotonic Solution
• concentration of solute
molecules is equal outside the
cell and inside
• “iso” = same
• Water molecules will move
into and out of the cell at
equal rates, maintaining
equilibrium.
30 %
solutes
30%
solutes
70%
water
70%
water
30 %
solutes
30%
solutes
70%
water
70%
water
The terms hypotonic,
hypertonic and
isotonic refer to the
concentration of
solutes, but it is the
water that moves.
Turgor Pressure
• the pressure against a plant
cell wall due to diffusion of
water into the cell
Plasmolysis
• a cell wilts due to loss of
water from the cell;
therefore, a loss of turgor
pressure
• “plasmo” = cytoplasm
• “lysis” = splitting
Cytolysis
• a cell swells up and bursts due
to too much water moving into
the cell
• “cyto” = cell
Contractile Vacuole
• to prevent cytolysis,
freshwater animal cells have a
special structure which
constantly pumps out excess
water that diffuses
constantly into the cell
Passive Transport
• no cell energy is used
• diffusion, osmosis are passive
transport
Facilitated Diffusion
• carrier molecules (proteins in
membrane) transport solutes
through the membrane
without using cell energy
• speeds up rate of diffusion
• a form of passive transport
Gated Channel
• protein-controlled passage
that permits the cell
membrane to be permeable as
needed
• a form of passive transport
Active Transport
• movement of a substance
across a cell membrane using
the cell’s chemical energy
• goes against the
concentration gradient---from lower to higher
concentration
Sodium-Potassium Pump
• chemical mechanism that uses
active transport to move Na+ out
of the cell and K+ into the cell
• Both ions move against their
concentration gradient
• 1/3 of the energy expended by an
animal at rest goes to the Na-K
pump
5%
Na+
3%
Na+
2%
K+
1%
K+
5%
Na+
3%
Na+
2%
K+
1%
K+
Both require
the use of
energy
because the
movement is
against the
concentration
gradient.
Endocytosis
• process by which cells engulf
substances too large to pass
through the cell membrane
• forms a pouch (vesicle) that
enters the cytoplasm
• “endo” = inside; “cyto” = cell,
“-sis” = process
Special Forms of
Endocytosis
• pinocytosis---bringing in fluids
“pino” = drinking
• phagocytosis---bringing in
large food particles “phago”
= eating
Exocytosis
• passage of large molecules out
of the cell
• “exo” = outside
• reverse process of
endocytosis