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Chapter 5
Homeostasis and Transport
Homeostasis
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The property of a system that regulates its
internal environment and tends to maintain
a stable, constant condition
Cell membranes help organisms maintain
homeostasis by controlling what
substances enter and leave cells
Passive Transport
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Substances crossing the cell membrane
without any input of energy by the cell –
move down their concentration gradient
Types:
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Diffusion
Osmosis
Facilitated Diffusion
Ion Channels
Concentration Gradient
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Concentration - how many of a
substance’s molecules are sitting in a
specific volume of a solution
Gradient - a measurement of how much
something changes as you move from
one region to another
Concentration Gradient - the difference
in the concentration of molecules across a
space
Concentration Gradient
Diffusion
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Movement of molecules from an area of
high concentration to an area of low
concentration
The process by which molecules
intermingle as a result of their kinetic
energy of random motion.
Diffusion
Diffusion
Add a sugar cube to
a beaker of water

It sinks to the bottom
making the concentration
of sugar higher there

Diffusion
As the cube dissolves
sugar molecules break away
and move from the bottom to
the top of the beaker

Diffusion
Because of their kinetic
energy, the molecules of
sugar are in constant motion
 They keep moving until
they hit something and then
they rebound

Diffusion
If no object blocks their
movement, molecules
continue on their path
 They move down their
concentration gradient from
areas of high concentration
to low concentration until
They reach equilibrium
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Do Now
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What is homeostasis?
Do forms of Passive Transport require cell
energy?
What is diffusion?
Equilibrium
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Diffusion will eventually cause the
concentration of molecules to be the same
throughout
Equilibrium – when the concentration of the
molecules of a substance is the same
throughout a space
The molecules are still moving, but they are
just as likely to move in one direction as the
other – they balance each other
Equilibrium
Diffusion Through Cell
Boundaries
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Cell membrane regulates movement of
dissolved molecules from the liquid on one
side of the membrane to the other
Cell membrane - selectively permeable some substances can pass across it and
some cannot
Nonpolar molecules can diffuse through the
lipid bilayer as well as small molecules
Diffusion Through Cell
Boundaries

If a molecule can pass through a cell
membrane, it will diffuse from an area of
high concentration on one side to an area
of low concentration on the other

The cell is not required to use energy for
diffusion.
Diffusion in a Cell
Osmosis
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Solution = solute + solvent
Osmosis
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Both solute and solvent molecules can
diffuse
In cells:
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the solutes are organic and inorganic
compounds
the solvent is water
Osmosis
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Osmosis - diffusion of water through a
selectively permeable membrane (cell
membrane)
Water moves down its concentration
gradient
It moves from areas of high concentration
of water to low concentration of water
Osmosis does not require energy
Osmosis
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When water is more concentrated on one
side of the membrane, water will move to
an area of lower concentration in order to
re-establish equilibrium.
Question??

If there is more salt outside of the cell, will
water move into the cell or out of the cell??
Osmosis
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The net direction of osmosis depends on
the concentration of solutes on the two
sides of the semi-permeable membrane
In a cell, this can have important
consequences
Types of Solutions
 Solutions
can be:
 Isotonic
 Hypotonic
 Hypertonic
Isotonic Solution
The concentration of solutes is the
same inside and outside cell
 Water will diffuse into and out of the
cell at equal rate
 No net movement of water

Hypotonic Solution
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The solution has a lower solute
concentration than the cell
Water diffuses into the cell until
equilibrium is established
Net movement of water – into the cell
Hypertonic Solution
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The solution has a higher solute
concentration than the cell
Water diffuses out of the cell until
equilibrium is established
Net movement of water is out of the cell
Osmotic Pressure
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Osmosis exerts a pressure (osmotic pressure)
on the hypertonic side of a selectively
permeable membrane. (This could cause water
to rush into cells and cells could bust)

This does not happen in animal cells because
they are usually in isotonic fluids

example: blood
Animal Blood Cells in
Different Solutions
Isotonic
Hypertonic
Hypotonic
The Effects of Osmosis on Animal
Cells
osmosis
Osmotic Pressure
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Plant and Bacteria cells are usually in
hypotonic environments (water wants
to diffuse into the cell:
 exposed to tremendous osmotic
pressure
 Rigid cell wall keeps plant and
bacteria cells from bursting.
Plant Cells in Different
Solutions
Hypotonic
Isotonic
Hypertonic
Facilitated Diffusion
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Some molecules easily pass through the cell
membrane because they dissolve in lipids
(alcohols) - others can not (glucose)
Specific carrier proteins allow these other
molecules to pass through the cell membrane
easily
This does not require energy (type of diffusion) only occurs when concentration is higher on one
side of the membrane than the other.
Facilitated Diffusion
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Carrier protein binds to molecule and
changes shape
Carrier protein shields molecule from
hydrophobic lipid bilayer
Carrier protein releases molecule inside
cell
Carrier protein returns to its original shape
Facilitated Diffusion
Ion Channels
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Some ions are important for cell functions
(Na+, K+, Ca2+ and Cl-)
Since they are charged and hydrophilic,
they can’t get across the lipid bilayer
Ion channels – specific membrane
proteins that help ions get across cell
membrane
Active Transport
Requires cell’s energy
 Cells must move substances up
their concentration gradient
 Substances go from areas of lower
concentration to areas of higher
concentration
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Types of Active Transport
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Cell Membrane Pumps
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Endocytosis
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Exocytosis
Cell Membrane Pumps
Carrier proteins also help out in active
transport and work against the
concentration gradient: Low  High
 Similar to facilitated diffusion, but
cell’s energy is required

Sodium (Na+) - Potassium (K+) Pump
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Many types of animal cells need to have:
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a high concentration of Na+ outside the cell
a high concentration of K+ inside the cell
The sodium-potassium pump uses cell
energy to maintain this concentration
difference
Sodium (Na+) - Potassium (K+) Pump
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Three Na in the cytoplasm bind to the
protein pump
At the same time, the protein splits a
phosphate from an ATP molecule (energy)
Protein carries the three Na across the
lipid bilayer and releases them out of cell
Sodium (Na+) - Potassium (K+) Pump
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Next the protein binds two K from outside
of cell
As K bind, the phosphate is released and
the protein changes shape again
Finally the protein releases the K into the
cytoplasm of the cell
This creates an electrical gradient across
cell membrane (nerve and muscle cells)
Endocytosis and Exocytosis
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Some substances (macromolecules and
food particles) are too large to enter cell
through membrane proteins
Endocytosis and exocytosis can move
these substances across cell membrane
Requires cell energy – active transport
Endocytosis
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Taking material into
the cell by means of
infoldings, or
pouches in the cell
membrane
Pouches pinch off
and form vesicles
(organelles) in
cytoplasm.
Two Types of Endocytosis
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Phagocytosis: cell membrane
engulfs food or whole cells (bacteria
and viruses)
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Pinocytosis: cell membrane engulfs
liquids
Exocytosis
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Membrane of
vesicle surrounding
material fuses with
cell membrane,
forcing contents
out of the cell
Reverse of
endocytosis
Release of proteins