Transcript Homeostasis and Transport

Homeostasis and Transport
Organisms respond to the environment
(this is a characteristic of life!)
Cells, tissues, organs, and
whole organisms must maintain
a biological balance with their
immediate environment
Each cell sustains this
balance (homeostasis) by
regulating what substances
can enter and/or leave
The organelle that regulates this is…
the CELL MEMBRANE
The transport of substances across the
membrane can occur in 2 different ways…
• PASSIVE TRANSPORT:
NO energy is required
• ACTIVE TRANSPORT:
Energy (in the form of ATP) is required
Passive Transport
Passive Transport: the movement of some substances
across the cell membrane without any energy input
Diffusion:
• A natural movement of molecules from an area of
high concentration to an area of low concentration
• In other words: the molecules move down their
concentration gradient
Passive Transport (cont.)
• Continues until the system has reached a
state in which the concentration of molecules
is equal everywhere in the system
→ this state is called EQUILIBRIUM
• Molecules are still in motion in a system
which has reached equilibrium, but there
is no concentration change
Diffusion across the cell membrane
• Molecules diffusing across the
cell membrane move DOWN
their concentration gradient
• Small, non-polar molecules
are able to simply diffuse
through the lipid bilayer
→ examples: CO2 and O2
• It is important to remember that
NO ENERGY is being used here
Facilitated Diffusion
... is used to transport molecules needed by the cell
that are very large and insoluble in lipids
Examples?
IONS, ORGANIC POLYMERS
• Their diffusion must be facilitated
(helped) by a part of the membrane
Ion Channels
• Some proteins embedded in the membrane create a pore
through which specific ions (charged atoms) can travel
• Gated channels open/close in response to stimuli
Carrier Transport
• Some proteins embedded in the membrane actually “carry”
a specific needed molecule across the cell membrane
• A molecule is “received” on one side, slides through
the carrier, and is “spit out” the other side
OSMOSIS
Osmosis: Diffusion of water molecules
through a selectively permeable membrane
Hypotonic:
• Describes an area of low solute concentration
(Therefore, solvent concentration is high)
Hypertonic:
• Describes an area of high solute concentration
(Therefore, solvent concentration is low)
Isotonic:
• Describes a situation in which there is
no difference in solute concentration
Rule of Thumb: Water moves towards high solute
Osmosis and Living Cells
If a living cell found itself in a
solution that was extremely
hypotonic to its internal conditions,
what problems could this cause?
→ Hint: you should first think
about which way water would
tend to move in this situation
The Paramecium
• Unicellular organism that lives
in fresh (hypotonic) water
• Has an organelle called a contractile
vacuole which pumps out water that
is constantly diffusing into the cell
*** Since this pumping action is being
done against the concentration
gradient, it requires energy!
Why do plants wilt?
QUESTIONS: In addition to the cell membrane…
What “barrier” do plants possess?
Where is it located?
• Plant cells, when watered regularly, find
themselves living in hypotonic solution
• Water, therefore, moves into the plant cells by osmosis
and the cell membrane swells, pressing it against the
cell wall (this force is called TURGOR pressure)
• Plant cells, when NOT watered regularly, find
themselves instead living in hypertonic solution
• Water, therefore, moves out of the plant cells by osmosis
and the cell membrane shrinks away from the cell wall
(this condition is called PLASMOLYSIS) → wilting plants!
REMEMBER:
Rule of Thumb: Water moves towards high solute
Evaluating Osmotic Scenarios
Solution
Hypotonic
Water will move…
Result
Into the cell
Animal cells undergo
cytolysis (burst)
Plant cells gain
turgor pressure
Animal cells shrivel/shrink
Plant cells undergo
plasmolysis
Hypertonic
Out of the cell
Isotonic
in/out of the cell Cells are happy
(at equilibrium) and healthy
HYPOTONIC
HYPERTONIC
Cells in Pure water
Cells in Salt-water