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 TransportPassive 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
OSMOSISOsmosis: 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 soluteRule of Thumb: Water moves towards high solute
Osmosis and Living CellsIf 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 soluteRule of Thumb: Water moves towards high solute
Solution Water will move… Result
Hypotonic Into the cell
Animal cells undergo cytolysis (burst)Plant cells gain turgor pressure
Hypertonic Out of the cell
Animal cells shrivel/shrinkPlant cells undergo plasmolysis
Isotonic in/out of the cell (at equilibrium)
Cells are happy and healthy
Evaluating Osmotic ScenariosEvaluating Osmotic Scenarios
HYPOTONIC HYPERTONIC
Cells in Pure water Cells in Salt-water