1. 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

3. 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

4. Passive Transport Diffusion:
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

5. 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

6. 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

7. 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

8. OSMOSIS Hypotonic: Describes an area of low solute concentration
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

9. 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!

10. 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

11. Evaluating Osmotic Scenarios
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/shrink
Plant cells undergo plasmolysis
Isotonic
in/out of the cell (at equilibrium)
Cells are happy and healthy

12. HYPOTONIC HYPERTONIC
Cells in Pure water Cells in Salt-water

13. Active Transport ACTIVE TRANSPORT Requires the input of energy (ATP)
Recall: Passive transport occurs down a concentration gradient, and therefore does not use energy!
ACTIVE TRANSPORT
Requires the input of energy (ATP)
Transports molecules from an area of lower concentration to an area of higher concentration

14. Cellular “Pumps”
Some membrane carrier proteins (like the ones involved in facilitated diffusion) also have a function in active transport
These proteins use ATP and act like pumps, which force specific molecules against (up) their concentration gradient

15. The Sodium-Potassium (Na+ / K+) Pump
Transports sodium and potassium ions UP their concentration gradients
A common example of a cellular pump found in many animal cells
Result:
Three sodium ions are moved out of the cell
Two potassium ions are moved into the cell
Cost: ATP

17. Endocytosis
Allows extremely large particles (such as food) entry into the cell without passing through the membrane
The cell membrane folds inward and pinches off, which results in the creation of a vesicle
Pinocytosis – the transport of bulk fluids into the cell
Phagocytosis – the transport of bulk solids into the cell

18. Exocytosis
Allows extremely large particles (waste, for example) to exit the cell without passing through the membrane
A vesicle inside the cell fuses with the cell membrane, releasing its contents outside
Picture endocytosis occurring backwards