1. Cells and Their Environment
Chapter 4

2. 4.1 Passive Transport Part 1: Diffusion
Remember: Living things need to maintain homeostasis (responds to external conditions to maintain a stable internal condition)
One way to do this is controlling the movement of things in and out of a cell

3. Passive Transport Requires no energy
Diffusion is the movement of a substance from an area of high concentration to an area of low concentration by random movement
A difference in concentration of a substance across a space is a “concentration gradient”

4. Equilibrium is where the concentration is equal
Since the interior of a cell is different from its external environment substances move back and forth through the cell membrane
The membranes lipid interior repels ions and polar molecules. WHY!!

5. The diffusion of small or nonpolar particles across the cell membrane is the simplest form of passive transport

6. Part 2: Osmosis
The diffusion of water through a selectively permeable membrane
Water moves down the concentration gradient
When something is dissolved in water its molecules are attracted to the water molecules
This causes a difference between “free water” and “non-free water”
This causes the concentration gradient

7. Hypertonic Solution
Fluid outside has higher concentration of dissolved substances than fluid inside the cell
Causes water to diffuse out of cell
Cell shrinks

8. Hypotonic Solution
Fluid inside has higher concentration of dissolved substances than fluid outside cell
Causes water to diffuse into cell
Cell swells

9. Isotonic Solution
Fluid inside has same concentration of dissolved substances as fluid outside cell
Causes no net water movement
Water moves in and out at equal rates
Equilibrium – no change in size

10. How does this affect you?

11. Cells have ways of dealing with swelling or shrinking
Cell walls keep cells from expanding too much
Some unicellular eukaryotes have contractile vacuoles to collect excess water and force it out
Animal cells remove dissolved particles to stop osmosis

12. Part 3: Crossing the Cell Membrane
Transport Proteins called channels help polar substances cross cell membrane
Each only allows 1 kind of substance through

13. Diffusion through ion channels
Many ions are important for cell functions
Important in heartbeat and nerve cells
Ion channels are transport proteins with a polar pore so ions can pass through membrane
Ions pass without touching nonpolar lipid tails

15. No use of energy so Passive Transport
Ion channel gates allow some to open and close in response with stimuli
Stretching of cell membrane
Change in electrical charge
Binding of specific molecules to channel
No use of energy so Passive Transport

16. Electrical Charge and Ion Transport
Positive or negative charge affects movement of charged particle across membrane
Inside of cells are negatively charged compared to outside
This causes positively charged particles to want to move in and the negative ones to want to move out (trying to reach equilibrium)

17. Facilitated Diffusion
Carrier Proteins (a kind of transport protein) bind to a specific substance and carry it across membrane and release it
Still moving down concentration gradient
Carrier protein attaches to substance on one side
Shape of protein changes bringing substance through
Substance protected from lipids and is released and protein goes back to original shape
Passive Transport

19. 4.2 Active Transport Part 1: Movement Against a Concentration Gradient
When concentration is higher inside than outside, cells must move things against gradient from low to high
Called Active Transport
Requires energy, usually ATP

20. Sometimes uses carrier proteins
Since moving against gradient, called membrane pumps

21. Sodium-Potassium Pump
3 sodium ions, Na+, out and two potassium ions, K+, in
Active transport of both
Sodium higher outside and potassium higher inside
Energy supplied by ATP

22. How Does ATP Work? ATP is Adenosine TriPhosphate
3 phosphate groups attached to the sugar Ribose and the base, Adenine

23. Phosphates are negative so they repel each other and are unstable
A little energy in causes one phosphate to break off and more energy is given off
This causes ATP to become ADP, Adenosine DiPhosphate
Cell uses energy given off from this reaction

24. Sodium-Potassium Pump
3 Na+ inside bind to pump and a phosphate group leaves ATP and binds to pump
Pump changes shape and 3 Na+ are released outside
2 K+ bind to pump and are transported across
2 K+ and P from ATP is released inside cell

25. Allows K+ to be brought in
Pump prevents Na+ from building up in cell since it constantly flows in due to diffusion
This build up would cause water to flow in due to osmosis, causing the cell to swell or burst
Allows K+ to be brought in
Many cells use Na+ pump to bring in things they need

26. Part 2: Movement in Vesicles
Some things are too large to use carrier proteins
Vesicles used instead

27. Endocytosis when moved in
Cell membrane forms a pouch around a substance, closes up and pinches off forming vesicle
Vesicle may fuse with lysosomes or other organelles

28. Exocytosis when moved out
Vesicles inside fuse with membrane and release contents
Used for proteins modified by Golgi apparatus
Nerve cells and gland cells release proteins