1. Passive Transport
Section 5.4

2. Learning Objectives Define passive transport
Contrast simple diffusion with facilitated diffusion
Define osmosis and predict changes to a cell based on various osmotic conditions
Identify solutions as hypotonic, hypertonic, or isotonic
Predict the movement of molecules across a selectively permeable membrane

3. Passive Transport
Passive transport = movement of molecules across a membrane without the use of energy (ATP) by the cell
3 Types:
Diffusion (simple diffusion)
Osmosis
Facilitated diffusion

5. Background Info.
Particles tend to spread out evenly in an available space
Things like to be equal & balanced
When they are not spread out evenly there is a concentration gradient
Concentration gradient = the difference in the amount of a substance between two different locations

7. Diffusion
Diffusion = the net movement of particles down a concentration gradient until equilibrium is reached
Molecules like O2 & CO2 can move through the cell membrane by diffusion

8. Dynamic equilibrium = particles move back & forth between the 2 locations, but at an equal rate
No NET change
Usually not found in organism

9. Osmosis
Osmosis = diffusion of water across a selectively permeable membrane
Water moves according to the concentration gradient of water
BUT….solute molecules interact with the water molecules, “binding” the water molecules
The more solute you have, the less “free” water molecules there are

10. Water moves from areas of higher effective water concentration (less solute) to areas of lower effective water concentration (high solute)

11. How Osmosis Affects Cells
Tonicity = the tendency of a cell in a solution to gain or lose water
Solutions can have 3 types of tonicity:
Isotonic
Hypotonic
Hypertonic
Must have 2 solutions to use tonicity labels (ex. A is hypertonic to B)
Remember that water will move according to its own concentration gradient

12. Isotonic = the concentration of water in the solution and the cell are the same
Water will move into & out of the cell at equal rates
Animal cell will not change size
Plant cell will wilt (central vacuole is not full)

13. Hypotonic = the solution has a higher concentration of water than the cell
Water will move into the cell from the solution
Animal cell will swell & eventually lyse (break open)
Plant cell will become turgid (what plants want)

14. Hypertonic = the solution has a lower concentration of water than the cell
Water will move out of the cell and into the solution
Animal cell will shrivel
Plant cell will shrivel (called Plasmolysis)

15. Solute concentration = the amount of a substance dissolved in the solvent
The higher the solute concentration of a solution, the lower the water concentration
May have to use solute concentration to figure out water concentration & direction of water movement for osmosis

16. Example Tonicity Problem: The sodium concentration in a cell is 10% and the surrounding solution has a sodium concentration of 30%. In what direction will water move?
Answer: water will move out of the cell

17. Osmoregulation Osmoregulation = the control of water balance
Organisms live in changing environments
Cells need a way to deal with diffusion & osmosis
Ex. Freshwater fish live in a hypotonic environment, so they have kidneys & gills that are constantly working to prevent excess buildup of water in the body

18. Plant cells have rigid cell walls
Actually prefer hypotonic environments
Helps to support the plant’s structure
Fungi & bacteria with cell walls also plasmolyze in hypertonic environments

19. Facilitated Diffusion
Some molecules can’t diffuse freely across membranes
Too big
Polar
Has a charge (ion)
Need the help of a specific transport protein in the membrane

20. Facilitated diffusion = the diffusion of a substance across the membrane through a transport protein
Molecules move down their concentration gradient, so it is passive transport
Ex. Sugars, amino acids, ions, water

21. 2 types of transport proteins
Channel proteins – forms a pore or tunnel for molecules to move through
Usually gated (can open and close)
Carrier proteins – binds to the molecule, changes shape, and released molecule to the other side
Protein is always specific to the molecule (like a lock and key)