Exchange Across Cell Membrane - Osmosis Flashcards
What is osmosis?
Diffusion of water molecules across a partially permeable membrane from an area of a higher water potential (high concentration of water - hypotonic) to an area of lower water potential (low concentration of water - hypertonic)
What is water potential?
Likelihood of water molecules to diffuse in or out of the solution
What does pure water have?
Highest water potential - 0
All other solutions have lower water potential
What is isotonic?
If two solutions have the same water potential - isotonic
What are the factors affecting osmosis?
- Water potential gradient
- Thickness of exchange surface
- Surface area of exchange surface
What does water potential gradient affect osmois?
Higher the water potential graident
faster rate of osmosis
difference in water potential on either side of membrane decreases
rate of osmosis levels off over time
How does thickness of exchange surface affect osmosis?
Thinner the exchange surface , faster rate of osmosis
How does the surface area of exchange surface affecting osmosis?
Larger the surface area the faster rate of osmosis
Instructions on doing serial dilution technique of different known concentrations
- five test tube in rack
- 10cm^3 of inital 2M sucrose solution to first and add 5cm^3 of distilled water to other
- Use pippette to draw 5cm^3 of solution of first and add to second ND MIX
- Repeat three times
How to find water potential of potato cells
- Use cork borer to cut potatoes in identically sized chips
- Divide of three and measure mass
- Leave them in solutions for 20 minutes - same amount of time
- Remove and dry
- Weight each group
- Calculate % change in mass
- Use calibriation curve
Potato chip mass
Potato chips will gain water (mass) with solutions of high water potential
lose water with solutions with lower water potential
What does the point of the curve crosses x-axis where % change in mass is 0
Point sucrose solution and water potential is same
Find concentration at point and look up water potential for the concentration of the sucrose solution
What does selectively permeable?
Permeable to water molecules and few water molecules and few other molecules but not to larger molecules
Why aren’t red blood cells placed in pure water?
Absorb water by osmosis because low water potential
Cell surface membrane is thin
Break , bursting the cell and release its contents
How to prevent cells from bursting?
They live in liquid with same water potential as cells
What happens if cell lives in external solution that is higher water potential ? (hypotonic)
- Net movement of water neters the cell
- Cell swells and bursts
What happens if cell lives in an external solution that has equal water potential?
- Net movement neither leaves nor enters cell
- No change
What happens if a cell has lower external water potential?
- Net movement of water leaves the cell
- Shrinks
Why is table above known as processed results?
Calculations made from raw data
Describe how you would use the student’s results in the table above to find the water potential of the potato tissue
- Plot graph with concentration (x)
- percentage change (y)
- Find point at which curve crosses x-axis where there is no percentage change of mass
- Find the concentration at this point and look up water potential for this by using another resource
Before reweighing the potato chips the student blotted dry the outside of each cylinder
Explain why:
Water moving in and out by osmosis
If not water affect mass
Ensures the same amount of water on the outside
Why use percentage change in mass than change in mass?
Cylinders have different starting weights
Use as a comparison
Why repeats are important
Reduce the chance of anomalous results
Line of best fit is more reliable
Why giving results as a raito?
Use comparison
Show proportional change
Why improve reliability by taking additional readings is important?
Reduce the change of anomalus results
Allows a mean to be calculated for the concentrations
How should students use cork borer to cut cylinders be accurate?
Cut down the root
Through the same part
Explain why bungs were placed in tubes?
Avoid change in water potential
Solution as water evaporate
The water potential of a plant cell is -400kPA.
Cell put in solution of -650kPA
Explain what happens?
Mmebrane/cytoplasm shrinks/pulls away from cell wall
plasmolysed
Water moves into the cytoplasm from the surrounding solution
Water moves from cytoplasm into contractile vacuole and is then expelled from the organism
Explain how water moves from cytoplasm into contractile vacuole
Water moves from cytoplasm to contractile vacuole by osmosis
High water potential in cytoplasm then moving into less high water poential in vacuole by diffusing through partially permeable membrane
Suggest why contractile vacuoles are not found in these species live in sea
Amobea is surrounded by water as cell is already rigid and maintains pressure
level of water potential is the same as cytoplasm
