Required Practical 3 Flashcards
Why is a ratio often used
Allows for comparisons for different starting masses
Shows proportional change
What is a serial dilution/dilution series
Step by step dilutions of a substance to create a range of different concentrations
Concentration on 1 part stock and 9 parts water
10%
Concentration of 1 part 10% solution and 9 parts water
1%
Advantage of serial dilutions
Allows you to make solutions with very low concentrations
Without having to measure very small volumes
Improving accuracy of measuring
Describe how you can make a 1 in 10 dilution and then go on to make a 1 in 1000 dilution series of an original solution
1 part stock to 9 parts solution (1 in 10)
Mix
Repeat using 9 parts water and 1 part 10-^1 then same with 10-^2
To make a 10-^3 dilution
Uses of serial dilutions
Counting bacteria
Too many to count under a microscope
Why do you need to mix
Even distribution and concentration throughout test tube
Dilution series equations
C1 x V1 = C2 x V2
V2 = V1 + Volume of distilled water to dilute with
C1 x V1 = C2 x V2 variables
C1: Concentration of stock solution
V1: Volume of stock solution used to make new concentration
C2: Concentration of solution you want to make
V2: Volume of new solution you are making
Why is it important to use potatoes of the same species
Potato species may affect the water potential of the cell/different species have different water potentials
Why is the skin removed
Impermeable to water
Why do we blot
Remove excess water
Which could contribute to mass
Giving an inaccurate representation of the change in mass
Why should potato cylinders have the same length
Same length means the same surface area
Surface area affects the rate of osmosis
Isotonic
Water potential of sucrose solution is the same as the water potential inside the cell
No net movement of water by osmosis in or out of cell
No change in mass
Crosses the X axis at this concentration
Or a ratio will be 1 at this concentration
Hazard
Scalpel is sharp, you could cut yourself
So cut on a mat and away from you
Hypotonic
Water potential of sucrose solution is greater than in cell Water enters the cell by osmosis Down a water potential gradient Increasing the mass of the cell Change in mass is positive Cytolysis
Hypertonic
Water potential of sucrose solution is less than in cell Water leaves the cell by osmosis Down a water potential gradient Decreasing the mass of the cell Change in mass is negative Plasmolysis
Why are the test tubes kept in a water bath
So they are all at the same temperature
Temperature affects the rate of osmosis
What are processed results
Data that has been calculated on
If in a table and it doesn’t show the initial and final mass then the data must be processed
How do you use the change in mass to determine the water potential of a cell
Plot a graph of concentration (x) against percentage change in mass (y)
Draw a line of best fit
X intercept/ratio of 1 used on another graph of concentration (x) against water potential (y) to read off value of water potential at the x intercept concentration
Or use another resource
How do you express things as a ratio
Something:1 = Something
e.g 10:2 = 5:1 = 5
Properties of water that make it important in biology
Metabolite Solvent Large/high specific heat capacity Large/high latent heat of vaporisation Cohesion due to hydrogen bonding
Method used to obtain results once cubes have been cut
Variables to control included
Control the temperature e.g r.t.p
Ensure all surfaces of the cubes are exposed to the solution
Blot dry on paper towel to remove excess water once removed
Measure the mass at given intervals/after 20 minutes
What affects validity of conclusions
Representative samples Random sampling Control groups Length of study Statistical analysis Controlled variables Number of repeats
