Required Practical 3

Question 1

Why is a ratio often used

Answer 1

Allows for comparisons for different starting masses

Shows proportional change

Question 2

What is a serial dilution/dilution series

Answer 2

Step by step dilutions of a substance to create a range of different concentrations

Question 3

Concentration on 1 part stock and 9 parts water

Answer 3

10%

Question 4

Concentration of 1 part 10% solution and 9 parts water

Answer 4

1%

Question 5

Advantage of serial dilutions

Answer 5

Allows you to make solutions with very low concentrations
Without having to measure very small volumes
Improving accuracy of measuring

Question 6

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

Answer 6

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

Question 7

Uses of serial dilutions

Answer 7

Counting bacteria

Too many to count under a microscope

Question 8

Why do you need to mix

Answer 8

Even distribution and concentration throughout test tube

Question 9

Dilution series equations

Answer 9

C1 x V1 = C2 x V2

V2 = V1 + Volume of distilled water to dilute with

Question 10

C1 x V1 = C2 x V2 variables

Answer 10

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

Question 11

Why is it important to use potatoes of the same species

Answer 11

Potato species may affect the water potential of the cell/different species have different water potentials

Question 12

Why is the skin removed

Answer 12

Impermeable to water

Question 13

Why do we blot

Answer 13

Remove excess water
Which could contribute to mass
Giving an inaccurate representation of the change in mass

Question 14

Why should potato cylinders have the same length

Answer 14

Same length means the same surface area

Surface area affects the rate of osmosis

Question 15

Isotonic

Answer 15

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

Question 16

Hazard

Answer 16

Scalpel is sharp, you could cut yourself

So cut on a mat and away from you

Question 17

Hypotonic

Answer 17

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

Question 18

Hypertonic

Answer 18

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

Question 19

Why are the test tubes kept in a water bath

Answer 19

So they are all at the same temperature

Temperature affects the rate of osmosis

Question 20

What are processed results

Answer 20

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

Question 21

How do you use the change in mass to determine the water potential of a cell

Answer 21

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

Question 22

How do you express things as a ratio

Answer 22

Something:1 = Something

e.g 10:2 = 5:1 = 5

Question 23

Properties of water that make it important in biology

Answer 23

Metabolite
Solvent
Large/high specific heat capacity
Large/high latent heat of vaporisation
Cohesion due to hydrogen bonding

Question 24

Method used to obtain results once cubes have been cut

Variables to control included

Answer 24

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

Question 25

What affects validity of conclusions

Answer 25

Representative samples
Random sampling
Control groups
Length of study
Statistical analysis 
Controlled variables
Number of repeats