salters-nuffield advanced biology practical work review

Salters-Nuffield Advanced Biology


Practical Work Review


1. Safety and use of apparatus

a) Students should:a) be able to comment critically on the suitability of apparatus for carrying out specific practical tasks in order to achieve measurements appropriate to the investigation

b) have an awareness of safety in the use of apparatus

c) during their practical work, consider the ethical issues arising from the use of living organisms and for the environment.


a) be able to comment critically on the suitability of apparatus…

Students should consider the choice of apparatus to measure a variable.– e.g. using a micrometer/vernier

callipers rather than a ruler because a ruler cannot be used accurately to measure lengths less than 1 mm.


b) have an awareness of safety in the use of apparatus

• Students should explain the reasons for using particular safety precautions or techniques.

• These should specifically relate to the practical investigation, or to groups of similar investigations, and should not be too general. – e.g. the use of protective goggles to prevent

accidental splashing of x into the eyes, because it is harmful to living tissue, but not, just wearing lab coats without explanation.

– e.g. the use of aseptic technique in all aspects of microbiology to avoid contamination. Details of the various aspects of aseptic technique would be needed.

– e.g. the use of a sand tray under the weighted fibres to prevent damage to toes when fibres break


c) during their practical work, consider the ethical issues ….

• Students should consider whether it is necessary to use living organisms in their experiments.

• If they do have to be used, it is necessary to ensure that their treatment is acceptable.

• If carrying out ecological investigations they must consider any possible effects on the environment.


2. Producing reliable and valid results

Students should: • a) appreciate that appropriate observations have to be

made to validly test a hypothesis or idea. A hypothesis should be supported by a VERY good reason or explanation. Eg when testing the thermal stability of membranes in the beetroot experiment, a detailed description of the behaviour of the phopholipid bilayer at higher temperatures is required. Reference to the fluid mosaic model would be helpful.

• b) appreciate that variables need to be identified, including both dependent and independent variables, and, where possible controlled or allowed for. Virtually all core pracs have an identifiable DV and IV


2. Producing reliable and valid results

Students should:• c) be aware that valid results are derived

through precise, repeatable measurements or observations, made with apparatus and experimental procedures that are suitable for the task. Eg the use of micropipettes or graduated pipettes rather than measuring cylinders.

• d) be aware that errors in readings can be systematic (values differing from the true value by the same amount) or random (values lying equally above or below an average value).


a) appreciate that appropriate observations have to be made …

• ‘Observations’ can be taken to include measurements. • Validity is the degree to which the data measure what they

were intended to measure - so results are only valid if measurements correspond to the true state of the phenomenon being measured. (They are therefore accurate).If observed measurements can be compared with some accepted standard, it is possible to judge the validity of data.

• So validity is the measure of confidence that can be placed in a conclusion.

• The measurements taken should also be valid. – e.g. volume of O2 collected in 30 seconds is a valid measure

of rate of catalase activity. Counting bubbles isn’t! – E.g.measuring change in light intensity with a light meter

and data logger is a valid measure of protease activity


b) appreciate that valuables need to be identified…

• The independent variable is the factor that is being varied.

• The dependent variable is the one that the student measures in some way.

• Any other variables that may affect the dependent variable should be controlled (kept constant) in order to produce results that are reliable.

• Students should say HOW each variable is being controlled (eg pH, by means of a buffer solution – with pH of that solution stated)


b) appreciate that valuables need to be identified…(continued)

Students should • say HOW each variable is being controlled (eg pH,

by means of a buffer solution – with pH of that solution stated, or temperature, by means of an electronic water bath)

• Explain the effects of NOT controlling a key variable – in detail. Eg not controlling temperature in an enzyme-related experiment will result in DENATURATION of the enzyme because it is a protein. If the protein is denatured, the shape of the active site is changed and neither ‘induced fit’ of enzyme and substrate, nor ‘lock and key’ will result in successful enzyme-substrate complexes


c) be aware that valid results are derived through precise, repeatable measurements …..

• Reliability is a measure of the confidence in a set of measurements or observations. – This depends on the number of measurements

or observations that were taken. – Ideally a large number of replicates (repeat

measurements) should be taken, and any results that vary considerably from the others should be repeated or discounted.

– A mean can be calculated to be representative of the set of results.


c) be aware that valid results are derived through precise, repeatable measurements …..

• To be reliable measurements should also be accurate.

• To be accurate a measurement should be close to the true value.

• Precision is that part of accuracy that is in the control of the student.

• Precision involves the choice of apparatus and the skill with which it is used.


d) be aware that errors in readings can be systematic or random

• Systematic error– values differing from the true value by the

same amount

• Systematic errors may be the result of the student not knowing how to use a piece of apparatus correctly. – e.g. parallax errors when reading scales on

pipettes or thermometers, or not calibrating a colorimeter before use.


d) be aware that errors in readings can be systematic or random

• Random error– values lying equally above or below an

average value

• Random errors can occur if the student’s concentration wanes during a long series of measurements. – The use of an appropriate sensor and a data

logger can reduce this and introduce objectivity.

• Random errors can also be the result of variation in biological material.


3. Significance of results /data

• Students should realise that the significance of differences or trends within data is dependent on the degree of error within the experiment.


realise that the significance of differences or trends within data …..

• In a set of replicate results, there is likely to be some variation.

• This can be quantified by the range, standard deviation or standard error

• The greater the variation in the replicates, the greater the ‘degree of error’.

• The greater the degree of error, the lower the significance of any trends in the results.


4. Presenting data

• Students should be able to discuss the most appropriate methods for presenting data in order to identify trends and patterns closely, and to an appropriate degree of accuracy.


be able to discuss the most appropriate methods for presenting

data in…..

• Tables• Graphs

– Bar chart– Pie chart– Histogram– Line graph

• See Biological Nomenclature IOB


Tables

• An informative title. • The first column containing the independent

variable. • The second and subsequent columns should

contain the dependent variables • Informative column headings. • Units in heading, not next to the numerical data.• Additional columns can be added to include

calculations based on raw data such as %, rates etc.


Graphs

• A graph is a visual representation of data. Trends and patterns are often more easily seen in a graph, and anomalies more easily identified.

• The student should decide on appropriate scales, and whether to start a scale at zero or some other value.

• The most appropriate type of graph should be chosen to accurately represent data, i.e. bar chart, histogram or line graph.


Bar chart

• A bar chart is to be used when the independent variable is non-numerical or discontinuous,

• e.g. the percentage of cells undergoing stages of mitosis

Percentage of cells in each stage of mitosis in Allium root tip squash

0

5

10

15

20

25

30

35

40

interphase prophase metaphase anaphase telophase

num

bers


Pie chart

• A pie chart can be used to display data that are proportions or percentages.

• There should be labels or a key.

• Ideally no more than 6-7 sectors, otherwise than can be confusing

Percentage of cells in each stage of mitosis in Allium root tip squash

interphase prophase metaphase

anaphase telophase


Histogram

• A histogram is to be used when the independent variable is numerical and the data are continuous, but classified into groups. – e.g. number of leaves

of different lengths.

Histogram of leaves of different lengths

0

5

10

15

20

90 100 110 120 130 140 150 160 170

leaf length / mm

nu

mb

ers


Line graph• A line graph is

used to show the relationships in data, which are not immediately obvious from tables.

• Both the dependent and independent variables are continuous.

• LOBF or joining points (as the case may be) should be justified

The effect of catalase concentration on breakdown of hydrogen peroxide

0

10

20

30

0 10 20 30 40 50

[catalase] / %

oxyg

en e

volv

ed

/ cm

3 min

-1


5. Using results to draw conclusions

• Students should appreciate that statements, explaining the trends and patterns in data from an investigation, should be supported by evidence from the data and their own biological knowledge.

• The reliability of a conclusion is dependent of the experimental method used and the validity of the results obtained.

salters-nuffield advanced biology practical work review

Documents

use of apparatus students

use of apparatus c

use of micropipettes

use of living organisms

use of aseptic technique

choice of apparatus

valid results students

use of protective goggles