Transcript Salters-Nuffield Advanced Biology

Salters-Nuffield
Advanced Biology
Practical Work Review
Salters-Nuffield Advanced Biology
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.
Salters-Nuffield Advanced Biology
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.
Salters-Nuffield Advanced Biology
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
Salters-Nuffield Advanced Biology
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.
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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
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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).
Salters-Nuffield Advanced Biology
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
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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)
Salters-Nuffield Advanced Biology
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 enzymerelated 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
Salters-Nuffield Advanced Biology
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.
Salters-Nuffield Advanced Biology
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.
Salters-Nuffield Advanced Biology
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.
Salters-Nuffield Advanced Biology
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.
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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.
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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.
Salters-Nuffield Advanced Biology
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.
Salters-Nuffield Advanced Biology
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
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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.
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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.
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Bar chart
numbers
• 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
40
35
30
25
20
15
10
5
0
interphase
prophase
metaphase
anaphase
telophase
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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 67 sectors, otherwise
than can be confusing
Percentage of cells in each stage of mitosis in
Allium root tip squash
interphase
prophase
anaphase
telophase
metaphase
Salters-Nuffield Advanced Biology
Histogram
n u m b ers
• 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.
H is to g ra m o f le a v e s o f d iffe re n t
le n g th s
20
15
10
5
0
90
100 110 120 130 140 150 160 170
le a f le n g th / m m
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Line graph
Th e e ffe c t o f c a ta la s e c o n c e n tr a tio n o n
b r e a k d o w n o f h yd r o g e n p e r o x id e
20
3
m in
-1
30
/ cm
ox y ge n e v ol v e d
• 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
10
0
0
10
20
30
40
50
[c a ta la s e ] / %
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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