c2 experimental design Flashcards
1
Q
What are the 5 key parts of a hypothesis?
A
IV, DV, causal relationship between IV and DV, has to be testable, and has to be a statement.
2
Q
Why can there only be one independent variable?
A
There is usually only one independent variable as otherwiseit’s hard to know which variable has caused the change.
3
Q
Define the importance of controlling all other variables.
A
This helps to establish a correlational or causal relationship between your variables of interest and helps avoid research bias, as other variables can also affect the outcome. If the scientists do not control these other variables, they can distort the primary results of interest.
4
Q
Why is it necessary to have a control group?
A
When conducting an experiment, a control is an element that remains unchanged or unaffected by other variables.
It’s used as a benchmark or a point of comparison against which other test results are measured.
5
Q
Why is a large sample size required?
A
In most cases, a large sample of at least 100 is needed to get a more precise estimate of the treatment effect. It also allows researchers to control the risk of reporting false-negative or false-positive findings. The greater number of samples, the greater the precision of the results will be.
6
Q
Does repeating an experiment improve accuracy?
A
Repeating an experiment allows confirmation of whether the previous experiment was a fluke- errors causing a change in data. The more similar repeated measurements are, the more reliable the results.
7
Q
Define placebo effect.
A
A placebo group is told that they are receiving treatment during the experiment but in reality is not receiving anything. This is essential in medicinal testing in humans and acts as the control group.
8
Q
Define a double-blind procedure.
A
When neither the patient nor the experimenter are aware of who received an active drug or a placebo drug. This removes experimental bias.
9
Q
List 6 important aspects that founds an ethical experiment.
A
honesty, objectivity, carefulness, opennes, respect for intellectual property, confidentiality, legality, respect animals, respect humans, inform about placebo, ethics committee.
10
Q
What are the 3 key aspects that shows humans are respected in an experiment?
A
Voluntary participation; informed, ongoing, noncoherent consent; maintain participant confidentiality
11
Q
What is the 3Rs mechanism?
A
Replacement
Reduction
Refinement
12
Q
What are some abiotic factors that can impact experiment results?
A
Temperature, light, water, availability of nutrients, salinity, ocean currents
13
Q
What are some biotic factors that can impact experiment results?
A
Autotrophs, heterotrophs, detritivores
Movement
Respiration: conversion of lipids and fats into usable energy
Sensitivity: the way organisms respond to their environment
Growth
Reproduction
Excretion
Nutrition: acquiring energy by eating other organisms
14
Q
Concentration gradient
A
Concentration gradient refers to the gradual change in the concentration of solutes in a solution as a function of distance through a solution.
15
Q
What parts to answer if question mentions placebo effect?
A
- identify it is placebo
- Acts as a control
- this can be used to identify if results are due to psychological effects or actual impact of the treatment
16
Q
Omega 3 fatty acids, commonly found in oily fish, have been found to be very effective at lowering blood levels of low-density cholesterol which reduce the risk of heart disease. Dieticians now recommend eating fish three times a week. It is even possible to buy HiQ bread that has been enriched with omega-3 fatty acids.
As a result of eating more of these omega-3 fatty acids, people have claimed other benefits to their health such as decreased levels of hyperactivity in children, increased ability to learn, and relief from arthritic pain. Design an experiment to test one of these claims.
A
(*not including those on formula sheet- those are assumed to be written and they count 1 mark each)
- treatment of groups
- control groups must be matched for age, gender, health, etc; diet
- how DV will be measured (practical methods for quantifyin DV
- baseline data by measuring dv before commence expeirment
17
Q
Design an experiment to test _______ human involved. (list components and refer to information sheet)
A
- sample size, RANDOMLY ASSIGNED TO ACCOUNT FOR INDIVIDUAL VARIATION
- list trial groups and their treatment
- control group, WILL BE USED TO COMPARE RESULTS
- IV
- controlled variables: age, gender, health, no other source of DV
- experiment repeated for few times to ensure consistent results
- DV
- Method of quantifying DV
- establish baseline data by measuring DV before commencing experiment
- experimental period: 6-12 months, not too long to be impractical, but long enough for change of DV to be evident
- DV measured at the end of experimental period, tabulated, averaged, and compared to establish result
- in what case of result would hypothesis be supported
BONUS: ETHICS COMMITTEE, DOUBLE-BLIND, PLACEBO
18
Q
What are some possible errors in an experiment?
A
Instrumental
Environmental
Procedural
Human
19
Q
2018 pp q1b
For EACH of the other three designs, outline ONE unique reason as to why they are unsuitable.
A
A: Contents need enzyme and substrate to work (1)
C: temperature is not constant for different pH levels/ two independent variables (1)
D: pH is not varied and it is the independent variable for the hypothesis(1)
20
Q
In freshwater crayfish, water is pumped across the gills by small limbs that are found in front of each gill chamber. The limbs pump the water across the gills by a beating action. The
beating rate of the limbs was measured in crayfish kept at different temperatures. Their normal water temperature is around 10°C.
The table below shows the data that were recorded.
Water temp Av no. of beats per min
10 16
20 42
30 64
40 64
50 0
Outline how the investigators could have set up their experimental method in order to test the hypothesis that freshwater crayfish increase their average number of gill
beats per minute as the temperature increases. (6 marks)
A
- DV: Average number of beats per minute as measured by a computer/camera/observer. (1)
- IV: Water temperature (oC) as measured by a thermometer/controlled water bath attached to a computer. (1)
- Control: The control group would have been the group at 10C as this is their normal temperature (1/2) and provides a baseline comparison for the other groups. (1/2).
- Sample Size
Each group would be made up of a suitably large number of crayfish (20+) (1/2 mark), which would help avoid errors due to chance variation (1/2 mark). - The groups could be picked randomly from a larger group (1/2), or selected in such a way to avoid variations between groups (1/2) due to size, age, sex, health etc. (1/2) to minimise impact of differences due to these variables (1/2).
- Alternatively, you could use the same group and gradually change the temperature (1/2), giving them time to get settled in the changed temperature (1/2), this would avoid any variables between groups that could adversely affect the results (1/2).
- Replication of more than one water tank/bath at each temperature increment (1/2) could be used to verify data collected (1/2).
- Temperature baths would be set up and maintained at a constant temperature for each test group (1/2).
-The other fixed variables in the temperature baths would be maintained at a constant level as much as possible (1/2), e.g. water used, salt concentration, light, size of the baths (1/2). - Observations would be made of gill beats over a period of around 10 minutes and averaged (1/2) or measured at different intervals of a minute over a period of time and averaged (1/2) to limit chance variation (1/2), any outlier results could be discarded (1/2).
21
Q
In freshwater crayfish, water is pumped across the gills by small limbs that are found in front of each gill chamber. The limbs pump the water across the gills by a beating action. The
beating rate of the limbs was measured in crayfish kept at different temperatures. Their normal water temperature is around 10°C.
The table below shows the data that were recorded.
Water temp Av no. of beats per min
10 16
20 42
30 64
40 64
50 0
Discuss the results and how well these support the hypothesis. (3 marks)
A
- Hypothesis supported until 30 bpm
- As beats increase from 16 to 64
- Negates hypothesis at 30 to 40 as results remained constant at 64 bpm
- At 50 degrees, there is a sudden decrease to zero (1/2), where presumably the crayfish have died (1/2).
- Overall the results negate the hypothesis. (1/2)
22
Q
In freshwater crayfish, water is pumped across the gills by small limbs that are found in front of each gill chamber. The limbs pump the water across the gills by a beating action. The
beating rate of the limbs was measured in crayfish kept at different temperatures. Their normal water temperature is around 10°C.
The table below shows the data that were recorded.
Water temp Av no. of beats per min
10 16
20 42
30 64
40 64
50 0
State TWO things that you would change in a follow-up experiment and briefly explain the reasons for your suggestions.
A
- The conditions used should not overly stress or harm the crayfish, certainly not kill them, so the maximum temperature should be limited to 40 degrees or less (1).
- Start a lower temperature, say 5 degrees, to see if any difference is found at a temp that is lower than their normal temp (1).
23
Q
2018 pp q3b
State a likely hypothesis that this experiment could be testing. (3 marks)
A
Increasing the Light Intensity will increase the rates of P/S differently for the two species due to differences in compensation points (or different chlorophyll or etc.)
- IV
- DV
- cause/effect relationship
24
Q
2018 pp q3c
Write a brief conclusion about the optimum light intensity for photosynthesis in each species of plant. (2 marks)
A
- Species A photosynthesises better at lower light conditions up to point X (½), but not at higher light intensities (½) as its rate of photosynthesis drops to zero.
- Species B, rate of photosynthesis increases steadily with increased light intensity up to point Y (1).
- The optimum level for species B cannot be confirmed with the data provided, however, it is likely to be beyond point Y on the graph.
25
Pat investigated how changes in environmental temperature affected oxygen (O2) and carbon dioxide (CO2) levels in the air around a cockroach. Three digital probes linked to a computer, a closed animal chamber and a heat lamp were used in the experimental set-up shown.
Before placing the cockroach in the chamber, Pat decided to measure the temperature, carbon dioxide and oxygen levels for four minutes. The following results were recorded.
Time (minutes) CO2 (%) O2 (%) Temperature (oC)
0 0.04 22.3 29.5
1 0.04 22.1 29.8
2 0.04 22.0 30.0
3 0.04 22.0 30.0
4 0.04 22.0 30.0
Explain ONE good reason why Pat recorded the data for four minutes and not just one minute. (1 mark)
The 4 minute period is to allow the set-up to stabilise at 30o
C (½) to make sure that the temperature, O2 and CO2 levels were accurate for the start of the experiment (½).
26
Pat investigated how changes in environmental temperature affected oxygen (O2) and carbon dioxide (CO2) levels in the air around a cockroach. Three digital probes linked to a computer, a closed animal chamber and a heat lamp were used in the experimental set-up shown.
Before placing the cockroach in the chamber, Pat decided to measure the temperature, carbon dioxide and oxygen levels for four minutes. The following results were recorded.
Time (minutes) CO2 (%) O2 (%) Temperature (oC)
0 0.04 22.3 29.5
1 0.04 22.1 29.8
2 0.04 22.0 30.0
3 0.04 22.0 30.0
4 0.04 22.0 30.0
After the initial four-minute period, Pat quickly placed the cockroach in the chamber and began recording the data from the digital probes. After 10 minutes, ice packs were placed around the sides of the animal chamber to slowly bring the temperature of the chamber down to 10 °C.
The data was recorded using the digital probes for a further 20 minutes. The experiment was then repeated once every day for the next six days with the same cockroach. At all times, care was taken to ensure that the cockroach showed no signs of stress.
(c) Identify TWO control measures that could be made to improve the experimental design (other than repeating the entire experiment). Explain the significance of each of these
control measures.
Ensure that the oxygen, CO2 & temperature levels are the same level at the start of each trial each day, allowing comparability of results.
- Build in a control with identical conditions without the cockroach for comparison
- Test using a number of cockroaches and get an average to account for individual variation.
- Keep the cockroaches’ diet constant so they have equal capacity for cellular respiration
- Use an electric cooler instead of ice packs to avoid variation in the cooling process
- Run each experiment at the same time each day in case the activity of the cockroaches varies on a daily rhythm
Extensions to the experiment (such as use other temperatures or use other species of
cockroaches) were given ½ mark.
27
2018 pp q4di
(i) Using the graphical data above, describe the changes in the levels of carbon dioxide and oxygen when the temperature in the chamber was kept constant compared to when the temperature was decreasing. (2 marks)
- The level of carbon dioxide rose more steeply (from 0.04 to 0.07 over a 10 minute period) when the temp was constant at 30 degrees than at decreasing temperatures (from 0.07 to
0.09 over a 20 minute period).
- The consumption of oxygen was at a higher rate also at the
higher temperature and decreased as the temperature fell.
28
2018 pp q4dii
What conclusion do you think Pat can draw from this investigation?
You should refer to each of the following in your response.
* The cellular process involved.
* The adequacy of the control measures.
* The data collected during Pat’s experiments.
(3 marks)
A maximum of 2 marks was given for:
- the data supports cellular respiration (use of O2 and
production of CO2) is occurring (½) and that the rate of respiration decreases with decreasing temperature (1) as shown by:
- At 30° C the CO2 increases by 0.03% in 10 minutes while the O2 decreases from 22.5 –17.5% (½)
- At 10°C over 10 to 30 minutes the CO2 levels increased by 0.02% and the O2 levels decreased by 2% (½)
- However, while the experiment was repeated six times, only one cockroach was used, which could be argued is not a representative sample (1).
- Sample size should be larger (replicas should
be included) (½)
- one cockroach does not consider any genetic variation in the species (½).
29
Investigators gathered data on a group of 100 smokers for a period of 10 years.
During this time 12 people in the group developed lung cancer, two died in traffic accidents, and three died of heart attacks. The investigators used this data to state that smoking caused lung cancer.
Outline several ways in which this investigation could be improved, with reasons. (4 marks)
Any 4 points from the following (or other logical responses):
- Sample size needs to be much larger and represent a suitably wide cross section of people, so other factors need to be considered, such as age, diet, health, gender, genetic background (1).
- A control group of people who don’t smoke but have the same average backgrounds need to be included. (1)
- To give a definitive link between smoking and cancer (i.e. lung cancer is not caused by other factors or naturally occurring in normal populations) (1)
- Exposure to passive smoke or other carcinogens needs to be taken into account (1) as well as the level number of cigarettes smoked currently as well as length of time smoking needs to be factored in, to show whether the effect of smoking on lung cancer is dose related. (1)
- Conclusions regarding cause can’t be drawn from one small study when there are so many other variables involved, it would need to be repeated (1)
30
2021 pp q1 a-c
a) state IV
b) state DV
c) state hypothesis
A. The independent variable is the surface area of the liver. (1mark)
B. The dependent variable is the amount of oxygen produced. (1mark)
C. The greater/increased the surface area of the liver the increase/ faster the catalase
breakdown of H2O2
31
2021 pp q1d
Identify roles of test tubes D and E. Provide a reason. (3 marks)
- D and E are controls for the experiment.
- D provides a control to show that the reaction comes from the liver and not the other substances such as sand
- E provides a control that shows that it is an enzymatic reaction as enzymes are denatured
at high temperatures, such as boiling.
32
2021 pp q 1e
Explain the results of the experiment, and indicate if the results supported the hypothesis. (3 marks)
- The greater the surface area of the liver (as in cutting and grounding Test tubes A to C)
- the more oxygen is produced because more of the enzyme is released by the cutting and grinding process. (1)
- The ground liver gave the greatest amount of oxygen. These results confirm the hypothesis. (1)
33
2021 pp q2a
Each student followed the same basic procedure:
▪ A Daphnia was placed on some cotton wool fibres in a cavity microscope slide as shown below.
▪ A few drops of 1 mg cm−3 caffeine solution were immediately added to the slide.
▪ The Daphnia was observed using the low power objective lens of a microscope.
▪ The number of heart beats of the Daphnia was counted.
a) Suggest a reason why the Daphnia was placed on cotton wool fibres.
To hold the Daphnia in place so that the heart could be seen, and the heartbeats counted. (1)
* To stop the Daphnia from moving and increasing its heartrate (1)
* The white cotton wool would provide contrast and allow the Daphnia to be easily observed (1)
* It makes the Daphnia easier to locate on the slide (1)
* To keep the Daphnia at the correct focal distance for the low power lens (1)
34
2021 pp q2b
Name 4 variables that would need to be fixed for this investigation. (2 marks)
B. Any 4 variables, clearly identified, for 2 marks:
* The concentration of the food source in the water.
* The species/ size / age / health of Daphnia.
* The water temperature.
* The time allocated to measure heart rate e.g. 15s, 30s (The microscope light would
heat up the slide and increase the heart rate or possibly kill the Daphnia.)
* Salinity of water.
* Same food source.
* Dissolved oxygen concentration.
* The concentration of the caffeine solution.
* The number of drops of the 1mg cm3
caffeine solution.
* Purity/brand of caffeine tablet used.
* Amount / type of cotton wool us
35
2021 pp q2c
Suggest a suitable control for this experiment.
Daphnia on a cavity slide with drops of water added instead of the caffeine solution. (2)
* The heartrate of the Daphnia in water on a slide with no caffeine added. (2)
* The Daphnia in pure water would act as a control group whose heartrate could be compared with those in the caffeine treatment. (2)
* The Daphnia in the same solution as the treatment group with the caffeine removed. (2)
for full marks, the response needs to include the daphnia/ heartrate, an indication of lack of caffeine, and the need for water to be added on the slide.
36
2021 pp q2d
Suggest three features of the procedure which may likely be sources of errors and suggest how they could be improved. (6 marks)
"A few drops of caffeine" is not a controlled amount as "few" is open to interpretation and drop size is variable.
- Use a pipette/ accurately weigh the solution to ensure each slide has the same amount of caffeine solution.
Heat from light in microscope could increase
heartrate
- Turn lamp on only when needed / use a heat
shield.
- Record heartrate under lamp before adding
caffeine
Evaporation of water from slide due to microscope heat, could increase caffeine concentration.
- Use a cover slip to prevent evaporation or keep slides on the microscope for limited time, but the same amount of time.
Daphnia become stressed when handled and left out of water on cotton wool this will increase heartrate.
- Handle only when needed / minimize time in experimental conditions.
- Test the addition of caffeine in their natural environment or give them time in an aquatic environment before adding caffeine.
If the same Daphnia was used by several students, the cumulative effect of caffeine (on one Daphnia), could influence heartrate.
- Allow recovery to time / use different Daphnia for each trial.
Counting heartbeats is difficult, due to the
small size, the use of a microscope, its manual focus and human error
- video recording and slow film down/have a heartrate monitor
detect heart movement / same person to count all treatments to avoid variation/ a number of different people counting each
trial and averaging their data
No time allowed for caffeine absorption, the drops were added, and heartrate counted immediately unlikely to have an immediate effect.
- Set a specific time interval in between the addition of the caffeine and the start of heartbeat recording
There was no control group in the
procedure, different caffeine treatments the hypothesis
could not be tested
- Include a control group which received no caffeine or a number of different caffeine treatment so that heartrate could be measured against different caffeine levels.
Sample size too small (a class set of Daphnia). The individual differences in the heart rates of Daphnia, chance occurrences and one-off errors will have a large impact on the overall
trends/data
- Use a large sample size (200 Daphnia) to minimize the influence of uncontrolled variables/ Repeat the experiment several times with fresh biological material. Use similar Daphnia that are matched for age, sex, health.
Genetic differences in the Daphnia with respect to their tolerance for caffeine will influence heartrate
- A large sample size is needed, and the results averaged to minimize the influence of individual variation. Genetically similar Daphnia need to be bred to use in the experiment.
37
2021 pp q3a
Identify two variables other than time and incubation temperature which could be fixed.
The type/ amount of nutrient medium used (and petri dish size).
* The volume/type/batch of antibiotic added.
* Size of wells.
* Sterile conditions.
* Volume / concentration of bacteria.
38
2021 pp q3b
Describe general trend indicated in the graph.
In general, the greater the concentration the greater the effectiveness of the antibiotic
(or vice versa - the less antibiotic the less inhibition/effect) (1 mark).
39
2021 pp q3c, d
Species X at 0.5g dm-3
At 0, 1, and 2 units of antibiotic there is no inhibition zone which is what would be expected for this reading too / the antibiotic is having very little effect at low concentrations until 4 g.dm-3 and even then, the inhibition zone is minimal.
Species Y at 6.0g dm-3
Species Y at 6 g.dm-3 is much higher than the trend line for a
linear decrease from left to right.
Species Y at 0.0g dm-3
Species Y at zero should have zero effect like all the other species' results as no antibiotic has been applied.
Species Z at 8.0g dm-3
Species Z at 8 g.dm-3 appears to be too low, almost the same
area of inhibition as at 6g dm-3. Below the trend line.
---
1 mark for each irregularity - ½ for stating above/below expected and ½ mark for
identifying the change in relationship to a trend.
40
In the 18th century, cowpox was a non-fatal disease while smallpox was fatal to about 30 percent of people who contracted it. Dr Edward Jenner tested the idea that having had cowpox afforded protection against smallpox. Jenner took material from a cowpox sore and inoculated it into a boy's arm. Months later, Jenner exposed the boy a number of times to the small pox virus, but the boy did not develop smallpox. More experiments followed, and, in 1801, Jenner published his discoveries that smallpox could be eliminated through vaccination.
Evaluate the design of jenner's investigation and suggest improvements. (6 marks)
Validity
* No control group included (1)
* Include a control group that did not receive the cowpox vaccine/ received smallpox only (1)
* Control group ensured it was the effects of cowpox that were being investigated (1)
* The use of a control group would have been unethical/ risked the health of participants (1)
* Consideration of other controlled/fixed variables e.g. age/gender/ethnicity (½) unless there was an explanation of why this was a weakness/ improvement then (1)
* Placebos were ignored unless described in the context of a control
Reliability
* Only one individual used/ small sample size (1)
* Needed a much larger sample size (1)
* Use more people (½) unless there was an indication of a suitable size eg 100
then (1)
Risk
Assessment
* There was a high chance of harm/ death to the participant (1)
* The use of material from cowpox sores may expose the participant to other
harmful pathogens (1)
Ethics
* No [mention of] consent (1)
* Consent required from participant/ parents (1)
* Should test on animals/ mammals before human trials (1) (non-human/
other organisms ½ only)
Other Aspects
* A strength was that the boy was exposed several times to smallpox/ more
experiments followed (1)
* A reference to the limitations in knowledge/ facilities in 18th century science/ medicine (1)
41
Watch what you eat before surgery. Rodents given a high-fat diet were more likely to experience chemical imbalances and inflammation after an operation than those fed normally. Calories restriction may be an inexpensive way to help protect the body from the stress of an operation.
State IV and DV.
IV: Fat intake of diet before operation.
DV: Chemical imbalance caused by stress
42
C2 test q2b
state four factors not already mentioned that should be kept constant in this experiment.
- Temperature of water- too high/low temp could impact rate of photosynthesis
- Source of water plant
- size of plant
- dissolved co2 in water
43
c2 test 3ii
- baby may not eat the same amount may impact experimental results
- hard to measure crying
44
c2 test 3c)
- replacing distilled water with actual cure that is safe and clinically proven
- also adjusting the amount of lactase-> how much lactase makes a difference?
45
c2 test 4b
- David should control amount of gas leakage
- r.p. amount of glucose in the yeast
46
c2 test 4ci
20 degrees celsius, optimum temperature
- act as baseline to show effects of other treatment
47
c2 test 4cii
- count of 30 at 60 degrees during the third count
48
c2 test 4e
- 50 degrees celsius
- not confident
- 3 sample size-> not enough to put weight on this conclusion
49
Tasc 2022 q1bi photos
The student hypothesised that very small organisms are more able to carry out efficient gas
exchange than larger organisms that might have gills or lungs.
i. What is the data suggesting about the relationship about SA:V and size? (1 mark)
The smaller the size of the potato, the larger the SA:V and therefore the faster the
rate of diffusion (1) or a relationship between SA:V and size cannot be interpreted
from the data (1).
50
tasc 2022 q1bii photos
The student hypothesised that very small organisms are more able to carry out efficient gas
exchange than larger organisms that might have gills or lungs.
ii. ii. Does the data support the student’s hypothesis? (1 mark)
Yes (½), small organisms have a high SA:V and can efficiently exchange gas with
environment. Large organisms require complex systems to increase SA:V to carry
out gas exchange (½) OR no (½), the data shows that a high SA:V is efficient for
diffusion with environment, however larger organisms have modified structures
that increase SA:V to carry out efficient gas exchange, so the data does support a
comparison between systems.
51
tasc 2022 q2b
It depends on the sample size. If the populations were large enough the mean would
be a reliable comparison as it will reinforce trends (1). Using averages makes each
group comparable (½). Having a large sample size decreases the chance of outliers
effecting the result as smaller populations can skew the results due to uncontrolled
variables/chance.
52
tasc 2022 q2c
Result R appears to be an anomaly, but it is included in the data.
Give one (1) reason as to why it could be included and one (1) reason why it should not be included (2 marks)
Included
* As it represents a certain section of the population (½) with a higher risk factor
than others (½)
* As it represents a large sample (½) and needs to be researched further to
understand why (½)
Not Included
* As these seem like Outliers (½) they might skew the results if included (½).
* As it is a result of experimental error (½)
* It would be unethical to remove/change results (½)
In all cases it makes it difficult to link cause and effect (½).
53
tasc 2022 q2d
d) Based on this study, the Australian Heart Foundation recommends that Australians reduce their
daily salt intake. Based on the information given, explain if this recommendation is justified. (2 marks)
Two possible answers
Yes, it is justified (½). If salt intake is limited to less than 4 units (or other data) (1). It
significantly reduces the likelihood of High Blood Pressure (½). Having such many data
points would validate results (½).
If student suggested it was unjustified (½), they needed to repeat the experiment to
validate (½) and mention factors such as unknown sample size and other uncontrolled
variables (1)
54
In the 1860’s, Pasteur conducted an experiment to investigate if spontaneous generation of
microorganisms does actually occur. The hypothesis he tested was that:
“Microbes come from cells of organisms in the air; not the air itself.”
a) Evaluate Pasteur’s hypothesis.
Your evaluation must outline your position as to whether it is suitable and include the following:
* Identification of the independent and dependent variables
* An explanation of the cause-and-effect relationship between the variables
* Any other factors that would determine if this is a suitable hypothesis (4 marks)
Marking breakdown:-
* The Identification of independent and dependent variables (1 mark in total)
o IV: Presence of organisms/cells in the air (½)
Also allowed: Break neck (from diagram) (½)
o DV: Growth of microbes in broth/liquid (½)
Also allowed Microbes (½)
* Explanation of cause-effect relationship: (up to 2 marks this was combined with the third dot point for a total of 3 marks possible)
o Giving appropriate cause and effect relationship that demonstrated both variables (1 mark) for example: The presence of cells of organisms in the air, lead to the microbial growth (½), the lack of cells of organisms in the air, lead to no microbial growth (½).
o Providing appropriate explanation that related to experiment: for example, cells must come from pre-existing cells (cell theory) (½) thus the sterilisation/boiling of broth killed any pre-existing cells and any new cells/microbes must come from the open flask being exposed to the air which contained cells (½) – or other reasonable explanation that demonstrated understanding.
* Other factors that would determine if this was a suitable hypothesis (up to 2 marks
– this was combined with second dot point for a total of up to 3 marks)
o Providing a statement that indicated this was a suitable or relatively suitable hypothesis (½) with evidence to support that claim (½ each for the evidence as it related to the given hypothesis); for example, Pasteur’s hypothesis was
suitable (½) as it demonstrated both an IV and a DV (linked to IV and DV in first dot point)( ½). The hypothesis is a testable statement (½) with a causeand-effect relationship (linked to second dot point) (½).
Students were given (½) if they noted that the clarity of the hypothesis could be improved because the question asked them to outline their position. Words that
meant the same thing in the appropriate context were awarded the (½) e.g.,
“unclear”, “could have been better written” etc.
55
tasc 2022 q3b
b) Analyse Pasteur’s experiment to explain if it was scientifically sound. (3 marks)
Experiment had a small sample size (½) with only one tube per condition, a larger
number of tests should be conducted and repeats performed (½) to increase validity
(½). It is not clear which variables were controlled (½), variables such as temperature,
boiling time should be controlled (½). Breaking the glass makes it difficult to control
variables (½) and could result in contamination (½). Flask one has been included as a
control (½) this allows for comparison (½).
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tasc 2022 q3c
c) Explain if the results given support or negate the hypothesis. (2 marks)
The results support Pasteur’s hypothesis (1) as bacterial growth was present in flask 2
(1) which was exposed to air containing microbes
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Question 4
The Tuskegee Syphilis Study, was a study into the natural progression of the disease syphilis in
African American men between 1932 and 1972 by the United States Public Health Service.
Key elements of the study are outlined below:
* 600 men living below the poverty line were involved in the study. 399 of these men had
syphilis at this time.
* The men were given free medical care, meals and free burial insurance in exchange for
their participation which they were told would only be to provide a blood sample at
regular intervals.
* The infected men were told they were being treated for “bad blood” (a local term that
included syphilis, anaemia and fatigue).
* They were never told they had syphilis.
* A cure for syphilis (penicillin) was discovered and was the standard treatment for syphilis
by 1947. None of the men were ever treated with it.
* Funding for the study ceased, however, the study continued without informing the men
they would never be treated.
* Nearly all of the men who had syphilis in the beginning of the study had died by the end;
40 wives contracted the disease; 19 children were born with syphilis.
Evaluate this information to discuss two (2) ethical issues and considerations that would need
to be addressed if this study was conducted today.
(4 marks)
Participants were not informed (½) of what the experiment entailed and therefore could not consent (½). If conducted today participants would need to be fully informed of the
experiment including objectives and possible side effects (½) and would need to sign
paperwork consenting to participate (½). Participants would also need to be allowed to pull
out at any time (½).
A cure for syphilis was available (½) and should have been provided (½) as studies are not
allowed to put their participants through unnecessary harm or suffering (½). If conducted
today participants would need to be provided with the cure as soon as it was available (½)
* Lack of information allowed the disease to spread to the wives and children (½).
* Withholding the cure led to preventable deaths (½).
* Participants were bribed (½) and coerced to participate (½) and therefore their consent
is not voluntary
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tasc 2022 q5a
a) Considering Florey’s experimental designs, identify its iv, dv, control, and 2 controlled variables.
(5 marks)
. Independent variable: dosage of penicillin given to mice, Dependant variable: survival
rate of mice, Control: Mice (1, 2, 3, 4) who were injected with Streptococci sp but did
not receive any penicillin, Controlled variables: mice to have no previous exposure to
pathogens (incl. Streptococci sp), mice to be from the same genetic line, mice to have
the same living conditions e.g., cage size, temperature of cage, mice to be provided
with the same amount of food/water. (1 mark for each box)
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2022 tasc q5b
b) Referring to the experiment explain why it is a laboratory test, a field test or a clinical trial.
This was an example of a laboratory test as it was conducted in very controlled
conditions. A clinical trial requires humans to volunteer and give consent. Mice can’t
give consent. Field test requires the product to be used in the field. Mice are not
properly contained in a field. (1 mark for laboratory test. ½ mark per explanation)
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2022 tasc q5c
Identify and analyse what else would need to be considered to make Florey’s experiment a
stronger experimental design. (4 marks)
A larger sample size of mice for all treatments is required to validate the findings of the
experiment. (1 mark). This allows outliers to be identified (removed in some cases)
and variation in the sample to be analysed. (1 mark) If there is a large variation in
survival rates of mice within each treatment, then Florey needs to consider whether
all other variables are controlled e.g., genetic line of mice, same sized separate
enclosures, same amount and type of food/water. (1 mark)
If necessary, he should then repeat the experiment with all the controlled variables
fixed. (1 mark) Additionally, mice should be randomly selected and assigned to
treatment groups to avoid scientist bias. (1 mark) The number of mice to receive an
injection of Streptococci sp with no penicillin treatment should be minimised due to
ethical reasons. (1 mark) Florey might choose to trial smaller, more frequent doses of penicillin, although that would only be in future experiments not his current
experiment. (1 mark)
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c2 test 1 part a,b,c,d
a) As a control (1 mark) to show that bacterial populations do not come from sources other than the hay.
(b) (i) independent variables = heat/no heat and grass/no grass (2 marks)
(ii) dependent variable = number/growth of microbial colonies (1 mark)
(c) ‘Heating of the grass kills micro-organisms’ (4 marks). Credit was given for incorrect hypotheses stated in an acceptable form (eg. ‘Heating increases the number of microbial colonies’).
(d) A range of possible answers, each gaining up to 2 marks depending on clarity of expression.
eg. broth goes cloudy
broth changes colour
changes in O2/CO2 levels
detection of wastes
staining
electron microscope
plating onto agar plates etc.
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c2 test 1 q1c
A control must be included so the experimental results can be compared with the control results (1) in order to establish their validity (½) or to establish that change in the independent variable was responsible for change in the dependent variable (½).
Each of the temperatures used could be considered a control (1), or one of them suggested as the control for the others (1) but it would be preferable to use a low temperature within the range at which beetroot normally grows (10oC – 20oC) (1).
Also acceptable: control using water without beetroot, or a series of such controls at the four temperatures. (1)
Information about variables that should be controlled/fixed, rather than a true experimental control (½)
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c2 retest q1d
Give 2 reasons why rodents were used in this study rather than human subjects.
photos
photos
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c2 retest q2b
list four factors not already mentioned that should be kept constant in this experiment.
photos
65
c2 retest q3b
photos
66
c2 retest 3c
photos
67
An American doctor, Dr William Bean, studied the growth of his fingernails for 35 years.
He filed a horizontal line on his thumbnail just above the cuticle (the strip of skin at the
base of the nail). By recording how long it took the mark to reach the tip of the
thumbnail he was able to calculate the growth rate. He was eventually able to
conclude:
A 35-year observation of the growth of my nails indicates the slowing of growth
with increasing age. The average daily growth of the left thumbnail, for instance,
has varied from 0.123 mm a day during the first part of the study when I was 32
years of age to 0.095 mm a day at the age of 67.
Is this a controlled experiment? Explain. (2 marks)
The use of the same subject (the doctor) (1) acts as a control for genetic factors that may influence nail
growth (1) carried out for many years ( ½ ). Single person doing the experiment should limit
experimental method differences (1).
ALSO ACCEPTABLE: A single trial is not controlled as it does not take various aspects of human
physiology into consideration (1) No mention of keeping other extraneous variables the same (1) Age
may have only been one factor, no mention of overall health (1). Other explanations, with justification,
were given ½ – 1 mark.
68
A scientist counted the number of chirps that crickets made in 10 seconds at three
different altitudes. He found that at sea level the crickets chirped 46 times in
10 seconds; at 250 metres above sea level they chirped 32 times in 10 seconds, and at
500 metres above sea level the crickets chirped 19 times in 10 seconds.
Suggest a hypothesis to account for the scientist’s findings. (3 marks)
Answer relates decrease in chirping rate (1) to decrease in oxygen concentration / air pressure /
temperature / any other valid abiotic variable (1); written in correct form (1).
Example: “The chirping rate of crickets is slowed as the rate of metabolism decreases”
Must have a “cause”.
If relate decrease in chirping rate to altitude, (1 ½ - 2)
If “As altitude increases, chirping rate decreases” (2)
OR
The chirping rate of crickets is slowed due to change in “any abiotic factor”
69
tasc 2016
A student put a leafy shoot from a laurel plant into a potometer, as shown in the diagram
below. The student put a second leafy shoot into another potometer.
(a) Explain how this potometer could be used to measure transpiration. Include in your
answer the significance of the oil layer. (3 marks)
Transpiration is water loss from the leaves of the plant (½) which can be measured by weighing the
apparatus at the start and end of specified period (1). The water level will reduce / go down. (1).The oil
prevents evaporation from the surface of the water (1) so that only water leaving through the shoot is
measured (1).
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tasc 2016 q2bi photos
What evidence in the tables shows that transpiration occurs:
mainly through the stomata of the leaves? (2 marks)
Shoot 1, treatment 2 upper surface covered and lower surface has most stomata therefore showing a
water loss of 65 units (1) compared to shoot 2, treatment 2 where the lower surface with most
stomata is covered therefore showing decrease in water loss to 18 units (1) .
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Tasc 2016 q3c
What measurements would you need to make using this apparatus in order to obtain
data on the rate of oxygen consumption of a bee? (1 mark)
The distance the drop moves ( ½ ) in a particular time period / specify a time ( ½ ), e.g. distance moved in 5
minutes.
72
Adam carried out an investigation into the effect of different environmental
temperatures on oxygen consumption in the honey bee, Apis mellifera. Rather than
using a single bee in each trial, he used a larger respirometer that could comfortably
hold 10 bees. At each temperature he measured the rate of oxygen consumption of the
bees. He did this three times for each temperature he used and then calculated the
average oxygen consumption for that temperature. He used a different group of 10
bees for each temperature.
(i) Adam believed that using a group of 10 bees improved his experiment. Explain
how. (2 marks)
Individual bees vary in many ways that may affect their rate of oxygen consumption (1) .e.g. size, activity,
gender etc. ( ½ for using examples providing species was not the example). Obtaining an average for
10 bees may reduce the impact of these differences (1) or by reducing natural variations ( ½ ).
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Tasc 2016 q3f
Adam was confident that he had measured the rate of oxygen consumption correctly in
each trial. Identify one experimental error that could have caused this unexpected
result. Explain. (2 marks)
Trial 3 at 25°C: This could be due to the temperature falling (to around 20 °C) (2), or the candidate forgot
to increase the temperature and left it at 20 °C (2), or some of the bees in the respirometer could have died
(2), or the bees were less active (2).
OR
Trial 1 at 35°C: This could be due to the bees being more physically active (1) or the temperature rising
above 35°C (1).
OR
Any other explanation that satisfactorily accounts for the anomaly.
74
tasc 2016 q3g
It was suggested that the temperature range should be extended to 55°C. What do you
predict would happen to the bees’ oxygen consumption at these temperatures and
would this make a good follow-up experiment? Explain with reasons. (3 marks)
The respiration rate would continue to rise ( ½ ) following the trend shown in the table ( ½ ) or the bees’
oxygen consumption may plateau ( ½ ) as other factors, such as amount of oxygen or glucose, limit cell
respiration ( ½ ) or the temperature would rise even more rapidly ( ½ ) as shown by a larger increase from
35°C to 40°C ( ½ ).
It is not a good follow-up experiment as bees are unlikely to experience such high temperatures in nature
(1). It is not ethical as the bees are likely to become stressed and die as the temperature is high enough to
start denaturing normal proteins (1).
OR
Bee oxygen consumption might fall, as one or more bees in the respirometer die from the high temperature
(1).
