Embed Size (px)
Citation preview
Edexcel Specification
Merchant Taylors’ School
BiologyBiologyBiologyBiology AAAA----LevelLevelLevelLevel
Core Practical Core Practical Core Practical Core Practical WorkbookWorkbookWorkbookWorkbook
2 | P a g e
3 | P a g e
1.1 The effect of caffeine on heart rate
1.2 The vitamin C content of fruit juice
2.1 The effect of temperature on membranes
2.2 Enzyme concentration and rate of reaction
3.1 Observing mitosis
3.2 Totipotency and plant tissue culture
4.1 The strength of plant fibres
4.2 Investigating plant mineral deficiencies
4.3 The antimicrobial properties of plants Some key expressions: Control Variable: A factor that is kept constant so that its
effects on the dependent variable are consistent throughout all experiments
Independent Variable: The factor that affects the dependent
variable. The factor you change. Dependent Variable: The factor that is affected by the
independent variable. The factor you measure.
Reliability: The same results are recorded if the
experiment is repeated. Standard deviation and / or standard error are an excellent measures of reliability.
Accuracy: There is little difference between your
results and the recorded “true” results
A-Level Core Practicals
4 | P a g e
Validity: A combination of accuracy and reliability. Valid results are representative and can be used to make accurate predictions.
Random Error: A mistake in the method or malfunction in
the equipment which leads to the production of a single anomalous result, inconsistent with the trend. Once spotted, a random error should be either repeated or ignored.
Systematic Error: Usually down to an uncontrolled factor, a systematic error affects the entire experiment, usually shifting the results by a consistent amount each experiment. Systematic errors always produce inaccurate results, but in some cases the data produced may still be reliable; and, as a trend may still be observable, valid to a degree.
Null Hypothesis: The opposite of your working hypothesis: i.e. that the independent variable has no effect on the dependent variable. You aim to disprove this hypothesis in your experiment.
Experimental Hypothesis: Your working hypothesis that the
independent variable does have an effect on the dependent variable. By disproving the Null Hypothesis you can accept your Experimental Hypothesis1.
1 Note: it is virtually impossible to prove something correct, yet very simple to prove something
incorrect. Therefore, scientists aim to disprove their Null Hypothesis, which then allows them to accept
their Experimental Hypothesis according to the principle of Occam’s Razor.
5 | P a g e
How Science Works
Part of Biology A-level is an assessment of “How science works”- an overview of the scientific process, neatly summarized into 12 criteria. You can be asked questions relating to these criteria in any written paper, so look at them carefully! Each Core Practical has been designed to introduce you to some of these criteria. As you complete the practicals you should make a note in the table of which criteria the practical meets.
Criteria Learning Outcome Practical
1) Use theories, models and ideas to develop and modify scientific explanations
a) Explain how the development of scientific theories involves hypothesizing, collecting and interpreting data and using creative thinking
b) Explain the importance of modeling as a way of developing scientific understanding
2) Use knowledge and understanding to pose scientific questions, define scientific problems, present scientific arguments and scientific ideas
a) Distinguish between questions that science can address, and those which science cannot address
b) Identify scientific questions or problems within a given context
c) Apply scientific theories to answer scientific questions or address scientific problems
3) Use appropriate methodology, including ICT, to answer scientific questions and solve scientific problems
Justify methods, techniques and processes used during scientific investigations, including use of ICT, to collect valid and reliable data and produce scientific theories for a chosen question or problem
6 | P a g e
Criteria Learning Outcome Practical
4) Carry out experimental and investigative activities, including appropriate risk management, in a range of contexts
Produce a risk assessment before carrying out a range of practical work
5) Analyse and interpret data to provide evidence, recognizing correlations and causal relationships
a) Analyze data including use of;
- Descriptive statistics (mean, mode and median, error bars, SD, identification of outliers and range)
- Graphic representation to identify patterns and relationships (e.g. correlation and cause)
b) Interpret data with reference to the methods of the analysis used
6) Evaluate methodology, evidence and data, and resolve conflicting evidence
Evaluate the validity of inferences made from data in terms of the methods, techniques and processes used to collect and analyze the data, recognizing any systematic or random errors present or conflicting evidence
7) Appreciate the tentative nature of scientific knowledge
Explain how scientific theories are developed, refined, supported or refuted as new data or new interpretations of data become available
8) Communicate information and ideas in appropriate ways using appropriate terminology
Present scientific information using text, graphics and other media as appropriate using scientific terminology with reference to data and credible sources
7 | P a g e
Criteria Learning Outcome Practical
9) Consider applications and implications of science and appreciate their associated benefits and risks
a) Evaluate activities in terms of their associated benefits and risks to humans, other organisms and the environment
b) Discuss the risk associated with an activity in terms of the actual level of the risk and its potential consequences, associated uncertainties, and the factors affecting people’s perception of the risk
10) Consider ethical issues in the treatment of humans, other organisms and the environment
a) Identify ethical issues arising from the application of science as it impacts on humans, other organisms and the environment
b) Discuss scientific solutions from a range of ethical viewpoints
11) Appreciate the role of the scientific community in validating new knowledge and ensuring integrity
a) Discuss the importance of critical evaluation of new data or new interpretations of data which challenge established scientific theories or propose new theories
b) Describe how the process of communication through journals and conferences and peer reviews contributes to the validation of new scientific theories by the scientific community
12) Appreciate the ways in which society uses science to inform decision-making
Discuss how science influences decisions on an individual, local, national or international level
8 | P a g e
•
•
•
•
Culture of Daphnia (water fleas)
3 cavity slides
3 dropping pipettes
Distilled water or pond water
• Caffeine solution • Stopclock
•
•
•
Cotton wool
Pipettes
Test tubes
• Paper towels or filter paper
• Microscope (+ lamp if
needed)
heart
gut
antenna
eye
legs
Daphnia
1.1 The effect of caffeine on heart rate
Plants produce caffeine as an insecticide. Cocoa in South America, coffee in Africa and tea in Asia have all been used for hundreds of years to produce ‘pick-me-up’ drinks. This is because in humans caffeine acts as a stimulant, causing increased amounts of stimulatory neurotransmitters to be released. In addition caffeine has a direct affect on the myocytes of the heart. At high levels caffeine consumption has been linked to insomnia, restlessness, anxiety, increased stress and hypertension. This can lead to heart and circulatory problems. Method:
Use a clicker, calculator or pencil and a stopclock to record the number of heartbeats in one minute
Repeat the process at least two more times and take an average
Repeat steps 2 and 3 until you have recorded the average heart rate of the Daphnia for at least five different concentrations of caffeine. Work in pairs using the blind technique
Place a few strands of cotton wool on a cavity slide to restrict the movement of the water flea. Using a pipette transfer one large water flea to a cavity slide, keeping plenty of water surrounding the flea. Do not use a cover slip. Focus on the Daphia’s heart, which can be seen through its translucent body
1
2))
4)
3)
9 | P a g e
Controls:
Results:
Caffeine
Concentration
/ %
Heart Rate
Repeat 1 /
bpm
Heart Rate
Repeat 2 /
bpm
Heart Rate
Repeat 3 /
bpm
Heart Rate
Average
bpm
Standard
Deviation
Factor Why you controlled it How you controlled it
10 | P a g e
Graph: Galton formulated standard deviation in the late 1860s. It is used to show dispersal of data and, therefore, reliability. Calculate the standard deviation of your 5 different caffeine solutions and use this to plot error bars on your graph. Evaluation:
Comment on the relationship between the IV and DV and the reliability of the experiment.
11 | P a g e
Ethics As a class, brainstorm the ethical implications of using animals in experiments. What precautions did you take to ensure their safety? Is it appropriate to use animals in scientific research? Does the Utilitarian argument apply to this practical?
Ethical Point Comment
12 | P a g e
Questions
1 What is the Independent Variable in this experiment?
2 What controlled variables did we fail to control?
3 What is the definition of a perfectly reliable experiment?
4 Are perfectly reliable experiments valid?
5 Why did we use Daphnia in this experiment?
Extension: caffeine affects neurotransmitter levels in the brain. Daphnia have no ‘brain’, so how did the caffeine produce it’s effect?
13 | P a g e
Notes, Comments:
14 | P a g e
1.2 The vitamin C content of fruit Fruit juice is recommended as a good source of the antioxidant
vitamin C and large volumes are sold every day in bottles and
cartons. In 2004, two high school students in New Zealand, who
were conducting an experiment to determine the vitamin C levels
of their favourite fruit drinks, found that the levels in one well-
known blackcurrant juice drink were much lower than those the
manufacturer claimed it contained. The manufacturer dismissed
the concerns saying the claim related only to the blackcurrant
fruit and not the product. However, the case was taken up by a
television consumer affairs show and, after further testing, it
was found that statements about the levels of vitamin C had
been misleading. 15 charges were brought under the Fair
Trading Act. In March 2007 the manufacturer pleaded guilty to
all 15 charges and was fined NZ$217 500.
ChooseChooseChooseChoose oneoneoneone ofofofof thethethethe followingfollowingfollowingfollowing questionsquestionsquestionsquestions totototo investigate.investigate.investigate.investigate.
• Which type of fruit juice provides the most vitamin C? • Is drinking fruit juice from a carton just as good as
eating fresh fruit to maintain high levels of antioxidant vitamin C in your diet?
(Remember that fruit juice sold in cartons is ‘long life’ and does
not require refrigeration because it has been heat treated.)
PlanningPlanningPlanningPlanning The quantity of vitamin C in food and drink can be determined
using a simple colour test. Vitamin C decolourises the blue dye
DCPIP (dichlorophenolindolphenol). Vitamin C is an antioxidant
and reduces the DCPIP. DCPIP changes from blue to colourless
(or slightly pink) as it becomes reduced.
15 | P a g e
You will be provided with the following equipment:
•
•
•
Range of fruit and or fruit juices • Pipette, syringe or burette to measure volumes
Standard 1% vitamin C solution
1% DCPIP solution
accurately
• Standard laboratory glassware and apparatus
Design an experiment to test the question you have chosen Make sure your plan;
• Includes the question or problem that you are testing
• Includes a procedure that uses suitable apparatus to test your question or problem
• Identifies the independent and dependent variables
• Identifies any other variables which may affect the outcomes of the experiment and, where possible, controls or allows for them
• Has a control, if appropriate, that is fully explained
• Includes repeats and an explanation of why these are necessary
• Says what measurements you will make, how they will be made, and the level of accuracy that you can expect from your measurements
• Includes an assessment of any risk Use the following headings for your plan; Introduction (explain what you plan to investigate) Method (a list of numbered points, include quantities, volumes) Diagram (surely you can work this one out for yourself?) Controls (use a table for this) Risk (identify any hazards and state precautions taken)
16 | P a g e
17 | P a g e
18 | P a g e
Results:
Analysis & Comments:
Volume required for colour change / ml
Test Reagent Repeat 1 Repeat 2 Average
19 | P a g e
Questions
1 What is an antioxidant?
2 What is a free radical and why are they so bad for DNA?
3 What else exposes us to free radicals?
Extension 1: free radical damage is a proposed mechanism for ageing. Do you agree with this theory?
Extension 2: vitamin C is also important in the formation of collagen. What is collagen and what is its role in scurvy?
20 | P a g e
You will need:
•
•
•
•
•
•
•
Raw beetroot
Size 4 cork borer
White tile
Knife
Ruler
Plastic beaker about 250 cm3
Forceps
• Water baths at 0, 10, 20, 30, 40, 50, • Stopclock
60, 70 °C
2 boiling tube racks
8 boiling tubes
Thermometer (one per water bath)
Colorimeter
Cuvettes
• Distilled water
• Pipette for measuring 2 cm3
• Small measuring cylinders
• Waterproof marking pen
•
•
•
•
•
2.1 The effect of temperature on membranes
Method:
1 Cut sections from a single beetroot using a cork borer. Cut eight 1cm length slices from these sections. Be careful: beetroot juice stains
2 Place the sections in a beaker of distilled water to wash away any excess juice
3 Pipette 5cm3 distilled water into three boiling tubes
4 Place a chip of beetroot into each tube and put into a waterbath at 20°C for 30min
5 Remove the beetroot sections and shake the tubes to disperse the dye in the solution
6 Switch on the colorimeter and calibrate it with a cuvette of 2cm3 of distilled water.
7 Pipette 2cm3 of the dye solution into a clean cuvette and take an absorbance reading. Repeat for the remaining two chips.
8 Repeat steps 4 to 7 with the other temperatures in the range
21 | P a g e
Controls:
Risk Assessment:
Risk 1: Precaution:
Risk 2: Precaution:
Risk 3: Precaution:
Risk 4: Precaution:
Risk 5: beetroot stains Precaution: washing beetroot chips carefully
Factor Why you controlled it How you controlled it
22 | P a g e
Results:
Graph:
Temperature
/ °C
Absorbance
1 / AU
Absorbance
2 / AU
Absorbance
3 / AU
Average
Absorbance / AU
23 | P a g e
Analysis:
Trend Explanation
A note on Systematic Errors (if applicable)
24 | P a g e
Questions
1 What is the effect of temperature on molecules?
2 What happens to membranes at ~50°C?
3 What is the effect of alcohol on membranes?
4 Give an example of an enzyme that does not denature at 50°C
Extension: What is the Maxwell-Boltzmann distribution? How does this help our understanding of the effect of temperature on rates of reaction?
25 | P a g e
Notes & Comments:
26 | P a g e
You are provided with the following equipment:
•
•
•
Standard amylase suspensions of different concentrations
Standard laboratory glassware and apparatus, including a ruler, stopclock and thermometer
Agar plates impregnated with iodinated starch
2.2 Enzyme concentration and rate of reaction
The aim of this activity is to investigate the effect of reduction in enzyme concentration on the rate of reaction by measuring the initial rate of reaction. In this case the reaction investigated is the breakdown of starch by amylase enzyme. We are going to use agar plates impregnated with starch and iodide solution. Remember, starch turns blue/black in the presence of iodide solution. As the starch is digested, the colour of the agar will change. Write a prediction for the relationship between the concentration of amylase and the clear circle of digested starch. Use scientific ideas to support your prediction. Design an experiment to test the answer you have suggested.
Design an experiment to test the question you have chosen Make sure your plan;
• Includes the question or problem that you are testing
• Includes a procedure that uses suitable apparatus to test your question or problem
• Identifies the independent and dependent variables
• Identifies any other variables which may affect the outcomes of the experiment and, where possible, controls or allows for them
• Has a control, if appropriate, that is fully explained
• Includes repeats and an explanation of why these are necessary
27 | P a g e
• Says what measurements you will make, how they will be made, and the level of accuracy that you can expect from your measurements
• Includes an assessment of any risk Use the following headings for your plan; Introduction (explain what you plan to investigate) Method (a list of numbered points, include quantities, volumes) Diagram (surely you can work this one out for yourself?) Controls (use a table for this) Risk (identify any hazards and state precautions taken)
28 | P a g e
29 | P a g e
Results:
30 | P a g e
Graph:
Diameter of clear circle / mm
Amylase conc / % Repeat 1 Repeat 2 Average
31 | P a g e
Trends and Explanation:
A note on Accuracy (if applicable)
32 | P a g e
Questions
1 Why is iodinated agar used?
2 How does iodide cause starch to turn blue/black? 3 Generally, why do scientists use as wide a range as possible? 4 We could have done a similar practical using protease enzyme and
agar impregnated with protein powder. Why might we prefer to use amylase?
5 Does the rate of reaction ever plateau? Why?
Extension: What is the induced fit hypothesis and why is it different to the lock and key theory?
33 | P a g e
Notes & Comments:
34 | P a g e
You will need:
•
•
•
•
•
•
•
•
Garlic roots
Scalpel or sharp knife
1 M hydrochloric acid
Ethanoic alcohol (acetic alcohol)
Orcein ethanoic stain (acetic orcein)
Ice-cold distilled water
Water bath at 60°C
2 watch glasses or small sample tubes
• Test tube
•
•
•
•
•
•
•
2 pipettes (and pipette fillers) or small measuring cylinders
Microscope slides and coverslips
Pair of fine forceps
Mounted needle
Filter paper or soft tissue paper
Microscope with magnifications of ×100 and ×400
Safety goggles
3.1 Observing mitosis
To see mitosis in action you need to look at living cells. Garlic bulbs grow roots that have actively dividing cells in their tips. Each cell has only eight chromosomes so it is relatively easy to see the chromosomes once they have condensed. In order to see the chromosomes inside the cells, the cells must be separated and spread out into a layer that is ideally just one cell thick. Plant cells are glued together by a middle lamella of pectin proteins. Hydrochloric acid will break down the pectins that hold the cell together. Use the method below to stain the chromosomes. Method:
1 Put a test tube containing 2 cm3 1 M hydrochloric acid into a water bath at 60 °C.
2 Cut off 1–2 cm from several root tips of some growing
garlic roots. Choose root tips which are white and have a firm rounded end; tips that are turning brown will give poor results.
3 Put the root tips in a watch glass containing
approximately 2 cm3 of acetic alcohol. Heat the watch glass gently for 5 min using a hot plate.
4 Remove the root tips and place them in a second watch
glass with approximately 5 cm3 ice-cold water. Leave for 4–5 minutes, then dry the root tips on filter paper. It is important to blot the tips well to remove the water at this stage or a precipitate may form when staining
35 | P a g e
5 Put the root tips into the pre-heated hydrochloric acid for exactly 5 minutes.
6 Transfer one of the root tips to a clean microscope
slide. Cut about 4–5 mm from the growing tip. Keep the rounded tip and discard the rest. The meristem tip is usually a denser white and more rounded than the cut end. If you take the wrong end, the presence of xylem will make maceration more difficult.
7 Gently break up the root tip cells with a mounted needle
(this is called maceration).
8 Add one small drop of orcein ethanoic stain and leave to stain for 2 minutes.
9 Cover with a coverslip, and blot firmly with several layers
of tissue or filter paper. Press gently to spread the root tip, or tap gently on the coverslip with the end of a pencil.
10 View under the microscope (×400 magnification) and look
for cells with chromosomes. If cells are overlapping, squash the slide again between two wads of filter paper. Avoid lateral movement of the coverslip.
11 Look for regularly shaped, actively dividing cells. DNA
stains dark red/black with acetic orcein stain so you should be able to see red/purple groups of chromosomes against a paler pink background.
Examine your preparation carefully for cells undergoing different stages of mitosis. Questions:
1 Identify cells in the following stages of mitosis: interphase, prophase, metaphase, anaphase and telophase. Draw one cell to illustrate each stage. Your drawings will be simple outlines of the cells and the groups of chromosomes in them as few other structures will be visible. Aim to show the relative sizes and positions of the chromosomes and the cell accurately. Annotate to describe what is happening.
36 | P a g e
2 Count the number of cells in the area visible under the microscope when viewed at ×400 (the field of view). Count the number of cells in each stage of mitosis. Record your results in the table.
Prophase
Metaphase
Interphase
Anaphase
Telophase
Spare (just in case)
37 | P a g e
3 Calculate the percentage of the cells in each stage of mitosis. Rank these values from highest to lowest. Given that your preparation freezes the process of mitosis at one point of time, what do these values suggest to you about the length of time a cell spends in each stage of mitosis?
4 If a group of cells is dividing rapidly, a high proportion of the cells will be undergoing mitosis. A group of cells that is not dividing will have all cells in interphase of the cell cycle. The amount of cell division occurring in a tissue can be quantified using the mitotic index. The mitotic index is used for studying tumour growth in cancer patients. Using the formula below, calculate the mitotic index for your root tip. If you have time, compare this value with the mitotic index of an area of cells away from the tip and comment on your findings.
Number of cells in this stage
Total Prophase Metaphase Anaphase Telophase Interphase
Percentage of cells in this stage (highest to lowest)
% % % % %
Mitotic Index = Number of cells containing visible chromosomes
Total number of cells in the field of view
38 | P a g e
5 Using a stage micrometer and eyepiece graticule make appropriate measurements to allow you to compare the size of interphase cells with those that are undergoing cytoplasmic division. Comment on your finding.
6 Explain how you ensured, or could ensure, that the results are reliable and valid.
7 Explain the safety precautions taken during this practical.
39 | P a g e
8 The cellulose walls of plant cells are held together by a cement called the middle lamella. Treatment with hydrochloric acid breaks this down. Why is this helpful in your preparation?
9 You may have found few dividing cells in your root tip(s). Suggest possible reasons.
40 | P a g e
You will need:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Seeds of white mustard (Sinapsis alba) or rapid-cycling brassica (Brassica rapa)
Agar
Distilled water
Damp sponge
Cling film
Short-necked test tubes or McCartney bottles
Weighing scales
Small plastic tray or lunchbox
250 ml beaker
Glass rod
Paper towels or ‘oven gloves’
Scissors
Wax pencil / permanent pen
Access to Bunsen etc.
Access to light bank or sunny windowsill
3.2 Totipotency and plant tissue culture Plant tissue culture refers to the growth of individual cells or as
in this case, organs, on an artificial medium. In this practical you
will take the tops of plant seedlings and grow them into
complete plants on agar. With most plant species this could take
many weeks, but you will use ‘rapid-cycling’ plants. (These are
plants which grow and complete their life cycles quickly, not
those that are speedy on a bike!) Over a week or two you should
see the explants (the parts of the seedlings you removed) grow
new leaves, a new stem and more leaves, and even new roots and
flower buds.
Plant tissue culture is used in industry to develop improved
plant and food crop species, increase the disease resistance of
plants and encourage plants to produce increased quantities of
phytochemicals used in drugs.
Some of these steps may have been done for you. In this case
start where your teacher indicates.
41 | P a g e
apex
cotyledons
hypocotyl
sponge
Removing the top of the seedling with sharp scissors
Method:
1 Sprinkle some seeds of white mustard or rapid-cycling Brassica onto a damp sponge places in a plastic tray. Cover with transparent cling film and place in a warm, light place to germinate. When the seedlings have just started to unfold their cotyledons (seed leaves) they are ready to culture.
2 Measure out 2.5g of agar powder and add to 250cm3 of distilled water. Boil and stir gently with a glass rod until the agar dissolves.
3 Whilst the agar is still molten, pour about 2cm depth into several short-necked test tubes or McCartney bottles. Allow to solidify.
4
5 Carefully push the cut end of each explant into the agar. Put
one explant into each test tube or bottle. Make sure the cotyledons do not touch the agar.
6 Cover the tubes with cling film. On each tube or bottle write your name and the date. Place the tubes in a rack under a light bank or on a sunny windowsill. Do not open the tubes again.
7 Observe the progress of your explants daily for ten days and record when anything of note develops. You will need to pop in at break / lunch on some days to do this.
With a sharp pair of scissors cut the tops off the seedlings just below the shoot apex (growing tip) as shown in the diagram. These are the explants. Leave the hypocotyls (the early stem) and roots behind on the sponge
42 | P a g e
Results: Questions:
1 What, if anything, would you expect the explants to obtain from the agar?
2 Why is it advised you use short-knecked test tubes or McCartney bottles? If you only had long-necked test tubes what should you do?
Day Observation
1
2
3
4
5
6
7
8
9
10
43 | P a g e
3 Why should you cover the tubes with cling film?
4 Why should you not open the tubes up again once you have set them up?
5 Suggest what measurements could be made as the explants grow
6 Explain why the explants can grow and develop new leaves, stem and roots
7 You could extend this experiment by just growing the shoot apex (no cotyledons), or isolated cotyledons on their own, and comparing your results with growing the shoot apex/cotyledons together. What further information would you gain by doing this?
44 | P a g e
Fibre Useful part of the plant Applications
Flax Stem of flax plant Linen for clothing
Cotton
Hairs on the seeds on plant belonging to the
mallow family
Cotton for clothing
Hemp Fibres from the stem/leaves of the hemp plant Used for ropes, backing for carpets
Coir Fibre from the husks of the fruit of the coconut Floor coverings, ropes
Jute Fibre from the stem of the jute plant Hessian, sacking and carpets
Manila Hard fibres from the leaves of a type of banana Marine cables and other ropes, nets and matting
Pulp Softwood trunks Paper, cardboard
4.1 The strength of plant fibres In this activity you will extract the fibres from nettles and then test their strength. ‘Fibres’ have been extracted from plant stems for centuries and used in the commercial manufacture of a wide range of textiles and paper. The term ‘fibres’ does not just refer to the sclerenchyma but is used to describe a range of ‘fibre-like’ structures. These plant fibres have been used for different purposes, as indicated in the table. Their use is dependent on their properties. Fibres can be removed from plant stems by retting. This can be done by either field retting or water retting. Field Retting: Plant stems are cut or pulled up and left in the field to rot; microbial action breaks down the stalks. Water Retting: Stems are cut or pulled up and immersed in water to rot; microbial action breaks down the stalks. This process produces more uniform, higher quality fibres, but is more expensive and produces nitrogen-rich waste water that must be treated before discharge. In both kinds of retting, bacteria and fungi breaks down the soft tissues of the stems leaving the cellulose intact. It is them relatively easy to remove the cellulose-rich fibres.
45 | P a g e
•
•
•
You will need:
Celery stems
Retort stand & clamp
10g and 50g masses
You need to know about the retting process. However, it is time-
consuming and very smelly. Therefore, for these reasons we are
going to do a simpler practical!
Method:
1. Carefully remove a fibrous string from a celery stem.
2. Check the string to ensure that it is of constant diameter down its entire length (look for tears and thin sections)
3. Tie one end of the string to the clamp
4. Record the length of the hanging string
5. Add 20g and record the length of the string
6. Repeat step 5 until the string breaks. Results:
Mass Added / g Start length / cm End length / cm Extension / cm
0
40
80
120
160
200
240
280
320
360
400
440
480
520
46 | P a g e
Graph:
47 | P a g e
Questions
1 What is the structure of cellulose? 2 Why is cellulose such a strong molecule?
3 Is the string perfectly elastic? Explain your answer
4 Does the diameter of the string make a difference?
Extension 1: Apply Hooke’s law (F = kx) to this practical to work out the spring constant for cellulose.
Extension 2: aside from strength, what other properties does cellulose have that make it a useful molecule to plants?
48 | P a g e
The following equipment will be available:
• a range of nutrient solutions, including
solutions with:
•
•
•
•
Geramium plants
Cotton wool
Aluminium foil
Standard laboratory equipment
–
–
–
–
–
–
–
all nutrients present
lacking nitrogen
lacking phosphate
lacking potassium
lacking magnesium
lacking calcium
lacking all nutrients
4.2 Investigating plant mineral deficiencies
Method: A series of Geranium plants have been grown in solutions lacking nitrogen, phosphate, potassium, magnesium, calcium and all nutrients. A control plant has also been grown for comparison.
Measure the height of the plants and make notes on the appearance of each plant.
Nutrient Height / cm
No NO3-
No PO42-
No K+
No Mg2+
No Ca2+
No Nutrients
Control
49 | P a g e
Nutrient Observation
No NO3-
No PO42-
No K+
No Mg2+
No Ca2+
No Nutrients
50 | P a g e
Questions
1 What do plants use Nitrate for? 2 What do plants use Phosphate for?
3 What do plants use Potassium for?
4 What do plants use Magnesium for?
5 What do plants use Calcium for?
6 What was the purpose of the control plant? Extension: Can you think of a more accurate method of measuring growth than recording the height of the plants?
51 | P a g e
Notes and Comments:
52 | P a g e
You will need:
•
•
•
•
•
•
Agar plate seeded with bacteria
Plant material (garlic cloves and mint leaves)
Pestle and mortar
10 cm3 industrial denatured alcohol
Pipette (sterile)
Paper discs (e.g. Whatman antibiotic assay paper discs)
• Sterile Petri dish
•
•
•
•
Sterile forceps
Tape
Marker pen
Incubator set at 25 °C
4.3 The antimicrobial properties of plants Plants are susceptible to infection by bacteria and fungi; they do everything to repel such attacks. Several plants are known to, or thought to, destroy or inhibit the growth of certain bacteria. A plant with this property is known as antimicrobial. Chemicals in their cells are toxic to bacteria or interfere with their metabolism in some other way. You can probably guess why there is mint in toothpaste, but would garlic be better? Mint may numb our gums but is it lethal to bacteria? In this activity you will investigate whether two plants contain antimicrobial chemicals and how effective they are by looking at the growth of bacteria on agar plates. Method:
1 Agar plates seeded with E.coli bacteria need to be prepared
2 Using a pestle and mortar crush ~3g of garlic with ~10cm3 of ethanol.
3 Using a pipette repeatedly dab a sterile paper disc with ethanol solution from the crushed garlic. After each dab, wait a few seconds for the ethanol on the disc to evaporate before repeating. By doing this slowly, you build up garlic on the filter paper.
4 Repeat steps 2 & 3 for mint
5 Prepare a disc soaked in deionized water and a disc soaked in dettol. Obtain pre-prepared discs soaked in antibiotic from the teacher
53 | P a g e
6 Leave all discs for 10min to dry out
7 Use sterile forceps to place each disc onto a bacterial plate. Maximum of 3 discs per plate. On the lid, draw a circle around the disc and label it with according to what the disc is soaked in.
8 Close each Petri dish and tape it as shown in the diagram. Do not tape all round the dish as this can promote the growth of anaerobic bacteria.
9 Write your name onto the dish and place in the incubator for 24hrs at 25°C
10 Observe the plates without opening them. Measure the diameter of the clear circles of dead bacteria. Use two other sets of results from the class to act as repeats.
11 Do not throw the plates away. They need to be autoclaved. Results:
Disc Width 1 / cm Width 2 / cm Width 3 / cm Average / cm
Dettol
Water
Mint
Garlic
54 | P a g e
Graph: Trends & Analysis:
55 | P a g e
Questions
1 What is an autoclave? 2 What is a bacterial lawn? Why is one important?
3 Why do schools use E. coli bacteria?
4 Why is ethanol used as a solvent when crushing the garlic & mint?
5 What is agar? Where does it come from? Why is it used as a nutrient medium?
Extension: What is antibiotic resistance? Why is it a problem and how are bacteria able to transfer it so quickly?
56 | P a g e
Notes & Comments: Make notes on the aseptic techniques you and your teacher are using during this practical. You could be asked about these in your AS exams!
Aseptic Technique Notes
57 | P a g e
There will definitely be at least one question on each AS paper about a core practical. Questions tend to fall into two categories;
1. Outline a method you could use to test… (i.e. write out the method for a core practical).
2. Analyze data from a practical…
The second type of question relies both on your ability to think like a scientist on the spot and also on your working knowledge of the theory behind each practical, so make sure you know exactly why the IV causes the DV to change as it does. The first type of question is more difficult. However, there are always marks available for the following;
1. Specific Procedure or Method 2. Specific Equipment 3. Safety & Risk 4. Controlled Factors you kept constant 5. A Control for comparison 6. Repeats & taking an Average 7. Stated Range
One way of remembering this is the mnemonic MERC CAR, like this one
Core Practical Revision
58 | P a g e
As you progress through the year try why not fill out the blank revision cards below? That way you have a complete record of what you need to learn in the summer before the summer!
1.1 - The effect of caffeine on heart rate
Method:
Equipment:
Risks:
Controlled factors:
Control for comparison:
Average:
Range:
Additional Notes:
One final comment for you to think about:
If you miss any of the Core Practicals this year it is
imperative that you catch up the work you have missed.
Arguably, they are more important than the theory you
complete in class because you know for certain that at
least one practical will appear on each paper, so each
practical has (in theory) ~ 1 in 4 chance of coming up.
See… important!
59 | P a g e
1.2 - The vitamin C content of fruit juice
Method:
Equipment:
Risks:
Controlled factors:
Control for comparison:
Average:
Range:
Additional Notes:
2.1 - The effect of temperature on membranes
Method:
Equipment:
Risks:
Controlled factors:
Control for comparison:
Average:
Range:
Additional Notes:
60 | P a g e
2.2 - Enzyme concentration and rate of reaction
Method:
Equipment:
Risks:
Controlled factors:
Control for comparison:
Average:
Range:
Additional Notes:
3.1 - Observing mitosis
Method:
Equipment:
Risks:
Controlled factors: none
Control for comparison: none
Average:
Range: none
Additional Notes:
61 | P a g e
3.2 - Totipotency and plant tissue culture
Method:
Equipment:
Risks:
Controlled factors: none
Control for comparison: none
Average:
Range: none
Additional Notes:
4.1 - The strength of plant fibres
Method:
Equipment:
Risks:
Controlled factors:
Control for comparison:
Average:
Range:
Additional Notes:
62 | P a g e
4.2 - Investigating plant mineral deficiencies
Method:
Equipment:
Risks:
Controlled factors:
Control for comparison:
Average:
Range:
Additional Notes:
4.3 - The antimicrobial properties of plants
Method:
Equipment:
Risks:
Controlled factors:
Control for comparison:
Average:
Range:
Additional Notes:
