I'm doing my Biology Evaluative coursework on thursday, and I just want to make sure all the stuff I say is correct and if not, whats wrong with it, and if I've left anything out please say!
So if I'm investigating how temperature affects the amount of pigment released from the cell surface membrane of the beetroot slices, and measuring this by putting a sample of the water with pigment in it in a calorimeter (assuming there is every limitation and control possible) the:
Independent variable would be: Temperature (degrees Celsius) 
Dependant Variable: (mean) transmission of light (%) 
Size and shape of beetroot slices/discs
Volume of water in the test tubes
The type of beetroot (all discs must come from the same source of beetroot)
Number of discs in each tubes
Time each of the beetroot discs are heated for
Discs held separately so all are equally exposed to thermal treatment (on a mounted needle)
Inconsistant stirring of slices while submerged in water. Beetroot slices were not stirred in the same way for each, meaning inconsistent rate of pigment leakage. Modification: Standardise timing and method of stirring for each slices.
Insufficient number of values of independent variables e.g.. only at 40,50,60,70,80 degrees Celcius, so lack of results between intervals, meaning you could miss changes so making conclusions about trends are less valid. Modification: Include intermediate temperatures within the range e.g. 45,55,65,75 degrees Celcius.
Insufficient range of independent variables (40-80 degrees) lack of results beyond the range investigated could make it difficult to identify a trend or pattern. Modification: extend the range e.g. 20-80
Discs were handout, so discs will have a different surface area to volume ratio of the slices, so pigment release will differ. Modification: use a mechanical cutter slicer to prepare beetroots
Species of beetroot differ, so the concentration of pigment may differ/vary with age/ species. Modification: take samples all from the same source (same beetroot)
Repeats, if below 4 they are unable to detect anomalous results thus decreasing reliability. Modification: perform at least 4 replicates
If not calorimeter was used, then judging by eye can show lack of consistency in judgement, may lead to less precise, less accurate and less reliable results. Modification: use a calorimeter.
How to improve reliability:
Compare your results with other peoples results, or take a lot of replicates and take the mean of them.
20-45 degrees Celcius, The phospholipids can move around and aren't packed together as tightly- the membrane is partially permeable. As the temperature increases the phospholipids move more because they have more energy- this increases the permeability of the membrane as pigment molecules are able to pass through the membrane. The higher temperature is the more kinetic energy the phospholipids have. This increases movement can disrupt the relatively weak forces holding the different parts of the polypeptide chains together, (like hydrogen bonds etc) and can make holes in the cell membrane inducing leakages, this allows substances that wouldn't normally enter of leave the cell, do so.
Above 45 degrees Celcius: The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable. Channel proteins and carries proteins denature as their tertiary structure unravels and their binding site changes shape and the membrane is no longer selectively permeable. They can't control what enters or leaves the cell - this increases the permeability of the membrane.
So as the temperature increases the % transmission of light decreases (as higher temperatures have higher pigment leakage therefore lower % transmission)
Okay I know no one is going to read all of this, but I like to get my thoughts down in a structure that makes sense to me, and this might help people who are doing the evaluative as well. If by any chance anyone reads this, it would be SO helpful if you could comment what you think, like if theres anything I should know for my evaluative. If any one has done the paper, is this all I roughly need to know, or have I missed anything out?
EDIT: Also what would be a couple of examples of sources of errors in this experiment?
You and your students may be familiar with the observation that colour leaks out of beetroot when it is cooked. Many cookbooks suggest that beetroot should be cooked with their outer skins on, and with a minimum amount trimmed from the top (by the leaves) and tail (by the taproot) to reduce the release of beet colour leaking into the water.
This procedure lends itself to detailed evaluation, and provides an opportunity to discuss how you would like students to write up a practical.
Cutting the cores to size and placing in water baths takes only a few minutes. During the thirty minutes heating (or chilling) time, you can discuss writing up or evaluating the procedure.
If you have access to only one colorimeter, one group could gather quantitative results while the others gather subjective, qualitative information. The number of beetroot cores used will depend on the number of water baths available.
Students can work individually or in pairs. To save time, reduce the number of temperatures used and collate results to provide repeats at each temperature.
Apparatus and Chemicals
For each group of students:
Beetroot cores, cut with a size 4 cork borer and soaked in distilled water overnight (Note 1)
Thermometers, 1 for each water bath
Kettle, to provide boiling water for the water baths
Ice bath (a beaker of water surrounded by ice)
Scalpel, 1, or sharp vegetable knife
Forceps or mounted needles to ‘handle’ beetroot cores
Ruler, up to 15 cm, 1
Distilled water, in wash bottle
Measuring cylinder, 10 cm3, 1
Test tubes, 1 for each temperature of water bath
For the class – set up by technician/ teacher:
Access to several water baths set at a range of temperatures, or beakers containing water at different temperatures (Note 3)
Health & Safety and Technical notes
Always carry cutting tools in a small tray. Cut away from you. Replace the cutting tool in the tray when not in use.
1 Beetroot must be raw, not cooked. Use a size 4 cork borer and cut with care using a cutting board. Cut enough cores to make eight 2 cm lengths per working group. Leave the cores overnight in a beaker of distilled water. The pigment from any cells that have been cut by the cork borer will leak into the water. Rinse away any pigmented water in the morning and replace with fresh water.
If you do not have a cork borer, cut the beetroot with a bread slicer (or onion slicer) to make even-sized slices, then cut the slices into even-sized chips. If beetroot is not available, use discs of red cabbage. You will need ten or more discs for each tube. If it is not possible to prepare beetroot in advance, students could cut the cores/ chips at the start of the lesson, wash in distilled water and blot dry.
2 Note: Beetroot juice will stain clothing (and, temporarily, skin) but is not hazardous. Students may wish to wear labcoats to protect their clothing from stains.
3 Check the temperature of the water baths regularly and top up with boiling water or add extra ice if the temperature has changed.
SAFETY: Take care carrying scalpels or knives around the laboratory. Always carry in a tray.
a Cut bores of beetroot with a size 4 cork borer and soak overnight in a beaker of distilled water (Note 1).
b Set up a series of water baths at different temperatures.
c Collect 3 or 4 beetroot cores from the beaker provided. Cut each core into 2 cm sections until you have enough for one core for each temperature of water bath that you will be using. Put your 2 cm sections into a test tube with plenty of distilled water.
d Label a set of test tubes (one for each temperature of water bath) with the temperature and your initials. Add exactly 5 cm3 of distilled water to each test tube and place the tubes, one in each water bath, for 5 minutes to equilibrate to the water bath temperature.
e Remove the beetroot cores from the distilled water and blot gently on a paper towel. Decide whether forceps or mounted needles are best for handling the tissue and what damage this might cause to the cores.
f Place one 2 cm beetroot core into each test tube and leave in the water bath for 30 minutes.
g After 30 minutes, shake the test tubes gently to make sure any pigment is well-mixed into the water, then remove the beetroot cores.
h Describe the depth of colour in each test tube. A piece of white card behind the tubes will make this easier to see. Arrange the tubes in order of temperature of the water bath. Describe any relationship between the amount of pigment released from the beetroot and the temperature.
i If you have access to a colorimeter, set it to respond to a blue/ green filter (or wavelength of 530 nm) and to measure absorbance. Check the colorimeter reading for distilled water.
j Measure the absorbance of each tube and plot a graph of absorbance against temperature. Describe any trends or patterns in your results.
The dark red and purple pigments in beetroot are located in the cell vacuole and are chemical compounds called betalains. The pigments cannot pass through membranes, but can pass through the cellulose cell walls if the membranes are disrupted – by heat (for example cooking), by surfactants, or after a long period pickled in vinegar.
Questions 1 and 2 on the student sheet ask the students to produce a hypothesis about the beetroot cells and make a prediction. Then they have to evaluate the procedure and see if they think it is a valid test of the hypothesis and will produce reliable results. They may wish to alter the procedure in the light of their thoughts.
The procedure allows for students to identify systematic and random variables. It is a good opportunity to practice graphical treatment of results, including standard deviation error bars to assess the variation in repeats.
Students can work individually or in pairs. To save time, it might be a good idea to suggest that the number of temperatures used is reduced and students combine results to provide repeats at each temperature.
Here is a sample of results obtained with a colorimeter – measuring transmission of light at 530 nm (rather than absorbance).
|Observation||Colorimeter readting (%transmission of light)|
|Sample A||Sample B||Sample C||Mean|
|22||very pale pink||93.9||95.0||96.0||95.0|
|42||very pale pink||80.1||77.0||76.9||78.0|
Health & Safety checked, May 2009
Download a student sheet with questions and answers
Investigating the effect of temperature on plant cell membranes (54 KB).