I have been posting comments about the questions that appear year after year on iGCSE Biology papers. Questions like the one below are found in every past paper we have. I call these the “Design an experiment to” questions for obvious reasons…..
“Rivers are sometimes polluted by warm water from power station outflows. This is known as thermal pollution and can affect the growth of plants. Design an experiment to investigate the effect of water temperature on the growth of plants. 6 marks. November 2010”
As you all know, the mark scheme for this kind of design an experiment question is based around the acronym CORMS.
C – how do you change the independent variable?
The independent variable is the thing you are going to change to see its effect. In this experiment it is the temperature of the water. So how are we going to change it? Well it might appear obvious but you need aquatic plants living in water baths at a range of temperatures, say 10,20,30,40,50,60 degrees. Try to make your independent variable continuous if it is possible – the range of temperatures above is much better than just one set of plants in hot water, another in cold water.
O – what organisms (or other biological material) will you use?
To get this mark you will need to say something about the plants you will use in your investigation. For the experiment to produce reliable results, there are many features of the plants that will need to be kept the same in each water bath. Same species, same age of plants, same starting size, same surface area of leaves etc. There are other factors too about the plants that need to be controlled. Can you think of any others?
R – reliability
In order to produce reliable results you will need to set up multiple repeats of each experiment so anomalous readings disappear as you average your results. How would you do this? Well in the example above, I would set up 5 identical water baths at each temperature. We are investigating six different temperatures so we will need 30 water baths. Don’t worry about this. For research as vital as this fascinating experiment, no expense should be spared……
M – how are you going to measure the dependent variable?
There are often two possible marks for this and you will see M1 and M2 on the mark schemes. The key idea is often the same however (there’s a shock) The first mark is for identifying what you will measure about the plants to measure growth. There are lots of alternatives depending on what kind of plant you are using. I am picturing a small floating algae growing in my water baths so I would measure the mass of the plants. (Dry mass would be better but this would lead to destructive sampling – plants won’t grow further if you dehydrate them completely in an oven before weighing them……) You could measure the height of the stem of a plant, or the total surface area of water covered. It doesn’t really matter which thing you choose as long as it is a sensible measure of growth. What will M2 be awarded for? Well it is essential you leave all 30 waterbaths for exactly the same length of time between measurements. How frequently will you measure the growth of your plants? Every hour would be too often, so perhaps every day would be sensible. So a statement that says “use a mass balance to measure the total mass of the plants in each water bath every day for a period of 10 days” will be certain to get both M marks…
S – what factors do you need to standardise to make the experiment a fair test?
You will have mentioned some of these “fair test” factors in the mark point O above. Now it is time to show that you understand what factors other than the temperature of the water will effect the growth of your plants. Growth of plants is done by photosynthesis so I would be aiming to show you understand the other factors that will effect rates of photosynthesis: i.e. light intensity. light wavelength and carbon dioxide concentration. All three should be kept constant and I would say how: same lamp at the same distance from the water baths, carbon dioxide in water controlled by dissolving same mass of sodium hydrogencarbonate in the water. There are often two S marks but by stating all three important control variables this should guarantee we get both.
Now I have written this post without looking at the mark scheme. “Promise…. Honestly Sir I wouldn’t cheat myself like that…..” But here it is and look we would have got full marks. Full Marks = A* #result
There are a few topics which you can pretty much guarantee will be tested somewhere in the two iGCSE Biology papers. There will be a genetics problem to solve (see later post) and in almost every year there is a question about the process of natural selection. These questions tend to be based around either the evolution of antibiotic-resistant strains of bacteria or an animal example based around some adaptation.
Questions on evolution are usually worth four or five marks and I would suggest you always answer them with bullet points. Mark schemes for these questions are often similar and once you have revised the topic, some time spent with past questions and mark schemes would be time well spent.
Imagine you are set a question about cheetah and high speed running. (Everyone knows cheetah can run for short distances at up to 70 mph: so can the gazelle of course – that’s a coincidence isn’t it?) How did modern-day cheetah evolve to run so fast?
Key ideas to include in your answer:
1) Variation in cheetah population: in any population of cheetah at any point in their evolutionary history, some cheetah will just happen to be able to run a little faster than others. This continuous variation could be due to environmental factors (diet, access to gyms etc.) or it could be due to the combinations of genes they happen to have inherited from their parents, or more likely to a bit of both. Environmental causes of variation are not inherited of course but the genetic ones can be and that’s the key to natural selection.
2) Competition: variation by itself cannot lead to natural selection. If all cheetah survived to breed however slowly they ran, then high-speed cheetah would never have evolved. In my example, cheetah are competing with other cheetah for access to prey species. Gazelle run pretty quick too (I wonder why?) so cheetah who are slower than average will get less food. Conversely if you are a cheetah who just happens due to random genetic variation to be a little quicker than your neighbours, you will get more food, be more healthy and more likely to survive to adulthood.
3) If your particular combination of genes makes you more likely to survive, then you are more likely to breed and pass these genes onto future generations of cheetah. This process is called Natural Selection and it results in certain alleles becoming more frequent in a population over time. In this example, the alleles that produce aerodynamic, long-limbed and muscle-bound cheetah become more frequent over time while alleles building lethargic, over-weight and peaceful cheetah tend not to be passed on as well to future generations.
4) This produces a gradual change in the population over time. Selection is a cumulative process: small changes from one generation to the next can add up to big changes over thousands of generations.
NB This answer does not contain the word mutation and this is quite deliberate on my part. You want to make absolutely clear in your answer that at no point in the history of cheetah, did two slow-running cheetah parents give birth to a “mutant” cheetah with Usain Bolt like qualities. Mutation is a random change in the DNA of an organism and much of the genetic variation described above comes not from new mutations appearing but from the shuffling up of alleles into new combinations in meiosis. These new combinations of alleles can produce new phenotypes and these are the features on which selection can act.
But…… If you are writing about the evolution of antibiotic resistance in bacteria, this can be due to a single mutation. One altered gene can produce an enzyme that will breakdown antibiotic molecules or pump them out of a bacterial cell, so in this example it would be correct to talk about a random mutation occurring in the bacterial population. The key thing here is that these mutations have been occurring randomly for billions of years in bacteria. The mutation is totally independent of the use of antibiotics. All that has happened differently in the past 50 years is that for most of evolutionary history these random mutations would have been harmful to the survival chances of the bacterium unfortunate enough to acquire them. Now in an environment particularly in hospitals flush with antibiotics, these once harmful mutations can give the bacteria a massive selective advantage. Hence the evolution of strains of bacteria in hospitals resistant to a variety of different antibiotics e.g. MRSA and C. difficile
This is an essential topic to get your head around for the exam. Please comment on the blog post if you have any questions or contact me via Twitter.