Writing longer answers and essays

Belated congratulations to everyone who got what they wanted from the results last week. I haven’t been into school yet so I don’t know how individuals did, other than an odd few bits that Mr Murray forwarded me.

I thought I’d suggest a few guidelines for writing out longer answers (or what we might call scientific essays). It’s not something we spend a lot of time in schools overtly teaching, and you will come across different opinions and styles for this, but I’m going to stick to ways to improve your writing for scientific purposes. Since a part of the homework for the holiday was an essay about oxidative phosphorylation, you may well get some clues here as well.

Rather than give too much away, I’ll use Krebs cycle as an example. Imagine you had a 9 mark question about Krebs Cycle on a question paper. My suggestion would be to start by getting down as many things you could think of to do with krebs, forget structure for a moment and brain-storm for a minute.

Let’s say you wrote down citrate, 654444, ATP, NAD, FAD, pyruvate, maybe you also draw a rough version of the cycle. Once you have some ideas down, you can build a structure from there. For example, drawing out the cycle might have made you think of the link reaction, perhaps you then remembered about the CO2 being removed which leads you to decarboxlase, maybe some other enzymes. Many of the structured (or long answer) questions you will come across in biology are based around step by step processes, which gives you a ready-made structure. The problem people often have is they start too quickly, forget a part of the process and then find it difficult to think backwards to what has been missed out. A good tip is to give a description for every scientific term used (within reason!), e.g. ‘…two hydrogen are removed from the citrate by NAD. NAD is a coenzyme that is reduced in this reaction, and used to carry electrons’ . You described the role of NAD which is relevant to the question, but not the coenzyme part, which would be more detail than is necessary.

When it comes to writing an essay, the process is similar but you have longer to plan things out. The first paragraph of your essay should always give the overview. A good example of this is to look at this description of the Cori cycle (don’t worry about what it is yet) from wikipedia.  The information about who it is named after is not very relevant, but notice how it encapsulates what the article is about. The bulk of the article then goes on to talk about the details; notice how you are expected to understand what most words (oxygen, glucose, pyruvate) mean. Because of the nature of wiki articles, important words are linked, remember you will not have that luxury so make sure that you describe anything that is important.


Super mice and species

Several newspapers today carry a story about genetic changes in mice. The basic issue is that species of mice common in europe have cross-bred with a species of Algerian mouse. This is of interest because the Algerian mice have a genetic resistance to many types of poison.

A commonly used poison on rodents is warfarin. You may remember it being mentioned in GCSE – it is an anticoagulant; this means it can prevent blood clotting. In high concentrations it kills mice and rats by causing excessive bleeding, but some populations are becoming resistant to it. This was identified back in the 1960s, and is a good example of how rapidly animals can evolve.

Returning to the Algerian mice, the warfarin resistance (and other rodenticides) is common in this species. The interesting part is that two different species of mice have potentially interbred to produce viable offspring. If you remember how we defined species as organisms that can breed to produce fertile offspring, how is it possible that two different species can interbreed? The answer is that our definition of ‘species’ is too simple (for example, what about species that don’t reproduce sexually?) and that genes can be passed between different species quite regularly, known as horizontal gene transfer. It is however more commonly observed in plants and bacteria rather than animals. Some scientists suggest that it may be more common in animals than we suspect, particularly in marine animals. Scientists have identified genes crossing between phyla (e.g. fungi to animals) as well.

What does all this mean for you? Apart from showing evolution in action, it is a nice reminder that very often in biology things are a little more complicated than they may first appear, and things don’t always fit into neat boxes as we may like.

And eventually…alcohol dehydrogenase

I said I’d do this sometime back in response to a question from, um…someone. Now we are into the glorious long summer and I have more time, some words on alcohol dehydrogenase.

Its role on humans is to convert ethanol to less harmful substances. It does this by oxidising ethanol CH3CH2OH to the toxic chemical acetaldeyde CH3CHO (notice what has happened in the oxidation reaction, look at the enzyme name as well). This is then oxidised further into harmless acetic acid (bonus points for working out the name of that enzyme).

However, the place we first encountered alcohol dehydrogenase was in the anaerobic respiration pathway of yeast, where pyruvate was decarboxylated to ethanal, which was then reduced further to ethanol with hydrogen from reduced NAD. Remember that the advantage of this step is to re-oxidise NAD so that more H can be accepted during glycolysis. This presents a few questions.

1) what is the difference between acetaldehyde and ethanal?

Nothing, it is the same chemical. Ethanal is an internationally accepted name, whereas acetaldehyde is a name in common usage by some people. I’m afraid it’s an example of scientists from different areas using different names for the same thing. I’d suggest sticking with ethanal (if in doubt, go with the syllabus).

2) If enzymes are specific, how come it is doing two different things in two different places?

This is really the same reaction in reverse (although there are actually quite a lot of different versions of alcohol dehydrogenase, we’ll leave that aside). Enzymes speed up the rate of a reaction, but they do not alter the equlibrium. In simple terms, in the case of yeast and human liver we have different substrate concentrations, leading the equilibrium in one direction more than the other.

Ethanol production in yeast gives the fungus an advantage because it is toxic to other organisms. What is the advantage of alcohol dehydrogenase in humans? Ethanol occurs naturally when fruit begins to ferment. Our ancestors diets included fruits, so anyone with this enzyme would have had a natural advantage in removing the toxin.

You asked part 2

Jack raised the point about theories in the lesson this week. In science, a theory has a different meaning to how we use the word in everyday language. Theory can be used to mean a conjecture, idea, speculation or simply a guess about why something happens. This use is fine as long as you realise that words can have multiple meanings, and the context the word is used in is important.

In science, a theory denotes something else. It used to describe statements or principles that not only describe what is happening, they also provide an explanation. It has predictive power, in other words a theory can be used to predict an event or observation that has not yet happened. Theories are well tested and have evidence to support the statements. If you wanted to show that a theory is incorrect, you could show that the predictions it makes do not happen under experimental conditions.

To give an example, let’s take the good old theory of evolution. Broadly speaking, it would predict that organisms closely related would share more genetic similarities than more distantly related organisms. If you found a monkey more closely related to a fish than a gorilla, you could start questioning the theory. Older fossils should be found in older rocks – if you found a human remains mixed in with a T rex’s bones then you may have a problem. So far these things have not occurred, but if they did and were shown to be valid (e.g. not faked) then the theory would be changed. Theories are changed in reponse to new evidence, that’s how science works.

In the example Jack brought up (ATP synthase mechanism) the theory is incomplete because it doesn’t provide an explanation for all observations. Although there is no hierachy of theories, some have been tested more than others so we often consider them to be more robust. Many accepted theories are used simply because they work well enough for the time being, in other words they hold up to testing so far.

Scientific theories are often described as falsifiable. This means that you could potentially cause a theory to be changed or abandoned by evidence that shows its predictions do not hold up in experiment or observation. Knocking down established theories is a good way to get a Nobel prize. But there’s a reason why the big theories are rarely abandoned…