1. Hi I think the Electrify person is certainly talking some real fact - Edexcel are a very strange kettle of fish - not only is their mark scheme sometimes completely unpredictable from the Qs, but they are often downright incorrect. Whether this is a result of poor control by the Govt (who are, as everyone knows, the worst crooks!) - anyone want to sue me, please try!) or an unfortunate follow-up of their purely money-minded political decisions is a matter of opinion.
As far as poor 11ashh and other poor victims of schools who have, in my opinion, very unwisely or due to a wide temporal width, selected Edexcel, are concerned, the problem to be addressed is how to tackle to the best possible level this extremely illogical and unfair method of assessing knowlege of a very logical and normally easily predictable subject, invented by the Tetragammaton himself (who actually I do not believe in!).
My advice to 11ashh and other students of biology (including those doing AQA and OCR, etc.) is to read the Q carefully twice, think about the wording, and try to work out what the examiner is looking for (very difficult with Edexcel when the examiner him/herself does not know hat they are talking about - apologies!!) - the ways to do this are:
1. Look at the words used: "Describe" means "what can you see or observe directly?" - if it is a graph you are looking at:-
a) what is the overall trend
b) is the graph bottom left to top right or other way round i.e. do the dependent values (y axis) increase or decrease, respectively, as the independent values (x axis) increase?
c) how steep is this rise or fall?
d) What x value does it start at?
e) what x value does it end at?
f) does it level out at any point?
g) if yes, where?
h) is there any change in direction anywhere?
i) If so, which way?
g) where?
h) does the line go though the origin?
2. "Explain" means "Why?"
a) try and work out reasons for any/all of the points in part [1] or for the particular point asked?
b) think logically what effect the values of x could have ion the values of y. e.g. Why does a graph of enzyme action plotting product conc (y axis) against time (x axis) level out after a certain time and why is this time different if the initial substrate concentration is different? If the substrate is all used up, of course no more product can be formed - less substrate to start with = levels out at a lower y axis height = less product formed at equilibrium. If a different curve is shown for a higher temperature e.g. at 40 degrees, why does it level out at the same height as the lower temp curve but earlier? Yes, because enzyme action is speeded up by faster collisions, so more substrate/active site interactions; reaches peak earlier but at same height as lower temp = same final amount of product - If temp = 65 degrees, say, OK because the denaturation of the enzyme by heat means that after a certain period there is no more active enzyme left to collide with and act on further molecules of substrate, the highest point reached is lower (on y axis) because it is likely some substrate will never be converted to product as there is no more viable enzyme remaining.
c) On a table of data, first, as with a graph, have a quick scan of the data (30-40 secs) to quickly glimpse what the variables are and how they have been measured from the description of the experiment. Then take a glance at the Qs followed by a deeper look at the data with a view to working out the points mentioned in [1] and [2] a) and [2] b) above.
d) The last part is usually a suggest Q where the mark scheme has a wider scope, and if what you say is correctly using the data AND makes rational sense WITH relevance to the Q, you will get your mark(s).
3. The number of marks assigned to each part of the Q (always given these days) usually tells you how many different correct and relevant points/ideas/factors/reasons/observations you need to gain full marks (the mrk scheme will almost always have more than the assigned no. of marks). It is a good idea to put down one more of these ideas/points etc. than the number of marks assigned to the Q as an insurance measure, as long as whatever you put down is not incorrect. If it is incorrect, you may be penalized; however, if it is irrelevant but correct, you will get zero for that item - nothing gained, nothing lost!
Check out ALL the other techniques (the ones referring to essay Qs for the 7 or 8 mark Q only) in the attached article that I wrote a short while ago. Best of luck! M (Specialist Biology Tutor)