Turbidity of yeast cells using colorimeter

Discussion in 'Supporting Biology' started by kazzag123, Jun 6, 2019.

  1. Hello

    Has anyone recently carried this practical out? Any success? It seems the students need to take up to 5hrs of samples every 20-30 mins to obtain enough data to get a decent graph ?

  2. Yes this practical has been badly planed. The doubling time for Yeast in log phase growth is 90 minutes which would mean that it may take longer than 5 hours to get a decent growth curve. Why not use a bacterial source such as E. coli K-12 where the doubling time of 20 minutes:)
    Jaytee likes this.
  3. Because, despite being told all this, and despite being told by Edexcel that you can modify experiment, teachers want it exactly as it is in book & on worksheet!
    We now just end up showing the equipment & giving them results!
    The 'proper' one with the light sensor etc, has to run over 24 hrs :eek::mad: ( I tried the one with colorimeters & was told it would confuse the students.
    ( Give me strength)
    We asked Edexcel if they actually send these experiments out to schools to test in 'real' conditions, they said they send them to a tech to test.
    Methinks the tech just sits & reads paper for a couple of weeks then says 'Yup, all good'!:confused:
  4. I think that you are most probably right.:D
  5. The issue is that such teachers are fixated on teaching students how to regurgitate what is in the textbooks, rather than the more important point which is proof of concept that can then be applied to non familiar scenarios. With the new syllabus, it's more common for questions to pop up that test understanding by applying knowledge to solve a new problem rather than simply testing memory.
    Teachers should focus on making sure the underlying scientific concepts of a practical are understood well and not be afraid to challenge them with new situations that they will inevitably encounter during an exam.

    Edexcel have told me themselves that the core practicals are merely a suggestion or a guide to tick the required skills. They are happy for teachers to use alternate practicals as long as the theory is related and the necessary skills are taught.
    Sharon Watson, ClaireS and Jaytee like this.
  6. Edexcel uses that as a stock response - the bottom line is the authors should have checked that the experiment works using the equipment they recommend before they publish - unfortunately some don't, particularly the physics ones (in my experience). On the positive side the vast majority do work with no changes required.
    Emil Retaw likes this.
  7. Oh I absolutely agree with you. I'm not justifying Edexcel for their lack of quality control with regards to some practicals. My point is that if a technician proposes changes or an alternative which works better than the original, teachers should consider it and not be terrified at the thought of deviating from something that isn't satisfactory. It's a complete waste of a lesson and everyone's time to provide something which doesn't work properly.

    There are plenty of amazing resources out there, Bob Worley's Microchemistry, just to name one example and it's such a shame such positive advances in school practicals are being ignored due to the fear some teachers have regarding exploration of alternatives.
    Techno likes this.
  8. I completely agree and have made many changes to the edexcel versions - but I try to keep it as close the recommended version as possible.
    Having been an A level Biology teacher for 11 years back in the 70s and late 80s, I soon found out that less able students easily lost the point/plot if the experiment was significantly different from the one in the text book or experiment sheet - of course, in those days, we just got an A5 syllabus with no other support so we just used what we had designed ourselves.

    Teachers have a very valid point about sticking to the scheme and asking for the equipment as per instructions - I see it as my job to point out it's faults then try and meet that request as much as possible or at least make it look like the one in the book.

    Producing an 'all singing', 'bangs and wizzes' version which is completely different from the text book serves to confuse some students and has the opposite effect from what we want. The bright ones will probably benefit though.

    Here is an example (all be it physics) where I have made an edexcel experiment that doesn't work, work by adapting the recommended 'ripple tank'
    Emil Retaw likes this.
  9. Absolutely, if you can make the original work with as few changes as possible then that is always the ideal scenario. My approach is exactly the same as yours. However, I was just sharing in Jaytee's frustration where some teachers are willing to accept something which doesn't work for the sake of it being in the book, when there are similar alternatives proposed which do work.
    Last edited: Jun 9, 2019
  10. I was actually talking about 'adapting' rather than huge changes Peter!
    Also, a lot of the experiments in the exams DO look very different from the ones in the syllabus anyway. It is about APPLYING the knowledge not learning it parrot fashion!
    I try to adapt practicals to make it look the same but sometimes the method, measurements, timing, even in some cases, equipment is just wrong.
    We have one in A level Biology that shows a drawing of non-existent equipment It took us a very long time to make something that resembled it, whereas the old equipment worked brilliantly but didn't look the same as picture.
    I am not talking about bangs & whizzes!:mad:
  11. My comment was not a personal comment aimed at you and I'm sorry you seem to have taken it that way - I was merely pointing out teachers have a very valid educational reason for mirroring the text book - I also agree with you about applying, but students have to know and understand first before they can hope to apply.
  12. Lol, I didn't take it personally. Maybe it's a teacher thing. :)
  13. So, back to your original question!
    Have you tried this out yet?
    One of our techs did it with the whole light sensor, hotplate stirrer, laptop, datalogger set up, & had it running overnight to get full graph. However he said it was a lot of fiddling & smoothing to get the graph right.
    I did the colorimeter one a couple of years ago as the teacher wanted the pupils to actually carry out the experiment. Although, we didn't get enough results in an hour for a decent graph.
    Not sure using bacteria in open cuvettes /test tubes is great idea. Students are not know for taking care!
    The teachers wanted to know why I couldn't do the one with light sensor etc as class practical! Completely ignoring the fact that we don't have the resources, or the budget to buy them :rolleyes:
    Did you have any luck with colorimeter way?
  14. How about this for a growth Curve? Turbidity with time was plotted as well as enzyme activity, total protein and pH. again it would not have been possible with Yeast, but relatively easy with a bacterial source.:);)

  15. Thank you everyone for your comments and suggestions. I will chat to the teacher and give them some suggestions!
  16. Why not use a cuvette cap?:)
  17. it says yeast in the title.
  18. NO, I know, George suggested using ecoli.
  19. Lol,oops caps lock.
  20. I think that we've established that producing a growth curve for a Yeast culture is not doable in the time allowed due the the 90 minute doubling time (unless of course you're planning to work until the early hours). I think that whoever devised this practical had a meltdown moment.:D:D