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A level Biology: Post your doubts here!

Discussion in 'International A And AS Level' started by XPFMember, Sep 29, 2011.

  1. Djlucifer

    Djlucifer

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    Can someone help me with serial sample and series dilution. Please provide me the step by step procedure for doing this
     
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  3. teachyourself

    teachyourself

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    check youtube/google and use your syllabus to find the most relevant video/notes, there were few links i posted regarding biology tutorials either in this thread or another
     
  4. Djlucifer

    Djlucifer

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    hmm well oka
     
  5. Metallic9896

    Metallic9896

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    Guys new here but I have this question that has REALLY bugged me. It's from Summer 2010, Question Paper 11, Question 2. The answer is C, and the examiner report
    says,

    "The relative difficulty of this item was due to many candidates failing to appreciate that animal cells are very
    difficult to see clearly using a light microscope and that the organelles are far too small to see."

    But it makes no sense to me since we learn that light microscopes CAN distinguish organelles larger than 0.2 micrometers or 200 nanometers, and organelles like mitochondria, lysosomes, etc. (Searching on google "size of cell organelles" and following the first link can show that.) Can someone care to explain? Or did CIE mess up here (which is what I think happened)? Thank you.

    EDIT:

    Just saw another question in summer 2012, which was:

    A microscope has a resolution of 200 nm. Which of the following organelles would not be
    resolved using this microscope?
    A chloroplasts
    B lysosomes
    C mitochondria
    D ribosomes

    The answer is of course D, but this imples that A B and C all CAN be resolved using a light microscope, which seems to contradict the answer in the question I'm confused about. Any input would be appreciated.
     

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    Last edited: Apr 8, 2017
  6. darks

    darks

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    The first question states that the magnification used is 400x. Which is very low to be able to see any cell contents except the nucleus.
    In the second question you have "resolution" given, 200nm resolution means that anything with size smaller than 200nm will not be seen. In this case it is only the ribosomes which have an average 25nm diameter
     
  7. Metallic9896

    Metallic9896

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    Finally. That makes sense. Thanks a lot.
     
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  8. darks

    darks

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    are details of IVF in the syllabus? I found long questions on it in past papers but the detail is not mentioned in our books... is it out? Its not mentioned in the syllabus like it was before....but its not mentioned in the "Out of syllabus" content at the end of the syllabus.
     
  9. I think that section was removed
     
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  10. AnonymousX9

    AnonymousX9

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    Hey guys, pls help me with this question:
    MJ 2013 P42 Q7 (the genetic cross and first part of the question).

    "7) Resistance to the poison warfarin is now extremely common in rats. Warfarin inhibits an enzyme in the liver, vitamin K epoxide reductase, that is necessary for the recycling of vitamin K. This vitamin is involved in the production of substances required for blood clotting.

    • Rats susceptible to warfarin die of internal bleeding.
    • Rats that are homozygous for resistance to warfarin do not suffer from internal bleeding when their diet provides more than 70 μg of vitamin K per kg body mass per day.
    • Heterozygous rats are resistant to warfarin when their diet provides about 10 μg of vitamin K per kg body mass per day.

    (a) Using appropriate symbols, complete the genetic diagram to show how two resistant rats can produce warfarin-susceptible offspring."


    It says rats that are homozygous need 70 micrograms of Vitamin K and heterozygous need only 10. But shouldn't it be the opposite? The allele is for resistance so if it is homozygous won't be it be more resistant to Warfarin?
    Also, what type of inheritance is this? Like how should I know what type of cross to do? Pls help.

    Thank you.
     
  11. Lee Qian Yi

    Lee Qian Yi

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    Interphase has 886 number of cells , prophase has 73 number of cells , metaphase has 16 number of cells , anaphase has 14 number of cells and telophase has 11 number of cells . What percentage of cells contain chromosomes that appears as two chromatids ? ANS:8.9%. Anyone can explain how to do ?
     
  12. darks

    darks

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    In interphase obviously no appearance as chromatids as chromatin not yet coiled up and not yet shorter and thicker.
    In anaphase, centromeres have replicated, chromatid pairs no loger exist.
    In prophase and metaphase only are the chromatids seen in pairs.
    So 73+16/1000=8.9
     
  13. Lee Qian Yi

    Lee Qian Yi

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    oh ok thx
     
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  14. darks

    darks

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    in P5, is method of serial/ any dilution as written in marking schemes enough? or do we have to write in detail, the entire method??
     
  15. olevelstudent

    olevelstudent

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    Someone answer him. I have the same query
     
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  16. Bah...

    Bah...

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    Hello,
    Does anyone have classified section B questions?
    Thank you
     
  17. student2

    student2

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    ny1 knws y farmers need 2 buy GM seeds every yr??
     
  18. Dhyani Amalia

    Dhyani Amalia

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    Can anyone pls help me with this question?
    upload_2017-5-10_20-32-44.png
     
  19. sunstar

    sunstar

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    People any ideas for AS paper 3 varient 2 please help
     
  20. Metallic9896

    Metallic9896

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    Have my 33 practical tomorrow. All seems set except one confusion. When making plan diagrams, what do I do at the "boundary" of the image I'm drawing? I'm referring to situations where they ask us to make the bottom 1/4 of an image for example. That would be a perfect rectangle or a square so should there be an external boundary within which I make the diagram? Or in cases like the attached picture. When drawing the plan diagram, do I seal the edges or leave it open? I'm referring to the highlighted top right area. Thank you.
     

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  21. darks

    darks

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    i'm giving 34. But leaving it open is what i was taught..
     
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