• We need your support!

    We are currently struggling to cover the operational costs of Xtremepapers, as a result we might have to shut this website down. Please donate if we have helped you and help make a difference in other students' lives!
    Click here to Donate Now (View Announcement)

Search results

  1. I

    A level Biology: Post your doubts here!

    S is the Svedberg unit. It refers to the rate of sedimentation during centrifugion. In a 70S ribosome, there is a small ribosomal unit (30S) and a large unit (50S). If you add them up, you get 80S. If you take a little bit of physics, you will know that the size is not proportional to the...
  2. I

    A level Biology: Post your doubts here!

    If the air inhaled is less than the air exhaled, then you will exhale more than you inhale, so eventually, you should completely run out of air, right? In order to maintain a constant lung volume, the air inhaled must equal the air exhaled.
  3. I

    AS LEVEL BIOLOGY... discuss here AS students only :)

    When guard cells are turgid (full of water), they are open, and when they're flaccid (no water), they're closed. K+ ions are actively transported out of guard cells This decreases the water potential outside Water leaves from the guard cells to outside by osmosis The guard cells become flaccid...
  4. I

    A level Biology: Post your doubts here!

    Occasionally, they ask a question relating to the chi-squared test on paper 4, so I think it's best to be prepared.
  5. I

    A level Biology: Post your doubts here!

    Let's say I lost a bet to a friend of mine, so I had to give him $150. So I lost $150 and my friend gained $150. The total amount of money exchanged is $150 NOT $300. So the amount of money exchanged = amount of money I lost = amount of money my friend gained. Taking this example to the...
  6. I

    A level Biology: Post your doubts here!

    Introductory notes: Amino acids are translated from codons Each codon is made up of 3 bases Frameshift: Let's say you have a sentence (polypeptide) of three letter words (since each codon/word is made up of three letters/bases) THE FAT CAT ATE HIS RAT. (It's just a sentence I made up to...
  7. I

    A level Biology: Post your doubts here!

    Stabilizing selection = If you have a (bell-shaped) graph whose peak is near the centre, the graph will be squeezed so that it will be taller but thinner. Disruptive selection = You will end up with two peaks, with one peak on each extreme. Both extremes have a high chance of survival, whereas...
  8. I

    A level Biology: Post your doubts here!

    I think you mean disruptive selection. And no, it isn't. Stabilizing selection = the intermediate is preferred Directional selection = one extreme is preferred Disruptive selection = both extremes are preferred
  9. I

    A level Biology: Post your doubts here!

    Biodiversity is a really hard term to define. There is no exact definition of it. The closest you can get to the definition is the one provided in the mark schemes. From Oct/Nov '07: 1 ref. variety of, species / organisms; 2 genetic diversity within species / AW ; 3 genetic diversity between...
  10. I

    A level Biology: Post your doubts here!

    17) The cell does not grow by mitosis. It only divides by mitosis. It grows during interphase (when it's not dividing) 26) I really have no idea if cambium cells are included. In my school, we take AP courses, and it's included there. Not sure about the AS syllabus though. 35) Okay, if I breathe...
  11. I

    A level Biology: Post your doubts here!

    17) Mitosis does not fix a cell, but it replaces damaged cells in a tissue. The damaged cells will enter apoptosis (programmed cell death). Afterwards, they will be engulfed by phagocytes 26) Xylem vessels are thick and found closer to the centre of the stem (D). Cambium cells separate the xylem...
  12. I

    A level Biology: Post your doubts here!

    They're referring to the EAR (Estimated average requirement) If you have all the numbers, from 1 to 10, the average would be considered 5. 50% of the numbers are below 5 (the average) and 50% of the numbers are above 5 The choice was, "50% of the population require more than 25 mg of vitamin C...
  13. I

    A level Biology: Post your doubts here!

    If you mention any 9 points out of the 15, then you'll get the full mark. But those 9 points must be mentioned in a logical order.
  14. I

    biology paper 1 doubts

    Cholesterol is definitely correct. I'm 100% that tertiary is affected. Just like the sickle-cell example given above, an polar amino acid is replaced by a non-polar one (primary structure changed). This also means that hydrogen bonds cannot be formed with the new amino acid (since it is...
  15. I

    biology paper 1 doubts

    And that's why the examiner's report took into consideration every single candidate who applied. As mentioned, the average score for may/june '10 was 23/40 and the GT was 36/40.
  16. I

    biology paper 1 doubts

    Okay, let's take a look at may/june '10: GT for Paper 11 was 36/40 Mean score was around 23/40. Six questions were difficult (less than 40% of candidates were able to answer)
  17. I

    biology paper 1 doubts

    I found the paper quite easy compared to may/june, with several repeated questions, so I don't think Cambridge will be lenient with the GT this time. So I'm honestly expecting above 35.
  18. I

    biology paper 1 doubts

    It's definately not cytokinesis. In fact, phospholipids are the main requirement for cytokinesis.
  19. I

    biology paper 1 doubts

    Cytokinesis is when the cell itself is splitting in two (split at the contractile ring) which comes after telophase. Telophase is when the nuclear envelope and nucleolus are reforming. Phsopholipids actually allow oxygen and carbon dioxide to flow freely through it. Although they are very...
  20. I

    biology paper 1 doubts

    In the xylem, it just moves upwards through the lumen. However, as soon as water enters the endodermis, it must go through the symplast pathway. The pathway which involves water moving through plasmodesmata is the symplast pathway.
Top