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  1. I

    Edexcel Chemistry Unit 5 (19th June 2013). Revision and Post Exam Discussions.

    HBr reacts in an addition reaction, so the double bond breaks and H and Br atoms are added to it.
  2. I

    A level Biology: Post your doubts here!

    There's the BCG vaccine. Although it's not 100% effective, it's the best we've got so far.
  3. I

    biology paper 1 doubts

    Cholesterol as well :P
  4. I

    A level Biology: Post your doubts here!

    Nitrogen fixation is a reduction reaction, so no oxygen is required. This means that they can live well in boggy and clogged soils. As far as I know, diazotrophs are facultative anaerobes. This means that they can grow without oxygen. But they cannot fix nitrogen under aerobic conditions (when...
  5. I

    A level Biology: Post your doubts here!

    I'll try to prove (or refute) all options: Option 1: If you have two isotonic solution (with the same water potential) that are separated by a partially permeable membrane, water will definitely move both ways, but the NET water movement is zero. Also, when you have to solutions of different...
  6. I

    A level Biology: Post your doubts here!

    100 ml of arterial blood carries 19 ml of oxygen while the same amount of venous blood carries only 12.5 ml of oxygen. When the blood passes through the lungs, (19 - 12.5) = 6.5 ml of oxygen is absorbed. In one minute, 5L of blood passes through the lungs. 5L = 5,000 ml. If 100 ml of blood...
  7. I

    A level Biology: Post your doubts here!

    21) I'm just as confused as you are. I believe the answer should be D as well. 34) You're right. But that's not what the question's asking. They asked for the disease which is least likely to be passed from parent to child. A: you already mentioned it B: HIV can be passed through the placenta...
  8. I

    A level Biology: Post your doubts here!

    16) As far as I know, cholesterol keeps a balance of fluidity. At low temperatures, the phospholipids should crystallize (become like a solid). Cholesterol has an irregular shape, so it doesn't fit perfectly in the membrane. This prevents phospholipids from going too close to each other, so the...
  9. I

    A level Biology: Post your doubts here!

    It's supposed to be 6%. Sorry!
  10. I

    A level Biology: Post your doubts here!

    It's definitely not only 10% between trophic levels. As you go deeper in the chain, the efficiency increases greatly. This allows maximum absorption of a low quantity of energy at higher levels. If this did not happen, then every food chain would only have two or three trophic levels.
  11. I

    A level Biology: Post your doubts here!

    Next year, you'll see that the cristae of mitochondria contains the enzyme ATP synthase which obviously makes ATP (that's what we use for energy). A high surface area means more ATP synthase molecules can be placed within the membrane. So in Oct/Nov '02, "enzyme reactions" would be the correct...
  12. I

    A level Biology: Post your doubts here!

    25) Xylem doesn't have any cytoplasm. It is also found to the inside of vascular bundles (closer to the centre of the stem). 26) Solute potential is always negative, and pressure potential is always positive. The water potential is a sum of the two 'potentials', so it would be found somewhere in...
  13. I

    A level Biology: Post your doubts here!

    If there is no light, then I agree that no photosynthesis can take place. The levels of oxygen should remain unchanged, because one small area will have a very little effect on the entire atmosphere. For example, if one person died, we can't really say that the concentration of carbon dioxide in...
  14. I

    A level Biology: Post your doubts here!

    0.005 mm x 3.5 units = 0.0175 mm = 17.5 micrometers which is approximately equal to 18 micrometers
  15. I

    A level Biology: Post your doubts here!

    Volcanoes release a ton of nutrients into the soil. Although the lava initially kills all plants and animals, after it cools down, the nitrifying bacteria can thrive easily with the abundance of nutrients.
  16. I

    A level Biology: Post your doubts here!

    If you look at the graph closely, you can see that approximately 80% of men used to smoke in 1980, but that fell to around 35%. This is a 45% decrease (not 60%)
  17. I

    A level Biology: Post your doubts here!

    18) They tell you that the cell is at the late stages of prophase. When it enters the NEXT prophase, you'll have two cells with the same number of chromosomes. 27) Option 2 is wrong because arteries have narrow lumens and veins have wide lumens.
  18. I

    A level Biology: Post your doubts here!

    I'll try to explain this without complicating things too much. But you have to know that the actual explanation goes much deeper. A change in pH implies a change in the concentration of H+ ions. If the pH is too low (too acidic), there are too many H+ ions that form hydrogen bonds with the...
  19. I

    A level Biology: Post your doubts here!

    An active muscle produces a lot of carbon dioxide (an acidic gas) which decreases the pH. This is the basis of the Bohr shift.
  20. I

    A level Biology: Post your doubts here!

    I'm not sure if you're supposed to know this, but denitrifying bacteria are obligate anaerobes. This means that they die when oxygen is present, so if such bacteria were to populate a region, that region must have a very low oxygen content.
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