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    Attachment 539305539307Name:  image.jpg
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Size:  506.8 KBHey guys,
    This is a question (B) from the official Ocr textbook and it's mark scheme. It makes no sense to me :/ could anyone explain it to me?
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    Sorry in advance if I don't get the 'terminology' or 'wording' or whatever part of the question because I do AQA bio not OCR (Though I do OCR physics...my god it's the worst)

    I think to understand the question you should be fairly comfortable with both:
    -Aerobic respiration pathway
    -Structure of a mitochondria

    Just to recap for you though to make it easier:

    Structure of a mitochondria:
    - Has one outer (mitochondrial) membrane, and an inner (mitochondrial) membrane also known as the cristae. The matrix of the mitochondria is the 'liquidy' part inside the inner mitochondrial membrane. The stuff in between the two membranes is known as the intermembranal space

    - Aerobic respiration pathway: Glycolysis (in cytoplasm) -> Link reaction (in matrix of mitochondria) -> Kreb's cycle (in matrix of mitochondria -> Electron transport chain (on the cristae/inner mitochondrial membrane, sort of requires both the matrix and intermembranal space)

    As you know with aerobic respiration: Normally NAD+ is reduced in glycolysis, link reaction and the Kreb's cycle. This means it accepts protons and electrons, or just H for hydrogen, to form NADH/reduced NAD. Since the link reaction and Kreb's cycle occur in the matrix of the mitochondria, the NAD does not need to pass through the inner membrane to give up its H+ ions to the electron transport chain (they are already on the inside!).

    However, for reduced NAD made in for example glycolysis which occurs in the cytoplasm, the reduced NAD is unable to pass through the inner membrane to pass its H+ ions in the matrix (because the inner membrane is impermeable to it), so instead it passes the H+ ions to a chemical that is able to pass through the inner mitochondrial membrane

    Long story short: The reduced NAD in the kreb's cycle+link reaction are already inside the matrix and so don't need to cross any membrane to release their H+ ions. However, the reduced NAD made in glycolysis in the cytoplasm is on the outside of the impermeable membrane, so needs to use some other means (the chemical) to pass its protons into the matrix.

    Sorry for going overboard with writing! Hope i've helped a tiny bit!
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