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    ok so i've been revising ages for the subject along with chemistry which i've got the day after, and I wanted to know if anyone else if finding the same things as I am. I've been doing past papers for a while now, and i've realised how picky the OCR mark schemes are, I know that they have to be, because its a hard subject but can someone tell me why:

    you are not allowed to say that Cholesterol in the plasma cell membrane provides rigidity? it says ignore?

    can someone explain to me the relationship between plasma proteins and water potential in the formation of tissue fluid topic?

    and why can't you say lysosomes engulf foreign particles, it apparently has to be 'ingest'?

    and why when explaining the importance of mitosis, you aren't allowed to say that it is for repair of cells it has to be of tissues?

    Thanks
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    okay your first question:

    you are not allowed to say that Cholesterol in the plasma cell membrane provides rigidity?

    Rigidity is too vague, the examiners like saying controls membrane fluidity, and gives eukaryotic cells mechanical stability.However rigidity is a correct statement but they prefer the control of membrane fluidity.so your better of learning that and putting that down. (More cholesterol makes the membrane more rigid but less fluid)

    relationship between plasma proteins and water potential in the formation of tissue fluid topic?
    What you should know is how tissue fluid is formed due to the high hydro static pressure at the arterial end of the capillary which forces fluid out of the capillary. forming tissue fluid, taking along with it oxygen etc. But substances like red blood cells and white blood cells are too large to pass through the capillary wall. Now both the blood and tissue fluid have a negative water potential however the water potential of the blood is lower than that of the fluid. This is due to the solutes , rbc , wbc etc. however both tissue fluid and blood contain solutes. Since the blood has a lower water potential, the water from the tissue fluid will naturally move back into the blood via osmosis down a water potential gradient ( from high water potential to low water potential). Thus water moves back into the capillary. However this is not the only reason why water moves back it is also due to the tissue fluid having its own hydro static pressure which pushes fluid back into the capillaries.

    and why can't you say lysosomes engulf foreign particles, it apparently has to be 'ingest'?

    Ingest is to simply digest, lysosomes have digestive enzymes which break down the particles/molecules. however engulf is to just ''take in'' if you must say. Thus ingest is a better word to use due to the digestive enzymes that break down the molecules.
 
 
 
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