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Glycolysis (OCR Bio A2) Watch

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    Hi,

    Just wondering, my book says:
    'The energy from the hydrolysed ATP molecules activates the hexose sugar and prevents it from being transported out of the cell. We can refer to the activated,phosphorylated sugar as hexose 1,6-bisphosphate.'

    What is meant by the term activated and how does that prevent it leaving the cell?

    Also, out of curiosity, are most people learning glycolysis in the way that it is in heinneman book, or are your teachers giving you a simiplified version that omitts some details? I'm unsure as to what depth I need to learn everything in this section because my teacher keeps missing lots of details out, saying I don't need to know them for the exam? Is this the case with other people?
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    Glycolysis is a very complex process and is simplified at A-level. The more you learn the better you will be for the exam, but it would be quite hard at A-level to learn all the facts. Phosphorylation of the hexose sugar makes the molecule have an overall charge. When a molecule is polar or has a net charge it is unable to cross the lipid bylayer due to the interactions between the charges of the molecules and the lipids. Hence stopping it from leaving. The addition of phosphate also gives the molecules energy to undergo chemical reactions.

    Hopefully I've not over simplified, if you do want a more indepth answer please do quote me.....but I will have to lookg it up myself also
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    (Original post by joshed)
    Phosphorylation of the hexose sugar makes the molecule have an overall charge. When a molecule is polar or has a net charge it is unable to cross the lipid bylayer due to the interactions between the charges of the molecules and the lipids.
    Glucose is too big to ever cross the membrane directly - even water cannot get through (something they very much don't tell you at A-level!) - the phosphorylation stops it from using the GLUT transporters. Minor detail at A-level i guess but still
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    (Original post by nexttime)
    Glucose is too big to ever cross the membrane directly - even water cannot get through (something they very much don't tell you at A-level!) - the phosphorylation stops it from using the GLUT transporters. Minor detail at A-level i guess but still
    But in the book it says fructose-6-phosphate + Pi -> fructose 1,6-bisphohate
    then the energy from hydrolysing the ATP in the previous reaction si what activates the fructose-1,6-biphoshate into hexose 1,6-bisphosphate, no more ATP is hydrolysed when turning fructose into hexose. So I'm assuming no more phosphate groups are added, so why would fructose "" get through but not the hexose thing?

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    (Original post by kej817)
    But in the book it says fructose-6-phosphate + Pi -> fructose 1,6-bisphohate
    then the energy from hydrolysing the ATP in the previous reaction si what activates the fructose-1,6-biphoshate into hexose 1,6-bisphosphate, no more ATP is hydrolysed when turning fructose into hexose. So I'm assuming no more phosphate groups are added, so why would fructose "" get through but not the hexose thing?

    Ah i think you are confused - a "hexose" sugar is just a sugar with 6 carbons. Unactivated glucose and fructose are both hexose sugars - the textbook is simply using 'fructose' and 'hexose' interchangeably.

    To clarify - activating is the process whereby the first phosphate is added. As stated, this makes it physically unable to leave the cell and gives it the energy it needs to carry on through glycolysis (or other processes). Then a second phosphate is added as it continues through glycolysis (the step you showed in the post above). All of these involve hexose sugars.
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    (Original post by nexttime)
    Ah i think you are confused - a "hexose" sugar is just a sugar with 6 carbons. Unactivated glucose and fructose are both hexose sugars - the textbook is simply using 'fructose' and 'hexose' interchangeably.

    To clarify - activating is the process whereby the first phosphate is added. As stated, this makes it physically unable to leave the cell and gives it the energy it needs to carry on through glycolysis (or other processes). Then a second phosphate is added as it continues through glycolysis (the step you showed in the post above). All of these involve hexose sugars.
    So fructose 1,6 bisphosphate is the same thing as hexose 1,6-bisphosphate!?
    that would make a lot of sense lol.
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    (Original post by kej817)
    So fructose 1,6 bisphosphate is the same thing as hexose 1,6-bisphosphate!?
    that would make a lot of sense lol.
    Wrt to detail needed, I learnt that glucose --> pyruvate and then how many NAD and ATP were used/produced etc. I think I knew the first step in the process as well (i.e. glucose --> fructose 1-6 whatever it was) you have four stages to learn, and you're not expected to remember every tiny detail
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    (Original post by kej817)
    So fructose 1,6 bisphosphate is the same thing as hexose 1,6-bisphosphate!?
    that would make a lot of sense lol.
    Indeed - i think the book was trying to say that but kind of failed . I guess it was telling you that as hexose 1,6 BP would refer to both glucose 1,6 BP and fructose 1,6 BP, both of which are steps (as you can see from wikipeida).
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    (Original post by nexttime)
    Indeed - i think the book was trying to say that but kind of failed . I guess it was telling you that as hexose 1,6 BP would refer to both glucose 1,6 BP and fructose 1,6 BP, both of which are steps (as you can see from wikipeida).
    Thank you so much
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    (Original post by nexttime)
    Glucose is too big to ever cross the membrane directly - even water cannot get through (something they very much don't tell you at A-level!) - the phosphorylation stops it from using the GLUT transporters. Minor detail at A-level i guess but still
    Seriously?? I thought the membrane contained aquaporins which is what allows to water to enter and exit a cell by osmosis.
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    (Original post by sc0307)
    Seriously?? I thought the membrane contained aquaporins which is what allows to water to enter and exit a cell by osmosis.
    That's in the cells of the collecting duct, the aquaporins slot on the cell-surface membrane to reabsorb water by osmosis.
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    (Original post by Waqar Y)
    That's in the cells of the collecting duct, the aquaporins slot on the cell-surface membrane to reabsorb water by osmosis.
    What collecting duct? In kidneys?? I'm just saying I thought that other guy said water generally cannot travel through cells.
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    (Original post by Waqar Y)
    That's in the cells of the collecting duct, the aquaporins slot on the cell-surface membrane to reabsorb water by osmosis.
    (Original post by sc0307)
    Seriously?? I thought the membrane contained aquaporins which is what allows to water to enter and exit a cell by osmosis.
    I was correcting a minor inaccuracy in pointing out that you need glucose transporters for glucose to cross a cell membrane. Equally, water also needs specific channels - aquaporins. These are on most cells, although not all.
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    (Original post by nexttime)
    I was correcting a minor inaccuracy in pointing out that you need glucose transporters for glucose to cross a cell membrane. Equally, water also needs specific channels - aquaporins. These are on most cells, although not all.
    Oh ok, what are the channels called that transport glucose?
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    nexttime, that is incorrect. Water is able to move through the lipid bilayer. As the phospholipids slide around, they can create small openings where water can enter the cell. The cell membrane not only obtains water through aquaporins, but by this passive mechanism also.
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    (Original post by l4ith)
    nexttime, that is incorrect. Water is able to move through the lipid bilayer. As the phospholipids slide around, they can create small openings where water can enter the cell. The cell membrane not only obtains water through aquaporins, but by this passive mechanism also.
    The only place i cold find that specifically answers the question whilst not being aimed at A-level students says that "Diffusion of water through pure lipid bilayers occurs with high activation energy (Ea >10 kcal/mol). In contrast, for >40 years it has been known that the rapid flow of water through human red cell membranes occurs with Ea <5 kcal/mol". Fact is, if the membrane were permeable to a significant amount i don't see how organs like the kidney or bladder could work - trying to move water about would be futile (in the collecting duct for example).
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    (Original post by sc0307)
    Oh ok, what are the channels called that transport glucose?
    GLUT

    http://en.wikipedia.org/wiki/Glucose_transporter
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    (Original post by nexttime)
    The only place i cold find that specifically answers the question whilst not being aimed at A-level students says that "Diffusion of water through pure lipid bilayers occurs with high activation energy (Ea >10 kcal/mol). In contrast, for >40 years it has been known that the rapid flow of water through human red cell membranes occurs with Ea <5 kcal/mol". Fact is, if the membrane were permeable to a significant amount i don't see how organs like the kidney or bladder could work - trying to move water about would be futile (in the collecting duct for example).
    But the water will move in by osmosis. In the kidney, due to the active transport of sodium ions in the ascending limb of the loop of Henle, sodium enters the interstitial fluid and so lowers its water potential, meaning water can enter via osmosis into the tissue fluid of the kidney in the descending limb. Aquaporins are certainly a mechanism by which water can enter and exit, but they aren't the only way!
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    (Original post by l4ith)
    But the water will move in by osmosis. In the kidney, due to the active transport of sodium ions in the ascending limb of the loop of Henle, sodium enters the interstitial fluid and so lowers its water potential, meaning water can enter via osmosis into the tissue fluid of the kidney in the descending limb. Aquaporins are certainly a mechanism by which water can enter and exit, but they aren't the only way!
    Yes - that's why i mentioned the collecting duct as an example - the urine here can be very dilute - if cell membranes themselves were more than a tiny bit permeable, it would take a LOT of energy to keep it that way! Same goes for the bladder - unless these cell membranes are inherently different in some way like skin, it wouldn't be possible to create urine.

    I don't dispute that it is possible, just that it is so small its physiologically insignificant.
 
 
 
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