Glycolysis (OCR Bio A2)

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

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.

So fructose 1,6 bisphosphate is the same thing as hexose 1,6-bisphosphate!?
that would make a lot of sense lol.

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).

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.

That's in the cells of the collecting duct, the aquaporins slot on the cell-surface membrane to reabsorb water by osmosis.

That's in the cells of the collecting duct, the aquaporins slot on the cell-surface membrane to reabsorb water by osmosis.

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.

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.

Oh ok, what are the channels called that transport glucose?

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!