[Lo.Lee.Ta.]

Monosaccharide metabolism:

1. The monosaccharides circulating in the bloodstream enter cells to undergo cellular respiration.
Glucose requires insulin to enter cells, but fructose and galactose do not.
               
Is the above statement correct?
 
Questions:

*What if diabetics only ate fructose and galactose-would their blood sugar always be normal? They wouldn't need insulin, then, it seems...
         

*Is glucagon/epinephrine/cortisol required for weight loss?
*The only reason why a person loses weight is because the body has to use its own stored energy reserves to maintain proper blood sugar level, since food intake is not sufficient to maintain blood sugar?
*It seems to me that glucagon, epinephrine, or cortisol would be required for weight loss, because isn't glycogenolysis and subsequently, gluconeogenesis the only thing that causes energy reserves to get used?


*Even though an untreated type 1 diabetic has a high blood sugar, they still undergo glycogenolysis and gluconeogenesis?
*I know they lose a lot of weight- is this so that their normal body cells can get energy from ketone bodies, at least?


*I know the brain has a different type of GLUT receptor that doesn't require insulin, so the brain of an untreated type 1 diabetic is able to utilize glucose?


*Since most normal body cells do need insulin to take up glucose, do they instead have to run off of ketone bodies when insulin is not present?
*If this is the case, wouldn't weight loss in untreated type 1 diabetics actually make their blood sugar go even higher but also be necessary, since their other body cells can't function without ketone bodies for fuel?


*Do type 1 diabetics produce glucagon. It does not seem like they would be able to- otherwise, why is a glucagon injection necessary when their blood sugar goes too low?


*If they don't produce glucagon (even though it's produced by panceatic alpha cells, which should not be affected...), then how does their body know to lose weight and undergo gluconeogenesis?
*Is their weight loss only controlled by epinephrine and cortisol...?


*If an untreated type 1 diabetic were to completely fast, their blood sugar would never decrease to normal, would it?
*Is this because even though the brain uses up glucose, their fat/protein stores are always being broken down into more glucose + ketone bodies (to supply most body cells)?


*Do cells always take up glucose against the concentration gradient?
Is there ever a time when cells are too full of sugar and won't take up any more?

I know I've asked a ton of questions, but I have seriously tried to look up and figure this stuff out forever! I can't seem to find the answers anywhere!

Thanks so much! 

[writer]

After absorption, fructose is mostly taken by liver where it is converted into glucose, which can be then incorporated into glycogen or released into the blood and from there (by the help of insulin) can go to the cells.

In most diabetics, some glucose still can enter the cells and can be used as energy. When glucose cannot provide energy, body and dietary fats can be used as an energy source for the body cells except for the brain, and some fats can be converted into ketones, which can be used as source of energy by brain.

[Babcock_Hall]

"*Do cells always take up glucose against the concentration gradient?
Is there ever a time when cells are too full of sugar and won't take up any more?"

Much glucose transport across the plasma membranes (from the bloodstream into a cell or vice versa) is done by facilitated diffusion, via the GLUTi transporters, where i = i, 2, 3, etc.  The only glucose transport that I can think of that goes against its concentration gradient is from the intestinal lumen into the intestinal epithelial cells.  This is driven by cotransport of sodium ions.

"*Do type 1 diabetics produce glucagon. It does not seem like they would be able to- otherwise, why is a glucagon injection necessary when their blood sugar goes too low?"

I don't think your conclusion follows from this observation.  The need inject glucagon would be driven by a need to change (rapidly?) the ratio of one hormone to the other, presumably because too much insulin was injected in the first place.  I am not certain, however.