caters » 05 May 2014 12:50 am wrote:Well what would happen if you gave a non-muscle, non-liver cell fructose or really any other sugar besides glucose(including disaccharides and oligosaccharides and polysaccharides)?
For a cell to be able to to anything with a molecule, it needs to have the enzymes that can either modify or break down the molecule. This is, in a sense, similar to light and our ability to detect it. Light is visible to us is because we have the receptors in our eyes that can capture it. If we didn't have those receptors, we would not be able to detect photons in the EM spectral range we call "visible range". Similarly, any molecule that the cell has no enzymes to work on is chemically "invisible" to the cell from an energetic point of view. If a cell can't break the bonds of an absorbed molecule, then it won't be able to extract energy from it.
Quite often, cells add a moiety, such as a phosphate group, to molecules before they're broken down. The addition of the phosphate group can make the bonds adjacent to it easier to break, and since ATP is quite abundant, phosphate groups can be readily transferred under the action of enzymes called kinases.
In the case of hexoses such as glucose, fructose, and galactoses, either general (work on all three) or specific (work on just one) hexokinases can add the phosphate to generate the phosphorylated sugar. Then the phosphorylated sugar can be further processed.
Glucose can be broken down in glycolysis to yield pyruvate, ATPs, and NADH. Pyruvate can further go through the Kreb's cycle and produce lots of ATPs from oxidative phosphorylation. Thus, for any molecule (other than glucose) to go through this same pathway, it has to be first converted to glucose, or otherwise to some intermediate metabolite along the glycolytic pathway. Since only muscle and liver cells have the kinase that will phosphorylate fructose, only these two cell types can utilize fructose sugar. muscle cells have the right kinase to allow fructose to be converted to a metabolite of glycolysis, and thus muscle cells are the only ones that can directly utilize fructose to generate ATP. In the liver, hexokinase can only make a phosphorylated form of fructose that cannot enter glycolysis, but can be used to make glucose. Thus, in the liver, fructose can be used to make glucose, and then the glucose can be used to make glycogen. Once glycogen stores of the liver are fully replenished, all the fructose goes into fatty acid and glycerol synthesis, which together make triglycerides.
caters wrote:Would you find missing links in sugar metabolism?
I mean we know how glucose is used in cells and we know normally other sugars such as galactose are converted in the liver to glucose which in turn is converted to ATP in all cells + glycogen in liver and muscle or digested in the stomach and mouth if it is multiple sugar molecules that make up the sugar.
But for example what would happen if you put galactose inside a cell that is not a hepatocyte?
The reason the liver can interconvert these molecules is because it has the hexokinase enzyme. Any cell that doesn't have the enzymes that can phosphorylate galactose and break it down simply can't make use of it.
caters wrote:All these I don't think have been done. I suppose putting cellulose inside one of your cells revs up the ATP production because it is basically hundreds of glucose molecules in just 1 unit of cellulose.
Cellulose sure has a lot of energy locked up in it, but the cell simply can't access that energy unless it can first break down the cellulose into the monohexose units. We don't have the enzyme that breaks down cellulose, that's why fiber literally goes right through us. Only ruminating mammals can access the energy in cellulose (and inside cells wrapped by cellulose), and they do so by harboring bacteria in their digestive systems that can break down cellulose. That's why eating the meat of plant-eating animals is good for us. It allows us to acquire so many nutrients that we have a hard time acquiring directly from plants.