Fructose and galactose are transformed into glucose?

1) Cells use glucose for cellular respiration. The enzymes involved in the process favor glucose. And btw, it is not necessary for them to only use glucose either. They also use amino acids, and fats.

2) Because you only need 2 molecules of them to make it. That's why their simple sugars. The body can break them down faster.

3) Because bodies take longer time breaking down polysaccarides (like starch) than they do breaking di or monosaccarides.

Glucose is a carbohydrate (molecule made of carbon and hydrogen). Try to picture the carbohydrates in your head, glucose is a small monosaccharide made of just 6 carbons. Polysaccharides are huge branched structures, that literally dwarf glucose, they could be made of tens of thousands of carbons! Fructose and galactose are also monosaccharides like glucose. This is why polysaccharides are complex, but dissacharides and monosaccharides are simple.

Now the enzymes which metabolise glucose into ATP (energy) can only work on glucose, the others arent the right shape, so in order to metabolise other monosaccharides there are other enzymes which first make them into glucose (so it can enter glycolysis for breakdown into pyruvate, which can then produce energy by oxidative phosphorylation). ?-galactosidase  is such an enzyme, it produces glucose from galactose by the addition of a water molecule.

The other thing to bear in mind, is how these molecules get into the body in the first place. All sugars are absorbed in the gut by facilitated diffusion/co-transport. Glucose and galactose are both transported by a Na+/Glucose co-transporter (called a symporter). Fructose on the other hand is absorbed by its own transporter, called GLUT5.

Fructose metabolism is slightly more complicated, it is first phosphorylated into - Fructose-1-phosphate and then broken into two fragments. One is called DHAP and the other is Glyceraldehyde, which is made into Glyceraldehyde-3-phosphate (G3P). These can then glycolysis.

Ok, where to start?  Glucose is the type of sugar that binds.  It's the difference between gelatin and pudding, for example.

The complexity of a sugar or starch just means how it bonds.  If you looked at an expanded atomic "picture" of a potato, you'd see long chains of starches that are bonded by hydrogen.  If you looked at a picture of white bread, you'd find very short chains.  The longer chains take your body more energy (calories) to process and are less likely to be converted to fat.

Cells use glucose as an energy source, structural unit (meaning that they make things with it), or a precursor for other organic componds, such as cellulose, starch, and glycogen. Glucose is actually a simple sugar, or monosaccharide. A monosaccharide is a type of carbohydrate. Glucose is the smallest unit. Since glucose dissolves easily in water and the human body consists mostly of water, glucose is a pretty good candidate for us. Sucrose is actually made from glucose and fructose, so it's disaccharide. Lactose is also a disaccharide, and it's made from glucose and galactose. Disaccharides and polysaccharides are made of more than one sugar monomer, so they're not simple. Disaccharides have two sugar monomers and polysaccharides have thousands.