Carbohydrate 101
Carbohydrates are molecules which consist of carbon, hydrogen and oxygen atoms only. the carbon atoms are in chains or rings. A carbohydrate with a 2 carbon chain would be a simple sugar called a diose. I don't know any. A 3 carbon chain is a sugar called a triose and there are some which appear as intermediaries in metabolic pathways. There are some tetroses 4 carbons, pentoses(5) and hexoses(6).And some compounds up to 10 carbons have been synthesised. The best known hexose is glucose, also called dextrose, because it's a right handed version of the molecule. (there is also a laevose, which is left handed and not, as far as I know, biologically active).
All these single carbon chain sugars are called monosaccharides. if 2 monosaccharides bond together, you get a disaccharide. Sucrose, (table sugar) is a disaccharide. as are maltose ( in sprouting grain) and lactose (in milk). There are some trisaccharides, consisting of 3 monosaccharides chemically linked and some tetrasaccharides. the polysaccharides are made up of many monosaccharide units linked, often many hundreds in either straight or branched chains. They include Starch, which we break down to its constituent glucose molecules, after it's been cooked, cellulose, which we cannot break down and call dietary fibre. there are others, but they're only of interest to geeks like me. The important thing to learn is that starches are made up of 100% glucose monomers and break down to maltose or glucose in our metabolic processes to appear as glucose in the blood. this can happen very quickly. Sucrose, incidentally breaks down to glucose and fructose, which is a sugar that doesn't easily turn to glucose and has very little effect on blood glucose levels. There are other issues with it though
Theoretically therefore, 5 grams of table sugar would yield only 2.5 grams of glucose and 5 grams of starch would yield 5 grams of glucose. We can't process the starch unless it's been cooked or treated in some other way to make it accessible to the enzymes we have. We can't break down starch in its natural form.
That in a large cocnutshell is why it's not a good idea to eat anything which consists largely of starch, such as pasta, rice, potatoes and other flour based products.