A carbohydrate is basically water plus carbon. Somehow I managed not to learn this until I was in earth system science, a class that I took mostly for fun, where all but one of my classmates were majoring in earth science, physics, chemistry or biology (the other outsider was majoring in journalism). My classmates all knew the following stuff already, but it was new and fascinating to me- maybe it will be to you too.
Anyways- when plants use photosynthesis, they take six carbon dioxide molecules (1 carbon atom plus two oxygen atoms) and six water molecules(two hydrogen atoms plus one oxygen atom) and turn that into six carbohydrate molecules (each with one carbon atom plus two hydrogen atoms plus one oxygen atoms) plus six oxygen molecules (each with two oxygen atoms). The chemical formula looks like this: 6H2O (water) + 6C02 (carbon dioxide) -> 6CH2O (carbohydrate) + 6O2 (oxygen)
Animals do the reverse in respiration. We take carbohydrates and oxygen and turn that into carbon dioxide and water. That literally means that we need carbohydrates in order to breath.
That is also why we need sugar in our blood; because this respiratory process, by which we get energy, requires carbohydrates and oxygen.
I really wanted to write about types of sugar. I was thinking about all those things that end in -ose: dextrose, glucose, fructose, lactose, maltose, etc. I was hoping to find a definition of sugar and to be able to list them all. Apparently sugar is not a scientific term, so I can't do this.
So instead let me tell you what the wikipedia article on carbohydrates tells me.
First, there are four general types of carbohydrates. Monosaccharides are usually the sorts called sugars, and then are in the formula of CaH2aOa. Monosaccharides include glucose (which is the same as dextrose- I had no idea they were the same thing!), fructose, galactose, and others I never heard of.
Disaccharides are two monosaccharides smushed together minus a water molecule. They are often also called sugars. Disacharides include sucrose (table sugar- and it is made up of fructose plus glucose), maltose (made of two glucoses) and lactose (the sugar in milk- it is made of galactose and glucose).
Oligosaccharides are more monosaccharides smushed together. While we eat some oligosaccharides, others are in our bodies because of broken down polysaccharides.
Polysaccharides are lots and lots of monosaccharides smushed together (like hundreds). They include starch (as in the stuff you eat) and glycogen (as in the way the liver stores carbohydrates before turning into glucose).
So basically a sugar is a monosaccharide or a disaccharide, and a complex carbohydrate is an oligosaccharide or polysaccharide. And the reason that sugars raise your blood sugar faster is that your body has to convert these things into glucose- a monosaccharide- before it can be blood sugar. Phew!