First of all, sugar is technically classified as a carbohydrate because it has the elements carbon, hydrogen, and oxygen. More precisely, sugars are classified as simple carbohydrates. Yes, there are simple carbohydrates and complex carbohydrates. The difference is that the elements in complex carbohydrates are bonded in a configuration that won’t readily break down into substances that can be absorbed by the blood as sugars. Complex carbohydrates are broken down by enzymes and digestive juices into sugars to be absorbed into the bloodstream. The sugars listed above are the simplest substances that the blood can absorb, with glucose being the easiest, followed by fructose, sucrose, maltose and then the other sugars. Complex carbohydrates are at the back of the line.
Glucose is blood sugar, and blood sugar is glucose. Because it is readily soluble in liquid, it is absorbed into the blood.