All carbohydrates are made up of carbon, oxygen, and hydrogen.
The chemical formula for Glucose is

.
The chemical formula for Fructose is the same as that of Glucose. The difference between the two is how they are structured.
The chemical formula for Sucrose is

.
As you can see in the chemical formulas of Glucose, Fructose, and Sucrose, they are all made up of the elements carbon, hydrogen, and oxygen.
Plant cells have cell walls, while animal cells don't have cell walls.
Answer:
Classification system changes because the scientists find new evidence in their studies.
Explanation: The world is always changing and growing and dying as well as developing, so over time things change. For example, as you grow up do you stay the same? No, you notice the physical changes happening, well thats sort of like how the earth and its ways are always changing as well.