Why are carbohydrates an excellent source of energy for organisms?
Carbohydrate are composed of sugars
Carbohydrates are an excellent source of energy to almost all if not every organism on Earth. The reason that carbohydrates are so useful to organisms is because that carbohydrates are just sugars.
These sugars can be both complex and simple coming in many natural forms. Some sugars come from plants via photosynthesis and others come from animals such as lactose found in dairy. Long story short, carbohydrates are sugars.
Now the reason behind why they're so useful to the body. In the case of humans, our bodies take advantage of proteins, carbohydrates and fats, however carbohydrates are easy to break down and metabolize. The sugars are also very easy for the mitochondria to use and create ATP with(ATP is the basic form of cellular energy). Through many processes, the mitochondria breaks down the sugar in Carbohydrates to make the energy that powers out body.
In the end the simple answer is that the carbohydrates are the easiest form of nutrition for our bodies to metabolize and therefore is an excellent source of energy.