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Carbohydrates are a major source of energy in our body, often in a form of starch and sugars. They come mainly from grains, such as rice and noodles. Besides, fruit, root vegetables, dry beans and dairy products also contain carbohydrates. It participates in the formation of ATP, or adenosine triphosphate, which is the main form of energy store in our body, and that's why carbohydrates provide energy.

There are many forms of carbohydrates, starch, sucrose and glucose being the most common one, and they have a common name: saccharides. Different kinds of carbohydrates will have different characteristics when providing energy. For example, glucose can provide instantaneous energy (that's why we take it when at gyms), while starch provides a relatively long term source of energy.

Learn more: how does carbohydrates provide energy to our body?

In middle school, you must have heard of the word “respiration” and wrote the equation C6H12O6+6O2→6CO2+6H2O. Indeed, this is the equation of the aerobic respiration happening in our body, and it releases energy in the form of ATP, adenosine triphosphate, which is the most common type of energy storage in our body.

This picture shows how cellular respiration produce many ATPs for our body, during which the reaction goes through the Krebs cycle to yield up to 36 ATP in one complete reaction. What a reaction!

As you can see, glucose participates directly in the synthesis of ATP, which brings energy. Therefore, when we are exercising, we often directly take in glucose to obtain quick and effective power gain. However, there are other sources of carbohydrates such as sucrose (the main source of “sugar” we take in everyday) and starch. These are the siblings of glucose and need to be converted to glucose first to do the reaction above. Therefore, they provide a relatively long-term and steady energy supply.

Learn more about the ATP itself:

Here is my note on the function of ATP. ATP, or adenosine triphosphate, is formed from ADP (the two phosphates version “adenosine diphosphate”) inside mitochondria. Here, a free phosphate group Pi and a ADP molecule undergo dehydration to fuse the bond between the terminal phosphorus atom and the oxygen atom. 

Reversibly, when ATP is used outside mitochondria, it breaks its Phosphorus-oxygen bond to release energy for about 340 kilojoules per mole. The phosphorus-oxygen bond requires relativey low activation energy to break, therefore ATP is a very efficient energy transporter inside our body! (Remember to have a healthy diet to obtain enough phosphorus. They are required in very small amounts as ATP is reusable, but you still have to take in them from meat, beans and dairy products.)

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