The energy used for cycling comes from the chemical energy of food.

The process of cycling requires a constant supply of energy to power the muscles and enable movement. This energy comes from the chemical energy stored in the food we consume. Let's explore this in more detail:

1. The Digestive System:
When we eat food, it goes through a process called digestion in our digestive system. During digestion, the food is broken down into smaller molecules, including carbohydrates, fats, and proteins. These molecules can be used by our body to release energy.

2. Energy Production:
Once the food molecules are broken down, they enter the bloodstream and are transported to the cells in our body. Inside the cells, these molecules undergo a series of chemical reactions known as cellular respiration.

During cellular respiration, the carbohydrates, fats, and proteins are further broken down to release energy in the form of a molecule called adenosine triphosphate (ATP). ATP is the primary source of energy for cellular processes, including muscle contraction during cycling.

3. ATP and Muscle Contraction:
Muscle contraction is a crucial component of cycling. When we pedal, our leg muscles contract and relax in a coordinated manner, propelling the bicycle forward. This contraction requires energy, which is provided by ATP.

As the muscles contract, ATP is broken down into adenosine diphosphate (ADP) and inorganic phosphate (Pi), releasing energy in the process. This energy powers the cycling motion.

4. Continuous Energy Supply:
Cycling is an endurance activity that requires a continuous supply of energy. This is why it is important to maintain a balanced diet and consume enough food to provide the necessary fuel for the body.

Carbohydrates, such as pasta, rice, and fruits, are a primary source of energy during exercise. Fats also play a role in providing energy, especially during longer cycling sessions. Proteins are important for muscle repair and recovery after cycling.

Conclusion:
In summary, the energy used for cycling comes from the chemical energy stored in the food we eat. Through the process of digestion and cellular respiration, the carbohydrates, fats, and proteins in our food are converted into ATP, which fuels the muscle contractions required for cycling. Therefore, option 'A' - the chemical energy of food, is the correct answer.