As basic building blocks of life, all living organisms use nucleic acids, proteins, lipids, and carbohydrates, as well as a variety of small molecules such as metabolites, messengers, and energy carriers. All living things need a continuous supply of energy just to stay alive. The energy is produced in chemical reactions.     A chemical reaction is a process in which some substances, called reactants, change chemically into different substances, called products. Reactants and products may be elements or compounds.     Chemical reactions that take place inside living things are called... Show more

As basic building blocks of life, all living organisms use nucleic acids, proteins, lipids, and carbohydrates, as well as a variety of small molecules such as metabolites, messengers, and energy carriers.

All living things need a continuous supply of energy just to stay alive. The energy is produced in chemical reactions.    
A chemical reaction is a process in which some substances, called reactants, change chemically into different substances, called products. Reactants and products may be elements or compounds.    
Chemical reactions that take place inside living things are called biochemical reactions. Living things depend on biochemical reactions for more than just energy. Every function and structure of a living organism depends on thousands of biochemical reactions taking place in each cell. Some biochemical reactions are anabolic reactions. In these reactions, smaller molecules combine to form larger molecules. Anabolic reactions form chemical bonds and need energy. Other biochemical reactions are catabolic reactions. In these reactions large molecules break down to form smaller ones. Catabolic reactions break chemical bonds and release energy.    
Some of the most important biochemical reactions are the reactions involved in photosynthesis and cellular respiration.    
Together they provide energy to almost all living cells.    

- Photosynthesis is the process in which producers capture light energy from the sun and use it to make food.    
This involves anabolic reactions. The reactants of photosynthesis are carbon dioxide and water. The products of photosynthesis are oxygen and glucose.    
- Cellular respiration is the process in which energy is released from glucose and stored in smaller amounts in other molecules that cells can use for energy. This involves catabolic reactions. The reactants of cellular respiration are oxygen and glucose. The products of cellular respiration are carbon dioxide and water.    

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