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Cellular Respiration

Introduces the process of transforming sugars into a usable form of energy.

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Cellular Respiration

Why do you need food?

The main reason you need to eat is to get energy. Food is your body's only supply of energy. However, this energy must be converted from the apple (or any other food you eat) into an energy source that your body can use. The process of getting energy from your food is called cellular respiration.

What is Cellular Respiration?

How does the food you eat provide energy? When you need a quick boost of energy, you might reach for an apple or a candy bar. But cells do not "eat" apples or candy bars; these foods need to be broken down so that cells can use them. Through the process of cellular respiration, the energy in food is changed into energy that can be used by the body's cells. Initially, the sugars in the food you eat are digested into the simple sugar glucose, a monosaccharide. Recall that glucose is the sugar produced by the plant during photosynthesis. The glucose, or the polysaccharide made from many glucose molecules, such as starch, is then passed to the organism that eats the plant. This organism could be you, or it could be the organism that you eat. Either way, it is the glucose molecules that holds the energy.


Specifically, during cellular respiration, the energy stored in glucose is transferred to ATP (Figure below). ATP, or adenosine triphosphate, is chemical energy the cell can use. It is the molecule that provides energy for your cells to perform work, such as moving your muscles as you walk down the street. But cellular respiration is slightly more complicated than just converting the energy from glucose into ATP. Cellular respiration can be described as the reverse or opposite of photosynthesis. During cellular respiration, glucose, in the presence of oxygen, is converted into carbon dioxide and water. Recall that carbon dioxide and water are the starting products of photosynthesis. What are the products of photosynthesis?

The process can be summarized as: glucose + oxygen → carbon dioxide + water. During this process, the energy stored in glucose is transferred to ATP.

Energy is stored in the bonds between the phosphate groups (PO4-) of the ATP molecule. When ATP is broken down into ADP (adenosine diphosphate) and inorganic phosphate, energy is released. When ADP and inorganic phosphate are joined to form ATP, energy is stored. During cellular respiration, about 36 to 38 ATP molecules are produced for every glucose molecule.

Drawing of the structure of ATP

The structural formula for adenosine triphosphate (ATP). During cellular respiration, energy from the chemical bonds of the food you eat must be transferred to ATP.


  • Through the process of cellular respiration, the energy in food is converted into energy that can be used by the body's cells.
  • During cellular respiration, glucose and oxygen are converted into carbon dioxide and water, and the energy is transferred to ATP.

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Use the resource below to answer the questions that follow.

  1. What is cellular respiration?
  2. Do plant cells respire?
  3. What kinds of molecules are used for cellular respiration? Give specific examples.
  4. What is the use of ATP? Give three examples of how ATP is used in the cell.


  1. What is the purpose of cellular respiration?
  2. What is ATP?
  3. How much usable energy is extracted from one glucose molecule?

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ATP Adenosine triphosphate; usable form of energy inside the cell.
cellular respiration Process of breaking down glucose to obtain energy in the form of ATP.
glucose Simple sugar with the chemical formula C6H12O6; a product of photosynthesis.
monosaccharide Simple sugar, such as glucose, that is a building block of carbohydrates.
polysaccharide Large carbohydrate usually containing hundreds or thousands of monosaccharides.
starch Large, complex carbohydrate; found in foods such as vegetables and grains; broken down by the body into sugars that provide energy.

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