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Cellular Respiration

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

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Cellular Respiration

License: CC BY-NC 3.0


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 carbohydrate monomer. Recall that glucose is the sugar produced by the plant during photosynthesis. The glucose, or the polymer 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, glucose is converted into 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 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. The process can be summarized as: glucose + oxygen → carbon dioxide + water. During this process, the energy stored in glucose is converted into 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 32 ATP molecules are produced for every glucose molecule.

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


A mitochondrion (mitochondria, plural), is a membrane-enclosed organelle that is found in most eukaryotic cells. Mitochondria are called the "power plants" of the cell because they use energy from organic compounds to make ATP (adenosine triphosphate). ATP is the cell's energy source that is used for such things such as movement and cell divisionSome ATP is made in the cytosol of the cell, but most of it is made inside mitochondria. The number of mitochondria in a cell depends on the cell’s energy needs. For example, active human muscle cells may have thousands of mitochondria, while less active red blood cells do not have any.

(a): Electron micrograph of a single mitochondrion, within which you can see many cristae. Mitochondria range from 1 to 10 μm in size. (b): This model of a mitochondrion shows the organized arrangement of the inner and outer membranes, the protein matrix, and the folded inner mitochondrial membranes.

As Figure above (a) and (b) show, a mitochondrion has two phospholipid membranes. The smooth outer membrane separates the mitochondrion from the cytosol. The inner membrane has many folds, called cristae. The fluid-filled inside of the mitochondrion, called matrix, is where most of the cell’s ATP is made.

Although most of a cell's DNA is contained in the cell nucleus, mitochondria have their own DNA. Mitochondria are able to reproduce asexually, and scientists think that they are descended from prokaryotes. According to the endosymbiotic theory, mitochondria were once free-living prokaryotes that infected ancient eukaryotic cells. The invading prokaryotes were protected inside the eukaryotic host cell, and in turn the prokaryote supplied extra ATP to its host.


  • 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.


  • 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 ATP, carbon dioxide, and water.


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? How does this make them vital to the functioning of cells?


  1. What is the purpose of cellular respiration?
  2. What is ATP?

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