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Cellular Respiration

All organisms require energy to sustain life. We need energy to function and this energy is obtained from the food we eat. The energy is obtained through chemical reactions that are enzyme controlled. Plants can trap the energy from sunlight through the process of photosynthesis and it is stored in the chemical bonds of carbohydrate molecules. Living organisms are capable of carrying out reactions in a controlled manner so that energy is released in the right quantity at the right time. Control of energy-releasing reactions organisms are able to use the energy to carry out activities such as reproduction, movement, and growth. There are many reactions that take place inside he organism, in cellular respiration, organisms control the release of energy from  organic molecules, and use this energy for the activities to sustain life. 

Almost all organisms use organic molecules from their surroundings to obtain energy. Organisms capable of photosynthesis produce food molecules like carbohydrates. There are many forms of cellular respiration. Some organisms require oxygen to carry out respiration known as aerobic cellular respiration and the respiration that does not require presence of oxygen is called anaerobic cellular respiration.

Cellular Respiration Steps


Cellular Respiration Definition

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Cellular respiration is a series of set of metabolic reactions and processes. These reactions takes place in the cells of organisms, where the organisms convert biochemical energy from nutrients into adenosine triphosphate. Catabolic reactions are involved in respiration, where large molecules are broken into smaller ones, releasing energy in the process. Cells gain useful energy to fuel cellular activity through respiration. The reaction is considered to be an exothermic redox reaction, the process occurs in many separate steps.
Nutrients that are commonly used by animal and plant cells are sugar, amino acid and fatty acids and a common oxidizing agent which molecular oxygen. Through cellular respiration the cells harvest energy stored in food by a catab
olic pathway for the production of adenosine triphosphate (ATP). ATP is a high energy molecule which is spent by working cells. Cellular respiration takes place both in eukaryotic and prokaryotic cells. Cellular respiration can be aerobic respiration in the presence of oxygen or anaerobic respiration in the absence of oxygen. 

Types of Cellular Respiration

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There are two types of cellular respiration: Aerobic and Anaerobic respiration.

Aerobic Respiration
Aerobic respiration takes place in the presence of oxygen, by which ATP is is produced by the cells by oxidation of organic compounds. The end product of glycolysis, pyruvate enters the mitochondria to be fully oxidized by the Krebs cycle. The product of this pathway is carbon dioxide and water and energy in the form of ATP. Most of the ATP produced by aerobic respiration is made by oxidative phosphorylation. Aerobic respiration takes place in three stages glycolysis, TCA cycle and electron transport chain.
Glycolysis - In glycolysis molecules of glucose in the cytoplasm are broken into molecules of pyruvate. One glucose molecule yields two molecules of pyruvate. 

TCA cycle - It is also known as Kreb's cycle or Citric acid cycle. This cycle begins 
after the pyruvate molecules from glycolysis are converted to acetyl CoA. Through a series of steps high energy electrons are produced along with two ATP molecules in this cycle. This cycle occurs only in the presence of oxygen. 

Electron Transport Chain requires direct supply of oxygen. The ETC is a series of electron carriers in the mitochondrial membrane. Through the reactions high energy electrons are passed to oxygen, a gradient is formed and ultimately ATP is produced. 

The simplified aerobic respiration is 
C6H12O6 + 6O2  6CO2 + 6H2O + Energy (ATP)The word equation for this is
Glucose (sugar) + Oxygen → Carbon dioxide + Water + Energy (ATP)

Anaerobic Respiration
Anaerobic respiration is the process used in some microorganisms in which oxygen is not final electron acceptor. This process starts like aerobic respiration and stops part way through because oxygen is not available to finish the respiration process. This type of respiration makes fewer ATP molecules and releases byproducts of lactic acid or alcohol. Aerobic respiration takes place in mitochondria. Glycolysis is the first step in cellular respiration, it can take place without the presence of oxygen.  Lactic acid fermentation and alcoholic fermentation are two types of anaerobic fermentation. 

Lactic acid Fermentation 
This type of anaerobic respiration takes place in humans if there is shortage of oxygen. 

Alcoholic Fermentation 
This does not takes place in humans. The same process happening in mitochondria during lactic acid fermentation happens in alcoholic fermentation in the yeast. The byproduct of alcoholic fermentation is ethyl alcohol. 

Stages of Cellular Respiration

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Cellular respiration occurs in both eukaryotic and prokaryotic cells. It has three main stages glycolysis, the citric acid cycle and electron transport chain.


  • Glycolysis is the anaerobic catabolic reaction of glucose. 
  • Glycolysis occurs in almost all cells. 
  • This pathway takes place with or without the presence of oxygen. 
  • Aerobic conditions produce pyruvate and anaerobic conditions produce lactate as the end products of glycolysis. 
  • In the eukaryotic cells, glycolysis occurs in the cytosol
  • Glycolysis a process where a molecule of glucose is converted into two molecules of pyruvic acid.
  • There are ten intermediate compounds in this process and there are ten enzymes are needed for the process of glycolysis. 
  • Two energy rich ATP molecules are required to start the process of glycolysis. 
  • At the end, the process yields a pyruvate molecule, four molecules of ATP are made and two NADP molecules.
  • Both ATP and NADP molecules are energy-rich and are used in other cell reactions. 
  • The cells which use oxygen, the pyruvate is passed on to the second process known as the Kreb's cycle by which more ATP molecules are produced. 
Glycolysis Process

Citric Acid Cycle 

  • Citirc acid cycle is also known as the Krebs cycle and the tricarboxylic acid cycle.
  • In the presence of oxygen, the pyruvate produced at the end of glycolysis is converted to acetyl-CoA. 
  • In the presence of oxygen the mitochondria will undergo aerobic respiration which leads to Krebs cycle.
  • In the absence of oxygen fermentation of the pyruvate molecule will occur. 
  • When acteyl-CoA is produced, this molecule enters the citric acid cycle, which takes place in the mitochondrial matrix. 
  • This molecule gets oxidized to CO2 and reduced NAD to NADH. 
  • NADH is then again used in the electron transport chain to produce more ATP in oxidative phosphorylation.
  • The TCA cycle is a process with 8 steps and involves different enzymes and co-enzymes.
  • The net energy gain from a single citric acid cycle is 3 NADH, 1 FADH2 and 1 GTP. The GTP is subsequently used to produce ATP. 
Citric Acid Cycle

Electron Transport Chain

  • The electron transport system is found in the mitochondria and chloroplast of eukaryotic organisms and is seen in the plasma membrane of prokaryotes. 
  • This pathway consists of series of carrier molecules which pass electrons from a high energy molecule to the final low energy electron acceptor molecule.
  • Energy released during this process of oxidation and reduction produces ATP. 
  • The NADH and FADH2 molecules bring electrons to the electron transport system. 
  • This system contains membrane-bound electron carriers that pass electrons from one to another. 
  • When a carrier molecule reduces another molecule, the energy that is released is used to pump hydrogen ions across the membrane into the inter-membrane space. 
  • The remaining energy is used to reduce the next carrier molecule. 
  • As a result of this, hydrogen ions become concentrated in the inter-membrane space.
  • The ATP synthase enzyme uses the energy of this gradient to produce ATP. 
  • In the electron transport system, NADH and FADH2 are oxidized and the energy released in this process is used to produce ATP.

Photosynthesis vs Cellular Respiration

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Photosynthesis and cellular respiration are metabolic reactions that complement each other in the environment. The are the same reactions but occur in reverse. In photosynthesis carbon dioxide and water yield glucose and oxygen, respiration process glucose and oxygen yield carbon dioxide and water. 

Comparison chart Photosynthesis vs Cellular Respiration 

                       Photosynthesis                         Cellular Respiration 
 Process releases energy
requires energy? 
 Requires energy  Releases energy as ATP molecules
in a step-wise manner. 
 Chemical Reaction  In the presence of sunlight, carbon dioxide
and water combine to form glucose and oxygen.
 Glucose is broken down into water and
carbon dioxide and energy. 
 Production of ATP  Yes  Yes, theoretically 38 molecules of ATP are yielded.
But actual yield is 32 or 34 molecules.
 Reactants  6CO2 and 12H2O and light energy.   C6H12O6 (glucose) and 6O2.
 Products   C6H12O6 (glucose), 6O2 and 6H2O  6CO2, 6H2O and energy in the form of ATP. 
 Sunlight requirement  Occurs only in the presence of sunlight.  Occurs at all times. 
 Organelle in which it occurs  Chloroplasts  Mitochondrial cytoplasm. 
 Process  Organic carbon(glucose and starch) is produces
 from inorganic carbon(carbon dioxide) with
the expense of ATP and NADPH produced
in the light dependent reaction. 
 ATP is produced from the oxidation
 of organic sugar compounds. 
 Organisms in which the
process occurs
 It occurs in plants, algae and some bacteria.   Occurs in all living organisms, in plants
and in animals.
 Equation  6CO2 +  12H2O + light →  C6H12O6 + 6O+ 6H2O  C6H12O6 +  6O→ 6CO2 + 6H2O + energy
 Main function  Produces food and captures energy.   Breakdown of food, releases energy. 
 Stages  2 stages - Light dependent reaction and
Light independent reaction. 
 4 stages - Glycolysis, Pyruvate oxidation, Kreb's cycle,
Electron transport chain (oxidative phosphorylation).
 Final electron receptor  NADP+ forms NADPH  O2 molecule.

More topics in Cellular Respiration
Glycolysis Citric Acid Cycle
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