Cellular respiration is a metabolic process involving many different reactions taking place; organic compounds are oxidized in these reactions and energy is produced. These organic compounds are known as respiratory substrates. Respiratory substrates include:
When these substrates are oxidized cells can use the energy produced to carry out their normal processes. The main aim of cell respiration is to produce energy; this energy is in the form of ATP.
The many reactions which take place during cellular respiration can be divided into three stages:
- Glycolysis: This stage involves the oxidation of glucose forming pyruvate and takes place in the cytoplasm of the cell (2ATP’s produced)
- Krebs cycle: Breaks down pyruvate from glycolysis forming carbon dioxide and hydrogen (in the presence of oxygen) or forms ethanol/lactate (in the absence of oxygen). This stage takes place in the matrix of the mitochondria. (2ATP’s produced)
- Electron Transfer System: In this stage the hydrogen is oxidized by oxygen forming water and takes place in the inner membrane of the mitochondria. (34ATP’s produced)
|Cellular Respiration Image|
Somewhere along these stages numerous amounts of ATP are produced which can then be utilized by the cell itself.
The availability of oxygen is what determines the fate of pyruvate, therefore different products can be obtained from these stages depending on whether oxygen is made available or not. There are two types of respiration, these are:
- Aerobic Respiration
- Anaerobic Respiration
When glucose is broken down in the cytoplasm the product formed is known as pyruvate. From this point the pyruvate can take one of two routes depending on whether oxygen is available or not. In aerobic respiration oxygen is made available. The purpose of this oxygen is to oxidize the pyruvate from glycolysis to ultimately form carbon dioxide and water (ATP’s are also produced along the way). The first stage involves the Krebs cycle where the pyruvate was broken to form carbon dioxide and hydrogen. This hydrogen then moves along to hydrogen carriers in the second stage (electron transfer chain) where it goes through a number of carriers and finally gets oxidized by oxygen forming water. Combining the equations from glycolysis, Krebs cycle and the electron transfer chain we get:
C6H12O6 + 6O2 => 6CO2 +6H2O + 38ATP (Energy)
Note that 38 ATP’s are produced in aerobic respiration, compare this with the amount of ATP’s produced during anaerobic respiration and you should see why aerobic respiration is more efficient than anaerobic respiration.
Anaerobic Respiration (Fermentation)
In this type of respiration oxygen is absent which therefore means different products will be formed from the point where pyruvate left glycolysis. It might seem odd but there are organisms that can actually survive with and without oxygen, such organisms are termed facultative anaerobes. There are even organisms that can’t survive in the presence of oxygen; these organisms are called obligate anaerobes.
Remember from aerobic respiration that the purpose of the oxygen was to combine with hydrogen in the final stage to ultimately form water. However since oxygen is not present there is no acceptor at the end of the final stage (electron transfer chain) to combine with hydrogen. Since there is no final acceptor the hydrogen goes back and combines with the pyruvate preventing the release of any energy.
Anaerobic Respiration has two Pathways which depend on the type of organism:
1. Anaerobic Respiration in Fungi
In fungi such as yeast Pyruvate is ultimately converted into ethanol and carbon dioxide (Alcoholic Fermentation) with an overall production of 2 ATP’s, allot of energy however still remains locked within ethanol.
C6H12O6 => 2Ethanol + 2CO2 + 2ATP (Energy)
Despite being an inefficient source of energy production this process still has some uses. Uses of fermentation by Yeast include:
- The production of alcoholic drinks such as wine and beer.
- The Carbon dioxide produced is used in the manufacture of bread, because it allows for the dough to rise.
- Though the energy still remains trapped in ethanol in countries such as Brazil it is used to make gasohol which can then be used to fuel cars.
2. Anaerobic Respiration in Animals
In certain tissues such as muscle tissues in animals the pyruvate formed from glycolysis is ultimately converted to lactate with the formation of 2ATP’s, no carbon dioxide is produced in this type of anaerobic respiration. In animals buildup of lactate can result in a sensation of fatigue and cramps.
C6H12O6 => 2Lactate + 2ATP (Energy)
As is the case with ethanol in fermentation much of the energy remains locked within the lactate which therefore means this path is also inefficient. The energy can however be released from the lactate if oxygen is later made available.
|Image Showing Aerobic and Anaerobic Respiration|