چهارشنبه 20 اردیبهشت 1391 06:07 ب.ظ
Respiration is a metabolic process used by all organisms to break down fuel molecules in order to release the energy stored in them. Here, the glucose manufactured in photosynthesis is now ready to be cashed in for ATP molecules to drive the reactions of living cells. This is a very important metabolic process in cells.
Note: A plant may use its own glucose, but that glucose is also available to animals which eat the plants, and then to other animals.
Cellular respiration is the process of trading glucose for ATP since it takes place within the cell. Cellular respiration takes place in 3-stages:
- Krebs cycle
- Electron-transport chain
During the process of respiration the glucose molecule is split. Next, the hydrogen atoms are transferred from the carbon atoms to oxygen atoms. The energy released as a result of this transfer is used to convert ADP to ATP, which is the energy currency of the cell.
Glycolysis is the first phase of respiration where glucose is broken down with a small energy yield. This process takes place in the cell fluid or cytoplasm. Note what takes place during the above process:
- A 6-carbon glucose molecule is split up and changed into two molecules of a 3-carbon substance called pyruvic acid.
- Also during the process two ATPs are produced.
Two molecules of pyruvic acid are produced.
- Yeasts and other anaerobic organisms, those that do not utilize oxygen in their respiration, derive all their ATP from glycolysis.
- Pyruvic acid is not the end of the line in glycolysis. In yeasts, the respiratory products are carbon dioxide (CO2) and ethanol (CH3CH2OH).
- In animals, the product of respiration with oxygen is not alcohol, but lactic acid.
- In aerobic animals, some of their energy may be derived from glycolysis.
To see an animated form of glycolysis, click here.
The Krebs cycle is the phase of respiration in aerobic reactions where the fuel is broken down in CO2 and H2O. In aerobic respiration, glucose is broken down into CO2 and H2O, releasing the equivalent of 38 ATP in useable energy. Most of the energy is released in the form of hydrogen ions and electrons. Primitive organisms use anaerobic respiration. Anaerobic fermentation breaks down glucose into CO2, ethanol, and H2O. The rest of the energy is lost as heat. This is the process used to make alcoholic beverages and to raise bread.
More advanced animals, such as ourselves, can break down glucose into CO2, lactic acid, and H2O, releasing 2 ATP by a process called anaerobic muscle glycolysis. (Note: Build up of lactic acid is what causes muscle fatigue, muscle cramps commonly found within athletes.)
The last stage of aerobic respiration is the electron-transport chain. During this process, the electrons are passed along to a series of electron carriers similar to those referred to in photosynthesis. The energy that is released is used to form more ATP (See figure below).
To see an animation of the Krebs cycle, click here.
|Diagram of an electron transport chain. As the electrons flow downhill from one carrier to the next, (which is at a lower energy level) the energy which the electrons possessed when they were held by the NAD is released in a stepwise manner. That energy goes to form ATP from ADP. Finally the spent electrons combine with protons and are accepted by an oxygen molecule (now you can see why you breathe) and water is formed. Some organisms, such as certain insects and desert rodents, can derive all the water they need from this "metabolic water".|
To view an animation of the electron transport chain, click here.
Basic Life Cycle
|The basic cycle of life on earth. Chloroplasts capture the sun's energy, convert it to glucose. The mitochondria utilize glucose. The mitochondria utilize the glucose to form ATP. the CO2 and water formed by the mitochondria is utilized by the chloroplast.|
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