All living organisms eat food, but do you know why? Because we need that food to produce energy for the normal functioning of the body. This is precisely what we go through the process of respiration. Respiration is the biochemical process through which all the cells of living organisms get the energy to perform certain life processes that are essential for survival. What happens in the process is, glucose from the food that organisms consume, gets broken down into simpler substances and energy is released. Since respiration takes place in the cells of organisms, it is called cellular respiration.
Types of Respiration
Different organisms can obtain energy in different ways. Humans, for example, need air to live. The reason behind this is that we use oxygen from the air we breathe to release energy. As oxygen is required here, this process is called aerobic respiration. However, there are some species of bacteria and algae or some prokaryotes which do not need oxygen to generate energy. So, these organisms get energy through anaerobic respiration.
Here, the first step is the conversion of glucose, a 6 Carbon molecule into a simpler sugar called pyruvate, a 3 Carbon molecule in the cytoplasm. Then, in the absence of oxygen, pyruvate gets converted to other substances, and energy is released in this process. This is how the process of fermentation takes place. In yeast, pyruvate gets broken down into ethanol, and carbon dioxide and energy are released. This is how bread and alcoholic beverages such as beer and wine are made.
Glucose → Pyruvate + Energy → Ethanol+ Carbon Dioxide + Energy
C6H12O6 → C3H3O3– + Energy → C2H5OH + CO2 + Energy
It is important to note that anaerobic respiration could occur in humans too, in the absence of oxygen. This, however, only happens in emergency situations. For example, when we exercise too heavily, there is a lack of oxygen in our muscle cells. So, our body has a backup plan. It can temporarily generate energy in the absence of oxygen as well. However, lactic acid is released during this process which causes muscle cramps. Thus, we are always advised to not indulge in rigorous exercise.
Glucose →Pyruvate + Energy →Lactic acid + Energy
C6H12O6 → C3H3O3–+ Energy → C3H6O3 + Energy
Thankfully, we do not always suffer from painful muscle cramps. That is only a backup plan, as mentioned before. Normally, energy is released by the breakdown of glucose into carbon dioxide and water. And this happens in the mitochondria. Interestingly, the first step of aerobic and anaerobic respiration remains the same, i.e. the conversion of glucose into pyruvate. But the amount of energy released during aerobic respiration is a lot greater than that during anaerobic respiration.
Glucose → Pyruvate + Energy → Water + Carbon Dioxide + Energy
C6H12O6 → C3H3O3– + Energy → H2O + CO2 + Energy
The energy thus released is immediately used to synthesize a molecule called ATP. This is the energy currency of cellular processes like the contraction of muscles, protein synthesis, conduction of nervous impulses, etc in our body.
Process of Cellular Respiration in Humans
Just as different organisms have different ways to break glucose down to release energy, the organisms that perform aerobic respiration also take in oxygen in different ways. Organisms living on land can breathe in oxygen from the air, but organisms living underwater need to be able to take in oxygen that is dissolved in water. If you have ever owned fish as pets, you would have noticed that they open and close their mouths at a very fast rate. This is because the amount of oxygen dissolved in water is comparatively lesser than the amount of oxygen in the air. Thus, all aquatic animals need to breathe at a faster pace.
Humans have developed a complex respiratory system involving a lot of different organs to breathe in oxygen and breathe out the carbon dioxide that is toxic to us.
- Air is taken through our nostrils. This is lined with fine hair and mucous in order to prevent foreign substances from entering our body.
- This air enters an air passage, through the throat and reaches the lungs eventually. Rings of cartilage present in the throat prevent the passage from collapsing.
- The air passage on reaching the lungs divides into two tubes called bronchi, which further divide into smaller tubes called bronchioles.
- These bronchioles end in alveoli or structures that are shaped like balloons. These expand when we breathe in and contract when we breathe out, thus allowing the exchange of gases to take place.
- The surfaces of the alveoli have an extensive number of blood cells, which absorb oxygen from inhaled air and give away carbon dioxide when we breathe out. The oxygen is taken up by blood because of the presence of hemoglobin, a pigment which has a high affinity for oxygen. Carbon dioxide that is soluble in water is transported in the dissolved form in our blood. This is brought to the alveoli by blood from all over the body.
During the breathing cycle, the lungs always contain a residual volume of air so that there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released.
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