Respiration is a fundamental biological process that organisms employ to generate energy by metabolizing organic compounds such as sugars, lipids, and proteins. This energy, stored in the chemical bonds of these compounds, is released and subsequently utilized for various cellular activities. This process is occurred in the living cells and is also called cellular respiration. There are two types of respiration：aerobic respiration (oxygen is required) and anaerobic respiration (oxygen is not required).
Aerobic Respiration (oxygen is required)
Aerobic respiration is a type of respiration that occurs in the presence of oxygen (O₂). It’s an efficient energy-releasing process predominantly observed in eukaryotes, where glucose is oxidized to produce carbon dioxide (CO₂), water (H₂O), and energy in the form of adenosine triphosphate (ATP). But not all the energy stored in glucose is transformed to ATP (only about 40%), a lot of energy is dissipated as thermal energy.
This can be summarized by the equation:
C₆H₁₂O₆ (glucose)+6O₂→6CO₂+6H₂O+energy(ATP)
This process encompasses several stages including glycolysis, the Krebs cycle (citric acid cycle), and oxidative phosphorylation.
Aerobic Respiration vs Combustion（burning）
Aerobic respiration and combustion of organic matter are both processes that involve the breakdown of organic molecules into simpler substances, such as water and carbon dioxide. However, there are significant differences between them. Combustion is a rapid and exothermic reaction that occurs when organic matter is exposed to oxygen in the air. It generates a lot of heat and emits light during a short time. It also cannot occur in aqueous environments.
Respiration is a slow and controlled reaction that occurs inside living cells（it takes place in aqueous conditions）, where oxygen don’t contact with the organic matter directly. Organic compound is gradually broken down by a series of enzymes, and electrons continuously release energy along the electron transport chain. In every step, a little energy is released, so it is a peaceful and continuous biochemical process and no visible light is emitted during respiration.
Anaerobic Respiration and Fermentation (oxygen is not required)
Anaerobic respiration and fermentation are two metabolic pathways that organisms use to get energy in the absence of oxygen. However, they differ significantly in their processes and products.
Anaerobic respiration uses other electron acceptors to take the place of oxygen in an electron transport chain, leading to a complete breakdown of glucose. For example, some anaerobic bacteria utilize sulfate as final electron acceptors in the electron transport chain. The foul smell of rotten eggs you detect when passing by a sewer is a byproduct of anaerobic respiration known as hydrogen sulfide.
On the other hand, fermentation also operates in the absence of oxygen, but unlike anaerobic respiration, it does not involve an electron transport chain. In fermentation, the cells partially break down glucose to produce ATP, and various end products such as ethanol or lactic acid are formed. In fermentation, electrons are transferred to NAD⁺ to generate NADH and pyruvate during glycolysis. Then the intermediate product is reduced by NADH to become ethanol or lactic acid, and NAD⁺ is present again.