what is cellular respiration equation

The overall goal of cellular respiration is to use the energy contained in a carbohydrate called glucose, which is converted into a usable form of energy called atp. Cellular respiration provides the energy needed for an organism to grow, move, maintain body temperature and function. The process allows for the transfer of the chemical energy of food into the chemical energy of atp. It is a very efficient way to capture energy. Although, some of the energy is lost as heat; however, a great deal is captured in the chemical bonds of the atp. Glucose can be thought of as the high-octane fuel and atp as the charged battery that powers a device.

The overall equation for aerobic respiration is:
C₆h₁₂o₆ + 6 o₂ → 6 co₂ + 6 h₂o + energy (atp + heat)
Let’s look at what each part of the above equation means.

C₆h₁₂o₆: this is glucose, a six-carbon sugar. Glucose is the fuel that makes cellular respiration work.
6 o₂: there are 6 molecules of Oxygen gas. Oxygen is the last electron acceptor in the electron transport chain. The electron transport chain is a critical area where energy is made.
6 co₂: there are 6 molecules of carbon dioxide. Carbon dioxide is a byproduct of cellular respiration that is expelled by animals and plants.
6 h₂o: there are 6 molecules of water. Water is also a byproduct of cellular respiration.
Energy (atp + heat): this is the energy produced by cellular respiration. Of this energy, approximately 30-32 ATP are created per glucose molecule. The remainder of the energy created during cellular respiration is lost as heat.

The equation for aerobic respiration shows us three major parts of the overall reaction. Glycolysis occurs first. During glycolysis, glucose is broken down into pyruvate. The next step in cellular respiration is the Krebs cycle. The Krebs cycle begins with pyruvate, which has been converted from glucose through glycolysis. The Krebs cycle then converts pyruvate into various organic acids and releases electrons. These electrons pass through the electron transport chain. Oxidative phosphorylation occurs while the electrons are passing through the electron transport chain. As they pass, energy is made, which results in the creation of atp.

Anaerobic respiration – Life without Oxygen
There exists another type of cellular respiration, anaerobic respiration (also referred to as fermentation). Anaerobic respiration does not require Oxygen. Because anaerobic respiration doesn’t have access to Oxygen, it cannot use Oxygen as its final electron acceptor. Instead, anaerobic respiration uses other substances as its final electron acceptors. Depending upon the specific type of anaerobic respiration occurring, different types of substances may act as the final electron acceptors. Therefore, different types of anaerobic respiration result in varying reactions.

Lactic acid fermentation
During vigorous physical activity, when the muscles need more Oxygen than they are receiving, muscles begin using anaerobic respiration. Specifically, they start using lactic acid fermentation. Lactic acid fermentation results in a few notable outcomes.

Glucose still serves as the original fuel for lactic acid fermentation.
Two molecules of lactic acid result from lactic acid fermentation.
only 2 atp molecules are created as a result of lactic acid fermentation.

Because only 2 atp molecules are created per glucose molecule, this process is only good for short-term bursts of energy. Over time, excess lactic acid builds up within muscles and leads to fatigue and soreness after physical activity.

Ethanol fermentation
Yeasts and certain bacteria perform ethanol fermentation to make alcohol-based drinks such as beer, wine and champagne. Ethanol fermentation creates Two key outcomes.

Glucose again serves as the original fuel for ethanol fermentation.
Two molecules of ethanol result from ethanol fermentation.
Two molecules of carbon dioxide and 2 atp molecules result from ethanol fermentation.

Again, because only 2 atp molecules are created per glucose molecule, ethanol fermentation is only suitable for creating short-term bursts of energy. The leftover energy from glucose fermentation can potentially be utilized if Oxygen were present.

Why does aerobic respiration produce so many more atp molecules than anaerobic respiration?

Aerobic respiration is capable of breaking down glucose completely so that all of the potential energy can be harnessed. Oxygen’s ability to attract electrons and facilitate their passage through the electron transport chain enables a large concentration gradient of protons across the inner mitochondrial membrane. This concentration gradient drives atp synthase to generate many atp molecules. On the other hand, anaerobic respiration only partially breaks down glucose. Thus, much of the energy remains tied-up in lactate or ethanol and would be extractable should Oxygen be available to continue metabolism. For this reason, anaerobic respiration is regarded as an antiquated and less-efficient metabolic pathway compared to aerobic respiration.

Photosynthesis vs. Cellular respiration

Cellular respiration can also be viewed in relation to photosynthesis. Photosynthesis can be seen as an inverse process of cellular respiration. Photosynthesis produces Oxygen (an end product of cellular respiration) and glucose (the reactant in cellular respiration). Photosynthesis also utilizes sunlight to synthesize glucose and release Oxygen. In contrast to photosynthesis, cellular respiration utilizes the products (glucose and Oxygen) from photosynthesis and synthesizes carbon dioxide (a reactant in photosynthesis) and water (another reactant in photosynthesis).

As previously mentioned, plants contain chloroplasts containing the pigment chlorophyll. Chlorophyll absorbs solar radiation and initiates a series of redox reactions involving a variety of coenzymes and ultimately produces atp, nadh and fadh₂. These products are then used in cellular respiration to produce glucose and release Oxygen.

Photosynthesis: 6 co₂ + 6 h₂o + solar radiation → c₆h₁₂o₆ + 6 o₂

Aerobic respiration: c₆h₁₂o₆ + 6 o₂ → 6 co₂ + 6 h₂o + energy (atp + heat)

Photosynthesis creates the conditions under which Life can thrive on our planet. Through this cycle, plants regulate atmospheric gases and provide a source of energy for living organisms.