# Cellular respiration
## Introduction

Cellular Respiration is a set of metabolic reactions that cells undergo in order to transform C$_6$H$_{12}$O$_6$, or glucose (food) into a form of energy that can be used to fuel cellular processes: adenosine triphosphate (simply known as ATP). ATP is a vital element in our lives. How else do you expect to walk, run, jump, etc,.? When the fastest man in the world, Usain Bolt, sprints his regular 9-second 100 meter races--ATP provides Bolt the necessary energy in order to sprint that fast! When football (soccer) players, American football players and other sportsmen sprint/run, ATP provides them the necessary energy for their athletic needs. ATP is so important that various systems in the body are dedicated to producing massive amounts of it!
So, how does this whole "ATP" thing work? How is ATP, the universal energy carrier molecule, used in an organism? First, let's review the chemical equation:
Cellular respiration's chemical equation is the opposite of photosynthesis' chemical equation (the products and reactants are switched), minusing sunlight (energy from the sun):

### Photosynthesis

   6CO$_2$ + 6H$_2$O + energy from the Sun → C$_6$H$_{12}$O$_6$ + 6O$_2$

### Cellular Respiration

   6O$_2$ + C$_6$H$_{12}$O$_6$ → 6CO$_2$ + 6H$_2$O
Cellular respiration takes place in two parts: Anaerobic respiration and aerobic respiration.

## Anaerobic Respiration
### Glycolysis

Cellular respiration has to continue to one of two different processes:
* If there is oxygen present, then cellular respiration will continue to the aerobic portions of the cycle, the citric acid cycle and the electron transport system, which occurs in the mitochondria. The oxygen used in this process will be transformed into carbon dioxide (CO2) which is important for the photosynthesis of plants.
* If oxygen is not present then the cell will undergo another anaerobic process in the cytoplasm called fermentation.

### Fermentation

Fermentation breaks down the 2 pyruvates that were formed as a result of glycolysis into either lactic acid or ethyl alcohol, depending on the organism. Lactic acid is typically formed in mammalian eukaryotic organisms while ethyl alcohol is mostly formed in microscopic eukaryotic organisms such as yeast.3[citation needed]
The site that fermentation most commonly occurs at in mammalian eukaryotes is the muscle cells.1[citation needed] This happens because the muscles of these organisms are frequently exerting a large amount of energy, which expends a lot of oxygen. Once these muscle cells run out of oxygen, lactic acid begins to form as a result of an anaerobic environment. The more water a person drinks before, during, and after a workout, the less sore the person will be the next day because less lactic acid was synthesized during the workout.1[citation needed] For microscopic eukaryotic organisms such as yeast cells, ethyl alcohol is the byproduct of fermentation.4[citation needed] This alcohol is often harvested by people in order to create a number of different things. The two most common uses for ethyl alcohol are for psychoactive purposes and fuel. The psychoactive purposes are typically carried out through the consumption of the alcoholic beverages that are created with the ethyl alcohol. The consumption of ethyl alcohol is very common amongst people even though it possesses numerous health risks, which can include death.3[citation needed]
The glucose that is used for all of this to occur is consumed into the body through different food sources. This food can be in the form of carbohydrates such as; monosaccharides, disaccharides, or polysaccharides. Monosaccharides are the simplest of the three sugars for the body to metabolize, but it yields the least amount of energy. Disaccharides take longer to break down than monosaccharides, but they give off more energy. Polysaccharides take the longest to digest and they emit the highest amount of energy.2[citation needed] Since polysaccharides take a while to digest, yet they yield the highest amount of energy, it is common for people to load up on them before performing strenuous activities that require lots of energy. An example of a polysaccharide would be starch.
During the course of cellular respiration, redox reactions are constantly occurring. Redox reactions include reduction reactions, the gaining of electrons, and oxidation reactions, the loss of electrons. The reduction of coenzymes called NAD+ and FAD into NADH and FADH2 is a pivotal reaction in the cellular respiration process. This reduction allows these coenzymes to shuttle electrons over to the electron transport system where they will be used to synthesize ATP.

## See also/Sources

* Mheducation.com - PDF of Cellular Respiration
* Simplified version of Cellular Respiration - Atcovi/Science/Cellular Respiration
* Introduction to cellular respiration | Cellular respiration | Biology | Khan Academy
* Glycolysis - Khan Academy