# Structural Biochemistry/Aerobic Respiration
## Introduction

In the aerobic respiration is a release of energy from glucose or another organic substate in the presence of Oxygen. Also, aerobic respiration is in the absent of air. Aerobic respiration is a process of cellular respiration that uses oxygen in order to break down molecules, which then release the electrons and also create the energy. It creates substances known as ATP. ATP stands for adenosine triphosphate. ATP's role is to store and carry most of the energy to other body cells. Besides, there are two main byproducts of aerobic respiration. They are water and carbon dioxide. Aerobic respiration is also contains three stages: glycolysis, Kreb's cycle, and the third stage is the electron transport phosphorylation.
The pyruvate produced in glycolysis undergoes further breakdown through a process called aerobic respiration in most organisms. This process need oxygen and yields much more energy than glycolysis. Aerobic respiration is separated into two processes: the Krebs cycle, and the Electron Transport Chain, which produces ATP through chemiosmotic phosphorylation. The energy conversion is as follows:
C6H12O6 + 6O -> 6CO2 + 6H2O + energy (38ATP)

## Krebs Cycle

The Krebs cycle occurs in the mitochondria of a cell. Before entering the Krebs cycle, the pyruvic acid molecules are altered. During the process, the pyruvic acid molecule is broken down by an enzyme, one carbon atom is released in the form of carbon dioxide, and the remaining two carbon atoms are combined with a coenzyme called the coenzymes. After the glycolysis takes place in the cell's cytoplasm, the pyruvic acid molecules travel into the interior of the mitochondrion. Once the pyruvic acid is inside, carbon dioxide is enzymatically detached from each three-carbon pyruvic acid molecule to form acetic acid. The enzyme then syndicates the acetic acid with coenzyme A to produce acetyl CoA. Once acetyl CoA is formed, the Krebs cycle begins. The cycle is split into eight steps, picture of this cycle is included in this link: http://notesforpakistan.blogspot.com/2011/02/krebs-cycle-or-citric-acid-cycle-or-tri.html
Prior to entering the Krebs Cycle, pyruvate must be converted into acetyl CoA (pronounced: acetyl coenzyme A). This is achieved by removing a CO2 molecule from pyruvate and then removing an electron to reduce an NAD+ into NADH. An enzyme called coenzyme A is combined with the remaining acetyl to make acetyl CoA which is then fed into the Krebs Cycle. 1. Citrate is created when the acetyl group from acetyl CoA combines with oxaloacetate from the previous Krebs cycle. 2. The citric acid molecule experience an isomerization. A hydroxyl group and a hydrogen molecule are detached from the citrate structure in the form of water. The two carbons form a double bond until the water molecule is added again. Only now, the hydroxyl group and hydrogen molecule are reversed with respect to the original structure of the citrate molecule. Thus, isocitrate is formed. 3. Isocitrate is oxidized to form the 5-carbon α-ketoglutarate. This step releases one molecule of CO2 and reduces NAD+ to NADH2+. 4. The α-ketoglutarate is oxidized to succinyl CoA, yielding CO2 and NADH2+. 5. Succinyl CoA releases coenzyme A and phosphorylates ADP into ATP. 6. Succinate is oxidized to fumarate, converting FAD to FADH2. 7. An enzyme adds water to the fumarate molecule to form malate. The malate is formed by adding one hydrogen atom to a carbon atom and then adding a hydroxyl group to a carbon next to a terminal carbonyl group. 8. Malate is oxidized to oxaloacetate, reducing NAD+ to NADH2+.
Unlike anaerobic respiration, in this process, where oxygen appears in sufficient amount, pyruvates are transported into mitochondria, where the largest ATP-producing facilities will provide energy for the cell. Aerobic Respiration consists of three main steps:
* Pyruvate dehydrogenase complex
* Krebs Cycle (Nitric Acid Cycle)
* Oxidative Phosphorylation Cycle (Electron Transport Chain cycle)

## Reference

http://incolor.inetnebr.com/mcanaday/Krebs%20Phases.htm http://www.cbu.edu/~seisen/AerobicRespiration.htm http://www.wisegeek.com/what-is-aerobic-respiration.htm