A Beginner's Guide to Aerobic Cellular Respiration and Its Stages

Aerobic Cellular Respiration
Aerobic cellular respiration is a part of cellular respiration, and it plays an important role in producing the energy that is required for various functions of a cell.
All organisms are made up of tiny cells which carry out various functions. Energy is required for processing these functions. This energy is provided by the cells, and is produced when the cells break down the chemical composition of food molecules and convert them into energy, i.e. convert glucose to energy. This is made possible with the process called cellular respiration, which takes place in the mitochondrion - the power house of a cell. During this process, cells break down glucose molecules and release energy. This energy that is released from the glucose is used to produce ATP. Therefore, cellular respiration is the process by which energy from glucose is transferred to ATP. It is a part of metabolism and all organisms go through cellular respiration.

Cellular respiration is of two types - anaerobic respiration and aerobic respiration. Here, we shall discuss aerobic respiration.

Aerobic Cellular Respiration
Cellular respiration is vital for the survival of all organisms, as energy from food (glucose) cannot be used by a cell until it is converted to ATP. Hence, it is a continuous cycle that takes place in all organisms. Aerobic respiration plays a crucial role in the production of ATP, where glucose and oxygen are vital elements. This process takes place only if oxygen is available. Take a look at the chemical formula given here.

C6H12O6 + 6O2 = 6CO2 + 6H2O + Energy (ATP)

In simple words - Glucose + Oxygen = Carbon Dioxide + Water + Energy (ATP)

Three Stages of Aerobic Respiration

Aerobic Cell Respiration

Aerobic respiration takes place in three phases - Glycolysis, Krebs Cycle, and Oxidative Phosphorylation (also called electron transport chain). The end result of these stages is ATP.

ATP is an abbreviation for Adenosine-5'-triphosphate, composed of: 3 phosphate groups, 5 carbon sugar (also called ribose), and Adenine. It is a multifunctional nucleotide or a chemical compound that releases energy to help perform important functions in a cell.

Glycolysis

The process of Glycolysis (glyco means 'sugar' and lysis means 'breaking' or 'to split') takes place in the cytosol or cytoplasm of a cell. This process can take place without oxygen. The objective in this process is to break down glucose and form ATP, NADH and pyruvates (pyruvates or pyruvic acid is the end product of glycolysis, which can be converted to different biomolecules). Glycolysis uses 2 ATP molecules as energy for fueling this entire process.
In this stage, glucose is partially oxidized. 1 molecule of glucose (C6H12O6) is broken down into two molecules of 3 carbon sugar. 2 NAD are added to these carbon sugar molecules. Simultaneously, a phosphate group is also added to each 3 carbon molecule.
Thus this process of glycolysis produces energy - 2 ATP (net) molecules, 2 NADH (nicotinamide adenine dinucleotide), and 2 pyruvates. Each NADH molecule carries 2 energy electrons. The cells later use these electrons. The main purpose of NADH electrons is to transport electrons to the electrons transfer chain, for more energy to be harvested from them.

Therefore, at the end of glycolysis, we have: Glucose ---- 2 pyruvates + 2 ATP (net) + 2 NADH

Krebs Cycle

This is the next stage of aerobic cellular respiration. This process takes place in the mitochondria of a cell. With a net gain of 2 ATP only in the previous stage, that is 'glycolysis', there is the need to harvest more energy. Hence, the main objective of this stage is to use the pyruvates to produce more ATP. It is in this stage that oxygen plays a vital role. The first process aims at converting pyruvate in a chemical form that will help it enter the next stage.

Pyruvate enters the mitochondrion, in this stage it also loses an atom of carbon, which is released as carbon dioxide.
NAD is reduced to NADH, after losing a carbon atom.
Now an enzyme called CoA, (an enzyme involved in the metabolism of carbon sugars), joins the remaining 2 carbon molecules in pyruvate.
After this fusion, a molecule called acetyl-CoA (also known as the activated form of acetic acid) is formed.
Now this molecule enters the citric acid cycle. The 2 carbon atoms in acetyl-CoA combine with 4 more carbon atoms, which are already present in this cycle. So, we have a total of 6 carbon atoms, 2 from acetyl-CoA and 4 that were already present. These 6 atoms form citric acid.
2 NAD (that were produced from breaking down of glucose in glycolysis), further get reduced and form 2 NADH. Here, we lose 2 more atoms of carbon (out of 6 in citric acid), which is also released as carbon-dioxide.
Now a process called substrate-level phosphorylation occurs. Phosphoryl (PO3) or phosphate is added to ADP. This converts ADP (adenosine diphosphate) to ATP (adenosine triphosphate).
In the next set of chemical reactions, the remaining 4 carbon atoms (out of 6 atoms, 2 were released as carbon-dioxide) are re-synthesized. This leads to another NAD present in the cycle to form NADH and FAD, which forms FADH2. We now have 1 ATP, NADH and FADH2.
Each CA (cycle) uses 1 pyruvate out of the 2 pyruvates formed during glycolysis. So, this means 2 cycles of CA take place for a breakdown of 2 pyruvates.
At the end of this cycle, we have a total of 4 ATP - 2 from glycolysis and 2 from the citric acid cycle or Krebs cycle.

Electron Transport Chain

This is the final stage of the aerobic cellular respiratory cycle. During glycolysis and the Krebs Cycle, the entire energy is not released from the glucose. In this stage of aerobic respiration, the remaining energy from the glucose is released by the electron transport chain. The electrons are stepwise transported in a pathway, which is termed as the electron transport chain.
From the Krebs Cycle and glycolysis, we have a total of 4 ATP, 2NADH and 2FADH2. In this step, the 2 NADH and 2 FADH2 work with the enzymes, and a process called oxidation reduction takes place. Here, NADH and FADH2 (we can call them electron donors, in this stage) contribute their electrons to the enzymes (electron acceptors) (already present in the membrane of a cell) through an electrochemical gradient or path. This is termed as the electron transport system.
After this, the NADH and FADH2 lose their electrons and are reduced to NAD and FAD. These return for processing again to the Krebs Cycle or citric cycle.
The electrons lose some part of their energy as protons (hydrogen ions), that are pumped in the inter membrane space of outer mitochondrion. This spins a gradient of protons formed by the release of hydrogen ions in the inter membrane space. It is this gradient of protons that fuels the synthesis of ATP.
How is this done? Well, NADH and FADH, both lose electrons, in the mitochondrion, thus lowering energy (H+) concentration in the mitochondrion. In the outer compartment of the membrane or inter-membrane space, constant formation of protons (hydrogen ions) is taking place. This creates high concentration of H+ (protons) in the inter membrane space.
This state of high and low energies in the cell has a very high potential of producing energy. This enables them to travel from the high energy gradient (outer membrane) to the low energy gradient which is the mitochondrion. In this process, they pass through the ATP synthase.
ATP synthase (also called F1 particle) harnesses this potential energy of the protons, and a process called oxidative phosphorylation takes place. This helps the conversion of ADP to ATP, which is called chemiosmosis.
Oxygen plays a major role in aerobic cellular respiration, because it is a great electron acceptor. It plays an active role in preventing the electrons from building up in the electron transport system. Oxygen draws the electrons from the last stage of the electron transport system. So, the electrons combine with the protons and form hydrogen. This further combines with oxygen which produces water (H2O).
Every 2 electrons donated by NADH passing through F1 (ATP synthase) creates 1 molecule of ATP. Therefore, each NADH that passes 6 electrons in the electron transport chain, gives us 3 ATP.
Similarly, FADH2 donates 4 electrons in the electron transport chain. This is because, FADH2 enters the electron transport system later than or after NADH donates electrons. So it generates less energy. From the 4 electrons that it donates, 2 ATP is produced.
The maximum number of ATP is generated by the electron transport chain through chemiosmosis (i.e. the process by ATP synthase). It gives the cells a total of 32 - 34 ATP.

A point worth mentioning here is, while glycolysis takes place in the cytoplasm of a cell, the Krebs Cycle and electron transport takes place in the mitochondria of a cell. Also, oxygen is the most important component of aerobic cellular respiration. Without oxygen, the electrons will remain stagnant in the electron transport chain, putting the production of ATP at halt. Eventually, the cell will die, and the organism too! Hence, aerobic respiration is a vital process for cell functioning, and the life of an organism.
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