Aerobic respiration is a process that generates ATP, also known as energy, by using oxygen and glucose. Glucose is a respiratory substrate that cells break down to produce energy in the form of ATP. The crucial thing to remember is that aerobic respiration needs oxygen to happen. This is unlike anaerobic respiration, which doesn't require oxygen and produces less ATP.

Where does aerobic respiration take place?

In animal cells, three of the four stages of aerobic respiration take place in the mitochondria. Glycolysis occurs in the cytoplasm, which is the liquid that surrounds the cell’s organelles. The link reaction, the Krebs cycle and oxidative phosphorylation all take place within the mitochondria.

Figure 1 shows how the structure of the mitochondria explains its role in aerobic respiration. The mitochondria has two membranes, an outer and an inner one, which create five distinct components. Each of these components helps with aerobic respiration in a different way. Here are the main adaptations of the mitochondria:

1. The outer membrane creates the intermembrane space, which holds protons that are pumped out of the matrix by the electron transport chain. This is essential for oxidative phosphorylation.
2. The inner membrane contains ATP synthase, which helps convert ADP to ATP. It also organises the electron transport chain.
3. The cristae are infoldings of the inner membrane that expand the surface area of the mitochondria. This helps it produce ATP more efficiently.
4. The matrix is where ATP synthesis takes place and where the Krebs cycle occurs.

By having these adaptations, the mitochondria can effectively carry out its role in aerobic respiration.

How does aerobic respiration occur in humans and animals?

There are four stages of aerobic respiration.

Glycolysis

Glycolysis occurs in the cytoplasm and involves splitting a single 6-carbon glucose molecule into two 3-carbon pyruate molecules. There are four stages in glycolysis, which involve multiple, smaller, enzyme-controlled reactions:

1. Phosphorylation of glucose - Glucose needs to be made more reactive before splitting into two 3-carbon pyruvate. molecules are added through phosphorylation. This stage lowers the activation energy for the next enzyme-controlled reaction by splitting two ATP molecules into two ADP molecules and two inorganic phosphate molecules (Pi) using hydrolysis.
2. Splitting of phosphorylated glucose - Each glucose (with the two added Pi groups) is split into two, forming two molecules of triose phosphate, a 3-carbon molecule.
3. Oxidation of triose phosphate - Hydrogen is removed from both triose phosphate molecules and transferred to a hydrogen-carrier molecule, NAD+. This forms reduced NAD or NADH.
4. ATP production - The two newly oxidised triose phosphate molecules are then converted to another 3-carbon molecule known as pyruvate. This process regenerates two ATP molecules from two molecules of ADP.

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The link reaction

During the link reaction, the 3-carbon pyruvate molecules produced during glycolysis are actively transported into the mitochondrial matrix, where they undergo a series of reactions:

1. Oxidation - Pyruvate is oxidised into acetate. During this reaction, pyruvate loses one of its carbon dioxide molecules and two hydrogens. NAD takes up the spare hydrogens, producing reduced NAD.
2. Acetyl Coenzyme A production - The new 2-carbon molecule formed from pyruvate, called acetate, combines with a molecule called coenzyme A to form 2-carbon Acetyl Coenzyme A (acetyl-CoA).

This link reaction prepares the acetyl-CoA molecule for entry into the Krebs cycle, where it will be further oxidised to release energy.

The Krebs cycle

The Krebs cycle, also known as the citric acid cycle, is the most complex of the four reactions involved in cellular respiration. It is named after the British biochemist Hans Krebs and takes place in the mitochondrial matrix. The Krebs cycle consists of a sequence of redox reactions that can be summarised in three steps:

1. The 2-carbon acetyl coenzyme A, which was produced during the link reaction, combines with a 4-carbon molecule to form a 6-carbon molecule.
2. This 6-carbon molecule then loses a carbon dioxide molecule and a hydrogen molecule through a series of different reactions, producing a 4-carbon molecule and a single ATP molecule as a result of substrate-level phosphorylation.
3. This 4-carbon molecule is then regenerated and can combine with a new 2-carbon acetyl coenzyme A, allowing the cycle to begin again.

reactions also result in the production of ATP, reduced NAD, and FAD as by-products.

Oxidative phosphorylation

The final stage of aerobic respiration is the electron transport chain, which is responsible for producing the majority of ATP during cellular respiration. During this stage:

1. Reduced coenzymes such as reduced NAD and FAD donate the electrons that hydrogen atoms are carrying to the first molecule of the electron transfer chain.
2. These electrons move along the electron transfer chain using carrier molecules. A series of redox reactions (oxidation and reduction) occur, and the energy that these electrons release causes the flow of H+ ions across the inner mitochondrial membrane and into the intermembrane space. This establishes an electrochemical gradient in which H+ ions are flowing from an area of higher concentration to an area of lower concentration.
3. The H+ ions build up in the intermembrane space. They then diffuse back into the mitochondrial matrix through the enzyme ATP synthase, a channel protein with a channel-like hole that protons can fit through.
4. As the electrons reach the end of the chain, they combine with these H+ ions and oxygen, forming water. Oxygen acts as the final electron acceptor.
5. ADP and Pi combine in a reaction catalysed by ATP synthase to form ATP.

Overall, the electron transport chain produces a large amount of ATP by using the energy released from the movement of electrons to establish a proton gradient across the inner mitochondrial membrane. This gradient is then used to ATP through the action synthase, with the final electron acceptor in the process.

Aerobic Respiration - Key Takeaways

The overall equation for aerobic respiration is:

glucose + 6O2 -> 6CO2 + 6H2O + 36-38 ATP

During glycolysis, glucose is converted into two molecules of pyruvate, producing a net gain of two molecules of ATP. In the link reaction, pyruvate is converted into acetyl coenzyme A and carbon dioxide is released. The acetyl coenzyme A then enters the Krebs cycle, where it undergoes a series of redox reactions to produce ATP, reduced NAD, and FAD as byproducts. Finally, during oxidative phosphorylation (the electron transport chain), the reduced NAD and FAD produced during the Krebs cycle their electrons to the electron transport chain, which produces a large amount of ATP through chemiosmosis. The end result of aerobic respiration is the production of water, carbon dioxide, and a total of 36-38 molecules of ATP.

Aerobic Respiration

What is aerobic respiration?

Aerobic respiration refers to the metabolic process in which glucose and oxygen are used to form ATP. Carbon dioxide and water are formed as a byproduct.

Where in the cell does aerobic respiration occur?

Aerobic respiration occurs in two parts of the cell. The first stage, glycolysis, occurs in the cytoplasm. The rest of the process occurs in the mitochondria.

What are the main steps of aerobic respiration?

The main steps of aerobic respiration are as follows: Glycolysis involves the splitting of a single, 6-carbon glucose molecule into two 3-carbon pyruvate molecules. The link reaction, in which the 3-carbon pyruvate molecules undergo a series of different reactions. This leads to the formation of acetyl coenzyme A, which has two carbons. The Krebs cycle is the most complex of the four reactions. Acetyl coenzyme A enters into a cycle of redox reactions, which results in the production of ATP, reduced NAD, and FAD. Oxidative phosphorylation is the final stage of aerobic respiration. It involves taking the electrons released from the Krebs cycle (attached to reduced NAD and FAD) and using them to synthesise ATP, with water as a by-product. 

What is the equation for aerobic respiration?

Glucose + Oxygen ----> Water + Carbon dioxide

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