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Aerobic Respiration

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Biology

During aerobic respiration, ATP is generated from oxygen and glucose. Glucose is known as a respiratory substrate, and is broken down by cells to produce energy in the form of ATP.

The key part of aerobic respiration is that it requires oxygen to occur. It is different from anaerobic respiration, which does not require oxygen to occur and produces far 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. Mitochondria struture.

As displayed in Fugire 1, the mitochondria’s structural features help to explain its role in aerobic respiration. The mitochondria has an inner membrane and an outer membrane. This double membrane structure creates five distinct components within the mitochondria, and each of these aids aerobic respiration in some way. We will outline the main adaptations of the mitochondria below:

  • The outer mitochondrial membrane allows for the establishment of the intermembrane space.
  • The intermembrane space enables the mitochondria to hold protons that are pumped out of the matrix by the electron transport chain, which is a feature of oxidative phosphorylation.
  • The inner mitochondrial membrane organises the electron transport chain, and contains ATP synthase which helps convert ADP to ATP.
  • The cristae refers to the infoldings of the inner membrane. The cristae’s folded structure helps to expand the surface area of the inner mitochondrial membrane, which means that it can produce ATP more efficiently.
  • The matrix is the site of ATP synthesis, and is also the location of the Krebs cycle.

How does aerobic respiration occur in humans and animals?

There are four stages of aerobic respiration.

Glycolysis

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

  1. Phosphorylation of glucose - Prior to being split into two 3-carbon pyruvate molecules, glucose needs to be made more reactive. This is done through adding two phosphate molecules, which is why this step is referred to as phosphorylation. We get the two phosphate molecules from splitting two ATP molecules into two ADP molecules and two inorganic phosphate molecules (Pi). This is done via hydrolysis, which means that water is used to split ATP. This then provides the energy needed to activate glucose, and lowers the activation energy for the next enzyme-controlled reaction.
  2. Splitting of phosphorylated glucose - In this stage, each glucose molecule (with the two added Pi groups) is split into two. This forms two molecules of triose phosphate, a 3-carbon molecule.
  3. Oxidation of triose phosphate - Once these two triose phosphate molecules are formed, hydrogen is removed from them both. These hydrogen groups are then transferred to a hydrogen-carrier molecule, NAD+. This forms reduced NAD or NADH.
  4. ATP production - Both of the triose phosphate molecules, newly oxidised, are then converted into another 3-carbon molecule known as pyruvate. This process also regenerates two ATP molecules from two molecules of ADP.

The overall equation for glycolysis is:

The link reaction

During the link reaction, the 3-carbon pyruvate molecules produced during glycolysis undergo a series of different reactions after being actively transported into the mitochondrial matrix. The following reactions are:

  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 and reduced NAD is produced. The new 2-carbon molecule formed from pyruvate is called acetate.
  2. Acetyl Coenzyme A production - Acetate then combines with a molecule called coenzyme A, which is sometimes shortened to CoA. 2-carbon Acetyl Coenzyme A is formed.

Overall, the equation for this is:

The Krebs cycle

The Krebs cycle is the most complex of the four reactions. Named after the British biochemist Hans Krebs, it features a sequence of redox reactions that occur in the mitochondrial matrix. The reactions 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. This produces a 6-carbon molecule.
  2. This 6-carbon molecule loses a carbon dioxide molecule and a hydrogen molecule through a series of different reactions. This produces a 4-carbon molecule and a single ATP molecule. This is a result of substrate-level phosphorylation.
  3. This 4-carbon molecule has been regenerated and can now combine with a new 2-carbon acetyl coenzyme A, which can begin the cycle again.

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

Oxidative phosphorylation

This is the final stage of aerobic respiration. The hydrogen atoms released during the Krebs cycle, along with the electrons they possess, are carried by reduced NAD and FAD (cofactors involved in cellular respiration) into an electron transfer chain. The following stages occur:

  1. After the removal of hydrogen atoms from various molecules during glycolysis and the Krebs cycle, we have a lot of reduced coenzymes such as reduced NAD and FAD.
  2. These reduced coenzymes donate the electrons that these hydrogen atoms are carrying to the first molecule of the electron transfer chain.
  3. 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.
  4. 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.
  5. 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, and ADP and Pi combine in a reaction catalysed by ATP synthase to form ATP.

The overall equation for aerobic respiration is the following:

Aerobic Respiration - Key Takeaways

  • Aerobic respiration occurs in the mitochondria and the cytoplasm of the cell. It is a type of respiration that requires oxygen to occur, and produces water, carbon dioxide and ATP.
  • There are four stages to aerobic respiration: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation.
  • The overall equation for aerobic respiration 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.

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.

The main steps of aerobic respiration are as follows: 


  1. Glycolysis involves the splitting of a single, 6-carbon glucose molecule into two 3-carbon pyruvate molecules.
  2. 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. 
  3. The Krebs cycle is the most complex of the four reactions. Acetylcoenzyme A enters into a cycle of redox reactions, which results in the production of ATP, reduced NAD, and FAD. 
  4. 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.

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

Final Aerobic Respiration Quiz

Question

What are the names of the four stages of aerobic respiration?

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Answer

Glycolysis, the link reaction, the Krebs cycle, oxidative phosphorylation.

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Where does aerobic respiration take place?


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The cytoplasm and the mitochondria.

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What are the main products of aerobic respiration?


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Water, carbon dioxide, and ATP.

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What is the name of the 3-carbon molecule produced in glycolysis?


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Pyruvate.

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What is the name of the 2-carbon molecule produced in the link reaction?


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Answer

acetyl coezyme A

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What three molecules are produced during the Krebs cycle?


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ATP, reduced NAD, and reduced FAD.

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What two molecules are produced during oxidative phosphorylation?


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ATP and water.

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Where within the mitochondria does ATP synthesis occur?


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The matrix.

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Where within the mitochondria is the electron transport chain?


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The inner mitochondrial membrane.

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Where in the mitochondria are H+ ions pumped into?


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The intermembrane space.

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What is a structural advantage of cristae?


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The cristae’s folded structure expands the surface area of the inner mitochondrial membrane, which allows it to produce ATP more efficiently.

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What are the four stages of glycolysis?


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Phosphorylation of glucose, splitting of phosphorylated glucose, oxidation of triose phosphate, ATP production.

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When does glycolysis occur during respiration?

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Glycolysis is the first stage of respiration.

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Where in the cell does glycolysis occur?

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In the cytoplasm.

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Glycolysis is only necessary for aerobic respiration. 


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False

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What are the main products of glycolysis?


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Pyruvate, ATP, and NADH.

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How many ATP molecules are used for glycolysis?


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Two.

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What is the name of the sugar molecule that glucose is split into after it has been phosphorylated?


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Triose phosphate.

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How is NADH formed during glycolysis?


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One molecule of hydrogen is removed from each triose phosphate molecule. These hydrogen groups are then transferred to a hydrogen-carrier molecule, NAD. This forms reduced NAD/NADH.

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Is triose phosphate/glyceraldehyde-3-phosphate oxidised or reduced during glycolysis?


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Oxidised.

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Which of the enzymes listed below are not involved in glycolysis? 



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Maltase

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Which molecule provides the phosphate group in order to phosphorylate glucose during the first stage of glycolysis?


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ATP.

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How many ATP molecules does glycolysis use in total?


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Four.

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Why is it essential that NADH is produced during glycolysis?


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NADH produces more ATP during oxidative phosphorylation.

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What happens to pyruvate after glycolysis if oxygen is not present?


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Pyruvate stays in the cytoplasm and undergoes fermentation. It is turned into either ethanol or lactate.

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What happens to pyruvate after glycolysis if oxygen is present?


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It enters the mitochondrial matrix and enters the link reaction.

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Where does the link reaction take place in the cell?

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It takes place in the mitochondrial matrix.

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Where does the Krebs cycle take place in the cell?

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It takes place in the mitochondrial matrix.

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What is the main reactant and main product of the link reaction?


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Main reactant is pyruvate, main product is acetyl CoA.

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How many molecules of ATP are produced for every acetyl CoA molecule in the Krebs cycle?


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1

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How many molecules of ATP are produced during the link reaction?

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0

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What is the name of the 2-carbon molecule formed when pyruvate is decarboxylated?


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Acetate

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What is the name of the molecule that acetate combines with to form acetyl CoA?


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Coenzyme A

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For every glucose molecule broken down during aerobic respiration, what is produced during the link reaction?


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2 molecules of carbon dioxide, 2 molecules of acetyl CoA, and 2 molecules of NADH.

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What are NAD, FAD, and NADP all examples of? 


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Coenzymes

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What is the name of the 4-carbon molecule that acetyl CoA combines with in the first stage of the Krebs cycle?


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 Oxaloacetate

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What is the 6-carbon molecule formed from oxaloacetate and acetyl CoA?


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Citrate

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What is the name of the 5-carbon molecule that is formed from oxaloacetate?


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alpha-ketoglutarate

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 How many molecules of carbon dioxide are produced for every molecule of acetyl CoA during the Krebs cycle?


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Two molecules

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How many molecules of NADH are produced for every molecule of acetyl CoA during the Krebs cycle?


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3 molecules of the NADH

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What carries the potential source of energy in the electron transport chain?


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Reduced NAD and reduced FAD (NADH and FADH2).

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  • Transport of electrons by NADH and FADH 

  • Proton pumping and electron transfer

  • Formation of water 

  • ATP synthesis

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Question

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Oxygen acts as the terminal hydrogen atom acceptor. Without this, electrons are unable to be transported down the electron transport and will back up along the chain.

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Oxidation involves the initial loss of electrons from reduced NAD and reduced FADH2 to the protein complexes in the electron transport chain. Phosphorylation occurs when a phosphate group combines with ADP to generate ATP, which is driven by the flow of H+ through ATP synthase.

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Cristae describes the highly convoluted inner mitochondrial membrane. The advantages of this means the membrane can hold a greater amount of electron transport chain protein complexes and ATP synthase.

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Oxygen is required in oxidative phosphorylation and this process regenerates the coenzymes NAD+ and FAD. Without the regeneration of these coenzymes, the electron transport chain and the Krebs cycle cannot take place.

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Question

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Answer

Adenine 

Ribose 

3 phosphates

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Answer

ATP synthase. This uses the flow of H+ to facilitate the binding of ADP to Pi to generate ATP.

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Chemiosmosis produces ____ of ATP made during cellular respiration. 


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80%

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Within the inner mitochondrial membrane.

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60%

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