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AQA A2 Biology Unit 4: Chapter 4 - Respiration

A summary of Chapter 4 - Respiration in the AQA A2 Biology Unit 4 exam.

Ciaran Elliott

on 17 November 2012

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Transcript of AQA A2 Biology Unit 4: Chapter 4 - Respiration

The Link Reaction The Link Reaction occurs inside the matrix of a mitochondrion, and involves this equation... Glycolysis Glycolysis occurs in the cytoplasm of a cell, and involves the breakdown of a hexose (6-carbon sugar, like glucose) into two molecules of pyruvate.
It is broken into four main stages... Aerobic Respiration Aerobic Respiration is the breakdown of glucose using oxygen.
It produces water, carbon dioxide and a lot of ATP.
Aerobic Respiration is broken down into four main
stages... The Krebs' Cycle The Krebs' cycle involves a series of redox reactions which take place inside the matrix of the mitochondria. Using Acetyl CoA, it undergoes this cyclic reaction... The Electron Transport Chain (ETP) The ETP occurs on the folds of the mitochondrial membrane, called the cristae. The cristae are also in the matrix. The ETP involves a series of redox reactions to produce ATP... Anaerobic Respiration When oxygen is not available to a mitochondrion, the pathway of respiration changes. This is known as anaerobic respiration, and it occurs in these steps... The conversion of glucose into energy in the form of ATP. Respiration 1) Activation by phosphorylation of glucose:

Glucose is activated by the addition of two phosphate molecules, P'.
These come from the hydrolysis of ATP into ADP and P'. This occurs twice for two molecules of P'.
The addition of the two P' molecules lowers the activation energy for an enzyme to split the molecule. 2) Splitting of phosphorylated glucose:

Each glucose molecules is broken down into two molecules of triose phosphate. 3) Oxidation of triose phosphate:

Hydrogen is removed from each of the triose phosphate molecules and transferred to a hydrogen-carrier molecule.
This molecule is called NAD. NAD accepts the hydrogen molecule and is reduced.
This forms Reduced NAD. 4) The production of ATP:

Enzyme-controlled reactions breakdown triose phosphate into pyruvate.
Due to this, P' can bond with ADP to form ATP. Therefore, ATP is regenerated. Pyruvate + NAD + CoA --> Acetyl CoA + Reduced NAD + Carbon Dioxide Oxidation of Pyruvate:

Pyruvate is oxidised by the removal of hydrogen.
This hydrogen is accepted by NAD, forming Reduced NAD. Formation of Acetyl CoA and Carbon Dioxide:

A two carbon molecule called an acetyl group binds with Coenzyme A to form Acetyl Coenzyme A (Acetyl CoA).
A carbon dioxide molecule is removed from each pyruvate. Remember that this reaction occurs twice! This is because each glucose molecule produces two molecules of pyruvate! 2C Acetyl CoA + 4C molecule --> 6C Molecule:

Acetyl CoA from the Link Reaction, a 2-carbon molecule binds with a 4-carbon molecule to produce a 6-carbon molecule. 6C Molecule ---> 4C Molecule + Carbon Dioxide + Coenzymes + ATP:

The 6-carbon molecule is oxidised. It loses a molecule of carbon dioxide and hydrogen at each redox reaction.
When this 6-carbon molecule is oxidised, NAD accepts the hydrogen molecule which is lost, reducing it to Reduced NAD.
FAD is also reduced to Reduced FAD.
From these reactions, ADP and P' can bind to form a molecule of ATP. This is a result of substrate-level phosphorylation. Renewal of the Cycle:

The 4-carbon molecule formed reacts with another molecule of Acetyl CoA, beginning the cycle again. Remember! For every glucose molecule, two molecules of pyruvate are formed, meaning two molecules of Acetyl CoA are formed as well. Therefore, the Krebs' Cycle occurs twice for each molecule of glucose broken down by glycolysis! Products from the Krebs' Cycle:

The hydrogen atoms produced during glycolysis and the Krebs' cycle have combined with the coenzymes NAD and FAD, producing Reduced NAD and Reduced FAD. These move to the cristae, the folds of the inner mitochondrial membrane.
Reduced NAD and FAD donate the electrons of the hydrogen atoms to the first carrier in the ETP.
As the hydrogen have lost their electrons, they become positive hydrogen ions (H+), also called protons. These are actively transported across the membrane. Redox Reactions:

The electrons are transported across the ETP, via a series of redox reactions. This is known as oxidative phosphorylation. The first carrier, Carrier A is reduced.
The electron is passed from Reduced Carrier A to Carrier B. This reduces Carrier B, and oxidises Carrier A, returning it to its normal state to accept more electrons from the reduced coenzymes.
The electron is passed from Reduced Carrier B to Carrier C. This reduces Carrier C, and oxidises Carrier B, returning it to its normal state to accept electrons from Carrier A.
As this electron is passed from carrier to carrier, it loses energy. This energy allows P' and ADP to form ATP. Oxygen as the final electron acceptor:

Across the membrane, protons (H+ ions) accumulate, and are pumped back out.
Reduced Carrier C transfers its electrons to an atom of oxygen.
This allows the pumped out H+ ions to bind with oxygen to form water, completing the ETP. They bind via this equation...

4 Hydrogen Ions (H+) + 4 Electrons (e-) + Oxygen (O2) ---> 2 Water Molecules (H2O)

More ATP can be made. ATP synthase, an enzyme which binds ADP and P' is found in a protein complex, called the ATP synthase complex.
The proton motive force generated from the movement of H+ ions across the membrane pass energy along this complex, which allows ADP and P' to form ATP. (This is not required to know for A-Level.) Summary of the ETP:

R.NAD and R.FAD donate their hydrogen atoms from the Krebs' Cycle and Glycolysis.
The hydrogen atom is seperated into H+ and e-. The H+ ions are transported across the membrane, and the e- are transported along the membrane via electron carriers.
The electron undergoes a series of redox reactions as it passes from carrier to carrier. This is known as oxidative phosphorylation.
The electron loses energy from each redox reaction, and this energy is used to form ATP.
Oxygen is the final acceptor in the ETP. H+ ions are also pumped back through the membrane. The electrons and protons combine with oxygen to form water. 1) Glycolysis:

Glucose is broken down into two molecules of pyruvate, an acid.
Reduced NAD and ATP is produced from the reactions. 2) Build up of Pyruvate:

As oxygen is not present, the link reaction, nor the Krebs cycle can occur. In this case, pyruvate builds up.
However, the build up of pyruvate acts as an inhibitor to glycolysis. Therefore it must be removed to keep producing ATP. 3a) Pyruvate removal in animal cells:

Pyruvate is reduced by reduced NAD, and an enzyme converts pyruvate into lactate.
This process is known as lactation.
Lactate is lactic acid, and interferes with muscle contraction. This causes cramp.
Lactic acid is removed into the blood and transported into the liver, and is stored as glycogen.
If oxygen is available, lactate is oxidised back to pyruvate and undergoes aerobic respiration. 3b) Pyruvate removal in yeast and in some plant tissue:

Pyruvate is reduced by reduced NAD, and loses a carbon dioxide molecule. An enzyme converts the resulting product into ethanol.
This is known as fermentation.
Ethanol is poisonous to both yeast and plants, and must be excreted.
This is vital for the brewing industry, as yeast are kept in anaerobic conditions to produce ethanol for alcoholic drinks.
Anaerobic conditions can occur for plants too, as roots can become waterlogged. It is important to realise the energy yields from both aerobic and anaerobic respiration.

Anaerobic respiration only yields ATP from glycolysis via substrate-level phosphorylation.

Aerobic respiration yields ATP from Glycolysis and the Krebs' cycle via substrate-level phosphorylation, as well as ATP from the ETP via oxidative phosphorylation.

Overall, aerobic respiration has a higher energy yield than anaerobic respiration and is thus favoured by cells more.
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