# Cell respiration

The cellular respiration is a metabolic process , in which by oxidation of organic substances energy is recovered. In the most common case, aerobic respiration, which occurs in all eukaryotes and in many prokaryotes , oxygen (O 2 ) is used as an oxidizing agent . Carbon dioxide (CO 2 ) and water (H 2 O) are released as end products . The energy gained is available in the form of adenosine triphosphate (ATP) for energy-consuming life processes.

In prokaryotes there is also anaerobic respiration, in which other substances serve as oxidizing agents instead of oxygen.

## Overview

Cell respiration corresponds, albeit only formally, to a 'cold' combustion. The process takes place in many small individual steps, and a large part of the energy released can be stored. The most commonly used 'to burn' substance in the organism is glucose (grape sugar). The sum equation in this case is:

${\ displaystyle \ mathrm {C_ {6} H_ {12} O_ {6} +6 \ O_ {2} \ longrightarrow 6 \ CO_ {2} +6 \ H_ {2} O}}$
One molecule of glucose and six molecules of oxygen become six molecules of carbon dioxide and six molecules of water.

Glucose is oxidized to carbon dioxide and oxygen is reduced to water at the same time - a redox reaction broken down into many individual steps . A redox reaction can formally be regarded as a transfer of electrons , whereby the oxidation of one reaction participant represents a release of electrons and the reduction of another participant represents an uptake of electrons. At the same time, however, there are also transfers of protons , which in total - but separately in the process - corresponds to a transfer of hydrogen .

## procedure

Cell respiration includes the following sub-processes:

1. the glycolysis ,
2. the citric acid cycle and
3. the end oxidation in the respiratory chain .

### Glycolysis

Glycolysis

Main article: glycolysis

Glycolysis (= sugar breakdown) takes place in the cytoplasm . During this process, glucose is split. First, there is a double phosphorylation , whereby glucose-6-phosphate is formed first, which is converted into fructose-1,6-bisphosphate via fructose-6-phosphate . For these processes, 2 molecules of ATP are dephosphorylated to 2 molecules of ADP. Due to the phosphorylation, the sugar is now in an activated state. This C 6 body is then split into two C 3 bodies, one molecule of dihydroxyacetone phosphate (DHAP) and one molecule of glyceraldehyde-3-phosphate (GAP). Only the glyceraldehyde-3-phosphate is broken down further, which is why the DHAP is isomerized into it.

Another molecule of inorganic phosphate is attached and GAP is oxidized, producing 1,3-bisphosphoglycerate (1,3bPG). The electrons are transferred to the hydrogen carrier NAD + ( nicotinamide adenine dinucleotide ). In the next step, a phosphate residue (P i ) is transferred to ADP, so that ATP and glyceric acid 3-phosphate (3-PG) are formed. 3-PG is isomerized to glyceric acid 2-phosphate (2-PG). Phosphoenolpyruvate (PEP) is formed from it by splitting off water . In this last step of glycolysis, the last residue of phosphate is also transferred to ADP, creating pyruvate and ATP. On the way from GAP to pyruvate, two molecules of ATP are formed per molecule of GAP by phosphorylation of ADP.

Glycolysis net balance:

${\ displaystyle \ mathrm {Glucose + 2 \ NAD ^ {+} + 2 \ ADP + 2 \ P_ {i} \ longrightarrow}}$ ${\ displaystyle \ mathrm {2 \ Pyruvat + 2 \ NADH + 2 \ mathbb {H} ^ {+} + 2 \ ATP + 2 \ H_ {2} O}}$

### Citric acid cycle

Main article: Citric acid cycle

The citric acid cycle takes place in the matrix of the mitochondria in eukaryotes and in the cytoplasm in prokaryotes . It is named after the first intermediate product, citrate , the anion of citric acid.

First, the pyruvate from glycolysis is converted into acetate , the anion of acetic acid , by oxidative decarboxylation , whereby CO 2 is split off (decarboxylation) and 2 H atoms are transferred to NAD + (redox reaction). The acetate is bound to coenzyme A (CoA), so that acetyl-CoA is formed.

Balance of the oxidative decarboxylation of pyruvate:

${\ displaystyle \ mathrm {2 \ pyruvate + 2 \ NAD ^ {+} + 2 \ CoA \ longrightarrow}}$ ${\ displaystyle \ mathrm {2 \ Acetyl {\ text {-}} CoA + 2 \ CO_ {2} +2 \ NADH + 2 \ H ^ {+}}}$
Citric acid cycle

The acetyl-CoA is then introduced into the citric acid cycle by condensing with oxaloacetate to citrate - by absorbing water and splitting off coenzyme A. In the process, coenzyme A is regenerated again. This is followed by two further oxidative decarboxylations. In the first, CO 2 is split off and hydrogen is taken over by the hydrogen carrier NAD + (formation of NADH), so that α-ketoglutarate is formed. In the next step, CO 2 is split off again with the help of coenzyme A and hydrogen is transferred to NAD + . The following steps are only used to regenerate oxaloacetate so that the cycle can start all over again. This is done via the molecules succinyl-CoA , succinate , fumarate and malate .

Balance of the citric acid cycle (runs twice per molecule of glucose, since 2 moles of pyruvate and thus also 2 moles of acetyl-coenzyme A are formed from 1 mole of glucose):

${\ displaystyle \ mathrm {2 \ Acetyl {\ text {-}} CoA + 6 \ NAD ^ {+} + 2 \ FAD + 2 \ GDP + 2 \ P_ {i} +4 \ H_ {2} O \ longrightarrow }}$ ${\ displaystyle \ mathrm {4 \ CO_ {2} +6 \ NADH + 6 \ H ^ {+} + 2 \ FADH_ {2} +2 \ GTP + 2 \ CoA}}$

### Respiratory chain

The previous process resulted in 4 ATP. Most of the ATP yield, however, is provided by the respiratory chain through oxidation of the hydrogen atoms bound to the hydrogen carriers NAD and FAD with oxygen (O 2 ). A total of 10 NADH (2 from glycolysis, 2 from oxidative decarboxylation and 6 (2 times 3) from the citric acid cycle) and 2 FADH 2 ( flavin adenine dinucleotide ) are available, i.e. 24 reduction equivalents .

An NADH releases 2 electrons (e - ), whereby the hydrogen bound to the NAD is released as a proton (H + ) and the remaining NAD molecule is positively charged: NAD + . Because the 2 electrons released in this way are at a very high energy level (very low redox potential of the redox couple NADH / NAD + ), 10 protons can be transported from the matrix into the intermembrane space with their help. This happens as follows: The 2 electrons of the NADH reduce the first complex ( complex I ) of several enzyme complexes of the respiratory chain, which are located between the matrix and the intermembrane space of the mitochondrion. Each electron is then passed on from one enzyme complex to the next via redox reactions. Due to the transfer of electrons from complex to complex, this process is also known as the electron transport chain. Through complex I, complex III and complex IV , H + ions (protons) are transported from the matrix into the intermembrane space. This creates a high concentration of hydrogen ions in the intermembrane space, which results in a pH value below 7 and an osmotic potential. The redox reactions and the creation of the osmotic potential together are called chemiosmosis : The redox reactions are chemical reactions, the difference in the H + concentration of the matrix and the intermembrane space represents an osmotic potential.

The protons finally flow back through the membrane-bound ATP synthase from the intermembrane space into the matrix space. This enzyme catalyzes the synthesis of ATP from a phosphate residue and ADP . The flow energy contained in the proton motor force is used to release the ATP synthase that has formed. The transport of a molecule of ADP from the cytoplasm into the matrix or, conversely, the transport of a molecule of ATP into the cytoplasm is catalyzed by an ATP / ADP translocase . For this transport, however, the proton gradient is also tapped, so that a proton is used up for the availability of ATP or ADP. This means that at least 4 protons must be calculated to generate one molecule of ATP.

The oxidation of one NADH thus produces 2.5 ATP. The two NADH from glycolysis are an exception. These are still in the cytoplasm and must first be transported into the mitochondria. If this is done with the help of the glycerine-3-phosphate shuttle , only 1.5 ATP are obtained from each of these. Since 8 + 2 NADH are oxidized, a total of 8 × 2.5 + 2 × 1.5 = 23 ATP is produced. If, however, cytosolic NADH is brought into the matrix by the malate-aspartate shuttle , 2.5 mol of ATP can be generated from it per reduction equivalent. This means that a maximum of 10 × 2.5 = 25 ATP can be generated.

With FADH 2 , the process is basically the same, only FADH 2 releases electrons at a higher redox potential and thus a lower energy level. Its electrons can only be introduced into the respiratory chain at an energetically lower level. Therefore, with the help of the two electrons of FADH 2, only 6 protons (instead of 10 protons as with NADH) can be pumped out of the matrix into the intermembrane space. As a result, only 1.5 ATP are formed with one FADH 2 . Since two FADH 2 are oxidized, 3 ATP are created.

The protons and electrons of NADH and FADH 2 (24 in total) are oxidized to 12 H 2 O together with 6 molecules of O 2 , which are transported through the membrane into the mitochondrial matrix. The electron and hydrogen carriers NAD + and FAD can be reduced again to NADH or FADH 2 by taking up 2 e - and 2 H + .

Balance of the respiratory chain (with maximum ATP yield through malate-aspartate shuttle):

${\ displaystyle \ mathrm {10 \ NADH + 10 \ H ^ {+} + 2 \ FADH_ {2} +28 \ ADP + 28 \ P_ {i} +6 \ O_ {2} \ longrightarrow}}$ ${\ displaystyle \ mathrm {10 \ NAD ^ {+} + 2 \ FAD + 12 \ H_ {2} O + 28 \ ATP}}$

### Energy balance

step Coenzyme Yield ATP yield ATP source
Glycolysis preparatory stage −2 Energy required to break down glucose into 2 molecules of glyceraldehyde-3-phosphate
Glycolysis yield level 4th Substrate chain phosphorylation
2 NADH 3 or 5 Oxidative phosphorylation (3 when using the glycerine-3-phosphate shuttle or 5 when using the malate-aspartate shuttle)
Oxidative decarboxylation 2 NADH 5 Oxidative phosphorylation
Citric acid cycle 2 Substrate chain phosphorylation (in the form of GTP)
6 NADH 15th Oxidative phosphorylation
2 FADH 2 3 Oxidative phosphorylation
Total yield 30 or 32 ATP per molecule of glucose

Since prokaryotes have no cell compartments, they do not have to spend energy for intracellular transport processes and can obtain 36 to 38 moles of ATP from one mole of glucose.

## literature

• Reginald Garrett and Charles M. Grisham: Biochemistry . 3rd edition (International Student Edition). Thomsom Learning Inc., 2005, ISBN 0-534-41020-0 , pp. 640-674
• Geoffrey Zubay: biochemistry . 4th edition. Mcgraw-Hill Professional, 1999, ISBN 3-89028-701-8
• Donald Voet and Judith G. Voet: Biochemistry . Wiley-VCH, 1994, ISBN 3-527-29249-7 , pp. 420ff.
• H. Robert Horton, Laurence A. Moran, K. Gray Scrimgeour, Marc D. Perry, J. David Rawn and Carsten Biele (translators): Biochemistry . 4th updated edition. Pearson Studies, 2008, ISBN 978-3-8273-7312-0
• David L. Nelson, Michael M. Cox, Albert L. Lehninger (first): Lehninger Biochemie . 4th, completely revised u. exp. Edition. Springer, Berlin 2009, ISBN 978-3-540-68637-8

## Individual evidence

1. a b c d Berg, Stryer, Tymoczko: Biochemistry . Spectrum Academic Publishing House, 2007, ISBN 978-3-8274-1800-5 .
2. ^ A b Reginald Garrett and Charles M. Grisham: Biochemistry . (International Student Edition). Thomsom Learning Inc .; 3rd edition 2005; ISBN 0-534-41020-0 ; P. 669f.