Aerobic respiration is the process of using oxygen to break down a respiratory substrate, such as glucose, to produce ATP. There are four main stages to aerobic respiration and these are glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. This summary explains these stages and describes the overall ATP yield of aerobic respiration.

Link reaction

Following glycolysis, the two molecules of pyruvate are transported, by active transport, from the cytoplasm to the mitochondrial matrix. Once in the matrix they will be converted to the two-carbon compound acetyl coenzyme A (CoA). This occurs through the following processes.

Biology; Energy for biological processes; KS5 Year 12; Aerobic respiration — 1. Pyruvate, 2. Acetate, 3. Acetyl CoA

1.	Pyruvate is decarboxylated. The carbon that is removed from the pyruvate will be oxidised to form $CO_2$ .
2.	Pyruvate is then dehydrogenated. Two hydrogen atoms are removed from pyruvate and are transferred to NAD. This produces reduced NAD.
3.	The two-carbon acetate combines with coenzyme A (CoA) to form acetyl coenzyme A (CoA).

Substrates other than glucose can also be used in aerobic respiration.

Example

Fatty acids and amino acids can be converted into acetyl CoA that can enter the Krebs cycle.

Krebs cycle

Following the link reaction, the Krebs cycle takes place in the mitochondrial matrix.

1.	Acetyl CoA enters the Krebs cycle.
2.	Each acetyl CoA combines with a four-carbon compound (oxaloacetate) to produce a six-carbon compound (citrate/citric acid).
3.	The six-carbon compound is decarboxylated as $CO_2$ is removed. It is also dehydrogenated as it loses two hydrogen atoms. The product of this step is a five-carbon compound ( $\alpha$ -ketogluterate).
4.	The two hydrogen atoms are accepted by NAD to form reduced NAD.
5.	The five-carbon compound is then converted to a four-carbon compound (oxaloacetate) as it is decarboxylated by the loss of $CO_2$ .
6.	ATP is produced through the substrate-level phosphorylation whereby a phosphate group is directly transferred from an intermediate compound to ADP.
7.	The five-carbon compound is also dehydrogenated as it loses six hydrogen atoms. Four of the hydrogen atoms form two molecules of reduced NAD and two of the hydrogen atoms convert FAD (flavine adenine dinucleotide) into reduced FAD.
8.	Oxaloacetate then recombines with acetyl CoA and the cycle starts again.

Note: Each glucose provides two pyruvate and therefore two acetyl CoA molecules. Therefore, this cycle turns twice for every molecule of glucose.

Oxidative phosphorylation

Oxidative phosphorylation uses the reduced NAD (or reduced FAD) from the Krebs cycle, the electron transport chain and chemiosmosis to synthesise ATP.

1.	Mitochondrial matrix
2.	Inner mitochondrial membrane
3.	Inter-membrane space
A.	Carrier A
B.	Carrier B
C.	Carrier C
D.	ATP synthase

1.	The reduced coenzyme (reduced NAD or FAD) pass their hydrogen atoms to an electron carrier fixed in the inner mitochondrial membrane.
2.	The hydrogen atom is split into a $H^+$ ion and an electron. The electron is passed to the next carrier in the transport chain in a series of redox reactions. *Note:* The carrier is reduced when it gains electrons and oxidised when it loses electrons.
3.	Energy is released which enables a $H^+$ ion to be pumped through the membrane into the inter membrane space. The $H^+$ ions will accumulate here and cause the pH of the inter membrane space to decrease. This forms an electrochemical gradient. *Note:* It is an electrical gradient because $H^+$ is a charged ion and it is a chemical gradient as the $H^+$ concentration is greater on one side than the other.
4.	At intervals in the cristae membrane, there are protein gates and channels within the stalked particles which allow $H^+$ ions to diffuse back into the matrix.
5.	As the $H^+$ diffuses back into the mitochondrial matrix, ATP is generated by ATP synthase enzyme at the gate. Hydrogen ions cause a conformational change in the enzyme’s active site, so the ADP can bind. *Note:* This is chemiosmosis.
6.	Inside the matrix at the end, the $H^+$ ions and the electrons recombine to form a hydrogen atom.
7.	The hydrogen atoms will combine with oxygen to form water.
8.	Oxygen is the final electron acceptor in the electron transport chain and it is reduced.
9.	This method of producing ATP is called oxidative phosphorylation.

Products from one molecule of glucose

Breaking down one molecule of glucose will yield the following products.

Stage of aerobic respiration	Molecules produced	Number of ATP produced
Glycolysis	$2$ ATP (net)	$2$
Glycolysis	$2$ reduced NAD	$5$
Link ( $\times 2$ )	$2$ reduced NAD	$5$
Krebs ( $\times 2$ )	$2$ ATP	$2$
Krebs ( $\times 2$ )	$6$ reduced NAD	$15$
Krebs ( $\times 2$ )	$2$ reduced FAD	$3$

Note: $2.5$ ATP are made from each reduced NAD and $1.5$ ATP are made from every reduced FAD.

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Aerobic and anaerobic respiration

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Respiration: definition and glycolysis

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