12.2.3
Aerobic Respiration
The Link Reaction
The Link Reaction
Pyruvate can be used in aerobic respiration by converting it to acetyl coenzyme A. This process is called the link reaction and it takes place in the matrix of the mitochondria.


Decarboxylation
Decarboxylation
- Pyruvate first diffuses from the cytoplasm across the mitochondrial membrane and into the matrix of the mitochondria.
- In the mitochondrial matrix, pyruvate is decarboxylated and dehydrogenated to acetate, which is a two-carbon molecule.
- CO2 is produced as a by-product.
- NAD is reduced to NADH.


Coenzyme A
Coenzyme A
- Acetate then combines with coenzyme A (CoA) to produce acetyl coenzyme A.
- Acetyl coenzyme A is used in the Krebs cycle (the next stage of aerobic respiration).
- The link reaction links glycolysis to the Krebs cycle.


Net gain
Net gain
- The net gain from the link reaction is:
- 1 CO2 molecule.
- 1 NADH molecule.
The Krebs Cycle
The Krebs Cycle
The Krebs cycle takes place in the matrix of the mitochondria. The products of the cycle are two coenzymes (NADH and FADH2), ATP and CO2.


Acetyl coenzyme A (coA)
Acetyl coenzyme A (coA)
- Acetyl coenzyme A acts as a carrier for the two-carbon acetyl group. It reacts with oxaloacetate (a four-carbon molecule) to produce citrate (a six-carbon molecule).
- CoA is now available to be recycled and reused in the link reaction.
- The production of citrate allows the Krebs cycle to begin.


6C → 5C
6C → 5C
- Citrate is converted to a five-carbon molecule (5C) by decarboxylation and dehydrogenation.
- CO2 is produced as a by-product.
- NAD is reduced to NADH.


5C → 4C
5C → 4C
- The five-carbon molecule is decarboxylated and dehydrogenated again to a four-carbon compound.
- CO2 is produced.
- NADH are produced.
- ATP is also produced by substrate-level phosphorylation.


Regeneration of oxaloacetate
Regeneration of oxaloacetate
- This 4C molecule is then dehydrogenated again to produce another molecule of NADH. FAD is also reduced to FADH2.
- No decarboxylation takes place at this stage.
- These intermediate reactions regenerate oxaloacetate. This allows the cycle to continue again.


Net gain
Net gain
- The net gain of the Krebs cycle is:
- 2 CO2 molecules.
- 3 NADH molecules.
- 1 ATP molecule.
- 1 FADH2 molecules.
- For each molecule of glucose, there are two cycles (this is because two molecules of pyruvate are produced in glycolysis).


Other respiratory substrates
Other respiratory substrates
- Fatty acids and amino acids can also be used as respiratory substrates in aerobic respiration.
- The substrates are converted to molecules that can easily enter the Krebs cycle.
Oxidative Phosphorylation
Oxidative Phosphorylation
Oxidative phosphorylation is the final stage in aerobic respiration.


Inner mitochondrial membrane
Inner mitochondrial membrane
- Oxidative phosphorylation takes place at the inner mitochondrial membrane.
- There are several features of the membrane that allows production of ATP on a large scale:
- Three electron carrier proteins (electron transport chain, ETC).
- ATP synthase enzyme.
- The space between the inner and outer mitochondrial membranes is called the intermembrane space.


Electron transport chain
Electron transport chain
- NADH (from the Krebs cycle) is oxidised by the first electron carrier protein in the inner mitochondrial membrane.
- FADH2 (from the Krebs cycle) is oxidised by the second electron carrier protein in the inner mitochondrial membrane.
- This initiates oxidative phosphorylation because NADH and FADH2 release two protons and two electrons each.
- The electrons are then transferred along the ETC.


Proton gradient
Proton gradient
- As the electrons move down the ETC, they lose energy.
- This energy pumps the protons from NADH and FADH2 into the intermembrane space.
- This creates a proton gradient (also known as an electrochemical gradient).


Chemiosmosis
Chemiosmosis
- The protons diffuse down the concentration gradient through the ATP synthase enzyme.
- As protons flow through the ATP synthase, energy is released.
- This energy converts ADP and inorganic phosphate to ATP.
- This process is called chemiosmosis.
 2.1.4.3 - Covalent bonding in oxygen (1),h_400,q_80,w_640.png)
 2.1.4.3 - Covalent bonding in oxygen (1),h_400,q_80,w_640.png)
The final electron acceptor
The final electron acceptor
- After the electrons have reached the end of the ETC and protons have flowed through the ATP synthase enzyme, they combine with O2 to form water (H2O).
- Oxygen is called the final electron acceptor for this reason.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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