Glycolysis occurs in the cytoplasm. Glucose is converted into pyruvate by oxidation. The net result of glycolysis = 2 x ATP, 2 x NADH. Glycolysis produces:
- Cellular energy sources- ATP
- Pyruvate for use in the Krebb's cycle
- Intermediate compounds e.g. hexose-1,6-bisphosphate, fructose-1-phosphate.
The link reaction occurs in the matrix of the mitochondria- it is actively transported here, meaning that some ATP that has been produced is used up.
The link reaction converts pyruvate to acetyl coenzyme A.
Overall reaction: 2pyruvate + 2NAD+ + 2CoA ---> 2CO2 + 2NADH + 2AcetylcoA
No ATP is formed in the link reaction
The Krebb's cycle occurs in the matrix. It occurs twice per glucose molecule as two pyruvate molecules are formed during glycolysis, resulting in two molecules of acetyl coenzyme A forming.
Per turn: 3 x NADH, 2 x CO2, 1 x ATP and 1 x FAD are formed.
Stage 1: Oxaloacetate + Acetyl CoA ---> Citrate (6C)
Stage 2: Decarboxylation and reduction occurs to form a 5C intermediate- CO2 and NADH produced.
Stage 3: Decarboxylation and reduction occurs to form a 4C intermediate.
Stage 4: Oxidation of the 4C compound occurs and oxaloacetate is made:FAD and NAD are reduced to form NADH and FADH. ATP is also formed.
Link reaction and Krebb's cycle summary
Oxidative Phosphorylation and Chemiosmosis
Oxidative Phosphorylation: The formation of ATP by adding a phosphate group to ADP in the presence of oxygen, the final electron acceptor.
1. Protons and electrons are produced when NAD and FAD are reoxidised.
2. First electron carrier is NADH dehydrogenase, where protons go into solution in the matrix.
3. Electrons are passed along a chain of electron carriers and donated to molecular oxygen.
Chemiosmosis: Energy is released and used by coenzymes as electrons flow along the electron transport chain to pump protons across the intermembrane space in a series of redox reactions.
- This builds up a proton gradient (also a pH gradient and an electrochemical gradient). So potential energy builds up in the intermembrane space.
- The hydrogen ions diffuse through ATPsynthase- protons flow through ATPsynthase and join ADP and Pi. Electrons from the electron chain join to molecular oxygen:
4H+ + 4e- + O2 ----> 2H2O
Anaerobic respiration in animals: Pyruvate is the hydrogen acceptor and is converted into lactate. This reaction is catalysed by lactate dehydrogenase.
- 2 x ATP and 2 x NADH formed from one glucose molecule.
2 Pyruvate + 2 NADH ---> 2 Lactate + 2NAD + 2ATP
Anaerobic respiration in yeast: Ethanal is the hydrogen acceptor. Catalysts used: pyruvate decarboxylase and ethanol dehydrogenase.
Respiratory Substrate Mean kJ
RQ= Volume of CO2/ Volume of O2
Despite providing the least amount of energy, carbohydrates are the main energy source because:
- The brain only uses energy produced by glucose except during starvation, as do red blood cells.
- It can be oxidised easily
- It is highly abundant
However, muscle cells and heart cells usually get a lot of their energy from fatty acids.