F2144 -4- Respiration

Question 1

The 4 steps of aerobic respiration (in order)

Answer 1

Glycolysis

Link Reaction

Krebs Cycle

Oxidative Phosphorylation

Question 2

Substrate-level phosphorylation

Answer 2

ADP + Pi –> ATP

Question 3

Glycolysis

Answer 3

Occurs in aerobic AND anaerobic respiration, in the cell CYTOPLASM.

Glucose -> Glucose-6-phosphate -> Fructose-6-phosphate -> Fructose-1,6-bisphosphate -> 2xTriose Phosphate ->->->->->-> Triose Phosphate

ATP is hydrolysed to provide phosphate for fructose-1,6-bisphosphate. Each triose phosphate molecule produces 2 ATP molecules, and has an inorganic phosphate added at one point. Each triose phosphate is also oxidised by removal of H2, producing reduced NAD.

So each glucose molecule produces:
2x Pyruvate
2x net ATP gain
2x net NADH2 gain

Question 4

The Link Reaction

Answer 4

ONLY in aerobic respiration, in the MITOCHONDRIAL MATRIX.

PYRUVATE is pumped into the MITOCHONDRIAL MATRIX where it undergoes OXIDATIVE DECARBOXYLATION (loss of H2 to NADH2 and loss of CO2 as waste product) to form a 2-CARBON FRAGMENT (an acetyl group) that binds to CO-ENZYME A to produce ACETYL COENZYME A.

Acetyl Coenzyme A hands the 2 carbon fragment to oxaloacetate in the Krebs cycle, forming citrate. This process replenishes CoA, which may then take part in the Link Reaction again.

Question 5

The Krebs Cycle

Answer 5

ONLY in aerobic respiration, occurs in the MITOCHONDRIAL MATRIX.

Acetyl CoA is oxidised by oxaloacetate to form citrate (releasing CoA to be reused in the Krebs cycle). Citrate undergoes oxidative decarboxylation - CO2 is released, and H2 is also released to produce reduced NAD - leaving a 5-Carbon intermediate. This 5C intermediate is oxidised to realease H2 again (forming NADH2), energy from reactions is used to combine Pi and ADP to ATP in a side reaction, and again CO2 is released through decarboxylation, leaving us with a 4-Carbon Intermediate. Dehydrogenase enzyme oxidises the 4C molecule twice, producing reduced FAD and reduced NAD, leaving us with oxaloacetate, the 4-Carbon acceptor molecule in the cycle.

One turn of the Krebs Cycle produces:
•3xNADH2
•1xFADH2
•1xATP
•2xCO2
•(one molecule of Coenzyme A is also replenished from Aceyl CoA)

Question 6

Oxidative Phosphorylation

Answer 6

ONLY in aerobic respiration, in the INNER MITOCHONDRIAL MEMBRANES.

• Oxidative Phosphorylation is the process where the ENERGY carried by ELECTRONS from REDUCED COENZYMES (NADH2 and FADH2) is used TO MAKE ATP in the presence of oxygen, the final electron acceptor.
• 2 processes are involved - the electron transport chain and chemiosmosis.

The Process Outlined:

1) H atoms are released from reduced NAD and FAD as they are oxidised to NAD and FAD for re-use in the Krebs cycle. The H atoms SPLIT INTO PROTONS AND ELECTRONS
2) The ELECTRONS move along the ELECTRON TRANSPORT CHAIN (made up of electron carriers) losing energy at each carrier.
3) The energy is used by the electron carriers to pump protons across the inner mitochondrial membrane, from the MITOCHONDRIAL MATRIX into the INTERMEMBRANE SPACE (space between inner and outer membranes).
4) The concentration of protons is now higher in the intermembrane space than in the mitochondrial matrix, producing an ELECTROCHEMICAL, and pH gradient (electrochemical gradient is a concentration gradient of ions).
5) Protons move down the electrochemical gradient through ion channels in the inner mitochondrial membrane that are attached to ATP SYNTHASE ENZYME. This movement provides energy, driving the synthesis of ATP from ADP and inorganic phosphate at the synthase enzyme.
6) The movement of H+ ions across a membrane, which generate ATP (through ATP synthase enzyme) is called CHEMIOSMOSIS.
7) In the MITOCHONDRIAL MATRIX, at the end of the transport chain, the protons, electrons and O2 (from the blood) combine to form water. Molecular oxygen is said to be the FINAL ELECTRON ACCEPTOR.