Citric Acid Cycle and Respiratory Chain Flashcards
What is glycolysis ?
Oxidation of glucose to generate pyruvate (net 2ATP + 2NADH + H+)
State what happens to pyruvate under aerobic conditions
Pyruvate & 2NADH & H+ transported to the mitochondria
State what happens to pyruvate under anaerobic conditions
Pyruvate reduced to lactate and NAD+ regenerated to enable glycolysis to continue (short term)
What are the stages in cellular respiration ?
Stage 1 : Glycolysis
Stage 2 : Citric Acid Cycle
Stage 3 : Oxidative Phosphorylation
Stage 1 of Cellular Respiration
Glycolysis
Generation of pyruvate to form acetyl coenzyme A
(fatty acids and some amino acids also generate acetyl -coenzyme A)
Stage 2 of Cellular Respiration
Citric Acid Cycle
Redox reactions to harness energy via electron carriers (NAD+ and FAD), producing CO2.
Stage 3 of Cellular Respiration
Oxidative Phosphorylation
Oxidation of co-enzymes : electron transfer and reduction of O2 & synthesis of ATP
(ADP phosphorylation)
Where does acetyl-coenzyme A synthesis occur ?
Occurs in the mitochondrial matrix
Describe how acetyl-coenzyme A is produced
Pyruvate (& fatty acids/amino acids) are degraded into acetyl groups
Acetyl groups are added to coenzyme A
This forms acetyl-coenzyme A
What is acetyl-coenzyme A ?
A high energy compound formed by hydrolysis of the thioester bond.
Used in the citric acid cycle.
What is the function of the mitochondrion ?
Site of eukaryotic oxidative metabolism = the cell’s “power plant”
After glycolysis and generation of pyruvate, metabolism takes place within the mitochondria.
Describe the structure of the mitochondrion
2 layered membrane
A number of cristae (varied depending on the cells metabolic requirements)
Function of the membranes of the mitochondrion
The selective permeability of the inner membrane to most ions & metabolites, enables generation of ionic gradients - key to ATP synthesis
What is the citric acid cycle ?
Common pathway by which all fuel molecules (carbohydrates, fats & protein) are broken down to CO2 and H2O.
Give a brief description of the Citric Acid Cycle
2-carbon acetyl-CoA combines with 4-carbon oxaloacetate.
6-carbon citrate broken down in stages to harness energy via electron carriers (NAD+/FAD)
4-carbon oxaloacetate regenerated
What is formed from each cycle of the citric acid cycle ?
2 x CO2
1 x GTP
3 x NADH & H+
1 x FADH2
What is oxaloacetate ?
A keto acid
What happens in the 1st step of the Citric Acid Cycle ?
Condensation of the acetyl group of acetyl coenzyme A with the keto acid OXALOACETATE by citrate synthase.
(acetyl group 2-carbon)
(oxaloacetate 4-carbon)
What type of reaction is the 1st step of the citric acid cycle ?
A highly exergonic reaction due to the thio-ester bond large -delta G, which is essential to drive the cycle forward.
The [oxaloacetate]mito concentration is normally very low.
What happens to the CoA liberated in step 1 of the citric acid cycle ?
Liberated CoA to participate in oxidative decarboxylation of another pyruvate.
State what happens in the middle stages of the Citric Acid Cycle
A number of dehydrogenation steps occur in the citric acid cycle, resulting in NADH & H+ formation.
Describe the keto acids of the citric acid cycle
They are quite reactive and can be decarboxylated = CO2 release
Describe what happens on conversion of succinyl coA to succinate
The high energy bond of succinyl-coA generates GTP on conversion to succinate
How is FAD (Flavin Adenine Dinucleotide) formed ?
From the vitamin riboflavin (vitamin B2)
What is special about FAD ?
FAD is bound to the enzyme succinate dehydrogenase, which is the only citric acid enzyme bound to the inner mitochondrial membrane.
Describe the conversion of succinate to fumerate
FAD is reduced to FADH2
Reoxidised via the electron transport chain
What happens to the NADH & H+ and FADH2 generated from the citric acid cycle ?
Reduced coenzymes NADH & H+ and FADH2 now enter the electron transport chain.
How is the citric acid cycle regulated ?
Inhibition of enzymes involved in these stages by levels of :
- ATP
- Acetyl-CoA
- NADH
- CO2
State how levels of ATP regulate the citric acid cycle
If there is plenty of ATP, we do not need to keep cycling through and generating huge amounts, so this feeds back and inhibits the enzyme.
State how NADH regulates the citric acid cycle
Acts as a limiting factor.
The reason energy production comes to an end in glycolysis is because we have reduced all of the NAD available.
We need pyruvate to then be reduced into lactate in order to free up NAD to then interact again.
State what fatty acids act as a precursor for
Citrate could potentially be farmed off from fatty acids
Give a summary of the Citric Acid Cycle
Stepwise chemical transformation of substrates
Oxidising substrates to harness molecular energy via reduction of coenzymes.
Delivering the reduced coenzymes to the electron transport chain.
What metabolic waste is produced from the Citric Acid Cycle ?
Carbon Dioxide
How much energy is produced from the reduced coenzymes of the citric acid cycle ?
Energy from each molecule of :
NADH & H+ = 2.5 ATP
FADH2 = 1.5 ATP
Where does the electron transport chain take place ?
Within the mitochondrial matrix and inter membrane space.
Describe what happens in the 2 phases of the electron transport chain.
Phase :
1 - Electron transport chain creates the proton gradient
2 - Chemiosmosis uses the proton gradient to synthesise ATP
State what happens in the 1st step of the electron transport chain
Reduced coenzymes deliver electrons to complexes 1 and 2
State what happens in the 2nd step of the electron transport chain
Electron transfer : each complex is then reduced, then oxidised (redox)
State what happens in the 3rd step of the electron transport chain
Energy released pumps H+ into intramembranous space
Coenzyme Q and Cytochrome C transfer electrons
State what happens in the 4th step of the electron transport chain
Complex 4 combines :
2H+ and 1/2 O2 —-> H2O
State what happens in the 5th step of the electron transport chain
Energy generated from proton gradient synthesises ATP
What is the function of complex 1 : NADH-Q reductase in the respiratory chain ?
Oxidises NADH and H+
Reduces Coenzyme Q
What is the function of complex 2 : Succinate-Q reductase in the respiratory chain ?
Oxidises FADH2
Reduces Coenzyme Q
What is the function of complex 3 : Q-cytochrome C oxioreductase in the respiratory chain ?
Oxidises Coenzyme Q
Reduces Cytochrome C
What is the function of complex 4 : Cytochrome C oxidase in the respiratory chain ?
Oxidises Cytochrome C
Reduces O2 to H2O
What happens to energy as electrons are passed along complexes ?
Energy is released in a stepwise manner as electrons are passed along complexes.
Protons (H+) are pumped into the intramembranous space
State the 2 electron transport chain inhibitors
Cyanide
Carbon Monoxide
[Both Poisons bind to cytochrome C oxidase ]
How does cyanide work ?
Binds to iron in the enzyme - prevents the electron transport chain from working.
Halts ATP production.
Function of the proton gradient in ATP synthesis
Creates :
- A pH gradient
(H+ concentration in matrix is lower than in the intramembranous space) - A voltage across the membrane
What do the conditions generated by the proton gradient cause ?
Both conditions strongly attract H+ back inside the matrix
Only freely permeable region is via complex V - ATP synthase
Describe energy flow during cellular respiration
Glucose –> NADH + H+ —> Electron transport chain –> Proton gradient energy –> ATP
Describe the energy yield of cellular respiration
Net 2+ ATP by substrate level phosphorylation
+2 ATP by substrate level phosphorylation
+ about 28ATP by oxidative phosphorylation
- 2ATP
= 30 ATP per glucose molecule
