Terminal respiration IA% (+

Question 1

Diagram summary 1

Answer 1

Question 2

Diagram summary 2

Answer 2

Question 3

Mitochondria

Answer 3

• The only site of oxidative phosphorylation in eukaryotes
• Allows the coupling of the oxidation of carbon fuels to ATP synthesis
• Utilises proton gradients to produce ATP (and lots of it)

Question 4

Locations

Answer 4

• For oxidation in the terminal respiratory system of eukaryotic cells, NADH and FADH₂ have to be in the mitochondrial matrix
• The majority of NADH and FADH₂ is formed there, already (citric acid cycle and β-oxidation of fatty acids), but some NADH is formed in the cytoplasm (gycolysis)
• A shuttle is used to move reducing equivalents across the mitochondrial membrane
• Cytoplasmic NADH cannot cross the membrane, but FADH₂ can pass it’s e-’s on to the electron transport chain within the mitochondria: ‘Glycerol phosphate shuttle’

Question 5

Glycerol phosphate shuttle

Answer 5

• The NADH is unable to cross the membranes of the mitochondria, but Glycerol-3-P can, passing it’s e-’s to FADH₂
• Oxidation of FADH₂ in the electron transport chain generates, per mol, less ATP than oxidation of NADH
• Thus, an energetic ‘price’ is paid for using cytosolic reduced co-substrates in terminal respiration

Question 6

The electron transport chain

Answer 6

• Complex I: NADH-Q oxidoreductase
• Complex II: Succinate-Q reductase
• Complex III: Q- cytochrome c oxidoreductase
• Complex IV: Cytochrome c oxidase

Question 7

Complex I: NADH-Q oxidoreductase

Answer 7

•Oxidises NADH and passes the e-’s to ubiquinone to give ubiquinol (QH₂) = reduced ubiquinone

Ubiquinone + e-‘s ⇒ Ubiquionol

• Utilises Fe-S centres and FMN (flavin mononuleotide)
• Pumps H+ ions into the intermembrane space

Question 8

Complex II: Succinate-Q reductase

Answer 8

• Oxidises FADH₂ and like complex I passes high-energy e-’s to ubiquinone, which becomes ubiquinol (QH₂)

Ubiquinone + e-‘s ⇒ Ubiquionol

• Utilises Fe-S centres to channel e-’s

Question 9

•Complex III: Q- cytochrome c oxidoreductase

Answer 9

• Takes the e-’s from ubiquinol (QH₂) and passes them to cytochrome c
• 1 QH₂ is oxidised to yield 2 reduced cytochrome c molecules
• Pumps H⁺ into the intermembrane space

Question 10

•Complex IV: Cytochrome c oxidase

Answer 10

• Takes the e-’s from cytochrome c and passes them to molecular O₂
• e-’s channelled through Fe-Cu centre
• Pumps H^{<strong>+</strong>} into the intermembrane space

Question 11

Electron flow

Answer 11

• Energy is conserved from the breakdown of food molecules and ultimately leads to the oxidation of NADH, FADH₂, ubiquinone and cytochrome c
• Energy is further conserved through the setting up of a proton gradient across the inner mitochondrial membrane

Question 12

Chemiosmosis and proton-motive force

Answer 12

• As e-’s pass through the complexes of the transport chain, protons move from the matrix to the outside of the inner mitochondrial membrane – chemiosmosis
• This movement has particular spatial directionality, so is classed as ‘vectoral’
• This is an example of an energy transformation
• The protons on the outside of the membrane act as a store of potential energy
• When these protons are ‘allowed’ to flow back down their gradient they release energy to do work – proton motive force

Question 13

ATP Synthesis

Answer 13

• Protons eventually flow down their concentration gradient, back into the matrix of the mitochondria
• Only a relatively small number of sites exist on the membrane were this happens
• At these sites a large multi-unit protein complex called ATP synthase (ATPase for short) is found
• ATPase has a mechanism that allows protons to pass through
• As they flow through ATPase, the energy stored in the gradient is used to convert ADP + Pi ⇒ ATP
• ATP then takes this potential energy to do work in the cells of the body
• This is the final step in metabolising the food molecules we eat into energy

Question 14

The stoichiometry of oxidative phosphorylation

Answer 14

• Oxidation of FADH₂ in the terminal respiration system generates less ATP than NADH oxidation
• As e-’s pass through the electron transport chain, complexes I, III and IV move a total of 8 H^{<strong>+</strong>} from the matrix to the outside of the membrane
• ATP synthase can produce 1 ATP for every 3 H⁺ it moves back into the matrix across the membrane
• NADH feeds in e-’s at Complex I, which means the e-’s pass through all 3 sites in the chain that allow protons to move across the membrane (Complexes I, III and IV)►
• FADH₂ feeds in at Complex II, so only 2 of the sites that move protons across the membrane are utilised (Complexes III and IV)
• Stoichiometrically, approximately 2.5 and 1.5 mol of ATP are generated per mol of NADH and FADH₂ respectively

Question 15

Diagram summary 3

Answer 15

• The food molecules are completely broken down to CO₂ and H₂O
• Some potential energy from food molecules is ‘saved’, after being transformed through reduced co-reactants and the terminal respiratory system, and is used to make ATP

Question 16

Coupling and uncoupling

Answer 16

• Electron transport is said to be coupled to ATP synthesis
• If the inner mitochondrial membrane becomes permeable to protons, the proton gradient cannot be generated
• If this happens the electron transport can still occur, with O₂ being reduced to H₂O, but no ATP is made
• The two processes are now uncoupled
• The energy released from e-’s passing along the terminal respiration system does not make ATP, and is released as heat
• Malignant hyperthermia is a disease caused by ‘leaky’ mitochondrial membranes that uncouple electron transport and ATP synthase

Question 17

ATP synthase

Answer 17

Has 2 parts:

•F₀ – membrane bound proton conducting unit

–10 subunits

–Separate subunit connects F0 to F1

•F₁ – protrudes into the mitochondrial matrix and acts as the catalyst for ATP synthesis

–Produces lots of ATP from the proton motive force energy collected by F₀