Metabolism 5- Oxidative Phosphorylation

Question 1

Where does oxidative phosphorylation take place

Answer 1

In the cristae ( folds of the inner membrane)

Question 2

What is the advantage of the cristae being folded

Answer 2

It increases the surface area in which oxidative phosphorylation can take place.

Question 3

What is the evidence that supports the endosymbiosis theory.

Answer 3

Mitochondria can only arise from pre-existing mitochondria.
They possess their own genome which resembles that of prokaryotes- single, circular loop of DNA with no associated histones
They have their own ribosomes, which are similar to prokaryotic ribosomes.
The first amino acid of mitochondrial transcript is formylated methionine residue (fMet) - same in bacteria- not methionine (Met) as in eukaryotes.
A number of antibiotics (Streptomycin) that act by blocking bacterial protein synthesis also block protein synthesis within the mitochondria. They do not interfere with protein synthesis in the cytoplasm of the eukaryotes.

Question 4

How many genes does mtDNA encode for

Answer 4

37 genes, and their can be several copies within the cell.

Question 5

Where is mtDNA inherited from

Answer 5

The ovum, and hence it is inherited from the mother, all mutations are passed on to the maternal offspring.

Question 6

Describe the re-oxidisation reactions of NADH and FADH2

Answer 6

NADH + H+ + 0.5O2 — NAD+ + H20

FADH2 + 0.5 O2 — FADH2 + H2O

Question 7

What are the two steps of the chemiosmotic model of oxidative phosphorylation

Answer 7

The translocation of protons from within the matrix of the mitochondria. This is controlled by the ETC.
The pumped protons are allowed back into the mitochondria through a specific channel which is coupled to an enzyme which can synthesise ATP.

Question 8

Describe the role of the NADH dehydrogenase complex in the ETC

Answer 8

Accepts electrons from NADH. The electrons are extracted in the form of a hydride ion, which is then converted into a proton and two high energy electrons- a reaction catalysed by the NADH dehydrogenase complex. The proton is pumped into the intermembrane space

Question 9

Describe the other events in the electron transport chain

Answer 9

The electrons from the NADH dehydrogenase complex are passed to a carrier protein ( ubiquinone) which transfers the electrons to the cytochrome c reductase complex, proton pumped into intermembrane space. Electrons are then transferred to cytochrome c (carrier protein) which transfers the electrons to the cytochrome c oxidase complex, which pumps protons into the intermembrane space.

Question 10

Why is the transfer of electrons energetically favourable

Answer 10

The electrons are passed from electron carriers with weaker electron affinities to those with stronger electron affinities, until they combine with a molecule of O2 to form water. Hence electrons lose energy as they pass through the ETC.

Question 11

Where else can ubiquinone accept electrons from

Answer 11

From the re-oxidation of FADH2, produced from the action of succinate dehydrogenase.

Question 12

Define redox

Answer 12

Reactions which involve electron transfer - involves a reduced substrate and an oxidised substrate.

Question 13

What is a redox couple

Answer 13

A substrate that can exist in both oxidised and reduced forms.

Question 14

What does a negative E’O value imply

Answer 14

The redox couple has a tendency to accept electrons and so has more oxidising power than hydrogen.

Question 15

What does a positive E’O value imply

Answer 15

The redox couple has a tendency to donate electrons, and so has more reducing power than hydrogen.

Question 16

What are the two consequences of proton pumping.

Answer 16

pH gradient created across the inner membrane- pH in matrix 7.9- pH in intermembrane space- 7.9
Voltage gradient created- matrix becomes negative- side facing the intermembrane space becomes positive.

Question 17

How do the pH gradient and membrane potential work together

Answer 17

They create a steep electrochemical proton gradient that makes it energetically very favourable for H+ ions to flow back into the mitochondrial matrix. The membrane potential contributes significantly to the proton motive force which pulls the H+ back across the membrane- the greater the membrane potential- the more energy stored in the proton gradient.

Question 18

What would happen if protons were simply allowed to flow back into the matrix

Answer 18

The energy would be lost as heat.

Question 19

Describe the structure of ATP synthase

Answer 19

Membrane bound part (F0)- H+ carrier
F1 head-part of protein where phosphorylation of ADP takes place.
Stalk connects F0 to F1.

Question 20

Describe how ATP synthase functions

Answer 20

The passage of protons through the carrier causes the carrier and its stalk to spin rapidly, like a tiny motor. As the stalk rotates it rubs against the proteins in the stationary head, altering their conformation and prompting the production of ATP. A mechanical deformation gets converted into the chemical bond energy of ATP.

Question 21

How can ATP synthase work in reverse

Answer 21

Energy can be used from ATP hydrolysis to pump protons uphill against their electrochemical gradient, functioning like the H+ pumps. Whether ATP synthase makes ATP or consumes it depends on the magnitudes of the electrochemical proton gradient/
Delta G needs to be large enough to drive ATP production.

Question 22

Consequences of accepting electrons from FADH2

Answer 22

One less proton pumped into intermembrane space, less ATP produced.

Question 23

How can the concentration of oxygen in solution be determined

Answer 23

Oxygen electrode- Pt cathode and AG anode underneath a Teflon membrane, which is permeable to O2.

Question 24

Describe how the oxygen electrode works

Answer 24

Oxygen is reduced to water at the Pt cathode
02 + 4H+ + 4e- — 2H20

Circuit is completed at the silver anode which is slowly oxidised to AgCl by the KCl electrolyte
4Ag+ + 4Cl- — AgCl + 4e-
The resulting current is directly proportional to the oxygen concentration in the sample chamber.