Oxidative Phosphorylation

Question 1

What are the two purposes of catabolic pathways?

Answer 1

○ Breakdown of larger molecules into smaller building units
○ Release and (temporary) storage of energy in high-energy molecules, like ATP/NTPs and reduced cofactors (NADH/FADH₂)

Question 2

What is meant by the statement that catabolic pathways are oxidative?

Answer 2

Metabolites are oxidized as cofactors are reduced. The re-oxidation of cofactors is used to generate ATP.

as your body breaks down food molecules (metabolites), helper molecules (cofactors like NAD⁺) capture energy by gaining electrons and becoming “charged up” (reduced). Later, these charged cofactors release their energy to power ATP production, which is your body’s main energy currency.

Question 3

What are the two processes of oxidative phosphorylation?

Answer 3

○ Oxidation of reduced cofactors (NADH, FADH₂) and reduction of molecular oxygen
○ Phosphorylation of ADP to ATP

Question 4

How are the oxidation of reduced cofactors and the phosphorylation of ADP to ATP linked?

Answer 4

The processes are linked through a proton gradient across the mitochondrial membrane.

Question 5

Where do the proteins associated with oxidative phosphorylation reside in eukaryotes?

Answer 5

The inner mitochondrial membrane.

Question 6

List the components of the electron transport chain.

Answer 6

○ Complexes I-IV (integral membrane proteins)
○ Coenzyme Q (lipid soluble coenzyme)
○ Cytochrome c (peripheral membrane protein)

Question 7

How is electrical current created as electrons travel along the electron transport chain (ETC)?

Answer 7

The movement of high-energy electrons along the proteins of the ETC allows some of these proteins to act as proton pumps, moving ions from the matrix of the mitochondria to the intermembrane space.

Question 8

Besides coenzyme Q, list the other cofactors that are reversibly oxidized/reduced during electron transport.

Answer 8

○ Flavin mononucleotide
○ Iron-sulfur clusters
○ Copper (Cu²⁺)
○ Cytochrome heme groups

Question 9

How do electrons move in relation to reduction potentials?

Answer 9

Electrons move from cofactors with lower reduction potential to those with higher reduction potentials.

Question 10

Describe the relationship between redox reactions and free energy.

Answer 10

Redox reactions have a free energy change related to reduction potential. A higher reduction potential change means a more negative ΔG. Electrons move from compounds with lower reduction potentials to those with higher reduction potentials. The free energy changes from redox reactions can be used to transport protons across the membrane.

Question 11

What is the terminal electron acceptor in the electron transport chain, and why?

Answer 11

Oxygen is the terminal electron acceptor because it has a very high reduction potential.

Question 12

For every NADH molecule re-oxidized, how many protons are moved out of the matrix?

Answer 12

10 protons

Question 13

What type of transport allows complexes in the electron transport chain to pump H+ ions?

Answer 13

Primary active transport, where energy is derived from a redox reaction.

Question 14

What is Complex II, and what is unique about it?

Answer 14

Complex II is succinate dehydrogenase, which is part of the citric acid cycle. It contains FAD as a prosthetic group and catalyzes the oxidation of succinate to fumarate. No protons are moved across the membrane at Complex II.

Question 15

For every FADH₂ molecule re-oxidized, how many protons are moved out of the matrix?

Answer 15

6 protons

Question 16

What is the proton motive force?

Answer 16

The proton motive force is the potential energy created by the proton gradient across the inner mitochondrial membrane. The intermembrane space has a higher concentration of protons (lower pH) than the matrix

Question 17

How is the potential energy of the H+ gradient converted into chemical energy?

Answer 17

The potential energy of the H+ gradient is converted into chemical energy in the phosphoanhydride bonds of ATP. ATP synthase uses the energy of the proton gradient to phosphorylate ADP to ATP.

Question 18

How many protons are needed for ATP synthase to synthesize one ATP molecule?

Answer 18

Approximately 3 H+ are needed per ATP synthesized by ATP synthase

Question 19

What are the two portions of ATP synthase, and what are their functions?

Answer 19

○ FO: Transmembrane portion, protons pass through it, triggering a conformational change in F₁
○ F1: Catalytic portion, responsible for the synthesis of ATP from ADP and P_i

Question 20

What determines the rate of proton movement and oxygen consumption in oxidative phosphorylation?

Answer 20

The rate of ATP synthesis determines the rate of proton movement and oxygen consumption

Question 21

How many ATP molecules are generated for every complete turn of the central shaft in ATP synthase?

Answer 21

3 ATP molecules are generated because there are 3 active sites that make ATP.

Question 22

Describe the process of exporting newly-synthesized ATP from the mitochondrial matrix.

Answer 22

Newly-synthesized ATP is exported from the mitochondrial matrix into the cytosol via the adenine nucleotide translocase. ADP and P_i produced in the cytosol are transported back into the mitochondrial matrix via the P_i-H+ symport.

Question 23

How are oxidation and phosphorylation coupled?

Answer 23

The rates of NADH re-oxidation, electron transport, and oxygen consumption are coupled to the rate of ATP consumption (and synthesis) through the magnitude of the H+ electrochemical gradient

Question 24

What is the P:O ratio?

Answer 24

The P:O ratio is the amount of ATP made (P) per oxygen atom reduced to water (O). It is non-stoichiometric and may vary with uncoupling. The P:O ratio is approximately 2.5 for NADH re-oxidized and approximately 1.5 for FADH₂ re-oxidized.

Question 25

What largely determines the rate of oxidative phosphorylation?

Answer 25

The relative concentration of ADP

Question 26

How does ADP concentration reflect the energy consumption of the cell?

Answer 26

Oxygen consumption, via electron transport, is connected to ATP production at the ATP synthase. Oxygen consumption increases when ADP concentration rises because ADP concentration reflects the energy consumption of the cell

Question 27

What happens during low energy use in oxidative phosphorylation?

Answer 27

Low [ADP] and [Pi] lead to low ATP synthase activity. This results in an increase in the H+ gradient, decreased electron transport, and a drop in O₂ consumption. [NADH] and [FADH₂] increase, inhibiting the citric acid cycle and pyruvate dehydrogenase (PDH).

Question 28

What happens during high energy use in oxidative phosphorylation?

Answer 28

High [ADP] and [Pi] lead to increased ATP synthase activity. This results in a decrease in the H+ gradient, increased electron transport, and an increase in O₂ consumption. [NADH] and [FADH₂] decrease, activating the citric acid cycle and pyruvate dehydrogenase (PDH).

Question 29

How does oxygen consumption in isolated mitochondria change when ATP synthesis is stimulated?

Answer 29

Oxygen consumption increases when ATP synthesis is stimulated by the addition of ADP.

Question 30

Describe the role of uncoupling proteins in oxidative phosphorylation.

Answer 30

Uncoupling proteins provide an alternate pathway for protons to enter the mitochondrial matrix, bypassing ATP synthase. This uncouples electron transport from ATP synthesis, generating heat instead of ATP.

Question 31

What is an example of mammals deliberately uncoupling oxidative phosphorylation?

Answer 31

Brown adipose tissue uncouples oxidative phosphorylation to generate heat.

Question 32

What happens to oxygen consumption in the presence of an uncoupler?

Answer 32

Oxygen consumption increases in the presence of an uncoupler, even when ATP synthesis is not occurring. This is because the proton gradient is dissipated faster, leading to increased electron transport.

Question 33

Provide an example of a poison that works by dissipating the proton electrochemical gradient.

Answer 33

2,4-dinitrophenol is an example of a poison that acts as an uncoupler. It dissipates the proton gradient, preventing ATP synthesis while allowing electron transport and oxygen consumption to continue. This can lead to a fatal increase in body temperature.