Glycolysis
Where in the cell does glycolysis occur?
In the cytoplasm (cytosol) of the cell.
Where in the mitochondrion does oxidative phosphorylation occur?
On the inner mitochondrial membrane (cristae).
In which organelle and compartment does the Krebs cycle take place?
It occurs in the mitochondrial matrix.
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Name the initial substrate and the final product of glycolysis.
The initial substrate is glucose (C₆H₁₂O₆), and the final product is two molecules of pyruvate (C₃H₄O₃).
What is the role of oxygen in oxidative phosphorylation?
Oxygen acts as the final electron acceptor in the ETC. It combines with electrons and protons (H⁺) to form water (H₂O), allowing the chain to continue functioning.
Name two electron carriers that are reduced during the Krebs cycle.
NAD⁺ and FAD are reduced to NADH and FADH₂, respectively.
During glycolysis, glucose is converted into pyruvate in the cytoplasm.
Explain why NAD⁺ is essential for glycolysis to continue.
NAD⁺ must be available to accept electrons and hydrogen ions when triose phosphate is oxidised to pyruvate. Without NAD⁺, oxidation stops, and glycolysis halts because no ATP is produced.
Explain why ATP is both used and produced during glycolysis.
Two ATP molecules are used in the energy investment phase to phosphorylate glucose and fructose-6-phosphate, but four ATP molecules are produced during the energy payoff phase by substrate-level phosphorylation — resulting in a net gain of two ATP.
Explain the function of the electron transport chain and ATP synthase in ATP formation.
The ETC transfers electrons from NADH and FADH₂ through protein complexes, releasing energy used to pump protons into the intermembrane space. This creates a proton gradient. ATP synthase then uses the flow of protons back into the matrix (chemiosmosis) to phosphorylate ADP into ATP.
Explain why the Krebs cycle is considered a cyclic pathway.
The cycle begins with acetyl-CoA combining with oxaloacetate to form citrate. Through a series of reactions, citrate is converted back to oxaloacetate, which is reused — making the process cyclic.
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Describe the energy investment and energy payoff phases of glycolysis, including the number of ATP molecules involved.
- Energy investment phase: 2 ATP are consumed to convert glucose into fructose-1,6-bisphosphate.
- Energy payoff phase: 4 ATP and 2 NADH are generated as two 3-carbon intermediates are oxidized to pyruvate.
Net yield: 2 ATP and 2 NADH per glucose.
In aerobic respiration, ATP is synthesised by substrate-linked reactions and by oxidative phosphorylation.
Outline the process of oxidative phosphorylation.
Electrons are released from NADH and FADH₂ and transferred to the electron transport chain (ETC) located on the inner mitochondrial membrane.
As electrons pass through a series of electron carriers (Complexes I–IV), energy is released.
This energy is used to pump protons (H⁺ ions) from the mitochondrial matrix into the intermembrane space, creating a proton gradient (electrochemical gradient).
Protons then diffuse back into the matrix through ATP synthase.
The movement of protons through ATP synthase provides the energy for ADP + Pi → ATP (chemiosmotic theory).
Finally, oxygen acts as the terminal electron acceptor, combining with electrons and protons to form water (H₂O).
Outline the main products formed from one turn of the Krebs cycle starting with acetyl-CoA.
One turn of the cycle produces:
- 3 NADH
- 1 FADH₂
- 1 ATP (or GTP)
- 2 CO₂ (released as waste).
Mitochondria isolated from a cell were supplied with oxygen but not NADH or FADH₂.
Explain why ATP synthesis did not occur.
Without NADH or FADH₂, no electrons can enter the electron transport chain. Therefore, no proton gradient is formed, ATP synthase cannot function, and no ATP is produced.
Sometimes the muscle cells of an athlete need to carry out respiration in anaerobic conditions.
Explain why the respiration of glucose in anaerobic conditions produces less ATP than in aerobic conditions.
In anaerobic conditions, oxygen is unavailable to act as the final electron acceptor in the electron transport chain. As a result, oxidative phosphorylation cannot occur, and the Krebs cycle stops because NAD⁺ is not regenerated.
The cell therefore relies only on glycolysis, which produces a net gain of 2 ATP per glucose molecule, compared to about 36–38 ATP in aerobic respiration.
Rotenone is used as an insecticide. Rotenone kills insects by inhibiting the transfer of electrons in the electron transport chain of the mitochondrion.
Explain how rotenone affects ATP synthesis in the mitochondrion.
Rotenone blocks the transfer of electrons from NADH to the first complex (Complex I) in the electron transport chain (ETC).
As a result, electrons cannot flow through the ETC, and protons (H⁺) are not pumped into the intermembrane space.
Without this proton gradient, ATP synthase cannot function because there is no chemiosmotic flow of protons back into the matrix.
Therefore, oxidative phosphorylation stops, and ATP synthesis in mitochondria greatly decreases or ceases.
Succinate dehydrogenase is an enzyme that converts succinate into fumarate during the Krebs cycle, producing FADH₂.
If this enzyme’s activity is reduced due to a mutation, explain how this would affect the Krebs cycle and the production of ATP in the mitochondrion.
If succinate dehydrogenase activity decreases, less succinate is converted into fumarate, and less FAD is reduced to FADH₂.
Because FADH₂ donates electrons to the electron transport chain (Complex II), fewer electrons are transferred, so less energy is available to pump protons across the inner mitochondrial membrane.
As a result, the proton gradient is weaker, ATP synthase produces less ATP, and overall energy yield from respiration decreases.
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The sky is blue because of a phenomenon called Rayleigh scattering, where air molecules in the atmosphere scatter sunlight in all directions.