5.1 Aerobic Respiration

What is cellular respiration? (2)

- It is the process where energy from food molecules is transferred to synthesise adenosine triphosphate (ATP).

- It involves a series of enzyme-controlled reactions that release energy from organic substances.

What is substrate-level phosphorylation? (2)

- It is a metabolic reaction that results in the formation of ATP.

- It involves the direct transfer of a phosphate group from a substrate molecule to ADP.

What is a respiratory substrate? (2)

- It is an organic substance that is broken down during respiration to release energy.

- The principal respiratory substrate used by cells is glucose.

What is the structure of ATP? (3)

- ATP is a phosphorylated nucleotide.

- It is composed of an adenine base attached to a ribose sugar.

- The ribose sugar is attached to a chain of three phosphate groups.

Why is ATP considered the energy currency of a cell? (2)

- It stores a significant amount of chemical energy in the bonds between its phosphate groups.

- The hydrolysis of ATP releases this energy, which is then used to power cellular processes.

How is ATP broken down and resynthesised? (3)

- ATP is hydrolysed into adenosine diphosphate (ADP) and an inorganic phosphate (Pi), releasing energy.

- This hydrolysis reaction is catalysed by the enzyme ATPase.

- The reverse process is a condensation reaction where ADP is phosphorylated back into ATP, which requires an input of energy.

Why is ATP not stored in large quantities within cells? (2)

- ATP cannot be stored in substantial amounts, but the raw materials for its synthesis are readily available.

- This allows the compound to be made quickly when needed, ensuring a constant supply of energy without the need for large reserves.

What are oxidation and reduction in the context of cellular respiration? (2)

- Oxidation is the removal of electrons or hydrogen from a substance, or the addition of oxygen.

- Reduction is the addition of electrons or hydrogen to a substance, or the removal of oxygen.

What is the role of hydrogen acceptors in cellular respiration? (2)

- During cellular respiration, hydrogen is removed from respiratory substrates.

- Hydrogen acceptors pick up this hydrogen and become reduced in the process.

What is the function of the electron transport chain? (3)

- Hydrogen is transferred to the next hydrogen acceptor along the chain in a series of redox reactions.

- The affinity for hydrogen increases down the chain.

- Each redox reaction releases energy that is used for the synthesis of ATP.

What are three examples of hydrogen acceptors found in the electron transport chain? (3)

- Flavoprotein.

- Coenzyme Q.

- Cytochromes.

What is the role of NAD in respiration? (3)

- NAD is the most common hydrogen acceptor in cellular respiration and acts as a coenzyme.

- It accepts hydrogen atoms from a metabolic pathway, becoming reduced to form NADH.

- The oxidised form of the molecule is denoted as NAD⁺.

What is the role of FAD in respiration? (2)

- FAD is another hydrogen carrier and coenzyme that operates within cellular respiration.

- It accepts hydrogen to form the reduced molecule, FADH₂.

What is glycolysis? (3)

- Glycolysis is a series of metabolic reactions that occurs in the cytoplasm.

- It involves the initial phosphorylation of glucose, a respiratory substrate.

- The phosphorylated glucose is subsequently broken down into two molecules of pyruvate.

How is glucose phosphorylated during glycolysis? (3)

- A molecule of ATP is used to phosphorylate glucose into glucose-6-phosphate.

- Glucose-6-phosphate is then converted into fructose-6-phosphate.

- A second ATP molecule phosphorylates this compound to form fructose-1,6-bisphosphate.

What is the role of ATP in glycolysis? (3)

- ATP is a key product of glycolysis and serves as the primary energy currency for the cell.

- For each molecule of glucose, glycolysis generates a net of two ATP molecules.

- This ATP is used to power a variety of cellular activities, such as active transport.

Why is glucose phosphorylated at the start of glycolysis? (2)

- Phosphorylation makes the sugar more reactive for the subsequent reactions.

- It also prevents the sugar from passing through the cell membrane, trapping it within the cell.

What happens during the oxidation of triose phosphate? (3)

- Each triose phosphate molecule loses two hydrogen atoms in an oxidation reaction.

- Dehydrogenase enzymes catalyse this, and the coenzyme NAD accepts the hydrogen, forming reduced NAD.

- The phosphorylation of two ADP molecules also occurs, resulting in the formation of two ATP molecules.

How is triose phosphate converted to pyruvate? (2)

- Each 3-carbon triose phosphate molecule is converted into a 3-carbon pyruvate molecule.

- This conversion also produces another two ATP molecules through the phosphorylation of two ADP.

What are the net products of glycolysis from one molecule of glucose? (3)

- 2 molecules of ATP.

- 2 molecules of reduced NAD.

- 2 molecules of pyruvate.

What is the fate of the products of glycolysis? (3)

- The reduced NAD molecules move to the inner mitochondrial membrane to enter the electron transport chain.

- The pyruvate molecules are actively transported into the mitochondrial matrix to undergo the link reaction.

- In anaerobic conditions, pyruvate can be converted to ethanol or lactate.

How does glycolysis contribute to cellular respiration? (3)

- Glycolysis is the first stage and provides the cell with an initial yield of ATP and NADH.

- The pyruvate produced is further metabolised in the Krebs cycle and the electron transport chain.

- These subsequent processes produce a much larger amount of ATP through oxidative phosphorylation.

Why do living organisms require energy? (2)

- All living organisms need to respire to produce the energy required for various life processes.

- Respiration synthesises ATP, which provides the energy for these processes.

What are some processes that require ATP? (3)

- Active transport.

- Exocytosis and endocytosis.

- Anabolism and cell division.

How is the structure of a mitochondrion related to respiration? (3)

- The link reaction and the Krebs cycle take place in the mitochondrial matrix.

- Oxidative phosphorylation occurs on the inner mitochondrial membrane.

- This membrane is folded into cristae to increase the surface area for the electron transport chain.

What is the link reaction? (2)

- It is a process that converts pyruvate, the end product of glycolysis, into acetyl-CoA.

- This reaction takes place in the mitochondrial matrix and links glycolysis to the Krebs cycle.

What are the main steps of the link reaction? (3)

- Pyruvate is actively transported into the mitochondrial matrix.

- It is decarboxylated by the enzyme pyruvate decarboxylase, releasing carbon dioxide.

- The remaining acetate is oxidised by pyruvate dehydrogenase to form reduced NAD, then combines with coenzyme A to form acetyl-CoA.

How is pyruvate converted to acetyl CoA? (3)

- Pyruvate undergoes decarboxylation, where a carbon atom is removed as carbon dioxide.

- The remaining 2-carbon molecule is then oxidised, losing hydrogen atoms which are accepted by NAD to form reduced NAD.

- The resultant two-carbon acetate group combines with coenzyme A, forming acetyl CoA.

What are the products of the link reaction per molecule of glucose? (3)

- 2 molecules of carbon dioxide.

- 2 molecules of acetyl-CoA.

- 2 molecules of reduced NAD.

What is the role of coenzymes in the link reaction? (2)

- They are molecules that transport electrons and hydrogen ions.

- They act as electron donors during the link reaction.

What is the fate of the products of the link reaction? (2)

- The reduced NAD molecules proceed to the electron transport chain to be used in ATP synthesis.

- The acetyl CoA molecules enter the Krebs cycle for further oxidation.

What is the Krebs cycle? (3)

- It is a series of chemical reactions, also known as the citric acid cycle, that occurs in the fluid matrix of the mitochondria.

- Its purpose is to produce energy through the oxidation of acetyl-CoA.

- The acetyl-CoA is derived from the breakdown of carbohydrates, fats, and proteins.

What is the main function of the Krebs cycle? (2)

- Its primary function is to produce energy in the form of ATP.

- This energy is then used by the body for various processes, such as muscle contraction and active transport.

How does the Krebs cycle interact with other metabolic pathways? (3)

- It is linked to glycolysis, as the products of glycolysis are converted into acetyl-CoA, which then enters the cycle.

- It is also linked to the electron transport chain.

- The energy-carrying molecules produced during the cycle are used by the electron transport chain to synthesise large amounts of ATP.

Why do two rounds of the Krebs cycle occur for each molecule of glucose? (2)

- Each molecule of glucose is broken down into two molecules of Acetyl-CoA during glycolysis and the link reaction.

- Each of these Acetyl-CoA molecules enters the Krebs cycle, resulting in two complete turns.

What is the role of enzymes in the Krebs cycle? (2)

- Enzymes act as catalysts for the reactions that take place within the cycle.

- They increase the rate of the reactions, ensuring that the cycle runs efficiently.

What are the consequences of an enzyme deficiency in the Krebs cycle? (3)

- The reactions of the cycle may not occur efficiently, or they may stop altogether.

- This can cause a buildup of toxic substances within the cells.

- It can also lead to a decreased ability to produce energy, resulting in a variety of health problems.

What are the main events of the Krebs cycle? (3)

- Acetate (2C) combines with oxaloacetate (4C) to form citrate (6C).

- The 6C citrate is decarboxylated and dehydrogenated twice to form a 4C molecule, producing reduced NAD.

- This 4C molecule is then converted back into oxaloacetate, producing ATP, reduced FAD, and more reduced NAD.

How is oxaloacetate regenerated from succinate? (3)

- Succinate (4C) is oxidised to form fumarate (4C), reducing one molecule of FAD to FADH₂.

- A molecule of water is added to fumarate to form malate (4C).

- Malate is then oxidised to regenerate the starting compound, oxaloacetate (4C), reducing NAD⁺ to NADH.

What are the products of one turn of the Krebs cycle? (3)

- Two molecules of carbon dioxide.

- Three molecules of reduced NAD and one molecule of reduced FAD.

- One molecule of ATP.

What is the role of NAD and FAD in the Krebs cycle? (2)

- NAD and FAD act as hydrogen carriers, accepting hydrogen atoms during oxidation reactions.

- The resulting reduced NAD and reduced FAD transport this hydrogen to the electron transport chain for further ATP synthesis.

What is the total yield of the Krebs cycle from one molecule of glucose? (3)

- Four molecules of carbon dioxide.

- Six molecules of reduced NAD and two molecules of reduced FAD.

- Two molecules of ATP.

What is the initial reaction of the Krebs cycle? (2)

- The 2-carbon acetyl group from acetyl-CoA combines with the 4-carbon molecule, oxaloacetate.

- This condensation reaction forms a 6-carbon compound known as citrate, and coenzyme A is released.

How does decarboxylation occur in the Krebs cycle? (3)

- Decarboxylation is the removal of a carbon atom in the form of carbon dioxide.

- The first decarboxylation occurs when isocitrate (6C) is converted to α-ketoglutarate (5C).

- The second decarboxylation occurs when α-ketoglutarate (5C) is converted to succinyl-CoA (4C).

How is ATP formed by substrate-level phosphorylation in the Krebs cycle? (2)

- This occurs when the 4-carbon succinyl-CoA is converted to succinate.

- During this conversion, enough energy is released to directly synthesise ATP from ADP and an inorganic phosphate group.

Why does the Krebs cycle require oxygen? (2)

- The Krebs cycle itself does not directly use oxygen.

- However, it requires a supply of NAD and FAD, which are regenerated by the electron transport chain, a process that is dependent on oxygen.

How many reduced coenzymes are produced in one turn of the Krebs cycle? (2)

- Three molecules of NADH are produced.

- One molecule of FADH₂ is produced.

What are the total products of the Krebs cycle from one molecule of glucose? (3)

- 2 molecules of ATP.

- 6 molecules of reduced NAD.

- 2 molecules of reduced FAD.

What is oxidative phosphorylation? (3)

- It is the process where cells generate ATP through the transfer of electrons from coenzymes like FADH₂ and NADH to oxygen.

- This process takes place across the inner membrane of the mitochondria.

- It involves chemiosmosis, where ATP synthase uses a proton gradient to produce ATP.

What is the role of Complex V? (2)

- Complex V is also known as ATP synthase.

- It harnesses the energy from protons flowing back down their concentration gradient to synthesise ATP.

How does the electron transport chain work? (3)

- Electrons are transferred from reduced NAD and reduced FAD down a series of electron carriers.

- The carriers, which include flavoproteins, quinones, and cytochromes, are arranged in order of increasing electron affinity.

- As electrons are passed along, each carrier becomes alternately reduced and then oxidised, releasing energy.

What is the process of oxidative phosphorylation? (5)

- Reduced NAD and FAD release hydrogen atoms, which separate into protons and electrons.

- The electrons move along the electron transport chain, releasing energy at each stage.

- This energy is used to actively transport protons from the mitochondrial matrix to the inter-membrane space, creating a proton gradient.

- Protons diffuse down the electrochemical gradient, back into the matrix, through the enzyme ATP synthase in a process known as chemiosmosis.

- This movement provides the energy for ATP synthase to join ADP and inorganic phosphate (Pi) to form ATP.

What is the role of cytochromes in the electron transport chain? (3)

- They are protein pigments that contain an iron group and act as electron carriers.

- They are reduced by accepting electrons from carriers like reduced FAD and reduced NAD.

- They are then oxidised when they pass the electrons on to the next carrier, releasing energy.

What is the role of oxygen in oxidative phosphorylation? (2)

- Oxygen serves as the final hydrogen and electron acceptor at the end of the electron transport chain.

- Upon accepting electrons and protons, oxygen is reduced, and water is formed as a byproduct.

How does reduced NAD from the cytoplasm contribute to the electron transport chain? (2)

- Reduced NAD produced in the cytoplasm cannot enter the mitochondrion, so it transfers its electrons via molecular shuttles.

- Different shuttle systems pass the electrons to either FAD or NAD within the mitochondrion, yielding different amounts of ATP.

What is the role of NAD and FAD before oxidative phosphorylation? (3)

- They act as electron acceptors in earlier stages of respiration, such as glycolysis and the Krebs cycle.

- During these oxidation reactions, they accept electrons and become reduced.

- They then transport these electrons to the inner mitochondrial membrane to enter the electron transport chain.

What is the role of reduced NAD and FAD in the electron transport chain? (2)

- They deliver high-energy electrons to the electron transport chain.

- They provide protons (H⁺) which are pumped across the inner mitochondrial membrane to establish a proton gradient.

How are electrons initially transferred in the electron transport chain? (3)

- Reduced NAD and reduced FAD first dissociate to release two protons and two electrons.

- Flavoprotein, the initial electron carrier, is reduced by accepting electrons exclusively from reduced NAD.

- The reduced flavoprotein is then oxidised when it transfers the electrons to the next carrier, coenzyme Q.

What is the overall role of the electron transport chain? (3)

- It is a series of proteins that facilitates the transfer of electrons from NADH and FADH₂ to oxygen.

- The transfer of electrons creates a proton gradient by pumping hydrogen ions across the inner mitochondrial membrane.

- This proton gradient is crucial for driving the synthesis of ATP through chemiosmosis.

What is the role of coenzyme Q? (2)

- Coenzyme Q is a component of the electron transport chain that receives electrons from both reduced NAD and reduced FAD.

- Upon being reduced, it transfers the electrons further down the chain to cytochrome B.

What is the sequence of electron transfer through the cytochromes? (3)

- Electrons are passed from coenzyme Q to cytochrome B, and then from cytochrome B to cytochrome C.

- Cytochrome C then passes the electrons to cytochrome A.

- Cytochrome A is the final cytochrome and is oxidised by oxygen.

Why does reduced FAD yield less ATP than reduced NAD? (2)

- Reduced NAD transfers its electrons at the beginning of the chain, allowing for the maximum production of ATP.

- Reduced FAD transfers its electrons to coenzyme Q, entering the chain at a later stage and bypassing the first ATP synthesis site.

How does the chemiosmotic theory explain ATP synthesis? (3)

- It describes how energy stored in a proton gradient across the inner mitochondrial membrane is used to produce ATP.

- Energy from the electron transport chain is used to pump protons from the matrix to the intermembrane space, creating the gradient.

- Protons flow back into the matrix through ATP synthase, and this movement drives the synthesis of ATP.

How is an electrochemical gradient established during electron transport? (3)

- As electrons are passed along the chain of carriers, energy is released.

- This energy is used for the active transport of protons from the mitochondrial matrix to the intermembrane space.

- This accumulation of protons creates a concentration gradient and an electrical potential difference across the membrane.

What is the proton motive force? (2)

- It is the potential energy stored in the form of an electrochemical gradient, generated by the pumping of hydrogen ions.

- It is a combination of the proton concentration gradient and the electrical potential difference across the inner mitochondrial membrane.

How does the proton motive force drive ATP synthesis? (3)

- The proton motive force causes protons to diffuse back down their gradient from the intermembrane space into the matrix.

- This diffusion occurs through channels within the enzyme ATP synthase.

- The flow of protons through ATP synthase drives its rotation, which stimulates the catalysis of ADP and inorganic phosphate into ATP.

What is the basic structure of the electron transport chain? (2)

- It is composed of five multimeric complexes located in the inner mitochondrial membrane.

- The transport of electrons between complexes I to IV is coupled to the pumping of protons from the matrix into the intermembrane space.

What is ATP synthase? (3)

- It is a protein complex that synthesises ATP from ADP and inorganic phosphate.

- It is located within the inner membrane of the mitochondria.

- It is powered by the flow of hydrogen ions moving through it down an electrochemical gradient.

What are the two main steps of oxidative phosphorylation? (2)

- The electron transport chain.

- Chemiosmosis.

What is the function of Complex II in the electron transport chain? (3)

- Complex II contains the enzyme succinate dehydrogenase, which oxidises succinate to fumarate.

- During this reaction, FAD acts as the hydrogen carrier and is reduced to FADH₂.

- FADH₂ is then oxidised by passing its electrons to iron-sulphide proteins and subsequently to Coenzyme Q.

What are three differences between oxidative phosphorylation and glycolysis? (3)

- Glycolysis occurs in the cytoplasm, whereas oxidative phosphorylation occurs in the inner membrane of the mitochondria.

- Glycolysis produces only two molecules of ATP per glucose molecule, while oxidative phosphorylation generates a much higher yield.

- Oxidative phosphorylation requires oxygen to act as the final electron acceptor, whereas glycolysis does not.

Why is oxidative phosphorylation significant? (3)

- It is the final and most efficient stage of cellular respiration.

- It generates the majority of the ATP from the breakdown of glucose and other fuel molecules.

- The energy it provides is essential for powering the metabolic activities required for the survival and growth of organisms.

What processes require ATP within the mitochondrion? (3)

- The active transport of pyruvate into the mitochondrion.

- The shuttle systems used to bring electrons from NADH produced in the cytoplasm into the mitochondrion.

- The transport of ADP into the mitochondrion and the transport of ATP out to the rest of the cell.

What is the theoretical ATP yield for each reduced coenzyme? (2)

- Each reduced NAD molecule can produce 2.6 molecules of ATP.

- Each reduced FAD molecule can produce 1.5 molecules of ATP.

Why is the actual ATP yield often lower than the theoretical yield? (2)

- Some reduced NAD and FAD are used for other reduction reactions within the cell.

- Some ATP is actively used to transport pyruvate into the mitochondria.

How is the theoretical total of 33 ATP produced from one molecule of glucose? (3)

- 10 reduced NAD molecules produce 26 ATP.

- 2 reduced FAD molecules produce 3 ATP.

- 4 ATP are produced directly during glycolysis and the Krebs cycle.

What is the respiratory quotient (RQ)? (2)

- The respiratory quotient is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed per unit time by an organism.

- It is calculated using the formula, RQ = Volume of CO₂ produced / Volume of O₂ used.

Why is the RQ for aerobic respiration of glucose 1.0? (2)

- During the complete aerobic respiration of glucose, six molecules of carbon dioxide are produced.

- For this process, six molecules of oxygen are consumed, resulting in a ratio of 6/6, which equals 1.0.

What are the typical RQ values for different respiratory substrates? (3)

- The RQ value for glucose is 1.0.

- The RQ value for triglycerides is 0.7.

- The RQ value for protein is 0.9.

When is an RQ value greater than 1.0 obtained? (1)

Values of RQ greater than 1.0 are obtained during anaerobic respiration.

How is the activity of acetyl CoA carboxylase controlled? (3)

- Citrate binds to an allosteric site on the enzyme, which activates it by changing the shape of its active site.

- This change allows the enzyme to bind more effectively to its substrate, acetyl CoA.

- Conversely, an increase in fatty acyl CoA molecules competitively inhibits the enzyme.

Why might inhibitors of acetyl CoA carboxylase be useful for treating obesity? (2)

- Inhibiting the enzyme would reduce the rate of conversion of acetyl CoA into fatty acids.

- This would lead to fewer fatty acids being made, resulting in less fat being stored in the body's tissues.

Why is hydrogen cyanide gas fatal? (3)

- Cyanide is a non-competitive inhibitor of cytochrome oxidase, the final carrier in the electron transport chain.

- This inhibition stops the transport of electrons, which prevents the synthesis of ATP via oxidative phosphorylation.

- A lack of ATP means vital processes like muscle contraction and active transport stop, leading to death.

Why can antibiotics that inhibit ribosome production prevent oxidative phosphorylation? (2)

- Inhibiting ribosomes prevents the synthesis of proteins, such as the enzymes and carrier molecules needed for the electron transport chain.

- Without these essential proteins, such as ATP synthase, the process of oxidative phosphorylation cannot take place.

Why is inhibiting both glycolysis and mitochondrial respiration an effective cancer treatment? (2)

- Inhibiting both of these major respiratory pathways would prevent the cancer cells from producing any ATP.

- Without a supply of ATP, the cells would lack the energy required to carry out vital metabolic processes, such as cell division.

How do hypoxia-inducible transcription factors (HIF) increase the rate of glycolysis? (3)

- Hypoxia is caused by the reduction in the transport of oxygen reaching the tissues of the body.

- HIF can bind to the promoter region of specific genes to switch on their expression and stimulate transcription.

- This leads to the increased synthesis of the enzymes and proteins that are required for the process of glycolysis.

Why do cells increase their rate of glycolysis during hypoxia? (3)

- In hypoxic conditions, an insufficient supply of oxygen causes the electron transport chain to stop operating.

- As a result, the production of ATP via oxidative phosphorylation is significantly reduced.

- Therefore, the cell increases the rate of glycolysis to produce a small but vital supply of ATP through substrate-level phosphorylation.

What happens to the levels of HIF-1 and HIF-2 during hypoxia? (2)

- During the onset of hypoxia, the levels of both HIF-1 and HIF-2 increase.

- After reaching a peak, the level of HIF-1 falls back towards its initial level, while the level of HIF-2 remains high.

Why do the levels of HIF-1 and HIF-2 change during hypoxia? (2)

- HIF-1 and HIF-2 switch on different genes, and it is likely that the products of both are needed for the cell's initial response to the early stages of hypoxia.

- The sustained high level of HIF-2 suggests its gene products are required for adapting to longer periods of hypoxia, for example, by sustaining the increased rate of glycolysis.

What is the role of the Krebs cycle? (3)

- The Krebs cycle completely oxidises the acetyl group from acetyl CoA to release as much energy as possible.

- It generates a small amount of ATP directly via substrate-level phosphorylation.

- It produces reduced coenzymes, such as NADH and FADH₂, which are used to produce a large amount of ATP in the electron transport chain.

How could a substitution mutation in the ATP synthase gene affect oxidative phosphorylation? (3)

- The mutation would change the amino acid sequence, and therefore the primary structure, of the ATP synthase enzyme.

- This could alter the shape of the enzyme's active site, preventing ADP from binding correctly.

- The overall structure of the enzyme's proton channel could also be changed, preventing or reducing the flow of hydrogen ions back into the matrix.

Why does lipid respiration generate more ATP than carbohydrate respiration? (2)

- A lipid molecule contains a higher proportion of hydrogen atoms compared to a carbohydrate molecule of a similar mass.

- This results in the production of more reduced coenzymes, which carry more hydrogen ions and electrons to the electron transport chain.

Why might an insect's respiratory quotient (RQ) change from 1.0 at rest to 0.7 during flight? (3)

- An RQ of 1.0 indicates that the insect is respiring carbohydrates while at rest for its basic metabolic needs.

- An RQ of 0.7 indicates that the insect is respiring lipids during flight.

- Flight is an energy-intensive activity, so the insect must switch to respiring lipids as they have a higher energy density.