cellular respiration

Question 1

what is the definition of catabolism?

Answer 1

catabolism includes the various pathways involving the breakdown of absorbed food substances into simpler, smaller molecules. there is an overall release of energy

Question 2

what is the definition of anabolism?

Answer 2

anabolism includes the various pathways involving the biosynthesis of complex molecules from simpler compounds. there is an overall energy requirement

Question 3

what ar the three most common ways in which a biomolecule can be oxidised or reduced?

Answer 3

1. decarboxylation, which is the removal of carbon atoms from a compound to form carbon dioxide
2. dehydrogenation, which is the oxidation of organic molecule which frequently involves the removal of electrons as well as hydrogen ions
3. oxidative decarboxylation, which is the removal of a carboxylate group, forming carbon dioxide

Question 4

what is the role of coenzymes in redox reactions in respiration?

Answer 4

1. electrons and protons which are removed from oxidised substances are transferred to one of several coenzymes, acting as transient carriers of electrons, hydrogen or other specific functional groups
2. upon reduction, these reduced coenzymes serve as reservoirs of electrons and protons to form ATP via oxidative phosphorylation

Question 5

how is glucose catabolism affected by the availability of oxygen?

Answer 5

1. in the presence of oxygen, this results in complete oxidation of glucose to carbon dioxide and water, termed aerobic respiration, which yields the maximum amount of 36/38 ATP molecules
2. in the absence of oxygen, this results in incomplete oxidation of oxygen, termed anaerobic respiration or fermentation, which can be identified as either ethanol fermentation or lactic acid fermentation. this process yields only net 2 ATP molecules.

Question 6

what are the 10 steps of glycolysis?

Answer 6

1. activation of unphosphorylated glucose to a phosphorylated fructose-1,6-biphosphate
2. hydrolysis of 2 ATPs to provide not just the phosphate groups, but also the energy
3. rate limiting step of glycolysis involving the enzyme phosphofructokinase
4. cleavage of fructose-1,6-biphosphate to two 3 carbon sugars, glyceraldehyde-3-phosphate (G3P)
5. each G3P molecule is dehydrogenated and NAD is reduced to NADH, resulting in a production of 2 NADH per molecule, each supplying 2 energised electrons, which directly drive ATP production by oxidative phosphorylation at the inner mitochondrial membrane
6. ADP is phosphorylated, coupled to the dephosphorylation of an organic substrate, producing 4 ATPs per glucose molecule, and an overall net gain of 2 ATPs per glucose molecule

Question 7

what is the importance and significance of glycolysis?

Answer 7

1. glycolysis, which occurs in most living cells, is the only catabolic reaction that can be completed in the absence of oxygen, hydrolysing one molecule of glucose into 2 molecules of pyruvate
2. in the presence of oxygen, pyruvate enters the mitochondrion and are completely oxidised to produce ATP by oxidative phosphorylation. reduced NAD and FAD supply energised electrons for ATP production
3. glycolysis supplies cells with essential biosynthetic precursors. the liver carries out glycolysis to provide precursors for a whole range of molecules it synthesises, including fats, cholesterol etc
4. once liver glycogen reserves are full, carbohydrate is converted to fats, and thus glycolysis is then predominantly associated with supplying the initial steps of fat biosynthesis with substrate

Question 8

how is glycolysis regulated?

Answer 8

1. as ATP accummulates, it acts as an allosteric inhibitor by binding to phosphofructokinase and thus slowing down glycolysis
2. phosphofructokinase is stimulated by AMP which the cell derives from ADP. it becomes active again as cellular work converts ATP to ADP faster than ATP is being regenerated
3. phosphofructokinase is also sensitive to citrate, as it passes into the cytosol and inhibits phosphofructokinase upon accummulation, synchronising the rates of glycolysis and the krebs cycle

Question 9

how does the mitochondrial structure relate to its function?

Answer 9

1. presence of outer membrane allows the mitochondria to be freely permeable to ATP and ADP
   2.

Question 10

how does the mitochondrial structure relate to its function?

Answer 10

1. presence of outer membrane allows the mitochondria to be freely permeable to ATP and ADP
2. presence of a selectively permeable membrane which is highly-folded to form cristae which increase surface area for embedding electron transport chain and ATP synthase complexes. it is not permeable to NADH, contains proteins for transporting H+, ATP and ADP, and contains members of the ETC and ATP synthase complexes. it is permeable to pyruvate
3. the matrix is a compartment enclosed by the inner mitochonrial membrane, and is the site of the link reaction and krebs cycle, and contains enzymes required for reactions in the krebs cycle

Question 11

what are the steps in the link reaction?

Answer 11

1. when molecule O2 is present, the pyruvate produced by glycolysis enters the mitochondrion, where the oxidation of glucose is completed in krebs cycle
2. pyruvate’s carboxyl group that is fully oxidised, is removed and liberated as CO2
3. the remaining 2C fragment is oxidised, forming acetate. two electrons and a proton removed are transferred to the coenzyme NAD, reducing it to NADH
4. coenzyme A is attached to acetate, forming acetyl CoA
5. this entire process is catalysed by pyruvate dehydrogenase, and occurs twice for each glucose molecule

Question 12

what are the steps in the krebs cycle?

Answer 12

1. condensation - acetyl coA adds its 2C acetyl group to oxaloacetate, producting citrate
2. citrate is converted to its isomer, isocitrate, by removal of 1 water molecule and the addition of another
3. oxidative decarboxylation NAD reduction - isocitrate is oxidised, reducing NAD to NADH, then the resulting compound loses a CO2 molecule. another CO2 is lost, and the resulting compound is oxidised, reducing NAD to NADH. the remaining molecule is then attached to coenzyme A by an unstable bond
4. substrate level phosphorylation - coA is displaced by a phosphate group, which is transferred to GDP, forming GTP, a molecule which functions similar to ATP. GTP can also be used to generate ATP
5. FAD reduction - two hydrogens are transferred to FAD, forming FADH2 and oxidising succinate
6. addition of a water molecule rearranges bonds in the substrate
7. NAD reduction - the substrate is oxidised, reducing NAD to NADH and regenerating oxaloacetate.

Question 13

how does 36/38 AP molecules arise from the complete oxidation of a glucose molecule?

Answer 13

the reduced coenzymes are high-energy compounds, which means that the transfer of electrons from these coenzymes to oxygen is highly exergonic, and thus accounts for the synthesis of most of the 36/38 ATP molecules produced during the complete oxidation of a glucose molecule

Question 14

how does the ETC transfer electrons from NADH and FADH2 to oxygen?

Answer 14

1. the ETC, embedded in the inner mitochondrial membrane, comprises a sequence of electron carriers that have the ability to be reversibly reduced and oxidised as electrons from NADH and FADH2 are passed down the mitochondrial ETC
2. several of the coenzymes and cytochromes in the ETC are bound to enzymes and other essential factors to form complexes I-IV
3. each subsequent member of the ETC has a higher affinity for electrons than its predecessor, but a lower affinity than its successor, ensuring a one-way transport of electrons down the ETC in order of increasing electron affinity
4. the electrons are eventually passed to molecular oxygen that has diffused into the mitochondrion, and is reduced in the mitochondrial matrix to produce a molecule of water
5. the energy released as the electrons flow through the ETC is sufficient to power the pumping of H+ ions across the inner mitochondrial membrane into the intermembrane space, establishing an electrochemical proton gradient

Question 15

what are the implications of proton pumping generating a proton gradient?

Answer 15

1. this establishes a H+ concentration gradient, as well as an electrical gradient due to the higher concentration of positively charged protons on one side of the membrane
2. with the build up of H+ concentration in the intermembrane space, there is a tendency for H+ to diffuse back into the matrix, creating a proton motive force
3. however, as the inner mitochondrial membrane is impermeable to H+, protons can only re-enter the matrix via an ATP-synthase complex that spans the inner mitochondrial membrane

Question 16

what is the mode of action of the ATP synthase complex?

Answer 16

1. the ATP synthase complex functions to couple the exergonic passage of H+ to the energonic phosphorylation of ADP to form AP
2. multiple copies of ATP synthase stud the inner mitochondrial membrane. ATP synthase is a multisubunit complex with four main parts, each made up of multiple polypeptides.
3. protons move one by one into binding sites on one of the part, causing it to spin in a way that catalyses ATP production from ADP.
4. for each pair of electrons stripped from NADH, 5 pairs are pumped across the inner mitochondrial membrane into the intermembrane space, which generates 3 ATP molecules
5. for each pair of electrons stripped from FADH, 3 pairs are pumped across the inner mitochondrial membrane into the intermembrane space, which generates 2 ATP molecules

Question 17

describe the glycerol phosphate shuttle

Answer 17

1. it is found in muscle and nerve cells
2. cytosolic glycerol-3-phosphate dehydrogenase converts dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate by oxidising one molecule of NADH to NAD
3. 2 ATP will be formed per molecule of cytosolic NADH oxidised, accounting for 36 molecules of ATP produced per glucose molecule

Question 18

describe the malate-aspartate shuttle

Answer 18

1. it is found heart and liver cells
2. the electrons and protons stripped from cytosolic NADH are passed to matrix NAD
3. 3 ATP will be formed per molecule of cytosolic NADH oxidised, accounting for 38 molecules of ATP produced per glucose molecule

Question 19

what are the 3 classes of respiratory poisons or inhibitors?

Answer 19

1. poisons that block electron flow - CO, cyanide and HS block the transport of electrons from cytochrome a3 to oxygen. this completely inhibits ATP production
2. poisons that inhibit ATP synthase - the antibiotic oligomycin, directly inhibits ATP synthase by preventing the influx of protons through ATP synthase. although the proton gradient becomes larger than normal, its potential energy cannot be tapped to make ATP
3. poions that make the inner mitochondrial membrane leaky to protons - 2,4-dinitrophenol and other acidic aromatic compounds are known as uncoupling agents, which carry protons across the inner mitochondrial membrane. as the protons leak back across the membrane, the proton gradient dissipates, and the proton gradient build-up are uncoupled. since no proton gradient is formed, no ATP can be made by oxidative phosphorylation, and the energy derived from electron transport is released as heat.

Question 20

describe the process of lactic acid fermentation

Answer 20

1. pyruvate is reduced directly by NADH to form lactate as a waste product, with no release of CO2
2. lactase is the ionised form of lactic acid
3. lactic acid can be reconverted to pyruvate when oxygen becomes available again
4. this occurs in animals and in certain fungi and bacteria
5. in animals, in the short term, the reaction satisfies the greater priority of regenerating NAD. in the long term, lactic acid is toxic and must ultimately be removed
6. in certain fungi and bacteria, lactic acid fermentation is used in the dairy industry to make cheese yoghurt

Question 21

describe the process of alcoholic fermentation

Answer 21

1. pyruvate is converted to ethanol in 2 steps
2. CO2 is released from pyruvate, which it itself is converted to acetaldehyde
3. acetaldehyde is reduced by NADH to ethanol
4. this process occurs in fungi and in most plant tissues

Question 22

how does substrate concentration and type affect the rate of respiration?

Answer 22

1. plants respire more rapidly after having been exposed to conditions favourable for photosynthesis during which carbohydrates are synthesised.
2. increase in respiration has also been observed to be associated with an increase in soluble sugars
3. nutrients such as amino acids and fatty acids are transformed into one of the metabolic intermediates that are fed into glycolysis or the citric acid cycle

Question 23

how does temperature affect the rate of respiration?

Answer 23

1. estimation of temperature coefficient Q10 of the process for a rise in temperature from 8-18 degrees celcius gives a Q10. of 2, indicating a chemical reaction
2. if the rise is at a much higher starting temperature, say between 20-30 degrees celcius, then the Q10 may fall below 2
3. in some cases the rate of respiration increases as a lower temperature, sometimes due to an increase in the quantity of substrate.

Question 24

what is the function of a compensation tube?

Answer 24

the compensation tube which contains an equal volume of inert materials to the volume of organisms used helps to compensate for changes in atmospheric pressure

Question 25

how is the respiratory quotient of an organism measured?

Answer 25

1. oxygen consumption at a particular temperature is found 2. then the respirometer is set up with the same organism at the same temperature, but with no chemical to absorb CO2 3. the manometer scale will show whether the volume of oxygen absorbed and CO2 produced are the same 4. the RQ can be measured by taking the volume of CO2 produced divided by the volume of oxygen