Chapter 18: Respiration

Question 1

Where does glycolysis occur?

Answer 1

• In the cytoplasm.

- Does not require O2 –> anaerobic.

Question 2

Outline the steps of glycolysis (anaerobic).

Answer 2

1. Phosphorylation:

• Two ATP molecules required.
• Two phosphate molecules released from the two ATP molecules attach to glucose to form hexose biphosphate.

2. Lysis:
   - Destabilises molecule causing it to split into 2 triose phosphate molecules.
3. Phosphorylation:

• Another phosphate molecule added to each triose phosphate molecule forming two triose biphosphate molecules.
• Phosphate molecules comes from free inorganic phosphate ions present in cytoplasm.

4. Dehydrogenation + ATP formation:

• Two triose biphosphate molecules oxidised by the removal of H atoms to form two pyruvate molecules.
• NAD coenzymes accept the two removed H atoms –> they are reduced forming two reduced NAD molecules.
• 4 ATP molecules formed using phosphates from the two triose biphosphate molecules.

Question 3

Define substrate level phosphorylation.

Answer 3

• Formation of ATP without involvement of ETC.

- ATP formed by transfer of phosphate group from phosphorylated intermediate (triose biphosphate) to ADP

Question 4

Describe the structure of mitochondria.

Answer 4

Matrix:

• Contains enzymes for Krebs cycle + link reaction.
• Contains mitochondrial DNA.

Intermembrane Space:

• Proteins pumped into here by ETC.
• Small space –> concentration builds up quickly.

Inner Mitochondrial Membrane:

• Contains ETC + ATP synthase.

Outer Mitochondrial Membrane:

• Separates contents of mitochondrion with from rest of cell.
• Provide cellular compartments with ideal conditions for aerobic respiration.

Cristae:

• Projections of inner membrane.
• Increase s.a. for oxidative phosphorylation.

Question 5

Outline steps for oxidative decarboxylation (link reaction)

Answer 5

1. In eukaryotic cells –> pyruvate enters mitochondrial matrix by active transport via specific carrier proteins.
2. Pyruvate undergoes oxidative decarboxylation –> hydrogen and CO2 removed.
3. Removed H atoms accepted by NAD.
4. NAD reduced to NADH (reduced NAD).
5. Resulting two carbon acetyl group bound by coenzyme A to form acetylcoenzyme A (acetyl CoA).
6. Acetyl CoA delivers acetyl groups to Krebs cycle.
7. Reduced NAD used in oxidative phosphorylation to synthesise ATP.
8. Acetyl groups are all thats left of original glucose molecule.
9. CO2 produced will either diffuse away + be removed from organism as metabolic waste or be used as a raw material of photosynthesis by autotrophic organisms.

Question 6

Where does the Krebs cycle take place?

Answer 6

• Mitochondrial matrix.

Question 7

Outline the stages of the Krebs cycle.

Answer 7

1. Acetyl CoA delivers acetyl group to Krebs cycle:
   - The two-carbon acetyl group combines with the four-carbon oxaloacetate to form six-carbon citrate.
2. Citrate molecule undergoes dehydrogenation + decarboxylation forming one reduced NAD + CO2:
   - Five-carbon compound produced.
3. Five-carbon compound –> undergoes further dehydrogenation + decarboxylation –> eventually regenerate oxaloacetate + cycle continues:

• More CO2, two NAD + one FAD produced.
• ATP also produced by substrate-level phosphorylation.

Question 8

State the importance of coenzymes in respiration.

Answer 8

• Used to transfer protons, electrons and functional groups between many enzyme-catalysed reactions.
• NAD + FAD –> accept protons + electrons released during breakdown of glucose in respiration.

Question 9

Outline the differences between NAD + FAD.

Answer 9

• NAD takes part in all steps of cellular respiration.
• FAD only accepts hydrogens in Krebs cycle.
• NAD accepts one hydrogen.
• FAD accepts two hydrogens.
• Reduced NAD oxidised at start of ETC releasing protons + electrons.
• Reduced FAD oxidised later along the chain.
• Reduced NAD –> synthesis of 3 ATP molecules.
• Reduced FAD –> synthesis of 2 ATP molecules.

Question 10

Outline steps of oxidative phosphorylation.

Answer 10

1. Hydrogen atoms collected by NAD + FAD delivered to ETC present in membranes of cristae of mitochondria.
2. H atoms dissociate into H+ ions + electrons.
3. High energy electrons used in ATP synthesis by chemiosmosis.
4. Energy released during redox reactions as the electrons reduce + oxidise electron carriers as they flow along ETC.
5. Energy used to create proton gradient leading to diffusion of protons through ATP synthase leading to ATP synthesis.
6. At end of ETC electrons combine with H+ ions and O2 to form water.
7. O2 –> final electron acceptor + ETC cannot operate unless O2 present.
8. Phosphorylation of ADP to form ATP dependent on electrons moving through ETC + requires O2.

Question 11

What are the 3 different categories organisms fall into based on their dependence on O2 or not?

Answer 11

• Obligate aerobe.
• Facultative anaerobe.
• Obligate anaerobe.

Question 12

Explain obligate anaerobe.

Answer 12

• Cannot survive in presence of O2.
• Almost all are prokaryotes.
• Some fungi.

Question 13

Explain facultative anaerobe.

Answer 13

• Synthesise ATP by aerobic respiration if O2 present.
• Switch to anaerobic respiration in absence of O2.
• E.g. yeast.

Question 14

Explain obligate aerobe.

Answer 14

• Can only synthesise ATP in presence of O2 –> e.g. mammals.

Question 15

Why can individual cells of some organisms (e.g. muscle cells in mammals) be described as facultative anaerobes?

Answer 15

• They can supplement ATP supplies by respiring anaerobically in addition to aerobic respiration when O2 conc. low.
• Only for short periods + eventually O2 required.
• Shortfall of O2 during periods of anaerobic respiration produces compounds that have to be broken down when O2 available again,
• Therefore organism as a whole is obligate aerobe.

Question 16

Define fermentation.

Answer 16

• Process by which larger organic molecules are broken down into simpler inorganic molecules without the use of O2 or ETC.

Question 17

What happens when there is no O2 to act as last electron acceptor at end of oxidative phosphorylation?

Answer 17

• Electron flow stops.
• No ATP synthesis by chemiosmosis.
• Reduced NAD + FAD –> not oxidised –> nowhere for electrons to go.
• NAD + FAD –> cannot be regenerated –> decarboxylation + oxidation of pyruvate + Krebs cycle comes to a halt –> no coenzymes accept H+ ions being removed.
• Glycolysis stops due to lack of NAD.

Question 18

Outline steps of lactate fermentation in mammals.

Answer 18

1. Pyruvate –> acts as hydrogen acceptor taking H from reduced NAD –> catalysed by lactate dehydrogenase.
2. Pyruvate converted to lactate (lactic acid) + NAD regenerated –> keep glycolysis going so small quantity of ATP synthesised.
3. Lactic acid removed from muscles + transported to liver in bloodstream where it is converted back to glucose:
   - O2 needed for this –> leads to oxygen debt + heavy breathing after exercise.
4. No O2 as final electron/hydrogen acceptor:
   - Link reaction/Krebs cycle/ETC cannot take place.

Question 19

Why can’t lactate fermentation occur indefinitely?

Answer 19

• Lactic acid build up decreases pH –> denatures proteins:
• Respiratory enzymes + muscle filaments made from proteins + will cease to function at low pH.
• Reduced quantity of ATP produced not enough to maintain vital processes for long periods of time.

Question 20

Outline steps of alcohol fermentation in yeast + plants.

Answer 20

1. Pyruvate converted to ethanal –> catalysed by pyruvate decarboxylase.
2. Ethanal accepts H atom from reduced NAD becoming ethanol.
3. Regenerated NAD continues to act as coenzyme + glycolysis continues.

Question 21

How do methanogens play essential role in digestive system of ruminants?

Answer 21

• Digest cellulose from grass cell walls into products that can be further digested, absorbed + used by ruminants.
• CO2 is the last electron acceptor in ETC and methane + water are produced.

Question 22

State + explain the biochemical adaptations of marine mammals to low O2 environments.

Answer 22

• More Hb + myoglobin in muscles used for swimming –> maximises O2 available and delays aerobic respiration.
• Whales –> high tolerance to lactic acid + high CO2 conc. –> can respire anaerobically and much longer without tissue damage.
• -> has blood buffer system that prevents large increases in pH.

Question 23

State + explain the physiological adaptations of marine mammals to low O2 environments.

Answer 23

• Modified circulatory systems:
• When they dive they undergo peripheral vasoconstriction which slows down heart rate by reducing blood flow to brain, heart and muscles –> bradycardia.
• Reduces energy demands of cardiac muscle.

Question 24

State + explain the physical adaptations of marine mammals to low O2 environments.

Answer 24

• Streamlining –> reduce drag due to friction from water whilst swimming –> reduce energy demand during dive.
• Limbs are fin-shaped –> maximise efficient use of energy in propulsion.

Question 25

How does a greater proportion of C-H bonds affect the RQ value of a lipid relative to a carbohydrate?

Answer 25

• More ATP produced in respiration.
• Require more O2 to break down.
• Release less CO2.
• RQ for lipid < 1 (around 0.7)
• RQ for carb = 1

Question 26

Outline respiration pathway of triglyceride.

Answer 26

1. Triglyceride broken down into fatty acid + glycerol.
2. Fatty acids undergo beta oxidation forming acetyl groups.
3. Acetyl groups taken into Krebs cycle by coenzyme A.
4. Glycerol converted to pyruvate which undergoes oxidative phosphorylation.

Question 27

Describe the difference between a respirometer and a spirometer.

Answer 27

• Both measure O2 uptake/CO2 release and so respiration rate.
• Respirometer is modified spirometer for smaller organisms.

Question 28

Why does aerobic respiration yield fewer molecules of ATP than the theoretical maximum?

Answer 28

• Some ATP used to actively transport pyruvate into mitochondrial matrix in link reaction.
• Some ATP used to actively transport H+ from reduced NAD, formed in glycolysis into the mitochondrion.
• Some energy released in ETC, released as heat, not used to transport H+ across membrane.
• Not all H+ movement back across membrane is used to generate ATP/is through ATP synthase.
• Not all reduced NAD used to feed into the ETC.

Question 29

Why does the incomplete breakdown of glucose in anaerobic respiration produce less ATP than aerobic respiration?

Answer 29

In anaerobic respiration:

• Glycolysis/conversion of glucose to pyruvate occurs.
• Produces two ATP molecules
• Only substrate-level phosphorylation occurs.
• O2 not available as final electron acceptor.
• Pyruvate/ethanal used to regenerate NAD for glycolysis.
• Krebs cycle + ETC + chemiosmosis + oxidative phosphorylation do not occur.

Question 30

The anaerobic respiration pathway in animal cells can be reversed, but the anaerobic pathway in yeast cells cannot be reversed. Explain why this is.

Answer 30

• Animals:
• Pyruvate converted to lactate.
• Lactate is the only product so can be reversed.
• Lactate dehydrogenase reverses reaction.
• Yeast:
• Pyruvate converted to ethanol + CO2.
• Cannot be reversed as CO2 is lost.
• Pyruvate decarboxylase cannot reverse reaction.

Question 31

Suggest why lactate is converted into pyruvate by hepatocytes rather than by the respiring tissues in which it is produced.

Answer 31

• Hepatocytes can tolerate low pH/lactate.
• Have enzymes to metabolise lactate.
• Conversion of lactate requires O2 + muscle cells do not have enough O2/O2 not available during anaerobic respiration.

Question 32

Explain what might happen to a person if the liver did not break down insulin.

Answer 32

• BGC would fall below normal level + would continue to be taken up by liver.
• Glucose continually converted to glycogen.
• Mitochondria cannot release enough energy/ATP.
• Coma/death.

Question 33

Describe + explain the role of ATP in the cells

Answer 33

• Transfer energy.
• Phosphate removed by hydrolysis to provide energy.
• Energy released for metabolism/active transport/glycolysis.
• ADP can attach to phosphate forming ATP during respiration/photosynthesis.
• Energy released in small quantities to prevent cell damage.

Question 34

Describe the way in which an endothermic animal, such as a mammal, normally prevents its body temp from decreasing when the external temp decreases.

Answer 34

• Peripheral thermoreceptors stimulated by decrease in temp.
• Impulses sent to hypothalamus/sensory cortex.
• Vasoconstriction of arterioles to reduce heat loss.
• Prevent heat loss by radiation, conduction, convection.
• Increased metabolic rate to generate heat.
• Release of adrenaline/thyroxine.
• Shivering to generate heat.
• Erector pili muscles raise to trap air/heat.