Studyledge

Energy and Respiration: Question 5

Syllabus 12.1, 12.2

Structured A2 8 marks

When oxygen cannot be supplied quickly enough to sustain aerobic respiration, cells can respire anaerobically instead, and the type of respiratory substrate used also affects the gases exchanged during respiration.

(a) Compare anaerobic respiration in mammalian muscle cells with anaerobic respiration in yeast cells, naming the products formed from pyruvate in each case, and explaining why both pathways are needed to allow glycolysis to continue. [4] (b) The respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in during respiration. State the equation for the aerobic respiration of glucose, use it to give the RQ for carbohydrate, and explain why the RQ for a lipid respiratory substrate is lower than this. [4]

Show worked solution Hide worked solution

Worked solution

Part (a): Comparing anaerobic respiration in muscle and yeast

When oxygen cannot be supplied quickly enough for the link reaction and Krebs cycle to keep up, pyruvate produced by glycolysis is instead diverted into a fermentation pathway. Both pathways start from the same problem: reduced NAD builds up in the cytoplasm during glycolysis, and this NAD must be regenerated so glycolysis can continue.

  • Mammalian muscle cells (lactate fermentation): pyruvate is directly reduced to lactate, using hydrogen from reduced NAD (catalysed by lactate dehydrogenase). This reoxidises reduced NAD back to NAD. No carbon dioxide is released in this pathway.
  • Yeast cells (ethanol fermentation): pyruvate is first decarboxylated to ethanal (acetaldehyde), releasing carbon dioxide; the ethanal is then reduced to ethanol, again using hydrogen from reduced NAD and regenerating NAD.

In both cases, the essential purpose is the same: regenerating NAD so that it is available again for the oxidation step in glycolysis (the conversion of triose phosphate to pyruvate). Without this regeneration, glycolysis would stop as soon as the small supply of free NAD in the cytoplasm ran out, and the cell would lose even the small net gain of ATP that glycolysis provides. Neither fermentation pathway produces any additional ATP itself; their role is purely to keep glycolysis running by recycling the coenzyme.

Part (b): The respiratory quotient (RQ)

The respiratory quotient is defined as the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in during respiration (equivalently, the ratio of the volumes of these gases exchanged, under the same conditions).

For the complete aerobic respiration of glucose:

glucose + 6 oxygen → 6 carbon dioxide + 6 water (C6H12O6 + 6O2 → 6CO2 + 6H2O)

Six molecules of carbon dioxide are produced for every six molecules of oxygen taken in, so:

RQ = 6 (CO2) ÷ 6 (O2) = 1.0 for carbohydrate.

Lipid molecules contain a much higher proportion of hydrogen (and carbon) relative to oxygen than carbohydrate molecules do (carbohydrates already contain a relatively large amount of oxygen bonded to their carbon skeleton). When a lipid is fully oxidised, extra oxygen has to be taken in to oxidise all of this additional hydrogen to water, in proportion to the amount of carbon dioxide produced from the carbon atoms. As a result, more oxygen is used per molecule of carbon dioxide produced than for carbohydrate, giving lipid respiratory substrates a lower RQ than carbohydrate, typically around 0.7.

Final answers

  • (a) Mammalian muscle: pyruvate → lactate (no CO2 released). Yeast: pyruvate → ethanal (+ CO2) → ethanol. Both regenerate NAD from reduced NAD so glycolysis can continue producing its net ATP.
  • (b) Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O; RQ for carbohydrate = 6 ÷ 6 = 1.0. Lipid RQ is lower (≈ 0.7) because lipids contain proportionally more hydrogen, requiring more oxygen per carbon dioxide produced.