Exercises: Cellular respiration

Write the overall equation for the aerobic respiration of glucose and state which substance is oxidised and which is reduced in the process.

Where in the cell does glycolysis occur, and what net products are obtained from one molecule of glucose in this stage?

Name the four stages of aerobic respiration and state where in the eukaryotic cell each one takes place.

What is pyruvate and what are its two main fates after glycolysis depending on whether oxygen is available or not? State what type of metabolic pathway it follows in each case.

Define fermentation and name the two main types with their final products, indicating an organism or tissue that carries out each one.

In the diagram of aerobic respiration, state which stage corresponds to each numbered box (1, 2, and 3), name the molecules labelled A and B, and indicate where in the cell each stage takes place.

In lactic acid fermentation, pyruvate is converted into lactate. Explain why this process allows $\text{NAD}^+$ to be regenerated and in what situations human muscles use it.

For each glucose molecule, glycolysis has a net balance of $2\ \text{ATP}$ and $2$ NADH. Explain why it is called a 'net' balance, detailing how many ATP are consumed in the first phase and how many are produced in the second.

Yeasts and our muscle cells can respire or ferment depending on the available oxygen. Explain how the cell 'decides' between one pathway and the other (the Pasteur effect) and what advantage it obtains when, oxygen being present, it reduces glucose consumption while maintaining the same ATP supply.

Mitochondria possess their own DNA, their own ribosomes and divide by binary fission. Explain what evolutionary origin these data suggest (the endosymbiotic theory) and why these organelles are the right place to house cellular respiration.

In an industrial bioreactor containing yeast (Saccharomyces cerevisiae) fed with glucose, substrate consumption and formed metabolites are monitored. In an initial anaerobic phase, the yeast consumes glucose at high speed, producing ethanol and $\text{CO}_2$. Upon continuously injecting oxygen (second phase), it is observed that the rate of glucose consumption decreases to one-fourth (Pasteur effect) and ethanol production ceases completely. a) Interpret and justify at a bioenergetic level why yeast consumes much more glucose per unit time in anaerobiosis than in aerobiosis to maintain the same rate of cell division. b) If, instead of yeast, a culture of skeletal muscle cells were subjected to these same two conditions, predict which organic metabolite would accumulate in the first phase and reason whether $\text{CO}_2$ would be released. c) Justify the common metabolic need that compels both yeast and muscle cells to reduce pyruvate during fermentation.

Carbon monoxide binds to haemoglobin with more affinity than oxygen. a) Explain what effect it has on the transport of $\text{O}_2$ to the tissues. b) Reason which stages of cellular respiration will be affected and why cells switch to fermentation even when they have glucose. c) Compare the toxicity mechanism of carbon monoxide with that of cyanide.

Muscle biopsies from a sprinter are analyzed during a maximum-effort race. In the first $2$ to $3$ seconds of the race, the cellular ATP concentration remains almost unchanged ($[\text{ATP}] \approx 5\ \text{mmol/kg}$), while phosphocreatine drops drastically from $25$ to $8\ \text{mmol/kg}$. From the fourth second onward, lactate increases rapidly from $1.5$ to $12\ \text{mmol/kg}$ and mechanical power begins to decrease.

1. a) Biochemically justify why the ATP concentration remains practically constant at the beginning despite massive hydrolysis by myosin-ATPases.
2. b) Predict what would happen to ATP levels, lactate accumulation, and performance in the first $3$ seconds if a selective creatine kinase inhibitor were administered to the muscle fibers.
3. c) Compare the rate of ATP resynthesis and the total energy production capacity of the phosphocreatine system versus mitochondrial respiration, justifying why an athlete cannot maintain their top speed throughout a $400\text{ meter}$ race.

In a class debate on cellular origins, a student claims: "The enzymes of glycolysis and those of the mitochondrial electron transport chain must have evolved at the same time in the first living cells to prevent cytosolic $\text{NAD}^+$ from being depleted."

1. a) Refute the student's statement by providing two geochemical arguments about the conditions of early Earth's atmosphere and one evolutionary argument based on the endosymbiotic theory.
2. b) If the first organisms had neither oxygen nor complex membrane-dependent electron transport systems, biochemically justify the unavoidable need to couple glycolysis to fermentation reactions for the continuity of cellular life.
3. c) Predict what would happen to an obligate anaerobic bacterium, whose only route for regenerating $\text{NAD}^+$ is lactic fermentation, if a mutation inactivated its lactate dehydrogenase in a medium where glucose is its only carbon and energy source.