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.

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.

Explain the role played by $\text{NAD}^+$ and FAD in cellular respiration and what happens to their reduced forms (NADH and $\text{FADH}_2$) in the electron transport chain.

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.

To experimentally test the chemiosmotic hypothesis, intact mitochondria are isolated and suspended in a medium at pH 8.0 without ADP or oxidizable substrates. After allowing the matrix to reach this pH, they are abruptly transferred to a medium at pH 4.0 containing ADP and $\text{P}_i$, in the total absence of oxygen and electron donors. a) Predict with justification whether ATP will be synthesized under these artificial conditions and describe the biophysical mechanism through which ATP synthase exploits this situation. b) What would happen to ATP synthesis if the inner mitochondrial membrane had previously been treated with an ionophore substance that allows the free passage of protons? Justify your answer. c) Reason why, under normal physiological conditions, oxygen depletion halts oxidative phosphorylation.

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.

Cyanide blocks the last complex of the respiratory chain (cytochrome c oxidase). Predict what will happen to oxygen consumption, ATP synthesis, and the fate of pyruvate in a poisoned cell, and reason why cyanide is lethal even if glucose is available.