Before a Gaelic football match in Croke Park, a player's cells must keep releasing energy from glucose for 70 minutes. A cell can obtain energy from glucose in two ways: by burning it all at once, as in a laboratory, or by breaking it down little by little, in dozens of linked steps, capturing the energy as ATP at each one. The second option is cellular respiration, and it is the one used by almost all living things.
Overview: the equation and the four stages
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Name the four stages of aerobic respiration and state where in the eukaryotic cell each one takes place.
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…
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.
If you grow yeast (as used in brewing) or muscle cells in a container with no oxygen at all and with glucose, you find that the sugar is used up quickly and the cells survive by producing ATP: how does the cell manage to extract energy from a carbohydrate without using oxygen?
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Where in the cell does glycolysis occur, and what net products are obtained from one molecule of glucose in this stage?
Where in the cell does glycolysis occur, and what net products are obtained from one molecule of glucose in this stage?
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…
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.
The Pasteur effect: when oxygen is present, yeast and muscle cells switch from fermentation to aerobic respiration. Respiration gives about 30 ATP per glucose instead of 2, so the cell needs far less glucose (about one-fifteenth) for the same ATP supply, and glucose consumption falls.
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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…
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.
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…
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.
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…
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.
The citric acid cycle (Krebs cycle): in the mitochondrial matrix
When mitochondria are supplied with glucose labelled with radioactive carbon, the $\text{CO}_2$ they give off contains that label, but the amount of oxaloacetate in the matrix stays constant after millions of molecules have been broken down: how can the fuel be completely oxidised within a pathway whose components never run out?
The mitochondrion and its origin
A mitochondrion has a smooth outer membrane, a folded inner membrane (the folds are cristae, where the electron transport chain and ATP synthase sit) and the matrix inside (link reaction and citric acid cycle). Mitochondria have their own circular DNA, ribosomes and divide by binary fission. This supports the endosymbiotic theory: they descend from aerobic bacteria engulfed by an ancestral cell. Early Earth's atmosphere had almost no oxygen, so the first cells relied on glycolysis and fermentation; oxygen-using chains evolved later.
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Mitochondria possess their own DNA, their own ribosomes and divide by binary fission. Explain what evolutionary origin these data suggest (the endosymbiotic theory) and…
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 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…
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."
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.
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.
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.
The respiratory chain and oxidative phosphorylation
Poisons and uncouplers: when the chain fails
Carbon monoxide (for example from a faulty boiler) binds haemoglobin more strongly than oxygen, so less O₂ reaches the tissues; the chain starves of its final acceptor and cells shift to fermentation. Cyanide instead blocks complex IV inside the mitochondrion itself.
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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…
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.
Back to the beginning: By breaking glucose down step by step instead of burning it all at once, the cell avoids an uncontrolled combustion that would destroy its structures and manages to convert more than $30\,\%$ of that chemical energy into 30-32 ATP bonds that can be used directly.
Summary
Let's practice! 5 medium exercises in this section