Cellular respiration
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.
Glycolysis: the first step, in the cytosol
If you grow yeast 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?
Write the overall equation for the aerobic respiration of glucose and state which substance is oxidised and which is reduced in the process.
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?
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 fate of pyruvate: respire or ferment
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.
Define fermentation and name the two main types with their final products, indicating an organism or tissue that carries out each one.
Define fermentation and name the two main types with their final products, indicating an organism or tissue that carries out each one.
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.
The 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?
What is the Krebs cycle? State which two-carbon molecule enters it coming from the oxidation of pyruvate and which gas is released in its reactions.
What is the Krebs cycle? State which two-carbon molecule enters it coming from the oxidation of pyruvate and which gas is released in its reactions.
Draw or describe a mitochondrion, pointing out its outer membrane, its inner membrane with the cristae and the matrix, and state in which zones the Krebs cycle and the…
Draw or describe a mitochondrion, pointing out its outer membrane, its inner membrane with the cristae and the matrix, and state in which zones the Krebs cycle and the respiratory chain occur.
You can do it on paper and send a photo with your answer.
In the Krebs cycle, acetyl-CoA joins oxaloacetate and, after a series of reactions, the oxaloacetate is regenerated. Explain why this pathway is called a 'cycle' and…
In the Krebs cycle, acetyl-CoA joins oxaloacetate and, after a series of reactions, the oxaloacetate is regenerated. Explain why this pathway is called a 'cycle' and detail what energy products and what waste product are generated per turn.
The respiratory chain and oxidative phosphorylation
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.
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…
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.
ATP synthase works like a rotary molecular motor. Explain what makes it spin, what reaction it catalyses and in which membrane it is located in the mitochondrion.
ATP synthase works like a rotary molecular motor. Explain what makes it spin, what reaction it catalyses and in which membrane it is located in the mitochondrion.
Poisons and uncouplers: when the chain fails
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…
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.
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…
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.
2,4-Dinitrophenol (DNP) is an uncoupler: it makes the inner mitochondrial membrane permeable to protons without ATP being produced. Predict, with reasoning, what will…
2,4-Dinitrophenol (DNP) is an uncoupler: it makes the inner mitochondrial membrane permeable to protons without ATP being produced. Predict, with reasoning, what will happen to oxygen consumption, to ATP synthesis and to the body temperature of someone who ingests it, and explain why it came to be used as a slimming drug with deadly consequences.
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.