Exercises: Energy yield

How many net ATP molecules are obtained in glycolysis per glucose molecule, and how many in total (classic theoretical figure) when completely respired aerobically?

Why is fermentation much less energy-efficient than aerobic respiration, even though both begin with glycolysis? Explain what happens to pyruvate in each case.

Rank from highest to lowest energy yield per glucose molecule: aerobic respiration, lactic acid fermentation, and alcoholic fermentation. Justify the ranking.

What does it mean that a metabolic pathway has an energy yield of $40\ \%$? Explain what happens to the rest of the energy contained in the food.

State how many ATP molecules are obtained, according to currently accepted values, for each NADH and for each $\text{FADH}_2$ that deliver their electrons to the respiratory chain.

The energy of foods is usually expressed in kilocalories. Explain what a kilocalorie is and what relationship it has with the energy that the cell finally stores in the form of ATP.

Why can organisms that respire aerobically maintain larger and more active bodies than those that only ferment? Relate your answer to the energy yield of each pathway.

The human body spends energy even in absolute rest (basal metabolism). Name two processes that consume ATP even when we are lying down and still, and explain why they are essential to stay alive.

In aerobic respiration, about $36$-$38\ \text{ATP}$ are obtained per glucose, of which only $2$ come from glycolysis. Calculate what percentage of the total yield is contributed by the mitochondrial stages (taking $36\ \text{ATP}$ as the total) and comment on the result.

Calculate how many glucose molecules a cell needs to break down to obtain $180\ \text{ATP}$ aerobically ($36\ \text{ATP/glucosa}$) and how many by fermentation ($2\ \text{ATP/glucosa}$). Comment on the implication of this difference for anaerobic organisms.

The graph compares the ATP molecules obtained per glucose molecule according to the metabolic pathway used. A yeast culture without oxygen degrades $100\ \text{mol}$ of glucose by alcoholic fermentation. Calculate, from the graph, how many moles of ATP it obtains, how many it would obtain if it respired that same glucose aerobically, and how many times more efficient the aerobic pathway is.

A person ingests $100\ \text{g}$ of glucose in a meal (molar mass $180\ \text{g/mol}$, that is, about $0,56\ \text{mol}$). Calculate how many moles of ATP their organism can obtain aerobically ($36\ \text{ATP}$ per glucose) and how many grams of ATP they are equivalent to (molar mass of ATP, $507\ \text{g/mol}$). Comment on the figure obtained.

In alcoholic fermentation, $2\ \text{mol}$ of ATP are obtained per mole of glucose. Calculate how many moles of glucose would need to be fermented to equal the ATP obtained by respiring a single mole of glucose, and reason what problem this poses for an anaerobic microorganism in a sugar-poor medium.

The combustion of $1\ \text{g}$ of glucose releases about $4\ \text{kcal}$ and that of $1\ \text{g}$ of fat about $9\ \text{kcal}$. Explain what that difference is due to and calculate how many grams of fat provide the same energy as $90\ \text{g}$ of glucose.

During a marathon, a runner performs about $10000\ \text{kJ}$ of useful work. If their organism has an overall yield of $25\ \%$, calculate how much chemical energy from food they have had to mobilise and reason why their body heats up so much during the race.

The NADH produced in glycolysis must transfer its electrons into the mitochondrion via shuttle systems: the malate-aspartate shuttle yields about $2,5\ \text{ATP}$ per NADH, and the glycerol-phosphate shuttle yields about $1,5\ \text{ATP}$. Explain why the total yield of respiration for one glucose molecule varies between about $30$ and $32\ \text{ATP}$ depending on the tissue, calculating both values from the remaining coenzymes (which yield $23\ \text{ATP}$) and the $4\ \text{ATP}$ obtained by substrate-level phosphorylation.

During a maximum speed $100\ \text{m}$ sprint, fast-twitch muscle fibers demand ATP at a rate of approximately $3{,}5\ \text{mmol}/(\text{kg}\cdot\text{s})$. In those same fibers, mitochondrial oxidative phosphorylation has a maximum production rate of barely $0{,}4\ \text{mmol}/(\text{kg}\cdot\text{s})$, whereas homolactic fermentation can generate up to $4{,}0\ \text{mmol}/(\text{kg}\cdot\text{s})$.

1. a) Based on these kinetic data, justify why it is energetically profitable for the muscle cell to massively activate lactic fermentation even though its stoichiometric yield per mole of glucose is about 16-18 times lower than that of aerobic respiration.
2. b) Predict what immediate metabolic and intracellular consequences (in relation to pH, substrate availability, and the activity of enzymes such as phosphofructokinase-1) would occur if an attempt were made to sustain this fermentative rate for several minutes, explaining why muscle fatigue develops.

Under standard biochemical conditions ($\text{pH} = 7{,}0$, $25\ ^\circ\text{C}$), the complete combustion of $1\ \text{mol}$ of glucose yields $\Delta G^{\circ\prime} = -2870\ \text{kJ/mol}$ and the hydrolysis of $1\ \text{mol}$ of ATP yields $\Delta G^{\circ\prime} = -30{,}5\ \text{kJ/mol}$. However, in the cytoplasm of a living hepatocyte, the actual concentrations of ATP, ADP, and $\text{P}_\text{i}$ mean that the phosphorylation of ADP requires around $+50{,}0\ \text{kJ/mol}$, with roughly $30\ \text{mol of ATP}$ net per mole of oxidized glucose obtained experimentally.

1. a) Calculate and compare the theoretical percentage energy efficiency under standard conditions (assuming $36\ \text{ATP/glucose}$) versus the actual intracellular thermodynamic efficiency. Justify the fate of the fraction of free energy that is not stored in the phosphate bonds of ATP and its significance for homeothermic animals.
2. b) 2,4-Dinitrophenol (DNP) is a lipophilic molecule capable of transporting protons across the inner mitochondrial membrane, dissipating the electrochemical gradient. Predict, with reasoning, how treatment with DNP will affect the net yield of ATP per glucose, the rate of $\text{O}_2$ consumption by the cell, and the body temperature of the organism.

The classic theoretical balance of cellular respiration predicts the synthesis of $36$ to $38\ \text{ATP}$ per glucose molecule, but in vivo measurements in mammalian mitochondria show that the actual yield rarely exceeds $30$-$32\ \text{ATP}$.

1. a) Justify at a molecular and thermodynamic level two physiological causes that explain this discrepancy, referring to the use and dissipation of the electrochemical proton gradient between the intermembrane space and the mitochondrial matrix.
2. b) The brown adipose tissue of hibernating mammals and newborns expresses the uncoupling protein thermogenin (UCP-1) in its inner mitochondrial membrane. Apply Mitchell's chemiosmotic hypothesis to explain how UCP-1 alters the ratio of $\text{ATP/O}_2$ consumed, and deduce what adaptive advantage it provides against cold thermal stress.

In a laboratory chemostat, a mixed microbial community composed of two species with identical affinity for glucose is cultured: a yeast with facultative aerobic respiration and a strict fermentative bacterium (obligate anaerobe).

1. a) If the bioreactor, initially kept without oxygen, begins to be continuously oxygenated while maintaining a constant supply of glucose, predict the evolution of the biomass of both populations and the change in specific glucose consumption per cell (Pasteur effect). Justify your prediction by relating the energy efficiency of both catabolic pathways to macromolecule synthesis.
2. b) A student argues that, since aerobic respiration is thermodynamically more efficient, fermentation is a primitive biological pathway destined to disappear through natural selection. Refute this statement by proposing two current physiological situations or ecological niches where fermentation constitutes the optimal or irreplaceable metabolic strategy, justifying why in each case.

To determine the atomic fate of carbon in cellular respiration, an animal cell culture is incubated with glucose radiolabeled with $^{14}\text{C}$ exclusively at its carbon 1 ($^{14}\text{C}_1\text{-glucose}$).

1. a) Deduce at which pyruvate carbon ($^{14}\text{C}_1$, $^{14}\text{C}_2$, or $^{14}\text{C}_3$) the radioactivity will be located at the end of glycolysis, justifying it based on the aldol cleavage of fructose-1,6-bisphosphate and the reaction catalyzed by triose phosphate isomerase.
2. b) A researcher predicts that the first gas released in the oxidative decarboxylation of pyruvate will be radioactive $^{14}\text{CO}_2$. Evaluate whether their prediction is correct or incorrect. Justify in which functional group of acetyl-CoA the label will remain, and reason why this labeled carbon is not released as $\text{CO}_2$ during the first turn of the Krebs cycle.

The adult human brain has a mass of about $1400\ \text{g}$ (around $2\ \%$ of body mass), but at rest it consumes $20\ \%$ of the oxygen and glucose of the entire body, degrading approximately $120\ \text{g}$ of glucose per day (molar mass $= 180\ \text{g/mol}$).

1. a) Justify at a molecular and membrane transport level which continuous physiological processes of neurons explain such a basal expenditure of ATP at rest.
2. b) Calculate the daily moles of ATP produced by the brain assuming a net yield of $30\ \text{mol of ATP}$ per mole of oxidized glucose. If the total intracellular reserve of ATP in brain tissue is $2{,}5\ \mu\text{mol/g}$, calculate how many seconds of activity this reserve would last if oxidative phosphorylation were stopped instantaneously.
3. c) Based on the results of b), justify why cerebral ischemia causes loss of consciousness in less than $10\ \text{seconds}$, and explain why the brain cannot sustain its energy viability through fermentation from endogenous glycogen, unlike skeletal muscle fibers.

A migratory bird weighing $30\ \text{g}$ must face an uninterrupted journey requiring a metabolic energy expenditure equivalent to $2\ \text{mol}$ of ATP.

Data:

• Glucose: $180\ \text{g/mol}$; yield = $36\ \text{mol}$ of ATP per mole of glucose. Glycogen is stored hydrated with $2\ \text{g}$ of water per gram of dry polysaccharide.

• Palmitic acid: $256\ \text{g/mol}$; yield = $106\ \text{mol}$ of ATP per mole of fatty acid. Triacylglycerols are stored in anhydrous form.

1. a) Calculate the total reserve mass (in grams) that the bird should incorporate into its body before the journey if it stored that energy exclusively as fat versus if it did so exclusively as hydrated glycogen.
2. b) Based on the results obtained, interpret the biomechanical impact on flight and predict what would happen to the viability of migration if glycogen were used.
3. c) Justify biochemically why one dry gram of palmitic acid yields more than double the ATP of one gram of glucose, considering the oxidation state of their carbon atoms.

An ecophysiology laboratory compares the metabolic expenditure of a homeothermic mammal (a $25\ \text{g}$ mouse) and a poikilothermic reptile (a $25\ \text{g}$ lizard) by measuring $\text{O}_2$ consumption (in $\text{mL}\ \text{O}_2\cdot\text{g}^{-1}\cdot\text{h}^{-1}$) at different ambient temperatures.

1. a) Predict how the consumption of $\text{O}_2$ will vary in each animal when the ambient temperature drops sharply from $22\ \text{°C}$ to $6\ \text{°C}$. Justify your answer by relating cellular respiration to the maintenance of body temperature in homeotherms.
2. b) In a situation of extreme food shortage for several weeks, the lizard survives by remaining inactive, while the mouse dies within 24 to 48 hours if its reserves are depleted. Justify this fact based on the basal metabolic rate and the fate of ATP in both animals.
3. c) In cold environments, mammals activate thermogenesis in brown adipose tissue via the uncoupling protein thermogenin (UCP-1) in the inner mitochondrial membrane. Explain how this protein alters oxidative phosphorylation to dissipate energy as heat without ATP synthesis.

Unit challenge. Design a bioenergetic audit connecting environmental solar flux with your own daily metabolic expenditure.

1. Data collection: Check the nutritional label of an everyday food rich in carbohydrates and lipids. Then, check on the AEMET portal the average global solar radiation ($15,4\text{ MJ/m}^2$, for instance) recorded in your province this week.

2. Autotrophic origin: Describe how oxygenic photosynthesis fixed that carbon in the plant ingredients and contrast it with the harnessing of inorganic energy in bacterial chemosynthesis within agricultural soil.

3. Catabolism and yield: Model the cellular breakdown of one serving: calculate and compare the net energy yield of glucose in cellular respiration (around $30,5\text{ ATP}$) versus the $\beta$-oxidation of fatty acids and the catabolism of amino acids from proteins (deamination and oxidation of their carbon skeletons), justifying the molecular differences in obtained ATP.

4. Heterotrophic anabolism: Develop a reasoned proposal explaining which heterotrophic anabolism pathways (glycogenesis or lipogenesis) your body will activate if you consume that food at rest compared to if you engage in intense exercise.

Present the results in a brief report with bioenergetic balance diagrams.

Assessment criteria:

• Accuracy in the comparative calculation of energy yield in ATP between substrates.

• Rigorous integration of autotrophic fixation processes (photosynthesis versus chemosynthesis).

• Metabolic consistency when interconnecting catabolic pathways and heterotrophic anabolism depending on the cellular context.