Exercises: Metabolism, anabolism, and catabolism

Define metabolism and explain the difference between anabolism and catabolism, giving an example of each type of process.

What is ATP and why is it known as the cell's "energy currency"? State what happens when it is hydrolyzed to ADP.

Classify the following processes as anabolic or catabolic and briefly justify your choice: protein synthesis, cellular respiration, photosynthesis, and food digestion.

What is an enzyme and why are the reactions of metabolism said to be catalyzed reactions? State what happens to the enzyme at the end of the reaction.

Describe the chemical composition of the ATP molecule (nitrogenous base, sugar, and phosphate groups) and state which bond is broken when it is hydrolyzed to ADP and why that bond is said to be energy-rich.

Explain why living things are said to be open systems from the point of view of the exchange of matter and energy, and what difference there is in this respect from an isolated machine.

Give two examples of processes in daily life that depend on the energy released by hydrolyzing ATP (muscle contraction, active transport, synthesis of molecules, transmission of the nerve impulse) and explain one of them briefly.

What is the activation energy of a reaction? Explain how enzymes manage to make metabolic reactions occur quickly at the temperature of the human body.

Look at the diagram of matter and energy exchange in metabolism. State which process (anabolism or catabolism) A and B represent, which molecule is indicated by ?, and explain, with an example, how the energy released by one process fuels the other.

Metabolic transformations occur in pathways consisting of many linked reactions, each catalyzed by an enzyme. Explain what a metabolic pathway is and the advantages of breaking down a transformation into numerous steps.

Distinguish between autotrophic and heterotrophic organisms from a metabolic point of view (carbon and energy source) and give an example of each type.

Photosynthetic organisms obtain their ATP from light and heterotrophs from nutrients, but all of them produce it by the same final mechanism: harnessing a proton gradient. Explain what chemiosmosis is and in which membranous structures it takes place in each type of organism.

Why is ATP, and not glucose, the immediate source of energy for the cell? Explain your answer by comparing how energy is released in each case (in small, usable amounts versus the rapid, uncontrolled combustion that releasing all of glucose's energy at once would involve).

Anabolic reactions need, in addition to ATP, reducing power in the form of NADPH. Explain what NADPH contributes to syntheses and how its function differs from that of NADH in metabolism.

Classify the following activities of a muscle cell according to whether they consume or release ATP, justifying each case: contraction, glycogen synthesis, glucose breakdown in glycolysis, and active transport of ions across the membrane.

During physical exercise, a person's oxygen consumption can be multiplied by ten or more. Explain what relationship this fact has with cellular metabolism and reason why oxygen is only needed for part of energy metabolism.

A student poses an apparent paradox: "The second law of thermodynamics dictates that spontaneous processes proceed with a $\Delta G < 0$. Since anabolism builds complex molecules through strongly endergonic reactions ($\Delta G > 0$), does the biosynthesis of macromolecules not violate the laws of physics?"

1. a) Refute the student's objection by justifying, through the principle of energetic coupling and the intermediate role of the ATP/ADP system, how the cell manages to make an overall anabolic pathway thermodynamically favorable.
2. b) If an uncoupling agent of oxidative phosphorylation (such as DNP) that dissipates the mitochondrial electrochemical proton gradient is added to a suspension of hepatocytes, reasonably predict what will happen to the rate of glucose catabolism, to the synthesis of biomolecules (anabolism), and to the temperature of the medium.

A poison blocks the hydrolysis of ATP without preventing its synthesis. Predict, with reasoning, what will happen to the cellular concentration of ATP, to anabolism, to catabolism and to cellular work, and explain why the cell will end up dying even though it is "full" of ATP.

A 70 kg adult human has at any given moment a constant total mass of only about 50 g of ATP. However, their basal and daily activity requires hydrolyzing and resynthesizing approximately 50 kg of ATP every 24 hours.

1. a) Calculate how many times a day on average each ATP molecule is recycled in this individual and deduce the average time (in seconds or minutes) that elapses between the synthesis of an ATP molecule and its subsequent hydrolysis.
2. b) If our body could not recycle ADP and had to store the required 50 kg for the day in advance as free ATP, justify the insurmountable consequences this would cause on cell volume and cellular osmotic pressure. Reason why neutral fats (triacylglycerols) and polysaccharides are physiologically ideal macromolecules as long-term energy stores compared to ATP.

In a bacterium, the synthesis pathway of the essential amino acid L from precursor P consists of three consecutive steps catalyzed by enzymes $E_1, E_2$, and $E_3$. Enzyme $E_1$ is allosteric and undergoes feedback inhibition by product L itself. a) Predict with justification what would happen to the concentration of the pathway intermediates and to the cellular energy balance if a mutation alters the allosteric site of $E_1$, preventing the binding of L but keeping its catalytic activity intact, when the bacterium grows in a medium with abundant exogenous L. b) If another mutation completely inactivates enzyme $E_2$, reason which compound will accumulate and whether the activity of $E_1$ will be maintained or slowed down in the absence of L in the medium. c) Justify the selective advantage of placing allosteric control on the first committed enzyme of the pathway and not on the last one ($E_3$).