Metabolism, anabolism and catabolism

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

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

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

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

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

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.

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

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.

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 ($E_3$).

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.

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 critic of evolutionary theory claims that the origin and maintenance of complex, ordered living beings from simple precursors is physically impossible because it contradicts the second law of thermodynamics, according to which disorder (entropy) must necessarily increase.

1. a) Using the formulation $\Delta S_\text{universe} = \Delta S_\text{system} + \Delta S_\text{surroundings} \ge 0$, refute this argument by distinguishing between an open system and an isolated one, and justify how cellular catabolism makes it possible to compensate for the local decrease in internal entropy generated during anabolism.
2. b) Design a thought experiment in which you enclose an animal and its food in a rigid, perfectly airtight, and adiabatic container (isolated from the outside with respect to matter and heat). Predict the vital and thermodynamic fate of the system over time and explain why Earth's biosphere does not suffer that same entropic collapse thanks to continuous solar flux.