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 hydrolysed to ADP.

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

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 catalysed 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.

Consider the following two independent metabolic reactions under physiological conditions:

1. $\text{Glucose} + \text{P}_\text{i} \rightarrow \text{Glucose-6-phosphate} + \text{H}_2\text{O} \quad (\Delta G^{\circ\prime} = +13.8\text{ kJ/mol})$

2. $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_\text{i} \quad (\Delta G^{\circ\prime} = -30.5\text{ kJ/mol})$

1. a) Justify at the molecular level why the hydrolysis of ATP has such a strongly exergonic $\Delta G^{\circ\prime}$ (refer to electrostatic charge repulsion and resonance stabilization of the products formed).
2. b) Write the equation for the net coupled reaction catalyzed by the enzyme hexokinase, calculate its overall $\Delta G^{\circ\prime}$, and explain how this energy coupling mechanism enables cellular processes that, in isolation, would be thermodynamically unfavorable.

In an investigation on muscle metabolism, a cell line is analyzed with a mutation that renders the enzyme glycogen synthase constitutively active and insensitive to inhibition by phosphorylation, so that it remains at 100 % activity even during intense adrenaline surges.

1. a) During a vigorous muscle contraction triggered by adrenaline, glycogen phosphorylase is activated by a phosphorylation cascade. Reasonably predict the immediate metabolic consequence of both enzymes (synthase and phosphorylase) acting simultaneously at maximum speed and justify why this phenomenon (futile cycle) is energetically detrimental to the cell.
2. b) Explain how coordinated reciprocal regulation (through reversible covalent modification and allosteric modulators such as AMP and ATP) prevents this conflict from occurring in normal muscle fibers during rest and exercise.

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.