Question

Select the correct statement. (a) When ATP is synthesized directly from metabolites, it is substrate level phosphorylation. (b) In Krebs' cycle, citrate undergoes 2 decarboxylations and 4 dehydrogenations. (c) Krebs' cycle is an amphibolic process. d) All of these.

Short answer

d) All of these.

Step-by-step solution

Analyze Option (a)

Consider the process described in option (a). ATP synthesis can occur via two primary pathways: oxidative phosphorylation and substrate-level phosphorylation. Substrate-level phosphorylation refers to the direct synthesis of ATP from an energy-rich substrate during a chemical reaction in the metabolic pathway.

Analyze Option (b)

Evaluate the changes that citrate undergoes during the Krebs' cycle. Citrate is first converted to isocitrate, followed by a series of reactions that include decarboxylations, where two molecules of CO2 are released, and dehydrogenations, where NADH and FADH2 are produced through four oxidation steps.

Analyze Option (c)

Determine whether Krebs' cycle is indeed an amphibolic pathway. An amphibolic pathway is one that serves in both the catabolic and anabolic processes - breaking down molecules for energy and building other molecules, respectively. The Krebs' cycle is involved in both the degradation of acetyl-CoA for energy production (catabolic) and the provision of precursors for various biosynthetic processes (anabolic).

Synthesize Understanding of Options

Having evaluated each option individually, options (a), (b), and (c) are accurate statements regarding cellular metabolism and the Krebs' cycle. Therefore, option (d), which states that all the options are correct, is the right selection.

Key concepts

Substrate Level Phosphorylation

Substrate level phosphorylation is akin to a relay race where the baton – in this case, a phosphate group – is directly passed to ADP to form ATP. Unlike oxidative phosphorylation, which requires a complex system of electron carriers and a gradient-driven turbine-like ATP synthase, substrate level phosphorylation is more straightforward. It involves the transfer of a phosphate group from a high-energy substrate molecule directly to ADP, producing ATP.

During the Krebs' cycle, specifically, substrate level phosphorylation occurs in two distinct steps. One instance is the conversion of succinyl-CoA to succinate, where the high-energy bond in succinyl-CoA is exploited to attach a phosphate group to GDP, forming GTP, which can be readily converted to ATP. This critical step is a prime example of the efficiency of direct phosphorylation in cellular energy cycles.

Understanding substrate level phosphorylation is essential because it highlights a method by which cells can rapidly replenish ATP without the need for oxygen, which is particularly crucial in anaerobic conditions where oxidative phosphorylation cannot occur.

Decarboxylation in Krebs' Cycle

Decarboxylation can be likened to pruning a tree, where excess branches, the carbon atoms in this analogy, are cut away to reshape the overall structure. In the Krebs' cycle, decarboxylation is a critical step where carbon dioxide (CO₂) is released, and the carbon skeleton of the substrate is remodeled. This happens not once, but twice, signifying two key transformations in the cycle.

The first decarboxylation occurs when isocitrate, a six-carbon molecule, is transformed into alpha-ketoglutarate, a five-carbon molecule. In this process, one CO₂ molecule is released into the atmosphere. Subsequently, another carbon is shed when alpha-ketoglutarate becomes succinyl-CoA, leaving a four-carbon compound behind.

These steps of the Krebs' cycle are not only important for reducing the carbon count but also for generating NADH, a crucial molecule that later donates electrons to the electron transport chain to drive the production of a significant amount of ATP. Hence, decarboxylation is a central aspect of cellular energy production, weaving together the breaking down of carbon chains with the harnessing of high-energy electrons.

Amphibolic Pathway

An amphibolic pathway is a biochemical crossroad that leads to both construction and deconstruction, depending on the metabolic demands of the cell. Just as a central hub in a railway system directs trains to various destinations, the Krebs' cycle or citric acid cycle operates as an amphibolic pathway, serving multiple metabolic purposes.

In its catabolic role, the Krebs' cycle is involved in the breakdown of acetyl-CoA derived from carbohydrates, fats, and proteins into CO₂ and energy-rich electron carriers like NADH and FADH2. These carriers will then enter the oxidative phosphorylation pathway to produce ATP. On the other side of the coin, the anabolic function of the Krebs' cycle is about the synthesis. It provides precursors for many biosynthetic processes, such as the formation of amino acids or gluconeogenesis, where new glucose is generated.

This dual functionality is vital for the adaptability of cellular metabolism. Whether a cell is in a state of starvation or plenty, the amphibolic nature of the Krebs' cycle allows it to toggle between tearing down molecules for immediate energy or constructing new molecules for future use. It symbolizes the intricate balance of breakdown and synthesis that is fundamental to life.