Question

In how many steps, \(\mathrm{CO}_{2}\) is released in aerobic respiration of pyruvic acid? (a) One (b) Six (c) Three (d) Twelve

Short answer

(c) Three

Step-by-step solution

Understanding aerobic respiration

Aerobic respiration is a process in which food (glucose) is broken down in the presence of oxygen to release energy. In eukaryotes, this process includes glycolysis, Krebs cycle, and oxidative phosphorylation.

Focusing on the Krebs cycle

The Krebs cycle is specifically where parts of the pyruvic acid are converted into \(\mathrm{CO}_{2}\). Here each molecule of pyruvic acid produces three molecules of \(\mathrm{CO}_{2}\). However, since glucose eventually results in two molecules of pyruvic acid, a total of six molecules of \(\mathrm{CO}_{2}\) are produced during the Krebs cycle.

Count the steps

Although the Krebs cycle does involve multiple steps, the question is specifically asking for the number of steps in which \(\mathrm{CO}_{2}\) is released. It is important to remember that \(\mathrm{CO}_{2}\) is released every time one of the carbon atoms in pyruvic acid is removed. Therefore, since each pyruvic acid molecule contains three carbon atoms, \(\mathrm{CO}_{2}\) is released in three steps.

Key concepts

Krebs cycle

The Krebs cycle, also known as the citric acid cycle, is a critical phase in aerobic respiration. It occurs in the mitochondria of cells and is central to converting biochemical energy from nutrients into adenosine triphosphate (ATP), the energy currency of the cell.

During the Krebs cycle, pyruvic acid, a product of glycolysis, is further broken down. Each molecule of pyruvic acid is transformed into acetyl-CoA. This process begins a cyclic series of reactions where acetyl-CoA integrates with oxaloacetate to form citric acid. The cycle goes through multiple steps, each catalyzed by specific enzymes.

• Citric acid undergoes a series of transformations, leading to release of carbon dioxide.
• In addition to producing carbon dioxide, the cycle generates high-energy electron carriers: NADH and FADH₂.
• The carbon dioxide's release indicates that carbon atoms are being removed and converted to CO₂, crucial for making ATP later in oxidative phosphorylation.

The Krebs cycle is vital as it links biochemical energy from carbohydrates, fats, and proteins with cellular energy mechanisms.

Pyruvic acid

Pyruvic acid plays a pivotal role in cellular respiration as an intermediary metabolite. It is formed during glycolysis, the first step in the breakdown of glucose.

Glycolysis splits a single glucose molecule into two molecules of pyruvic acid. This simple substance is then decarboxylated in the mitochondria, preparing for entry into the Krebs cycle.

• Each molecule of glucose is thus converted into two molecules of pyruvic acid.
• These molecules then undergo conversion to acetyl-CoA before they can enter the Krebs cycle.
• It's this conversion where the first molecule of CO₂ is actually released.

By forming a bridge between glycolysis and the Krebs cycle, pyruvic acid is a key player in the overall energy conversion process of aerobic respiration, facilitating the continued production of ATP.

Carbon dioxide release

The release of carbon dioxide during aerobic respiration is an essential process to understand. Carbon dioxide production occurs at different stages but is notably highlighted during the Krebs cycle.

During the transformation of pyruvic acid into acetyl-CoA and throughout the Krebs cycle, carbon atoms are systematically removed and released as CO₂.

• Each carbon atom removed from the pyruvic acid as CO₂ is a result of chemical reactions that cascade through the Krebs cycle.
• Every complete cycle of one molecule of glucose translates to the release of six CO₂ molecules.
• These reactions are stepwise, ensuring that carbon dioxide is expelled three times per molecule of pyruvic acid within the Krebs cycle.

The regulated release of CO₂ is instrumental in balancing cellular functions and energy production. Moreover, releasing CO₂ as a waste product serves to regulate cellular environment and maintains the acidity balance in blood and tissues during respiration.