CICLO DE KREBS | Biologia para o Enem | Cláudia de Souza Aguiar

Summary

In this video, the teacher explains the Krebs cycle, a crucial part of cellular respiration. It takes place in the mitochondrial matrix of eukaryotic cells, where acetyl-CoA combines with oxaloacetate to begin the cycle. The process generates energy molecules like NADH, FADH2, and ATP while releasing carbon dioxide. The cycle repeats twice, as it is fueled by two pyruvate molecules from glycolysis. The Krebs cycle produces essential substances for the next stage of cellular respiration, oxidative phosphorylation, which will be explored in a future class.

Takeaways

• 😀 The Krebs cycle is a critical stage in cellular respiration, also known as the citric acid cycle, and occurs in the mitochondrial matrix of eukaryotic cells.
• 😀 The cycle begins with the combination of pyruvic acid (pyruvate) and coenzyme A, forming acetyl-CoA, which initiates the process.
• 😀 The Krebs cycle occurs twice for each glucose molecule, as glycolysis produces two pyruvic acids, starting two cycles.
• 😀 The cycle involves a series of reactions, producing key molecules: 2 CO2, 3 NADH, 1 FADH2, and 1 ATP per cycle.
• 😀 While ATP is produced, the primary goal of the Krebs cycle is to generate NADH and FADH2, which are crucial for oxidative phosphorylation.
• 😀 The cycle begins with the formation of citrate (a 6-carbon molecule), which is then converted into isocitrate, undergoing oxidation to release carbon dioxide and form NADH.
• 😀 Alpha-ketoglutarate, a 5-carbon molecule, is formed after the oxidation of isocitrate, followed by another carbon dioxide release and NADH formation.
• 😀 Succinyl-CoA is produced from alpha-ketoglutarate after further oxidation, leading to another CO2 release and NADH generation.
• 😀 Succinyl-CoA is converted to succinate, releasing ATP and FADH2 in the process, with a reduction in carbon content to 4 carbons.
• 😀 The cycle concludes with the formation of oxaloacetate, which is regenerated to start a new cycle, making the Krebs cycle a continuous and cyclical process.
• 😀 The NADH, FADH2, and ATP produced in the Krebs cycle provide essential energy for the next stage of cellular respiration, oxidative phosphorylation.

Q & A

What is the Krebs cycle and why is it important?

The Krebs cycle, also known as the citric acid cycle, is a key part of cellular respiration that takes place in the mitochondria of eukaryotic cells. It helps produce energy-rich molecules like NADH, FADH2, and ATP, which are essential for the next stage of cellular respiration, oxidative phosphorylation.

Why is the Krebs cycle also referred to as the citric acid cycle?

It is called the citric acid cycle because the cycle begins with the formation of citrate (citric acid), a six-carbon molecule, when acetyl-CoA binds with oxaloacetate.

Where does the Krebs cycle occur in the cell?

The Krebs cycle occurs in the mitochondrial matrix of eukaryotic cells, which contain mitochondria. Prokaryotic cells can also undergo the cycle, but they do not have mitochondria.

Why does the Krebs cycle produce only one ATP per cycle?

The primary aim of the Krebs cycle is to generate NADH and FADH2, which are used in oxidative phosphorylation to produce more ATP. The one ATP produced in the Krebs cycle is a secondary outcome, as the focus is on producing electron carriers for later stages.

Why does the Krebs cycle occur twice for every glucose molecule?

Glycolysis, the process that occurs before the Krebs cycle, produces two molecules of pyruvate. Since each pyruvate molecule enters the Krebs cycle, the cycle must occur twice, once for each pyruvate.

What happens to the carbons during the Krebs cycle?

During the Krebs cycle, carbon atoms are released as carbon dioxide. Initially, citrate has six carbons, but through a series of reactions, the number of carbons is reduced, with the release of two molecules of CO2.

What role does NADH play in the Krebs cycle?

NADH is produced when NAD+ gains electrons (hydrogens) during oxidation reactions in the cycle. It acts as an electron carrier, which will later participate in oxidative phosphorylation to produce more ATP.

What is the function of FADH2 in the Krebs cycle?

FADH2 is produced when FAD gains electrons (hydrogens) during a reaction in the cycle. Like NADH, FADH2 acts as an electron carrier and is used in oxidative phosphorylation to help produce ATP.

What happens to succinyl-CoA during the Krebs cycle?

Succinyl-CoA undergoes a reaction in which it loses coenzyme A and forms succinate, releasing one molecule of ATP in the process. This also leads to the production of FADH2.

How is the Krebs cycle related to glycolysis?

The Krebs cycle is linked to glycolysis because glycolysis produces pyruvate, which is the molecule that enters the cycle. After glycolysis, pyruvate is converted into acetyl-CoA, which starts the Krebs cycle.