Cellular Respiration Part 2: The Citric Acid Cycle

Summary

This video explores the citric acid cycle, also known as the Krebs cycle, which is a key part of aerobic respiration in the mitochondria. After glycolysis generates pyruvate, it enters the mitochondrial matrix, where it is converted to acetyl-CoA. The cycle then goes through eight enzyme-catalyzed steps, producing energy carriers like NADH and FADH2. These products are essential for oxidative phosphorylation, which generates the bulk of ATP in aerobic organisms. The video emphasizes the importance of this cycle for producing more energy than glycolysis alone, enabling complex life forms to evolve.

Takeaways

• 🧪 Glycolysis is an anaerobic process, meaning it doesn't require oxygen, and it was sufficient for early organisms.
• 🌱 Higher organisms evolved aerobic respiration for more efficient energy production, thanks to the oxygen produced by photosynthesis.
• 🏭 Aerobic respiration occurs in the mitochondria, which likely evolved from independent organisms according to endosymbiotic theory.
• 🧬 Pyruvate from glycolysis enters the mitochondria and is converted into acetyl CoA, a crucial molecule for the citric acid cycle.
• 🔄 The citric acid cycle (also called the Krebs cycle or tricarboxylic acid cycle) is an 8-step process that occurs in the mitochondrial matrix.
• 🔧 Citrate synthase starts the cycle by combining acetyl CoA with oxaloacetate to form citrate.
• 🌀 The cycle produces 3 NADH, 1 FADH2, and 1 ATP (or GTP) per acetyl CoA, which doubles per glucose molecule due to the production of two pyruvates.
• 💧 Multiple enzymes, including aconitase, isocitrate dehydrogenase, and succinate dehydrogenase, play key roles in the various steps of the cycle.
• ⚡ The citric acid cycle itself produces a moderate amount of energy, but its real contribution is providing electron carriers (NADH, FADH2) for oxidative phosphorylation.
• 📈 Oxidative phosphorylation, the next stage after the citric acid cycle, generates the majority of ATP in aerobic respiration.

Q & A

What is glycolysis, and why is it considered anaerobic?

Glycolysis is the metabolic process that breaks down glucose into pyruvate, generating 2 ATP molecules. It is considered anaerobic because it does not require oxygen to occur.

Why was glycolysis sufficient for early organisms, but not for higher organisms like animals?

Early organisms, which were simple and had low energy requirements, could survive on the 2 ATP produced by glycolysis. However, higher organisms, like animals that need more energy to perform complex tasks such as running or swimming, required more efficient energy pathways like aerobic respiration.

How did the presence of oxygen in the atmosphere enable more efficient energy production?

The presence of oxygen, which became abundant after plants started producing it through photosynthesis, made aerobic respiration possible. This process, occurring in the mitochondria, produces much more energy than glycolysis alone.

What is the role of mitochondria in aerobic respiration?

Mitochondria are organelles in eukaryotic cells where aerobic respiration takes place. According to endosymbiotic theory, they were once separate organisms that were incorporated into eukaryotic cells due to their ability to generate energy through respiration.

What happens to pyruvate once it enters the mitochondria?

In the mitochondrial matrix, pyruvate undergoes decarboxylation and oxidation by NAD+, then attaches to Coenzyme A, forming acetyl CoA. This acetyl CoA then enters the citric acid cycle.

What is the first step of the citric acid cycle, and which enzyme is involved?

The first step of the citric acid cycle involves the enzyme citrate synthase, which removes the acetyl group from acetyl CoA and attaches it to oxaloacetate, forming citrate.

What is the significance of NAD+ and FAD in the citric acid cycle?

NAD+ and FAD are electron carriers that play key roles in the citric acid cycle by accepting electrons during oxidation reactions. NAD+ is involved in multiple steps, while FAD participates in the oxidation of succinate to fumarate.

What are the end products of the citric acid cycle for each acetyl CoA that enters?

For each acetyl CoA, the citric acid cycle produces 3 NADH molecules, 1 FADH2 molecule, and 1 ATP (or GTP).

How many NADH, FADH2, and ATP molecules are produced per glucose molecule in the citric acid cycle?

Since one glucose molecule produces two pyruvates, which then form two acetyl CoA molecules, the citric acid cycle produces 6 NADH, 2 FADH2, and 2 ATP per glucose molecule.

What is the significance of the citric acid cycle in terms of energy production?

While the citric acid cycle itself doesn't produce much ATP directly, it generates NADH and FADH2, which are crucial for oxidative phosphorylation. This final step in aerobic respiration produces the majority of ATP in the cell.

Outlines

00:00

🔬 Introduction to the Citric Acid Cycle

Professor Dave introduces the citric acid cycle, emphasizing its importance in aerobic respiration. He briefly recalls glycolysis, the anaerobic process used by early organisms to generate energy, and notes how this limited energy production led to the evolution of more complex metabolic pathways. With the advent of plants and oxygen production through photosynthesis, aerobic respiration became possible, allowing higher organisms to generate more energy.

🧬 Mitochondria and Their Role in Respiration

The mitochondria, referred to as the powerhouse of eukaryotic cells, are highlighted as the location of the citric acid cycle. According to endosymbiotic theory, mitochondria were once independent organisms absorbed into eukaryotic cells for their respiratory capabilities. The process starts with pyruvate molecules produced during glycolysis, which enter the mitochondria and react with Coenzyme A after decarboxylation and oxidation, forming acetyl CoA, the molecule that initiates the citric acid cycle.

⚙️ The Steps of the Citric Acid Cycle

Professor Dave dives into the detailed steps of the citric acid cycle, also known as the Krebs or tricarboxylic acid cycle. The eight-step pathway involves the enzyme citrate synthase forming citrate from acetyl CoA and oxaloacetate. The process continues with a series of oxidation, decarboxylation, and hydration reactions, each facilitated by specific enzymes, producing key molecules like NADH, FADH2, and ATP. These molecules are essential for the next phase of cellular respiration.

🔁 Overall Yield of the Citric Acid Cycle

For each acetyl CoA entering the cycle, the products include three NADHs, one FADH2, and one ATP. Since glycolysis produces two pyruvates per glucose, this doubles the output per glucose molecule. The summary concludes with a list of enzymes and their respective roles in the citric acid cycle. While the energy yield from the cycle itself is moderate, these products play a crucial role in the subsequent oxidative phosphorylation process, where the majority of ATP is generated in aerobic respiration.

📚 Conclusion and Further Learning

Professor Dave wraps up the tutorial by noting that oxidative phosphorylation will follow the citric acid cycle to produce the majority of ATP in aerobic respiration. He encourages viewers to subscribe to his channel and reach out via email for further questions, signaling the end of the educational video.

Keywords

💡Glycolysis

Glycolysis is the metabolic pathway that breaks down glucose into pyruvate, generating a small amount of energy (2 ATPs). It is an anaerobic process, meaning it does not require oxygen. In the video, it is described as the primary method early organisms used to generate energy before the atmosphere was filled with oxygen.

💡Anaerobic

Anaerobic refers to processes that occur without the need for oxygen. Glycolysis is given as an example of an anaerobic process. The video highlights how early life forms relied on anaerobic processes like glycolysis to generate energy in the absence of oxygen.

💡Mitochondria

Mitochondria are membrane-bound organelles in eukaryotic cells where aerobic respiration occurs. According to the video, they are thought to have originated as separate organisms that were incorporated into eukaryotic cells, as per the endosymbiotic theory, allowing complex life forms to produce more energy through oxygen-dependent processes.

💡Coenzyme A

Coenzyme A is a molecule that plays a crucial role in cellular respiration. It binds to pyruvate in the mitochondria to form acetyl CoA, which enters the citric acid cycle. In the video, this molecule is essential for transferring the acetyl group, enabling the continuation of energy production.

💡Citric Acid Cycle

Also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, the citric acid cycle is an eight-step metabolic pathway where acetyl CoA is oxidized to produce energy. The video outlines each step of this cycle and its importance in generating NADH, FADH2, and ATP, key components for aerobic respiration.

💡NAD+

NAD+ (Nicotinamide adenine dinucleotide) is a coenzyme that plays a key role in redox reactions, accepting electrons during various metabolic processes. In the video, NAD+ is repeatedly reduced to NADH, which is later used in oxidative phosphorylation to produce a significant amount of ATP.

💡Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of aerobic respiration where most of the ATP is generated. It uses the NADH and FADH2 produced in the citric acid cycle to drive ATP synthesis in the mitochondria. The video notes that although the citric acid cycle generates some ATP, the bulk of energy production comes from this process.

💡Acetyl CoA

Acetyl CoA is a molecule formed from pyruvate and Coenzyme A. It is the entry point for the citric acid cycle, where its acetyl group is combined with oxaloacetate to begin the cycle. The video emphasizes its importance as the starting molecule that fuels the citric acid cycle.

💡FADH2

FADH2 is a molecule that, like NADH, carries electrons to the electron transport chain during oxidative phosphorylation. It is produced in the citric acid cycle during the oxidation of succinate to fumarate. The video explains that FADH2, along with NADH, is critical for ATP production in aerobic respiration.

💡Endosymbiotic Theory

The endosymbiotic theory suggests that mitochondria were once independent prokaryotic organisms that became integrated into eukaryotic cells. The video references this theory to explain why mitochondria are essential for cellular respiration, having evolved to enhance the energy production capabilities of complex organisms.

Highlights

Glycolysis is an anaerobic process, meaning it doesn't require oxygen and allowed early organisms to generate energy.

The first organisms relied on glycolysis, but it only produces two ATP per glucose molecule.

Plants covering the earth and producing oxygen made aerobic respiration possible, leading to the evolution of higher organisms.

Aerobic respiration occurs in the mitochondria, which were once separate organisms incorporated into eukaryotic cells according to the endosymbiotic theory.

The pyruvate generated during glycolysis enters the mitochondrial matrix, undergoing decarboxylation and oxidation to form acetyl-CoA.

Acetyl-CoA enters the citric acid cycle, also called the Krebs cycle or tricarboxylic acid cycle, which is an eight-step pathway with eight enzymes.

In the first step, citrate synthase converts acetyl-CoA and oxaloacetate into citrate.

Aconitase helps transform citrate into its structural isomer, isocitrate, through removal and addition of a water molecule.

Isocitrate dehydrogenase catalyzes oxidation by NAD+ and decarboxylation, leading to the formation of alpha-ketoglutarate.

Further decarboxylation and oxidation by NAD+ leads to the formation of succinyl-CoA, with CoA again binding.

Succinyl-CoA synthetase displaces CoA with a phosphate group, forming succinate and producing one molecule of GTP, which is used to make ATP.

Succinate dehydrogenase oxidizes succinate using FAD, leading to the formation of fumarate and FADH2.

Fumarase catalyzes hydration of fumarate to form malate.

Malate dehydrogenase catalyzes a final oxidation by NAD+ to regenerate oxaloacetate, completing the cycle.

For each acetyl-CoA, the cycle produces three NADH, one FADH2, and one ATP, doubling these amounts for each glucose molecule.

Transcripts

Professor Dave again, let's talk about the citric acid cycle.

We learned about glycolysis, which is an anaerobic process, meaning it

does not require oxygen to occur. Since oxygen is not necessary,

the first simple organisms on earth were able to generate energy through

glycolysis for millions of years. But two ATPs per glucose just isn't that much.

In order for higher organisms like animals to evolve that can run, and jump, and swim,

they had to evolve additional metabolic pathways that generate far more energy

than glycolysis. This became possible once plants covered the earth, thereby

filling the atmosphere with oxygen, which is a product of photosynthesis,

thus the possibility for large-scale oxygen dependent aerobic respiration was born.

The location in the cell where this process occurs is the mitochondria, which

are eukaryotic cell organelles. According to endosymbiotic theory, mitochondria

seem to have been entirely separate organisms that were incorporated into

eukaryotes specifically for the respiratory abilities they possessed.

This activity begins with the pyruvate molecules that were generated in the

cytoplasm during glycolysis. These pyruvates will enter the mitochondrial

matrix to find Coenzyme A. In the presence of NAD+ pyruvate will undergo

decarboxylation, oxidation by NAD+, and then attachment to Coenzyme A, generating

acetyl CoA. Acetyl CoA will then enter the citric acid cycle, also known as the

Krebs cycle, or the tricarboxylic acid cycle. This is an eight-step pathway

requiring eight separate enzymes. In the first step, the enzyme citrate synthase

removes the acetyl group and tacks it on to oxaloacetate to form citrate.

Next, with help from aconitase, a water molecule is removed

and another one is added to generate a structural isomer of citrate called

isocitrate. Then, catalyzed by isocitrate dehydrogenase, isocitrate is oxidized by

NAD+ and then decarboxylates to form alpha-ketoglutarate. Next, another CO2

is lost and further oxidation by NAD+ takes place, with the help of

ketoglutarate dehydrogenase. The resulting molecule will join with Coenzyme A once

again to form succinyl-CoA. CoA is then displaced by a phosphate group to

form succinate, which is catalyzed by succinyl-CoA synthetase. This will

make one molecule of guanosine triphosphate, or GTP, in the process, which

can be used to make one ATP. Then with the help of succinate dehydrogenase

succinate is oxidized by a different molecule, FAD, which will result in

fumarate and FADH2. Next, fumarase will catalyze hydration which results in

malate, and lastly, one more oxidation by NAD+ takes place with the help of malate

dehydrogenase to give oxaloacetate, which will restart the cycle, reacting with a

new acetyl CoA. Overall, for every acetyl CoA that enters, this cycle

will produce three NADHs, one FADH2, and one ATP. Since one glucose will produce

two pyruvates in glycolysis and therefore two acetyl CoAs, we can double

these numbers to get the amounts per glucose molecule. Once again, for the ease

of memorization, here is a list of each step of the citric acid cycle with the

names of the respective enzymes. We can see that there still hasn't been a huge

payoff in terms of energy, but the products of the citric acid cycle will

then move on to oxidative phosphorylation, which will generate the

majority of the ATP produced in aerobic respiration.

Thanks for watching guys, subscribe to my channel for more tutorials, and as always, feel free to email me: