Cellular Respiration

Cellular respiration is the metabolic pathway used by humans and animals to create energy for everyday life. This process can be summarized by the equation:

C6H12O6 + O₂ → CO₂ + H₂O + ATP

The process of cellular respiration consists of three main stages:

1. Glycolysis: This stage involves breaking down glucose to create electron carriers.
2. Krebs cycle: This cycle creates more electron carriers.
3. Oxidative phosphorylation: This final stage uses electrons to create ATP.

Definition: Anaerobic respiration doesn't require oxygen, while aerobic respiration needs oxygen.

Glycolysis is the first stage of cellular respiration and occurs outside the mitochondria in the cytoplasm. It can be divided into two phases:

1. Energy investment phase: 2 ATP molecules are used to break down glucose into 2 G3P molecules.
2. Energy pay-off phase: G3P is used to create pyruvate, resulting in the production of 4 ATP and 2 NADH.

Highlight: The net gain from glycolysis is 2 ATP molecules and 2 NADH molecules per glucose molecule.

The overall result of glycolysis is:

1 glucose → 2 pyruvate + 2 NADH + 2 ATP $net gain$

This process sets the stage for the subsequent stages of cellular respiration, which will further break down the pyruvate molecules to generate more energy.

Pyruvate Oxidation and Krebs Cycle

After glycolysis, the next stages of cellular respiration take place in the mitochondria, beginning with pyruvate oxidation and followed by the Krebs cycle.

Pyruvate Oxidation:

1. Pyruvate reacts with coenzyme A to create Acetyl CoA.
2. This reaction also produces CO₂ and NADH.

Vocabulary: Coenzyme A is a crucial molecule that helps convert pyruvate into Acetyl CoA.

Krebs Cycle:

1. Acetyl CoA enters the mitochondrial matrix and loses the coenzyme A, leaving a 2-carbon acetyl group.
2. Oxaloacetate joins with the acetyl group to form citric acid.
3. Citric acid undergoes a series of reactions to reform oxaloacetate, producing CO₂, NADH, FADH₂, and ATP in the process.

Highlight: The Krebs cycle is a key process in cellular respiration, producing important electron carriers $NADH and FADH₂$ that will be used in the next stage.

The Krebs cycle is also known as the citric acid cycle due to the formation of citric acid in its initial step. This cycle is crucial for generating the electron carriers that will be used in the final stage of cellular respiration, oxidative phosphorylation.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of cellular respiration and takes place in the mitochondria. This process uses the electron carriers $NADH and FADH₂$ created during glycolysis, pyruvate oxidation, and the Krebs cycle to generate ATP.

The process involves several key steps:

1. Complex I: NADH donates electrons to Complex I, which pumps H⁺ ions from the mitochondrial matrix into the intermembrane space.
2. Electron Transport Chain: Electrons are passed through a series of carrier proteins, including Complex II $which receives electrons from FADH₂$, Complex III, and Complex IV.
3. Proton Gradient: As electrons move through the complexes, more H⁺ ions are pumped into the intermembrane space, creating a concentration gradient.
4. ATP Synthesis: The H⁺ ions flow back into the matrix through ATP synthase, driving the production of ATP.

Highlight: Oxygen serves as the final electron acceptor in this process, combining with electrons and H⁺ ions to form water.

Vocabulary: The inner mitochondrial membrane $IMM$ is where the electron transport chain complexes are located.

Oxidative phosphorylation is highly efficient, producing the majority of ATP generated during cellular respiration. This process demonstrates the importance of the electron carriers produced in earlier stages of respiration and the crucial role of oxygen as the final electron acceptor.

Fermentation

Fermentation is an alternative pathway for producing ATP when oxygen is not available. It occurs after glycolysis if O₂ is not present and serves to regenerate NAD⁺, which is necessary for glycolysis to continue.

There are two main types of fermentation:

1. Lactic Acid Fermentation: Glucose is broken down to pyruvate through glycolysis. Pyruvate is then converted to lactic acid. NADH is oxidized back to NAD⁺ in this process.
2. Alcoholic Fermentation: Glucose is broken down to pyruvate through glycolysis. Pyruvate is converted to ethanol and CO₂. NADH is oxidized back to NAD⁺ in this process.

Example: Lactic acid fermentation occurs in muscle cells during intense exercise when oxygen supply is limited.

Highlight: While fermentation allows for the continuation of glycolysis in the absence of oxygen, it produces much less ATP compared to the complete process of cellular respiration.

Fermentation is an important process in many industries, including food production $e.g., yogurt, cheese, bread$ and biofuel production. However, in terms of energy production for cellular processes, it is less efficient than aerobic respiration, which can produce a much larger amount of ATP through the complete oxidation of glucose.

Calvin Cycle

The Calvin cycle is a crucial part of photosynthesis that does not require light and can occur during both day and night. This process is responsible for carbon fixation, which is essential for plant growth and survival.

The Calvin cycle consists of three main stages:

1. Fixation of CO₂: In this stage, carbon dioxide is added to an organic molecule.
2. Formation of G-3P: This stage involves the addition of phosphate from ATP to form 1,3-biphosphoglycerate, which is then reduced by NADPH.
3. Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, while others are used to create sugar.

Vocabulary: RuBP $Ribulose-1,5-bisphosphate$ is a 5-carbon sugar that plays a crucial role in the Calvin cycle.

Definition: Carbon fixation refers to the process of adding CO₂ to an organic molecule.

Highlight: The enzyme Rubisco catalyzes the reaction between RuBP and CO₂, which is a critical step in the Calvin cycle.

The overall formula for photosynthesis, which includes the Calvin cycle, is:

CO₂ + H₂O → C6H₁2O6 + O₂

This equation represents the conversion of carbon dioxide and water into glucose and oxygen, driven by the energy captured from sunlight during the light-dependent reactions of photosynthesis.