Chap 9 slide info

Overview of Cellular Respiration and Fermentation

Cellular respiration and fermentation are crucial biological processes through which cells convert organic molecules into usable energy. This process is vital for maintaining life, as living cells require energy to conduct essential functions such as assembling complex polymers, facilitating membrane transport, and reproducing.

Energy in Ecosystems

Energy flows into ecosystems primarily in the form of sunlight, which is harnessed by photosynthetic organisms and is critical for the production of organic molecules. This energy eventually exits the ecosystem as heat during various metabolic processes. Moreover, while energy flows one way, essential chemical elements such as carbon, nitrogen, and oxygen are recycled within ecosystems.

Photosynthesis not only produces the crucial by-product of oxygen (O2) but also generates organic molecules, or food, which serve as substrates for cellular respiration. Cells utilize these organic molecules by breaking them down to generate adenosine triphosphate (ATP), the primary energy currency of the cell.

Catabolism and Energy Production

Catabolic Pathways

These pathways play a significant role in energy production by breaking down larger and complex molecules to release stored energy. In the absence of oxygen, fermentation occurs, which partially degrades sugars and produces ATP.

• Fermentation: This process allows glycolysis to continue producing ATP without oxygen, typically yielding just 2 ATP molecules per glucose molecule. It can take two main forms:

  • Alcohol fermentation: In this type, pyruvate is converted into ethanol and carbon dioxide, commonly seen in yeast.

  • Lactic acid fermentation: Here, pyruvate is converted into lactate (lactic acid), occurring in animal muscle cells during vigorous exercise.

• Aerobic Respiration: In the presence of oxygen, this process completely oxidizes organic molecules, resulting in a much higher yield of ATP – typically around 32 molecules of ATP from a single glucose molecule.

• Anaerobic Respiration: This occurs when there is a lack of oxygen, utilizing alternative electron acceptors other than O2 to facilitate energy production.

Cellular Respiration Process

Cellular respiration generally refers to aerobic respiration, involving several interconnected metabolic pathways. The overall chemical reaction for cellular respiration is summarized by the equation:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)

Redox Reactions in Energy Transfer

Redox reactions play a pivotal role in cellular respiration, facilitating the transfer of electrons and the synthesis of ATP:

• Oxidation: Refers to the loss of electrons during the metabolic process.

• Reduction: Indicates the gain of electrons, which is essential for transferring energy.

• In any redox reaction, the electron donor is considered the reducing agent, while the electron acceptor is termed the oxidizing agent.

Overview of Cellular Respiration Stages

Cellular respiration consists of several distinct stages:

1. Glycolysis: Occurring in the cytoplasm, this process breaks down glucose into two molecules of pyruvate, yielding a net gain of 2 ATP and 2 NADH.

   • Energy Investment Phase: The initial investment of 2 ATP is necessary to transform glucose into a more reactive form.

   • Energy Payoff Phase: Subsequently, this phase produces a total of 4 ATP, resulting in a net gain of 2 ATP, alongside 2 molecules of NADH and 2 pyruvate.

2. Citric Acid Cycle (Krebs Cycle): This cycle continues glucose degradation in the mitochondria, producing CO2, NADH, FADH2, and ATP. It connects glycolysis to further processing through the formation of acetyl CoA from pyruvate. This cycle involves a series of enzymatic reactions that release CO2 as a waste product while generating high-energy electron carriers and ATP.

3. Oxidative Phosphorylation: The final stage occurs across the inner mitochondrial membrane, involving the electron transport chain and chemiosmosis. Here, NADH and FADH2 donate electrons, creating a proton gradient as protons (H+) are pumped across the membrane. ATP synthase utilizes this gradient to synthesize ATP as H+ flows back into the mitochondrial matrix, an energy-efficient process known as chemiosmosis.

ATP Yield from Cellular Respiration

The complete oxidation of one glucose molecule typically results in the generation of approximately 32 ATP. This yields energy flows through multiple stages:

• Glucose → NADH → Electron Transport Chain → ATP, with some energy inevitably lost as heat, which is a natural consequence of metabolic efficiency.

Fermentation and Anaerobic Respiration

In environments devoid of oxygen, cells can still produce ATP through fermentation, which allows limited ATP production along with the regeneration of NAD+:

• Fermentation serves as an alternative pathway, yielding only 2 ATP per glucose molecule while recycling NAD+.

Regulation of Cellular Respiration

Metabolic control is crucial for maintaining balance in energy production, primarily occurring through feedback mechanisms. Feedback inhibition can enhance respiration rates when ATP levels are low and reduce the rates when ATP is sufficiently high. This strategic enzyme regulation ensures the efficiency of energy production, optimizing metabolic pathways according to cellular energy demands.