Lecture 14

Exam Overview

• Post-exam discussions: Continuous feedback and discussions will be provided soon via Teaching Assistants (TAs), ensuring students understand their performance and clarify any doubts regarding the exam.

• Grading system: The top five scores from the exam are averaged and considered as a perfect score of 100. This method allows for the recognition of consistent high performance among students while accounting for discrepancies in individual test difficulty.

• Grade adjustments: Adjustments to grades may be made based on individual performance relative to overall class performance, and the varying difficulty level of the exam will be taken into consideration to ensure fairness in grading.

Oxidative Phosphorylation

• What it is: Oxidative phosphorylation is a crucial biochemical process that generates adenosine triphosphate (ATP) from food molecules. This process occurs in the mitochondria and is the main pathway for ATP production in aerobic organisms.

• Mechanism: High-energy electrons derived from food molecules, such as glucose, are transferred through the electron transport chain (ETC). This flow of electrons is coupled with the pumping of protons (H+ ions) across the inner mitochondrial membrane, resulting in the formation of a proton gradient across the membrane.

• ATP Synthase: ATP synthase is a vital enzyme located in the inner mitochondrial membrane that harnesses this proton gradient to synthesize ATP. It is responsible for the majority of ATP generated in cells, using the energy from protons that flow back into the mitochondrial matrix.

Chemiosmotic Model Hypothesis

• Evidence: The chemiosmotic model is supported by various experimental findings, particularly those demonstrating that the flow of electrons through the ETC is tightly coupled with ATP synthesis.

• Key Experiment: When NAD or succinate is added to isolated mitochondria, oxygen consumption increases, indicating active electron transport. Furthermore, supplying ATP allows for continued electron transport; however, the introduction of oligomycin, an ATP synthase inhibitor, halts both ATP synthesis and electron transport, illustrating the direct connection between these processes.

Uncouplers and Their Mechanism

• Definition: Uncouplers, such as dinitrophenol (DNP), are compounds that disrupt the coupling between electron transport and ATP synthesis in oxidative phosphorylation.

• How it works: DNP acts as a hydrophobic weak acid that can traverse the inner mitochondrial membrane. It picks up protons from the intermembrane space and redistributes them into the mitochondrial matrix, allowing protons to bypass ATP synthase.

• Result: This uncoupling allows for the continued flow of electrons through the ETC without the accompanying synthesis of ATP, resulting in the release of energy as heat. This can lead to increased body temperature and potentially dangerous conditions for organisms, as it disrupts normal metabolic function.

ATP Synthase Structure

• F1 Subunit: The F1 subunit extends into the mitochondrial matrix and is responsible for synthesizing ATP from ADP and inorganic phosphate (Pi) utilizing the energy derived from the proton gradient.

• F0 Subunit: The F0 subunit is embedded in the inner mitochondrial membrane and facilitates the flow of protons across the membrane. This subunit functions like a motor, as protons moving through it induce conformational changes that drive ATP production.

Photosynthesis Overview

• Process: Photosynthesis occurs in plants, algae, and some bacteria, following oxidative phosphorylation, and is one of the most vital biological processes on Earth. It converts solar energy into chemical energy stored in glucose, enabling life to thrive.

• Carbon Fixation: This process involves the conversion of carbon dioxide (CO2) into organic compounds, predominantly glucose, through a series of biochemical reactions.

• Oxygen Production: Photosynthesis also produces oxygen as a byproduct, which is essential for the survival of aerobic organisms, including humans, as it is a critical component for cellular respiration.

Importance of Photosynthesis

• Sustaining Life: Photosynthesis is fundamental to life on Earth, capturing energy from sunlight and transforming CO2 into organic compounds that serve as food for various life forms.

• Energy Pyramid: Plants and photosynthetic microorganisms form the base of the energy pyramid, providing sustenance for herbivores, which in turn support carnivores and the overall food web.

• Carbon Cycle: Photosynthesis plays a crucial role in the carbon cycle by fixing atmospheric CO2 into organic matter, thus helping to regulate and maintain CO2 levels in the atmosphere.

Carbon Dioxide Levels

• Historical Context: Historically, atmospheric CO2 levels were around 300 ppm, but current measurements have surpassed 400 ppm. This increase has been largely driven by human activities, including fossil fuel combustion and deforestation.

• Urgency for Action: There is a pressing need to reverse the trend of rising CO2 emissions. Enhancing photosynthesis through agricultural practices and reforestation is being explored as one way to naturally absorb CO2 from the atmosphere.

Calvin Cycle

• Location and Requirements: The Calvin cycle, often referred to as the dark reactions of photosynthesis, occurs in the stroma of chloroplasts. It specifically requires ATP and NADPH, which are produced during the light-dependent reactions.

• Process Overview: In this cycle, ribulose-1,5-bisphosphate (RuBP) combines with CO2, catalyzed by the enzyme RuBisCO, to produce glucose and other carbohydrates.

• RuBisCO Enzyme: RuBisCO is the most abundant enzyme on Earth and plays a key role in carbon fixation, but it also catalyzes an unwanted side reaction known as photorespiration, which can reduce the efficiency of photosynthesis.

C4 Pathway

• Adaption Mechanism: The C4 pathway is an adaptive mechanism evolved by some plants to thrive in high-temperature environments while minimizing photorespiration.

• Process Details: Initially, CO2 binds to phosphoenolpyruvate (PEP) to form a four-carbon compound, which is then transported to bundle sheath cells. Here, CO2 is released for entry into the Calvin cycle, significantly reducing competition between CO2 and oxygen at the active site of RuBisCO.

Conclusion

• Interconnected Processes: Photosynthesis and oxidative phosphorylation are intrinsically linked processes that are vital for energy metabolism and the sustenance of life on Earth.

• Further Study: Ongoing research into these processes aims to deepen our understanding of cellular respiration, energy transfer, and the ecological balance that underpins our environment.