Cellular Metabolism: Energy and Reactions

Metabolism encompasses all chemical reactions in cells, converting substances to generate or consume energy. These reactions are broadly categorized into anabolic (building up) and catabolic (breaking down), with energy carriers like ATP playing a crucial role in cellular processes.

Core Principles

• Metabolism (stofwisseling) involves continuous biochemical reactions within cells.
• Energy is essential for metabolic processes, driving complex cellular organization.
• Energy exists in various forms: chemical, kinetic, heat, light, and potential.
• The law of conservation of energy applies: energy can be transformed but not created or destroyed.
• Systems naturally tend towards disorder; increased order requires energy input.
• Anabolic reactions build larger molecules from smaller ones, requiring energy (endergonic).
• Catabolic reactions break down larger molecules into smaller ones, releasing energy (exergonic).
• ATP (adenosine triphosphate) is the universal energy currency of the cell.
• Electron carriers like NAD+, NADP+, and FAD are vital for energy transfer.
• Aerobic respiration fully oxidizes glucose to CO2 and water, yielding significant ATP.
• Fermentation is an anaerobic process that regenerates NAD+ for glycolysis.
• Metabolism can utilize various fuel sources, including carbohydrates, fats, and proteins.

Action Steps

• Identify the type of metabolic reaction: anabolic (building) or catabolic (breaking down).
• Recognize energy transformations between chemical, kinetic, heat, light, and potential forms.
• Understand ATP's role as the primary energy currency.
• Trace the flow of electrons via carriers like NAD+ and FAD during redox reactions.
• Follow the stages of aerobic respiration: glycolysis, decarboxylation, Krebs cycle, and electron transport chain.
• Differentiate between aerobic respiration and anaerobic fermentation.
• Calculate energy yields from glucose metabolism under different conditions.
• Consider how fats and proteins can enter metabolic pathways.

Formulas

• $C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O + \text{energy}$
• $C_6H_{12}O_6 + 2 ADP + 2 P_i + 2 NAD^+ \rightarrow 2 \text{pyruvate} + 2 NADH + 2 H^+ + 2 ATP + 2 H_2O$
• $2 \text{pyruvate} + 2 \text{CoA} + 2 NAD^+ \rightarrow 2 \text{acetyl-CoA} + 2 NADH + 2 H^+ + 2 CO_2$
• $2 \text{acetyl-CoA} + 6 H_2O + 6 NAD^+ + 2 FAD + 2 ADP + 2 P_i \rightarrow 2 \text{CoA} + 6 NADH + 6 H^+ + 2 FADH_2 + 2 ATP + 4 CO_2$
• $10 NADH + 10 H^+ + 5 O_2 + 23(25) ADP + 23(25) P_i \rightarrow 10 NAD^+ + 10 H_2O + 23(25) ATP$
• $2 FADH_2 + O_2 + 3 ADP + 3 P_i \rightarrow 2 FAD^+ + 2 H_2O + 3 ATP$
• $C_6H_{12}O_6 + 6 O_2 + 30(32) ADP + 30(32) P_i \rightarrow 6 CO_2 + 36(38) H_2O + 30(32) ATP$
• $2 \text{pyruvate} + 2 NADH \rightarrow 2 \text{lactate} + 2 NAD^+$
• $2 \text{pyruvate} \rightarrow 2 \text{ethanol} + 2 CO_2$

Key Terms

• Metabolism: The sum of all chemical reactions occurring within a living organism.
• Anabolic Reaction: A metabolic pathway that builds complex molecules from simpler ones, requiring energy (endergonic).
• Catabolic Reaction: A metabolic pathway that breaks down complex molecules into simpler ones, releasing energy (exergonic).
• ATP (Adenosine Triphosphate): The primary energy currency of the cell, used to power most cellular activities.
• Aerobic Respiration: A metabolic process that uses oxygen to completely break down glucose into carbon dioxide and water, producing a large amount of ATP.
• Fermentation: An anaerobic metabolic process that breaks down glucose partially, regenerating NAD+ without oxygen.
• Glycolysis: The initial stage of glucose breakdown, occurring in the cytoplasm, producing pyruvate, ATP, and NADH.
• Krebs Cycle: A series of reactions in the mitochondrial matrix that further oxidizes acetyl-CoA, producing ATP, NADH, FADH2, and CO2.
• Oxidative Phosphorylation: The process in mitochondria where electron transport and a proton gradient are used to generate the majority of ATP.
• Electron Carrier: Molecules like NAD+ and FAD that accept and donate electrons during metabolic reactions.

Pro Tips

• Remember that energy is conserved; transformations are key.
• ATP hydrolysis is the direct source of energy for most cellular work.
• Electron carriers (NADH, FADH2) store energy harvested from glucose breakdown.
• The electron transport chain uses a proton gradient to drive ATP synthesis.
• Fermentation regenerates NAD+ to allow glycolysis to continue in the absence of oxygen.

Pitfalls to Avoid

• Confusing anabolic and catabolic processes.
• Forgetting that energy is required to build complex molecules.
• Assuming all energy released from glucose is captured as ATP; much is lost as heat.
• Overlooking the role of electron carriers in energy transfer.
• Thinking fermentation produces more ATP than aerobic respiration.

Myth vs Reality

• Energy can be created or destroyed.: Energy is conserved; it can only be transformed from one form to another or transferred between systems.
• Cells can directly use light energy for all processes.: Only photo-autotrophs can directly use light energy (for photosynthesis); other organisms rely on chemical energy derived from food.
• Fermentation is an inefficient way to produce energy.: While less efficient than aerobic respiration in terms of ATP yield per glucose molecule, fermentation is crucial for regenerating NAD+ and allowing glycolysis to proceed anaerobically.

Real World Examples

• Muscle exertion during intense exercise.: When oxygen supply is limited, muscle cells perform lactic acid fermentation to produce ATP quickly, leading to lactate buildup and muscle fatigue.
• Baking bread.: Yeast undergoes alcoholic fermentation, converting sugars into ethanol and carbon dioxide. The CO2 gas causes the dough to rise.
• Yogurt and cheese production.: Lactic acid bacteria ferment lactose (milk sugar) into lactic acid, which contributes to the texture and flavor of dairy products.
• Plant photosynthesis.: Plants use light energy to convert CO2 and water into glucose (anabolic process), storing chemical energy.

Timeline

• 1930s: Hans Adolf Krebs elucidates the citric acid cycle (Krebs Cycle).

People

• Hans Adolf Krebs: German biochemist who discovered the citric acid cycle.