2.4 Cellular metabolism and energy production

Cellular metabolism and energy production are the powerhouses of life. These processes break down nutrients, create energy-rich molecules like ATP, and fuel all cellular activities. Understanding them is key to grasping how cells function and survive.

Enzymes play a crucial role in these metabolic pathways, speeding up reactions and regulating cellular processes. The balance between aerobic and anaerobic respiration showcases how cells adapt to different energy needs and environmental conditions.

Cellular Respiration Pathways

Glycolysis and Pyruvate Oxidation

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• Cellular respiration breaks down glucose and other organic molecules to produce ATP, the primary energy currency of the cell
• Glycolysis, the first stage of cellular respiration, occurs in the cytoplasm and breaks down glucose into two pyruvate molecules
  • Produces a net gain of 2 ATP and 2 NADH per glucose molecule
• In the presence of oxygen, pyruvate enters the mitochondria and undergoes oxidative decarboxylation to form acetyl-CoA

Citric Acid Cycle and Electron Transport Chain

• Acetyl-CoA enters the citric acid cycle (Krebs cycle), a series of enzymatic reactions that further oxidize it
  • Generates 2 ATP, 6 NADH, and 2 FADH2 per glucose molecule
• The electron transport chain (ETC), located in the inner mitochondrial membrane, is the final stage of cellular respiration
  • Involves a series of redox reactions that transfer electrons from NADH and FADH2 to oxygen, creating a proton gradient across the membrane
• ATP synthase, an enzyme complex in the inner mitochondrial membrane, uses the proton gradient generated by the ETC to synthesize ATP through chemiosmosis
  • Produces the majority of ATP in aerobic respiration (up to 34 ATP per glucose molecule)

ATP in Energy Transactions

Structure and Function of ATP

• Adenosine triphosphate (ATP) is the primary energy currency of the cell, used to power various cellular processes (biosynthesis, transport, mechanical work)
• ATP consists of an adenosine molecule (adenine base and ribose sugar) and three phosphate groups
  • High-energy bonds between the phosphate groups store energy that can be released through hydrolysis
• ATP is generated through substrate-level phosphorylation (directly from high-energy compounds) and oxidative phosphorylation (via the ETC and chemiosmosis)

ATP Hydrolysis and Cellular Processes

• When ATP is hydrolyzed to ADP (adenosine diphosphate) and inorganic phosphate (Pi), energy is released and can be coupled to endergonic reactions, making them thermodynamically favorable
• The ATP/ADP cycle is a continuous process of ATP synthesis and hydrolysis, ensuring a constant supply of energy for cellular functions
• ATP is involved in various cellular processes:
  • Active transport of molecules across membranes (sodium-potassium pump)
  • Synthesis of complex molecules (proteins, nucleic acids)
  • Muscle contraction and cell movement (myosin-actin interaction)
  • Signal transduction and nerve impulse transmission (neurotransmitter release)

Enzymes in Metabolism

Enzyme Structure and Function

• Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process
• Enzymes lower the activation energy of reactions, allowing them to proceed at physiological temperatures and pressures
• Enzymes are highly specific, typically binding to a particular substrate and catalyzing a specific reaction due to their unique three-dimensional structure and active site

Regulation of Enzyme Activity

• Enzyme activity is regulated by various factors:
  • Substrate concentration: Increasing substrate concentration increases reaction rate until enzyme saturation is reached (Michaelis-Menten kinetics)
  • Temperature: Enzyme activity generally increases with temperature until the optimal temperature is reached, beyond which the enzyme denatures and loses its catalytic function
  • pH: Each enzyme has an optimal pH range in which it functions most efficiently; deviations from this range can alter the enzyme's structure and decrease its activity
• Enzymes are crucial for maintaining homeostasis and coordinating metabolic pathways, ensuring that cellular processes occur at appropriate rates and in a controlled manner
• Enzyme deficiencies or malfunctions can lead to various metabolic disorders (phenylketonuria (PKU), lactose intolerance)

Aerobic vs Anaerobic Respiration

Aerobic Respiration

• Aerobic respiration occurs in the presence of oxygen and is more efficient, producing a higher yield of ATP (up to 38 ATP per glucose molecule)
• In aerobic respiration, pyruvate enters the mitochondria and undergoes oxidative decarboxylation to form acetyl-CoA, which then enters the citric acid cycle and electron transport chain

Anaerobic Respiration

• Anaerobic respiration takes place in the absence of oxygen and is less efficient (2 ATP per glucose molecule)
• Anaerobic respiration, such as lactic acid fermentation in animal cells and alcohol fermentation in yeast, occurs in the cytoplasm and does not involve the mitochondria
  • In lactic acid fermentation, pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD+ for continued glycolysis; important in tissues with high energy demands and low oxygen supply (skeletal muscles during intense exercise)
  • Alcohol fermentation, common in yeast and some bacteria, involves the decarboxylation of pyruvate to acetaldehyde, which is then reduced to ethanol by alcohol dehydrogenase, regenerating NAD+
• Anaerobic respiration is less efficient and can lead to the accumulation of metabolic byproducts (lactate) that may cause fatigue or muscle soreness
• Organisms that rely solely on anaerobic respiration, such as certain bacteria and parasites, are typically found in environments with little or no oxygen (human gut, deep-sea sediments)