Study Notes on Metabolism and Cellular Reactions

Lecture Overview

  • Focus on metabolism and biochemical reactions in cells.

  • Background information related to energy needs, ATP synthesis, and preparation for first exam.

    • Two quizzes and handouts on relevant topics will form the basis of the exam.

Definition of Metabolism

  • Metabolism: The sum of all chemical reactions in an organism, whether it be a single-celled bacterium or complex organisms like humans.

    • Includes reactions necessary for survival, growth, development, and reproduction.

    • Cells continuously perform millions of reactions to maintain life.

Energy and Nutrients

  • Cells require nutrients to sustain energy needs.

    • Nutrients are broken down to release energy, which is stored in ATP (adenosine triphosphate).

    • ATP is crucial for various cellular processes:

    • Protein synthesis

    • DNA synthesis

    • Cellular movement and division

  • ATP cycling:

    • Cells continuously synthesize ATP from available nutrients while utilizing it for energy.

Types of Reactions in Metabolism

Types of Reactions

  1. Anabolic Reactions: Require energy to synthesize larger molecules from smaller ones.

    • Example: Building proteins, nucleic acids, and macromolecules.

  2. Catabolic Reactions: Release energy by breaking down larger molecules into smaller units.

    • Example: Decomposition of glucose into pyruvate.

  3. Exchange Reactions: Combines both anabolic and catabolic processes.

Energy Principles

  • Energy cannot be created or destroyed; it can only be transformed from one form to another.

  • Linking catabolic and anabolic processes is essential to carry out metabolic functions.

Nutrient Acquisition and Energy Production

  • Cells acquire nutrients from the environment (diet for humans, sunlight for photosynthetic organisms).

  • Nutrient breakdown involves:

    • Glycolysis: The breakdown of glucose into two molecules of pyruvic acid (C6H12O6 -> 2 C3H4O3).

  • Glycolysis occurs without oxygen and is followed by either:

    • Aerobic Cellular Respiration (requires oxygen)

    • Fermentation (anaerobic process)

ATP Cycle

  • ATP Formation:

  • ATP is made from ADP (adenosine diphosphate) and inorganic phosphate (Pi).

  • Energy from nutrient breakdown is used to regenerate ATP.

    • ATP Utilization:

  • When cells use ATP, they convert it back to ADP + Pi through catabolic processes.

    • Balancing act of ATP production and utilization:

  • ATP levels fluctuate based on nutrient availability.

  • Glycolysis

    • Glycolysis: Splitting of glucose (6 carbons) into two pyruvic acids (3 carbons).

  • Ten steps with various enzymes facilitating the process.

  • Results in:

  • Formation of NADH (electron carrier)

  • A net yield of 2 ATP.

    • End Products:

  • If oxygen is available: Converts to acetyl CoA for Krebs cycle.

  • If oxygen is not available: Converts to lactic acid or alcohol (in yeast).

  • Krebs Cycle

    • Krebs Cycle: Series of reactions occurring in mitochondria, initiated by acetyl CoA.

  • Continuous as long as acetyl CoA is available.

  • Produces NADH, FADH2, ATP, and CO2 during each cycle.

  • Main purpose: Generate electron carriers for the next stage (ETC).

  • Electron Transport Chain (ETC)

    • ETC: Final stage of aerobic respiration happening in mitochondria.

  • Final Electron Acceptor: Oxygen, which is converted into water.

  • Electrons transferred from NADH and FADH2 through proteins, releasing energy used to pump protons across the mitochondrial membrane, creating a gradient.

  • ATP Synthase: Protons flow back through ATP synthase, generating ATP from ADP and Pi.

    • Approximately 34 ATP generated from one molecule of glucose.

Anaerobic vs Aerobic Metabolism

  • Anaerobic Fermentation: Occurs when oxygen is unavailable, yielding fewer ATP and producing waste byproducts (e.g., lactic acid).

  • Examples of Fermentation:

    • In animals: Lactic acid fermentation during heavy exercise.

    • In yeast: Alcohol fermentation producing ethanol.

Metabolism of Lipids and Proteins

  • Lipids: Broken down through beta-oxidation to produce acetyl CoA, which enters the Krebs cycle.

  • Proteins: Deaminated to remove the amino group, with the remaining carbon skeleton entering glycolysis or Krebs cycle as acetyl CoA.

Photosynthesis (Brief Overview)

  • Process by which plants and some organisms convert CO2 and water into glucose using sunlight.

  • Essentially the reverse of cellular respiration:

    • Converts light energy into chemical energy stored in glucose.

    • Anabolic Process involving the formation of larger molecules from smaller ones.

Summary

  • Cellular metabolism involves a complex network of reactions including glycolysis, Krebs cycle, and the electron transport chain, each critical for ATP generation.

  • Energy production relies heavily on nutrient availability and proper transition between aerobic and anaerobic pathways as needed.