L

Chapter 8 Energy & Metabolism

Chapter 8: Energy & Metabolism

Energy in Life

  • Definition: Work is required for the process of life.

  • Units of Measurement: Expressed in units of work (kilojoules, KJ) or heat energy (kilocalories, kcal).

    • Conversion: 1 kcal = 4.184 KJ.

  • Energy Conversion: Energy can change forms through energy conversion.

  • Types of Energy:

    • Potential Energy: The capacity to do work.

    • Kinetic Energy: Energy of motion.

  • Storage: Organisms use chemical bonds to store and transfer potential energy.

  • Efficiency of Energy Conversion: No energy conversion is 100% efficient, necessitating a constant influx of energy because energy is lost in conversions.

Metabolism

  • Definition of Metabolism: Includes all biochemical reactions that occur within a living organism.

  • Two Components:

    1. Anabolism: Processes that build complex molecules from simpler ones.

    2. Catabolism: Processes that break down complex molecules into simpler ones.

  • Dynamic Equilibrium: A state where the relative concentrations of reactants and products are equal

    • Reaction Rates: Forward and reverse reaction rates are equal, and concentrations remain constant.

    • Manipulation: This state is rare; cells manipulate reactants and products in numerous ways.

Exergonic Reactions

  • Definition: Reactions that release energy.

    • Free Energy Comparison: Products have less free energy than reactants.

    • Thermodynamic Favorability: Exergonic reactions are thermodynamically favored and are spontaneous.

    • Examples: Most catabolic reactions are exergonic.

  • ATP Reaction: ATP + H_2O ightarrow ADP + P + ext{Energy}

    • Energy released is approximately 30 kJ/mol.

Endergonic Reactions

  • Definition: Reactions where energy is absorbed and stored in chemical bonds.

    • Free Energy Comparison: Products have more free energy than reactants.

    • Thermodynamic Favorability: Not thermodynamically favored and are non-spontaneous.

    • Examples: Most anabolic reactions are endergonic.

  • Coupling Reactions: Often, an endergonic reaction is coupled with an exergonic reaction to provide the needed energy to drive the endergonic reaction forward.

    • Net Nature: Coupled reactions must have a net exergonic nature.

Coupling Exergonic and Endergonic Reactions

  • Example:

    • Exergonic Reaction: A
      ightarrow B

    • Endergonic Reaction: C
      ightarrow D

    • Coupled Overall: A + C
      ightarrow B + D (Overall exergonic)

ATP: Adenosine Triphosphate

  • Composition: Consists of Adenosine, Ribose, and three phosphate groups.

  • Hydrolysis: The process of breaking down ATP with water.

    • Process:
      ATP + H_2O
      ightarrow ADP + P + ext{Energy}

    • Energy released approximately 30 kJ/mol.

  • Intermediates: Compounds formed when ATP hydrolysis is coupled to provide energy.

  • Example: ext{Glucose} + ext{Fructose} ightarrow ext{Sucrose} + H_2O

    • Requires energy of 30 kJ/mol.

    • Requires ATP:
      ext{Glucose} + ext{Fructose} + ext{ATP}
      ightarrow ext{Sucrose} + ext{ADP} + ext{Pi}

Energy Transfer in Reactions

  • Mechanism: Energy is often transferred through the transfer of a phosphate group.

  • ATP Formation: ATP is synthesized via an endergonic condensation reaction:
    ADP + P + ext{Energy}
    ightarrow ATP + H_2O

  • Energy Requirement: This reaction requires about 30 kJ/mol and needs to be coupled with an exergonic reaction (often from catabolic pathways).

  • ATP/ADP Ratio: Cells maintain a high ATP to ADP ratio (approximately 10:1), even though the total ATP concentration is low overall in the organism.

    • Supply Duration: Sufficient ATP supply lasts only a few seconds, necessitating continuous ATP production.

Redox Reactions

  • Definition: Reactions used for energy transfer involving the transfer of electrons.

    • Reduction: Gain of electrons.

    • Oxidation: Loss of electrons.

  • Energy Transfer:

    • The oxidized substance loses energy by losing electrons, while the reduced substance gains energy by gaining electrons.

    • Energy is released as electrons are transferred to an acceptor molecule, which can drive other chemical reactions.

  • Proton Transfer: Typically, a proton (H⁺) is also removed along with the electron.

  • Electron Carriers: Common electron carriers include NAD+/NADH, which are involved in the transfer of protons and electrons.

  • Example Reaction:
    NAD^+ + H^+ + 2e^{-}
    ightarrow NADH

Catabolism

  • Definition: Involves the removal of hydrogen atoms (protons) from nutrients and the transfer to intermediate electron acceptors.

  • Example Electron Acceptor:

    • Nicotinamide adenine di-nucleotide (NAD).

  • Oxidation Example: Pretend that XH_2 represents a nutrient molecule.

  • Reaction Example: XH_2 + NAD^+ ightarrow X + NADH + H^+

    • Where NAD is in oxidized form and NADH is in reduced form.

Law of Mass Action

  • Principle: States that the rate of a biological or chemical reaction is proportional to the product of the concentrations of the reactants.

Enzymes

  • Definition: Organic molecules (typically proteins) that act as catalysts in chemical reactions.

    • Catalyst Role: Increases the rate of a chemical reaction.

  • Function: Enzymes only alter the reaction rate, not the overall reaction itself.

  • Activation Energy:

    • Definition: The energy required to break existing bonds of the reactants for a chemical reaction.

    • Enzymes lower activation energy, facilitating easier reaction progression.

  • Mechanism: Enzymes work by holding reactants (substrates) together to form an enzyme-substrate complex, which is highly dependent on enzyme shape.

  • Active Site: Substrates bind to the enzyme's active site.

    • Induced Fit: Changes in the enzyme and substrate; the enzyme's active site flexibly changes shape for optimal fit.

  • Stability of Complex: The enzyme-substrate complex is short-lived and unstable.

Enzyme-Substrate Interaction

  • Reaction Sequence:
    ext{Enzyme} + ext{Substrate(s)}
    ightarrow ext{Enzyme-Substrate Complex}
    ightarrow ext{Enzyme} + ext{Products}

  • Enzyme Shape Dependency: Substrate binding depends on the active site shape of the enzyme; they must fit correctly for the reaction to occur.

  • Nomenclature: Enzymes typically end in "-ase" (e.g., catalase) or "-zyme" (e.g., lysozyme).

  • Optimal Conditions:

    • Temperature: Each enzyme has an optimal temperature range; high temperatures can denature enzymes.

    • Human Body Temperature: Optimal temperature around 37°C.

    • pH Levels: Enzymes thrive in optimal pH conditions, with extreme pHs often leading to denaturation.

  • Increasing Reaction Rate:

    • By increasing substrate amount, reaction rates can also be increased, provided the substrate concentration exceeds enzyme concentration. This increase only works up to a point, determined by the number of free enzyme molecules.