Energy, Enzymes, and Metabolism Flashcards

Key Concepts

• Energy and Chemical Reactions
• Enzymes and Ribozymes
• Overview of Metabolism
• Recycling of Organic Molecules

Chemical Reactions

• Chemical reactions are the making & breaking of chemical bonds
• Reactants → products
  • Reactants = The starting molecules of a chemical reaction
  • Products = the final molecules of a chemical reaction
• Properties of chemical reactions
  • Require a source of energy
  • In living organisms, they often require an enzyme as catalyst
  • Tend to proceed in a particular direction but will eventually reach equilibrium
  • Occur in liquid (water)

Energy and Chemical Reactions

• Energy = ability to promote change or do work
• Two forms of energy:
  • Kinetic Energy – associated with movement
  • Potential Energy – due to structure or location
    • Chemical potential energy, the energy in molecular bonds, is a form of potential energy

Types of Energy

• Light
  • Description: Light is a form of electromagnetic radiation that is visible to the eye. The energy of light is packaged in photons.
  • Biological example: During photosynthesis, light energy is captured by pigments in chloroplasts. Ultimately, this energy is used to produce organic molecules.
• Heat
  • Description: Heat is the transfer of kinetic energy from one object to another or from an energy source to an object. In biology, heat is often viewed as kinetic energy that can be transferred due to a difference in temperature between two objects or locations.
  • Biological example: Many organisms, including humans, maintain their bodies at a constant temperature. This is achieved, in part, by chemical reactions that generate heat.
• Mechanical
  • Description: Mechanical energy is the energy possessed by an object due to its motion or its position relative to other objects.
  • Biological example: In animals, mechanical energy is associated with movement due to muscle contraction, such as walking.
• Chemical potential
  • Description: Chemical potential energy is potential energy stored in the electrons of molecules. When bonds are broken and rearranged, energy may be released.
  • Biological example: The covalent bonds in organic molecules, such as glucose and ATP, store large amounts of energy. When bonds are broken in larger molecules to form smaller molecules, the energy that is released can be used to drive cellular processes.
• Electrical/ion gradient
  • Description: The movement of charge or the separation of charges can provide energy. Also, a difference in ion concentration across a membrane constitutes an electrochemical gradient, which is a source of potential energy.
  • Biological example: During a stage of cellular respiration called oxidative phosphorylation, H^+ provides the energy to drive ATP synthesis.

Laws of Thermodynamics

• First Law of Thermodynamics
  • “Law of conservation of energy”
  • Energy cannot be created or destroyed, but can be transformed from one type to another
• Second Law of Thermodynamics
  • Transfer of energy from one form to another increases the entropy (degree of disorder) of a system
  • As entropy increases, less energy is available for organisms to use to promote change

Biological Order and Disorder

• Cells and whole organisms create ordered structures from less organized starting materials
• They also replace ordered forms of matter and energy with less ordered forms

Thermodynamics and Entropy

• A brown bear can run at speeds up to 35 miles per hour (56 km/hr) as fast as a racehorse.
• First law of thermodynamics: Energy can be transferred or transformed but neither created nor destroyed. For example, chemical reactions in this brown bear will convert the chemical (potential) energy in the fish into the kinetic energy of running.
• Second law of thermodynamics: Every energy transfer or transformation increases the disorder (entropy) of the universe. For example, as the bear runs, disorder is increased around its body by the release of heat and small molecules that are the by-products of metabolism.
• Heat is a by-product of energy conversion
• After cycling up a hill would you feel warm or cold?
• Entropy: At each conversion some energy is transferred in the form of heat (2nd law of thermodynamics)

Entropy

• Living things must work to counter entropy
• Entropy is the amount of disorder in a system.
• To restore order, considerable energy must be expended
• With each energy conversion heat is released, so the disorder in a system increases.

Biological Order and Disorder

• Total energy = Usable energy + Unusable energy
• Free energy (G) is the amount of energy available to do work
  • Also called Gibbs free energy
• Entropy is a measure of the disorder that cannot be harnessed to do work

Laws of Thermodynamics (Continued)

• The total amount of energy before a transformation equals the total amount after a transformation. No new energy is created, and no energy is lost
• Although a transformation does not change the total amount of energy within a closed system, after any transformation the amount of free energy available to do work is always less that the original amount of energy
• Another statement of the second law is that in a closed system, with repeated energy transformation, free energy decreases and unusable energy increases – a phenomenon known as creation of entropy

Thermodynamic Equations

• H = G + TS
  • H = enthalpy or total energy
  • G = free energy or amount of energy for work
  • S = entropy or unusable energy
  • T = absolute temperature in Kelvin (K)
• Total energy (enthalpy) = free energy (work) + heat [unusable energy (entropy)]
• Change in free energy determines direction of chemical reactions
• \Delta G = \Delta H - T \Delta S

Change in Free Energy

• Change in free energy (ΔG)
• \Delta G = free energy of the final state (products) - the free energy of the initial state (reactants)
• \Delta G of a reaction tells us whether or not the reaction occurs spontaneously
• Key factor is the free energy change – if \Delta G is negative, then process is exergonic and spontaneous
• Spontaneous reactions:
  • Occur without input of additional energy
  • Not necessarily fast, can be slow
• \Delta G = \Delta H - T \Delta S

Spontaneous Reactions

• Exergonic = spontaneous
  • \Delta G < 0 (negative free energy change)
  • Energy is released by reaction
• Endergonic = not spontaneous
  • \Delta G > 0 (positive free energy change)
  • Requires addition of energy to drive reaction

Exergonic and Endergonic Reactions

• Exergonic reaction
  • In an exergonic reaction, energy is released as the reactants form lower-energy products.
• Endergonic reaction
  • Energy must be added for an endergonic reaction, in which reactants are converted to products with a higher energy level.

ATP Hydrolysis

• ATP hydrolysis: releases
  • Energy
  • ADP
  • Inorganic phosphate
• Bonds between phosphate groups store large amounts of chemical potential energy \Delta G= −7.3kcal/mole
• Hydrolysis: breaking covalent bond by adding water
• ATP hydrolysis: breaking the covalent bond which links phosphate groups by adding water

ATP Hydrolysis Drives Reactions

• Energy released by exergonic reactions (ex. ATP hydrolysis) is used to drive endergonic reactions
• An endergonic reaction can be coupled to an exergonic reaction
• The reactions will be spontaneous if the net free energy change for both processes is negative
• Glucose + Phosphate^{2-}\rightarrow Glucose-6-phosphate^{2-} + H_2O \Delta G = + 3.3 kcal/mol Endergonic
• ATP^{4-} + H_2O \rightarrow ADP^{2-} + Pi^{2-} \Delta G = -7.3 kcal/mol Exergonic
• Glucose + ATP^{4-} \rightarrow Glucose-6-phosphate^{2-} +ADP^{2-} + Pi^{2-} \Delta G = -4.0 kcal/mol$$ Coupled

ATP Synthesis and Hydrolysis

• The energy to synthesize ATP comes from chemical reactions that are exergonic.
• ATP hydrolysis provides the energy to drive cellular processes that are endergonic.

ATP Regeneration

• Cells regenerate ATP
• Each ATP undergoes 10,000 cycles of hydrolysis and resynthesis every day
• Other exergonic reactions in the cell: Breakdown of food molecules to form smaller molecules