< General Biology < Cells

General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material

Energy

  • The capacity to do work.
    • Kinetic energy: energy of motion (ex. jogging).
    • Potential energy: stored energy (ex. a lion that is about to leap on its prey).
  • Many forms of energy: e.g.,
    • Heat
    • Sound
    • Electric current
    • Light
    • All convertible to heat
  • Most energy for biological world is from sun
  • Heat (energy of random molecular motion, thermal energy)
    • Convenient in biology
    • All other energy forms can be converted to heat
    • Thermodynamics: study of thermal energy
  • Heat typically measured in kilocalories
    • Kcal: 1000 calories
    • 1 calorie: amount of heat required to raise the temperature of one gram of water one degree Celsius (°C)
  • Heat plays major role in biological systems
    • Ecological importance
    • Biochemical reactions

Oxidation–Reduction

  • Energy flows into biological world from sun
  • Light energy is captured by photosynthesis
    • Light energy raises electrons to higher energy levels
    • Stored as potential energy in covalent C-H bonds of sugars
  • Strength of covalent bond is measured by amount of energy required to break it
    • 98.8 kcal/mole of C-H bonds
  • In chemical reaction, energy stored in covalent bonds may transfer to new bonds. When this involves transfer of electrons, it is oxidation–reduction reaction
  • Always take place together
    • Electron lost by atom or molecule through oxidation is gained by another atom or molecule through reduction
    • Potential energy is transferred from one molecule to another (but never 100%)
  • Often called redox reactions
    • Photosynthesis
    • Cellular Respiration
    • Chemiosynthesis
    • Autotrophs
    • Heterotrophs

NAD+

  • Common electron acceptor/donor in redox reactions
  • Energetic electrons often paired with H+

Free energy

  • Energy required to break and subsequently form other chemical bonds
    • Chemical bonds: sharing of electrons, tend to hold atoms of molecule together
    • Heat, by increasing atomic motion, makes it easier to break bonds (entropy)
  • Energy available to do work in a system
  • In cells, G = H - TS
    • G = Gibbs’ free energy
    • H = H (enthalpy) energy in molecule’s chemical bonds
    • TS (T, temperature in °K; S, entropy)
  • Chemical reactions break and make bonds, producing changes in energy
  • Under constant conditions of temperature, pressure and volume, ΔG = ΔH - TΔS
  • ΔG, change in free energy
    • If positive (+), H is higher, S is lower, so there is more free energy; endergonic reaction, does not proceed spontaneously; require input of energy (e.g., heat)
    • If negative (–), H is lower, S is higher. Product has less free energy; exergonic; spontaneous

Activation energy

  • Reactions with –ΔG often require activation energy
    • e.g., burning of glucose
    • Must break existing bonds to get reaction started
  • Catalysts lower activation energy

Enzymes

  • Biological catalysts
    • Protein
    • RNA (ribozyme)
  • Stabilizes temporary association between reactants (substrates) to facilitate reaction
    • Correct orientation
    • Stressing bonds of substrate
  • Lower activation energy
  • Not consumed (destroyed) in reaction

Carbonic anhydrase

  • Important enzyme of red blood cells
  • CO2 + H2O → H2CO3 -> HCO3 + H+
  • Carbonic anhydrase catalyzes 1st reaction
    • Converts water to hydroxyl
    • Orients the hydroxyl and CO2

Enzyme mechanism

  • One or more active sites which bind substrates (reactants)
    • Highly specific
  • Binding may alter enzyme conformation, inducing better fit

Factors affecting enzyme activity

  • Substrate concentration
  • Product concentration
  • Cofactor concentration
  • Temperature
  • pH
  • Inhibitors
    • Competitive: bind to active site
    • Noncompetitive: bind to 2nd site, called allosteric site; changes enzyme conformation
  • Activators
    • Bind to allosteric sites, increase enzyme activity

Cofactors

  • Required by some enzymes
  • Positively charged metal ions
    • e.g., ions of Zn, Mo, Mg, Mn
    • Draw electrons away from substrate (stress chemical bonds)
  • Non-protein organic molecules (coenzymes)
    • E.g., NAD+, NADP+, etc.
    • Major role in oxidation/reduction reactions by donating or accepting electrons

ATP

  • Adenosine triphosphate
  • Major energy currency of cells, power endergonic reactions
  • Stores energy in phosphate bonds
    • Highly negative charges, repel each other
  • Makes these covalent bonds unstable
    • Low activation energy
  • When bonds break, energy is transferred
  • ATP → ADP + Pi + 7.3 kcal/mole

Biochemical pathways

  • Metabolism: sum of chemical reactions in cell/organism
  • Many anabolic and catabolic reactions occur in sequences (biochemical pathways)
  • Often highly regulated

Evolution of biochemical pathways

  • Protobionts or 1st cells likely used energy rich substrates from environment
  • Upon depletion of a substrate, selection would favor catalyst which converts another molecule into the depleted molecule
  • By iteration, pathway evolved backward

References

This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.

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