General Biology/Cells/Energy and Metabolism

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

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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.