General Biology/Cells/Energy and Metabolism

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

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

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

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

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

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

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

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

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

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