# General Biology/Cells/Energy and Metabolism

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
* CO$_2$ + H$_2$O → H$_2$CO$_3$ -> HCO$_3$ + H$^+$
* Carbonic anhydrase catalyzes 1st reaction
** Converts water to hydroxyl
** Orients the hydroxyl and CO$_2$

### 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.