Module 9 - Energy and Metabolism

included on exam 3

free energy

energy that can be converted to do work

if negative, a reaction is spontaneous (increases disorder of the universe)

reflects the degree to which a reaction creates a more disordered state of the universe

second law of thermodynamics

the universe tends toward disorder

e.g. increasing order in a cell generates heat → that heat dissipates and increases disorder outside the cell

overall, even though the cell is more ordered, disorder (entropy) of the entire universe overall increases

also applies to chemical reactions

energy

the capacity to perform work

exergonic

spontaneous, as a reaction

energy is released

notice: energy of the product is less than energy of the reactant

endergonic

nonspontaneous, as a reaction

energy is consumed

notice: energy of the product is greater than energy of the reactant

the second law of thermodynamics applying to chemical reactions

building a more complex/ordered molecule decreases entropy in a cell → is energetically unfavorable (e.g. condensation)

breaking bonds in a molecule increases energy in a cell → is energetically favorable (e.g. hydrolysis)

dehydration synthesis

a non-spontaneous that produces a water (H from one molecule, OH from another)

e.g. disaccharide synthesis

results in an increase in molecular order → requires energy

hydrolysis

a spontaneous reaction that consumes a water (H goes to one molecule, OH goes to another)

e.g. breaking down of a disaccharide into monosaccharides

results in a decrease in molecular order → more favorable

equilibrium

the state during which both directions of a reaction occur with the same frequency

eg. X→Y = Y→X

note: does NOT mean amount of X = amount of Y

DOES mean that no net change in the amount of reactant and product occurs

metabolism

the web of continuous and sequential chemical reactions that harvest energy and make cell components

very complex

in living cells: the end products are consumed (energy is spent) → metabolic reactions occur mostly in the forward direction

consists of catabolic and anabolic reactions

all reactions in a living organism cannot reach equilibrium, because then necessary gradients and differences would not keep the organism functioning

spontaneity vs speed

spontaneity does not mean a reaction will happen quickly

sometimes activation energy is so great that it hinders a reaction

enzymes are needed to speed up these reactions by lowering activation energy

e.g. breakdown of sucrose → glucose and fructose; glycosidase speeds it up vastly

enzymes

proteins that act as biological catalysts by accelerating specific chemical reactions

cause specific transformations to be favored

lower activation energy

different ones can be employed by the cell to catalyze different reactions in a pathway, to ensure that pathways proceed in the correct sequence

note: they DO NOT change the equilibrium point, only the rate at which reactions go toward equilibrium

forcing endergonic reactions

enzymes can catalyze a nonspontaneous reaction, but need an energy source, e.g. ATP

unfavorable reactions can be coupled with favorable reactions to make the overall multistep process favorable

e.g. conversion of Glu to Gln (condensation) is coupled with ATP hydrolysis (high-energy intermediate is made in the process) (see slide 37 for illustration)

ATP

adenosine triphosphate

a ribonucleotide

energy carrier: source of useful available energy, because of high-energy phosphoanhydride bonds

hydrolysis (ATP → ADP + P_i) is energetically favored

energy released from hydrolysis can be used to drive other (nonspontaneous) reactions

released P_is can make higher-energy bonds with many proteins and metabolic intermediates

respiration

the harvesting of the energy stored in macromolecules, e.g. sugars and macromolecules

energy from macromolecules is transferred to smaller carrier molecules (useful)

consumes oxygen

releases CO2 and water

catabolic reactions

the respiration side of metabolism

breakdown of food molecules into basic components

energy is released from food molecules and transferred to carrier molecules

anabolic reactions

the biosynthesis side of metabolism

products of food breakdown are used as building blocks for synthesis of macromolecules (cell constituents)

require energy from energy carriers

photosynthesis

how energy enters living organisms

energy from sunlight is used to build sugars

C is obtained from atmospheric CO2

water is consumed

oxygen is released

energy stores are generated as sugars, which can then be harvested by all organisms (via respiration)

relation between photosynthesis and aerobic respiration

reverse reactions of each other

products of photosynthesis are needed by respiration

how cells harvest energy from sugars

single-step process cannot be used, because all energy would be released in that one reaction and most would be wasted as dissipated heat

instead: "slow burning” occurs, gradually releasing small energy pockets

activation energies are lowered by enzymes, overcome by body temperature

small energy pockets can be stored in energy carrier molecules while minimizing energy lost as dissipated heat

oxidation

a reaction in which a molecule or atom loses electrons

energetically favored

always coupled with reduction

common molecules that get oxidized: NADH, NADPH, FADH2

reduction

a reaction in which a molecule or atom gains electrons

energetically unfavorable

always coupled with oxidation

e.g. intermediates that receive electrons and protons from oxidized NADH, NADPH, or FADH2

examples of high-energy molecules/carriers

ATP and GTP: phosphoanhydride bonds store energy pockets, can be used by cell to drive unfavorable reactions when hydrolyzed

NADH, NADPH, FADH2: can be oxidized; electrons and protons can be transferred to metabolic intermediates → intermediates get reduced

acetyl coA: intermediate of aerobic respiration; has a thioester bond that releases a lot of energy when hydrolyzed

NADH vs NADPH

difference: NADH is the electron carrier in cellular respiration, NADPH is the electron carrier in photosynthesis

similarities: both are electron carriers; both accept 2 electrons and 1 proton to become reduced from NAD+/NADP+ to NADH/NADPH (result in a new high-energy C-H bond)

note: reduction occurs to oxidize another target molecule

note: the reverse is also true: NADH/NADPH can be oxidized to NAD+/NADP+ in order to reduce another molecule

FAD/FADH2

FAD (oxidized form) can accept 2 electrons and 2 protons to be reduced to FADH2

vice versa also occurs