21 - Metabolism Flashcards
Metabolism
the set of chemical reactions that occur within a living organism in order to maintain life
ΔG°
Standard free energy change. Difference in energy between substrates and products in a reaction at physiological pH, temp and pressure
E’0
Standard reduction potential of redox half reactions
Δ E’0
Difference in E’0 between two redox pairs
Electron Transport Chain (ETC)
Series of linked half reactions having sequentially more positive E’0 values. Ensures that electrons are directed through chain to a terminal electron acceptor
Proton Motive Force (PMF)
Energy that is generated by the transfer of protons or electrons across an energy-transducing membrane. Creates a potential difference across the membrane
Electron carriers (ECs)
Molecules (NAD+, FAD) that transfer electrons to the electron transport chain (ETC)
first law of thermodynamics
Energy can be neither created nor destroyed
second law of thermodynamics
physical and chemical processes proceed in such a way that disorder (Entropy) increases
3 types of work of bacteria
- Chemical work (synthesis of complex biological molecules from much simpler precursors)
- Transport work (take up nutrients, eliminate wastes, and maintain ion balances)
- Mechanical work (cell motility and the movement of structures within cells)
Equilibrium constant (Eeq)
Keq = [C][D] / [A][B]
Chemical reaction
A + B ⇌ C + D
Keq greater than 1.0
Reaction is exergonic (favours forward reaction. C + D products)
Keq less than 1.0
Reaction is endergonic (favours reverse reaction A + B products)
Exergonic reaction
Net release of free energy (reaction is spontaneous). Negative ΔG. “earns ATP”
Example of exergonic reaction
Respiration (generates energy) termed fuelling reactions
Endergonic reaction
requires energy/energy is absorbed (positive ΔG). Requires a coupled exergonic reaction to supply energy. “spends ATP”
Example of endergonic reaction
Anabolism (requires energy) and 3 types of work
Cell energy cycle
Exergonic reactions power endergonic reactions through cycling ATP
2 main ways of forming ATP
- Substrate level phosphorylation
- Oxidative phosphorylation
Substrate level phosphorylation
Direct phosphorylation of ADP by substrates with high phosphate transfer potential
Oxidative phosphorylation
Indirect production of ATP by ATP synthase. ATP synthase powered by proton gradient
(proton motive force)
3 components of oxidative phosphorylation
- Capture and transfer of energy as electrons by NAD+ and FAD
- Transport of electrons to and along an electron transport chain
- ATP synthase
Why are electrons thought of as packets of energy in cells
- Many essential biochemical reactions occur are driven by the transfer of electrons between reactants
- Electron transfer is directly coupled to formation of electrochemical energy (the proton motive force, PMF)
Redox reactions
- Electrons move from an electron donor to an electron acceptor (the reactants in the reaction)
- Produces an oxidised form of the
donor and a reduced form of the acceptor
(the products of the reaction) - As electrons transfer from donor to
acceptor, the donor becomes less energy
rich and the acceptor becomes more
energy rich
Redox pair
Includes an electron donor and an electron acceptor.
the oxidation half-reaction
The donor and its oxidised product form
one half of a redox pair in a redox reaction
the reduction half-reaction
The acceptor and its reduced product form
the other half of a redox pair in a redox
reaction
Lose electron
Oxidation (LEO)
Gain electron
Reduction (GER)
Are redox reactions reversible
Yes. Compounds can also act as a donor in a certain reaction but an acceptor in another
What determines whether a compound acts as a donor or receptor
the standard reduction potential of the half-reactions (E’0). Measured in volts
redox pairs with more negative E’0
Spontaneously donate electrons in a pair with more positive E’0
ATP synthase
The proton motive force is used to generate ATP by the enzyme ATP snythase (ADP –> ATP)
Electron flow
Flow from electron carrier with the most negative reduction potential to the most positive reduction potential
Reduction potential
tendency of a chemical species to acquire electrons from or lose electrons to an electrode
3 forms of biochemical pathways
Linear ( 1–>1), branched (1–>2) and cyclic (continuous)
Different kinds of metabolites
- Metabolites that enter the pathway (substrates)
- Metabolites that exit the pathway (products)
- Metabolites in between are intermediates
- Metabolites from one pathway can serve as substrates in others
enzymes
Biological catalysts that speed up metabolic reactions by reducing the activation energy. Can be stand-alone or holoenzymes (apo-enzyme plus a cofactor)
proton motive force
- Electron transfer between each adjacent redox pair generates energy
- This energy is used to pump protons (H+) outside across a membrane to
generate a proton motive force - This creates a potential difference across the membrane
ETC in bacteria
ETC in bacteria are located in the plasma membrane (phospo bilayer)
