Chapter 8.
Concepts of Metabolism
- Some big ideas:
- Life is chemistry and
chemistry involves making and breaking chemical bonds (e.g. during
photosynthesis).
- As chemical bonds are
made and broken, energy is consumed and made available (respectively).
- Life requires a
continual supply of energy. The ultimate source of energy for
life on Earth is the sun and photosynthesis, which captures solar energy.
- Swimming upstream -
maintaining order in a universe becoming increasingly disordered
- First and second laws
of thermodynamics
- Plants have great
synthetic capacities; solar energy is used to support synthesis
- Metabolism = chemical
reactions where complex molecules are synthesized and destroyed
- Chemical reactions
- Making and breaking
covalent bonds
- Covalent bonds
"contain" potential energy
- Kinetic energy,
temperature, collisions
- Potential energy of
substrates, activated complex, products (Fig. 8.1, 8.2)
- Catalysts (enzymes)
speed reactions by lowering activation energy (Fig 8.3)
- Enzymes catalyze
specific reactions
- Enzyme active site
fits one set of substrates
- Some enzymes require
associated cofactors, e.g. those involved in carrying electrons
- Rates of reactions in
cell affected by:
- Number of collisions
between substrate molecules: temperature and concentration
- Enzyme activity,
which depends on:
- Gene expression
(synthesize enzymes only when needed)
- Regulating existing
enzymes
- Metabolic pathways
- Linked reactions: one
reaction's product is next reaction's substrate
- Feedback inhibition
common form of regulation of metabolism
- Which way will a reaction go?
- Reversible reactions
- Free energy of
substrates (reactants) and products, determined by:
- Initial concentration
of substrate and product molecules
- Relative stability of
bonds in substrates and products
- Temperature
- Downhill reactions:
spontaneous, result in drop in free energy (less energy in products than
in reactants). Shorthand: downhill
reactions release energy.
- Uphill reactions: not
spontaneous, result in increase in free energy (more energy in products
than in reactants). Shorthand:
uphill reactions require energy.
- Coupled reactions. Typically, an uphill reaction is driven
by a downhill reaction. Examples:
- Oxidation/reduction
- Hydrolysis/condensation
- Two important downhill
reactions power the cell (each has a corresponding uphill reaction). See Figures 8.9, 8.10
- Hydrolysis of ATP (releases
energy stored in ATP during condensation of ADP and phosphate)
- Oxidation of NADH and
NADPH (releases energy stored in NADH and NADPH during reduction of NAD+ and NADP+, respectively).