Essential Cell Biology 5th edition

Essential Concepts
113
ESSENTIAL CONCEPTS
• Living organisms are able to exist because of a continual input of
energy. Part of this energy is used to carry out essential reactions
that support cell metabolism, growth, movement, and reproduction;
the remainder is lost in the form of heat.
• The ultimate source of energy for most living organisms is the sun.
Plants, algae, and photosynthetic bacteria use solar energy to produce
organic molecules from carbon dioxide. Animals obtain food by
eating plants or by eating animals that feed on plants.
• Each of the many hundreds of chemical reactions that occur in a cell
is specifically catalyzed by an enzyme. Large numbers of different
enzymes work in sequence to form chains of reactions, called metabolic
pathways, each performing a different function in the cell.
• Catabolic reactions release energy by breaking down organic molecules,
including foods, through oxidative pathways. Anabolic
reactions generate the many complex organic molecules needed by
the cell, and they require an energy input. In animal cells, both the
building blocks and the energy required for the anabolic reactions are
obtained through catabolic reactions.
• Enzymes catalyze reactions by binding to particular substrate molecules
in a way that lowers the activation energy required for making
and breaking specific covalent bonds.
• The rate at which an enzyme catalyzes a reaction depends on how
rapidly it finds its substrates and how quickly the product forms and
then diffuses away. These rates vary widely from one enzyme to
another.
• The only chemical reactions possible are those that increase the
total amount of disorder in the universe. The free-energy change for
a reaction, ΔG, measures this disorder, and it must be less than zero
for a reaction to proceed spontaneously.
• The ΔG for a chemical reaction depends on the concentrations of the
reacting molecules, and it may be calculated from these concentrations
if the equilibrium constant (K) of the reaction (or the standard
free-energy change, ΔG°, for the reactants) is known.
• Equilibrium constants govern all of the associations (and dissociations)
that occur between macromolecules and small molecules in
the cell. The larger the binding energy between two molecules, the
larger the equilibrium constant and the more likely that these molecules
will be found bound to each other.
• By creating a reaction pathway that couples an energetically favorable
reaction to an energetically unfavorable one, enzymes can make
otherwise impossible chemical transformations occur. Large numbers
of such coupled reactions make life possible.
• A small set of activated carriers, particularly ATP, NADH, and NADPH,
plays a central part in these coupled reactions in cells. ATP carries
high-energy phosphate groups, whereas NADH and NADPH carry
high-energy electrons.
• Food molecules provide the carbon skeletons for the formation of
macromolecules. The covalent bonds of these larger molecules are
produced by condensation reactions that are coupled to energetically
favorable bond changes in activated carriers such as ATP and
NADPH.