An object not in motion (such as a diver on a platform) may possess
potential energy if it is capable of motion later.
The potential energy is converted to kinetic energy when the object is
in motion (as when the diver jumps off the platform).
After the dive, the diver has less potential energy than before, and
must expand more energy to get back on the platform.
Heat is released when energy is converted from one form to another.
Energy cannot be created nor destroyed (conservation of energy), but
can be transferred or transformed from one form to another.
For example, the chemical (potential) energy in food is converted to
the kinetic energy of the cheetah's movement through metabolism.
As open systems, organisms can increase their order by increasing
disorder (entropy) of their surroundings through metabolism.
This process releases energy that can be used for cellular work.
Phosphate group transfer provides energy for most cellular work.
After work is done, the phosphate is released as inorganic phosphate
The enzyme sucrase catalyzes the hydrolysis of sucrose.
The starting molecules ( sucrose and HO ) in a chemical reaction
are called reactants or substrates.
The molecules produced by the reaction ( glucose and fructose ) are
The enzyme does not take part in the reaction, but serves as a
catalyst, speeding up the reaction.
In an exergonic reaction, the products have lower free energy than the
reactants (DG < 0), and the reaction occurs spontaneously.
However, the reaction usually involves first breaking some bonds and
requires activation energy (E[A]).
The E[A] provides a barrier that determines the rate of the reaction.
An enzyme speeds up an exergonic reaction by reducing its activation
The free-energy change (DG) of the reaction is not affected.
An enzyme catalyzes the conversion of reactant (substrate) molecules
to product molecules.
The substrates bind to the active site of the enzyme by weak forces
such as hydrogen bonds.
The enzyme lowers the E[A] by orienting substrates properly within the
This change allows more weak bonds to form, causing the active site to
embrace the substrate and hold it in place.
They have 2 kinds of binding sites, the active site for the substrate
and the regulatory site for a signal molecule.
When a signal molecule binds to its regulatory site, the shape of the
This change may activate or inhibit enzyme activity.
The activity of an enzyme is influenced by both pH and temperature.
Most human enzymes, such as the intestinal trypsin work best at about
35°C - 40°C and neutral pH.
Gastric enzymes such as pepsin are adapted to the acidic environment
of the stomach.