100 CHAPTER 5 How Ecosystems Work
- Summarize how energy flows through a food web
and transfers between trophic levels. - Distinguish between gross primary productivity
and net primary productivity. - Define energy, and state the first and second
laws of thermodynamics. - Distinguish among producers, consumers, and
decomposers.
The Flow of Energy Through Ecosystems
LEARNING OBJECTIVES
*ÌiÌ>Ê>`ÊiÌVÊiiÀ}ÞÊUÊ}ÕÀiÊx°ÓÊ
E
nergy is the capacity or ability to
do work. Organisms require en-
ergy to grow, move, reproduce,
and maintain and repair dam-
aged tissues. Energy exists as stored energy—
called potential energy—and as kinetic energy,
the energy of motion. We can think of poten-
tial energy as an arrow on a drawn bow
( }ÕÀiÊx°Ó). When the string is released, this
potential energy is converted to kinetic energy
as the motion of the bow propels the arrow.
Similarly, the grass a bison eats has chemical
potential energy, some of which is converted
to kinetic energy and heat as the bison runs
across the prairie. Thus, energy changes from
one form to another.
The First and Second Laws
of Thermodynamics
Thermodynamics is the study of energy and
its transformations. Two laws about energy ap-
ply to all things in the universe: the first and
second laws of thermodynamics. According
to the first law of thermodynamics, an organ-
ism may utilize energy by converting it from
one form to another, but the total energy
content of the organism and its surroundings
is always the same. An organism can’t create
the energy it requires to live. Instead, it must
capture energy from the environment to use
for biological work, a process that involves
the transformation of energy from one form
to another. In photosynthesis, for example,first law of
thermodynamics
A physical law which
states that energy
cannot be created or
destroyed, although it
can change from one
form to another.
photosynthesis
The biological process
that captures light
energy and transforms
it into the chemical
energy of organic
molecules, which are
manufactured from
carbon dioxide and
water.Romeo Gacad/AFP/Getty Images © Max Rossi/Reuters/Corbisa bPotential energy is stored in the drawn bow (a) and is converted to kinetic energy (b) as the arrow speeds toward its target.
Photographed in Athens, Greece, during the 2004 Summer Olympics.