Biology (Holt)

Breaking Down Food for Energy
The chemical energy in organic compounds can be transferred to
other organic compounds or to organisms that consume food.
Organisms that must get energy from food instead of directly from
sunlight or inorganic substances are called (HEHT uhr
oh trohfs). Heterotrophs, including humans, get energy from food
through the process of cellular respiration. is a
metabolic process similar to burning fuel. While burning converts
almost all of the energy in a fuel to heat, cellular respiration releases
much of the energy in food to make ATP. This ATP provides cells
with the energy they need to carry out the activities of life.

Transfer of Energy to ATP
The word burnis often used to describe how cells get energy from
food. Although the overall processes are similar, the “burning” of
food in living cells clearly differs from the burning of a log in a
campfire. When a log burns, the energy stored in wood is released
quickly as heat and light. But in cells, chemical energy stored in
food molecules is released gradually in a series of enzyme-assisted
chemical reactions. As shown in Figure 2,the product of one
chemical reaction becomes a reactant in the next reaction. In the
breakdown of starch, for example, each reaction releases energy.
When cells break down food molecules, some of the energy in the
molecules is released as heat. Much of the remaining energy is
stored temporarily in molecules of ATP. Like money, ATP is a
portable form of energy “currency” inside cells. ATP delivers energy
wherever energy is needed in a cell. The energy released from ATP
can be used to power other chemical reactions, such as those that
build molecules. In cells, most chemical reactions require less
energy than is released from ATP. Therefore, enough energy is
released from ATP to drive most of a cell’s activities.

Cellular respiration

heterotrophs

SECTION 1Energy and Living Things 95

CO

CC

C HOH H C

H OH

CH 2 OH

H

OH

OH

H

6CO 2 + 6H 2 O

Carbon

dioxide

Reactant Products

Reactant Product

Starch

Glucose

Enzyme Enzymes

Water

AT P

Heat

Energy is released from starch in a series of enzyme-assisted chemical reactions.

Figure 2 Breakdown of starch

The words autotrophand

heterotrophhave the same

suffix, -troph,which is from

the Greek word trophikos,

meaning “to feed.” The pre-

fix auto-is from the Greek

word autos,meaning “self,”

and the prefix hetero-is

from the Greek word

heteros,meaning “other.”