Environmental Engineering Reference
In-Depth Information
Source: U.S. Department of Agriculture
Energy Flow
Photosynthesis and Cellular Respiration
Photosynthesis
is one of the most important biological processes for living organisms because. Through pho-
tosynthesis, most life on Earth obtains energy from the sun, directly or indirectly. The only exceptions are the
chemosynthetic organisms that live at heat vents in the oceans and geysers in terrestrial systems.
Photosynthesis occurs when plants take in carbon dioxide (CO
2
) and water (H
2
O) while absorbing energy
from the sun. The absorption takes place in their chloroplasts and produces glucose (a carbohydrate, C
6
H
12
O
6
)
and oxygen (O
2
).
Cellular respiration
(or just respiration) is the process of "burning" the carbohydrate gluc-
ose in the presence of oxygen to release the stored energy for use by the organism.
The balanced chemical reaction for photosynthesis is 6 CO
2
+ 6 H
2
O + light energy → C
6
H
12
O
6
+ 6 O
2
,
where glucose (C
6
H
12
O
6
) is the initial storage of energy in plants. This energy is then released by cellular res-
