Geoscience Reference
In-Depth Information
time the roots of the plant are absorbing water, the green leaves
of the plant absorb solar radiation (2). Compounds in the plant's
leaves called chlorophylls and carotenoids are important ingre-
dients in the process because they absorb sunlight of specific
wavelengths. Chlorophylls absorb wavelengths in the blue and
red part of the electromagnetic spectrum, whereas carotenoids
absorb blue-green light (carotenoids appear orange or yellow).
Figure 10.4 shows how these patterns of electromagnetic
absorption take place. Notice that plants do not absorb energy
in the green and orange parts of the spectrum. Instead, these
wavelengths are either reflected or pass through the plant's
leaves. The green wavelength is most actively reflected, which
is why plants appear to be green to the human eye. This energy
absorbed in the blue and red wavelengths are important to
the photosynthetic process because it excites chlorophyll and
carotenoid electrons within the plant.
Turn back to Figure 10.3. Stage 3 (upper left) shows the
photosynthetic process where carbon dioxide is absorbed by
the plant and converted to sugars by light energy in the water
medium. A by-product of this process is oxygen, which is re-
leased into the atmosphere (4). Excess CO 2 is also released to
the atmosphere or used in the photosynthetic process. Oxygen
that is released is then absorbed by animals, which produce CO 2
as a by-product of their own respiration (5). When plants and
animals die, their remains fall to the ground and are consumed
by decomposers (6). As the decomposers absorb oxygen through
respiration from the atmosphere or soil (7), they combine the
oxygen with decomposing carbohydrates, releasing energy
contained within CO 2 and water vapor that are emitted into the
atmosphere (8).
Although most of the solar energy absorbed by plants is
used for respiration, a great deal of it is used in photosynthesis
to produce glucose, which is retained in the plant to build and
sustain leaves, fruits, and seeds. In addition, plants convert
glucose to cellulose, which is the structural material used to
build cell walls. Although much of the produced glucose is
used to sustain the plant, a great deal of it is stored as starches
and other carbohydrates in roots, stems, leaves, and grass (9),
where it can be used later. Some of these carbohydrates sub-
sequently provide energy for animals after they are consumed.
Scientists who study plant production are called plant
ecologists . Plant ecologists are often interested in the net
amount of vegetation produced by photosynthesis. An impor-
tant indicator of plant productivity is biomass , which is the dry
weight of all living organisms in a given area. This measure is
important because it reflects the amount of chemical energy that
has been stored and is thus available for consumption. Biomass
is typically measured in kilograms per square meter of ground
or as metric tons per hectare. Given the distribution of global
climates, it should be no surprise that biomass varies greatly
across geographic regions. Forests have the highest biomass,
whereas grasslands have less.
In the context of global biomass, examine Figure 10.5,
which is a composite satellite image that shows the geography
of global biomass as constructed from data collected from 1978
to 1986. The ocean portion of the figure is a composite of more
Biomass The amount of living matter in an area, including
plants, animals, and insects.
Blue
Green
Orange
Red
Carotenoids
Chlorophylls
400
450
500
550
600
650
700
Wavelength of Light (nm)
Figure 10.4 Preferential absorption of solar wavelengths in plants. Note that blue and red
wavelengths are actively absorbed, whereas the green and orange parts of the spectrum are not.
 
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