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3. Which process produces more energy: gross primary production or net
primary production? Why?
4. What are the units to measure net primary production?
5. What factors affect the productivity in an ecosystem?
6. What portion of the available solar energy do photosynthetic organisms
use?
7. Why agricultural ecosystems are very productive?
8. What species dominates on Earth now? What is the term to denote this
phenomenon?
HOW ECOSYSTEMS WORK
In nearly all ecosystems, plants are the primary producers, the auto-
trophs that form the base of food chains. The leaves of plants contain chloro-
phyll, a pigment that can capture the Sun’s energy. Within chlorophyll, glu-
cose is manufactured from carbon dioxide and water vapor in the process of
photosynthesis. The equation for photosynthesis is:
6 CO2 + 6 H2O C6H12O6 + 6 O2
Plants obtain the carbon dioxide and water vapor for photosynthesis
through small openings in the leaves called stomata. The minerals and addi-
tional water they need are taken up through their roots. Glucose is used either
to perform work or to make the complex molecules that are found in all liv-
ing things, carbohydrates, proteins, lipids, and nucleic acids.
The rate at which the producers create useful energy is known as pri-
mary production. Gross primary production (GPP) refers to the amount of
carbon dioxide that is “fixed,” or converted from carbon dioxide gas to glu-
cose by photosynthesis. Some of this glucose is used to carry out cellular res-
piration, the process in which glucose is changed to energy to carry out life
processes. The equation for cellular respiration is:
6 O2 + C6H12O6 6 CO2 + 6 H2O
Net primary production (NPP) is the amount of primary production after
the cost of cellular respiration in plants is deducted. Measuring NPP tells you
how much organic material has been synthesized from inorganic compounds
and made available to the ecosystem. Measured in units of mass/area/time,
NPP in terrestrial ecosystems is usually expressed in grams of carbon per
square meter per year.
Ecologists are interested in NPP because it helps them understand the
balance of carbon dioxide in ecosystems. One way to estimate primary pro-
duction is by finding the leaf area of the ecosystem.
Energy flows through ecosystems, beginning with primary producers
and moving to herbivores, then carnivores. In terrestrial ecosystems, the
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