336 ■ CHAPTER 18 General Principles of EcologyECOLOGY
DecomposersPathways resulting from human activityNatural pathwaysDrilling and
extractionTransportationManufacturingAnimalsPlantsAtmospheric carbon (CO 2 )Marine
organismsPhotosynthesisPhotosynthesisRespirationRespirationAgricultureRespiration is the process
by which organisms take up
O 2 and release CO 2 during
energy metabolism.Photosynthesis is the process by
which plants and algae use energy
from sunlight to convert water and
CO 2 into sugars and O 2.Figure 18.14
The carbon cycle
Arrows indicate the direction of carbon flow.
Q1: What are three ways that carbon is released into the atmosphere?Q2: Are all of the pathways you listed for question 1 affected by human activity?Q3: What are two biotic reservoirs of carbon?carbon was released into the atmosphere when
the forests burned.
Carbon, in the form of CO 2 gas, makes up
only about 0.04 percent of Earth’s atmosphere,
although that percentage has been creeping
upward every year for the last 200 years and
causing global warming, as we’ve seen. Carbon
is also found in Earth’s crust, where carbon-rich
sediments and rocks formed from the remains
of ancient marine and terrestrial organisms.
Carbon is present in every living thing.
Living cells are built mostly from organic
molecules—molecules that contain carbon atoms
bonded to hydrogen atoms. After oxygen, carbon
is the most abundant element in cells by weight;
every one of the large biomolecules in an organismhas a backbone of carbon atoms. Living organ-
isms, in both aquatic and terrestrial ecosystems,
acquire carbon mostly through photosynthe-
sis. Aquatic producers, such as photosynthetic
bacteria and algae, absorb dissolved CO 2 and
convert it into organic molecules using sunlight
as a source of energy. Plants, the most import-
ant producers in terrestrial ecosystems, absorb
CO 2 from the atmosphere and transform it into
food with the help of sunlight and water. This is
how a plant builds itself—its leaves, stems and
branches, flowers, and other structures.
The transfer of carbon within biotic commu-
nities and their physical surroundings, the
abiotic world, is known as the global carbon
cycle (Figure 18.14). (For more on nutrient