Producers for most ecosystems.
organisms use light energy to drive the synthesis of organic molecules
from carbon dioxide and (usually) water. they feed not only themselves,
but the entire living world.
On land, plants are the predominant producers of
food. Three major groups of land plants - mosses, ferns, and flowering
plants - are represented in theis scene. In oceans, ponds, lakes,
and other aquatic enviroments, photosynthetic organisms include
(B) multicellular algae, such as kelp; (C) some unicellular
protists, such as Euglena; (D) and the prokaryotes called
cyanobacteria; and (E) other photosynthetic prokaryotes, such
as these purple sulfur bacteria (C, D, E; LMs).
and other autotrophs are the producers of the biosphere
nourishes almost all of the living world directly or indirectly.
An organism acquires the organi, compounds it uses for energy and
carbon skeletons one of two major modes: autotrophic or heterotrophi
nutrition. At first, the term autotrophic (Gr. autos, "self," and
trophos, "feed") may seem to contradict the principle that organisms
are open systems, taking in resource from their environment. Autotrophs
are not totally self sufficient, however; they are
self-feeders only in the sense that they sustain themselves without
eating or decomposing other organisms. They make their organic molecules
from inorganic raw materials obtained from the environment. It is
for this reason that biologists refer to autotrophs as the producers
of the biosphere. Plants are autotrophs; the only nutrients they
require are carbon dioxide from the air, and water and minerals
from the soil. Specifically, plants are photoautotrophs, organisms
that use light as a source of energy to synthesize lipids, proteins,
and other organic substances. Photosynthesis also occurs in algae,
including certain protists, and in some prokaryotes (FIGURE io.1).
In this chapter, our emphasis will be on plants. Variations in photosynthesis
that occur in algae and bacteria will be discussed in Unit Five.
A much rarer form of self-feeding is unique to those bacteria that
are chemoautotrophs. These organisms produce their organic compounds
without the help of light, obtaining their energy by wigdizing inorganic
substances, such as sulfur or ammonia. (We will postpone further
discussion of this type of autotrophic nutrition until Chapter 25.)
obtain their organic
material by the second major mode of nutrition. Unable to make their
own food, they live on compounds produced by other organisms; heterotrophs
are the biosphere's consumers. The most obvious form of this "other-feeding"
(hetero means "other, different") is when an animal eats plants
or other animals. But heterotrophic nutrition may be more subtle.
Some heterotrophs do not kill prey, but instead decompose and feed
on organic litter-such as carcassesces, and fallen leaves-and thus
are known as decomposers. Most fungi and many types of bacteria
get their nourishment this way. Almost all heterotrophs, including
humans, are completely dependent on photoautotrophs for food, and
also for oxygen, a by-product of photosynthesis. Thus, we can trace
the food we eat and the oxygen we breathe to the chloroplast.