Chemistry Reference
In-Depth Information
transformed the atmosphere of the earth into an oxidizing environment in
which there is a relatively high proportion of free oxygen. The precipitation
of solids has created much of the sediments in the outer crust of the earth.
It can be said, therefore, that the biosphere is that part of the planet's surface
which is regulated by a flow of energy, mediated by the process of
photosynthesis.
8.1.
LIVING ORGANISMS AND CELLS
Living organisms are made up of
cells
, organized aggregations of associate
substances arranged in more or less complex structures adapted to fulfill spe-
cific living processes (see Textbox 50). Cells are the fundamental units of life.
Primitive living organisms, such as bacteria and some algae and fungi,
consist of just one cell. Advanced and complex organisms, for example, the
animals and the green plants, are made up of millions of cells grouped into
organized arrangements that make up the tissues, organs, and organ systems
in their bodies.
TEXTBOX
50
THE LIVING CELL
The
cell
is the smallest organized structure of living matter and the basic
constituent of all living organisms. To a large extent, the cell is also the
building block as well as the factory of all living matter. A cell contains all
the components necessary to sustain life:
carbohydrates
,
lipids
,
proteins
,
nucleic acids
, and
minerals
. Depending on its function, a cell may have a
variety of shapes and sizes, although most cells are relatively small; their
size may vary, however, from less than one micrometer (0.001 millimeter)
in diameter in some bacteria up to 75 millimeters in the eggs of ostriches.
A human cell is about 100 micrometers (0.1 millimeters) across (Alberts
et al. 1998). The diagram in Figure 59 illustrates the major features in an
animal cell.
Each cell consists primarily of a
membrane
, which separates it from the
environment, preserves its structural integrity, and keeps it apart from
other cells or from the surrounding environment. Plant cells, unlike animal
cells, also have, in addition to a cell membrane, a
cell wall
, composed of
cellulose and lignin. The cell
wall
provides structural strength not only to
the vegetable cell itself but to all plant tissues as well. Inside the mem-
brane, the interior of the cell, known as the
protoplasm
, includes two main
