228 Green Chemistry, 2nd ed
Eukaryotic cells all are enclosed by plasma membranes (cell membranes), are filled
with cytoplasm, and contain a variety of organelles that are enclosed by membranes.
Figure 9.2 shows only a few of the most important organelles contained in eukaryotic
cells. The genetic material in eukaryotic cells is contained in a nucleus that is enclosed
by a membrane. This DNA is associated with proteins and RNA forming chromosomes.
Mitochondria in eukaryotic cells are bodies in which oxidative metabolism, the process
by which the cells use oxygen to gain energy from “burning” food nutrients, is carried
out. Lysosomes are bodies that contain enzymes capable of breaking down cellular
macromolecules (proteins, carbohydrates, nucleic acids, and lipids). This very important
process destroys waste macromolecules that otherwise would accumulate and stop
the cells from performing their necessary functions and in so doing recycles the small
molecules that cells need for their metabolic processes. In a sense, therefore, lysosomes
do green chemistry at the cellular level. The system of internal membranes composing the
endoplasmic reticulum contains surface-bound enzymes that synthesize proteins, such
as proteinaceous enzymes, that are exported from the cell. The endoplasmic reticulum
is the feature that most clearly distinguishes eukaryotic from prokaryotic cells. Golgi
bodies act to export materials from the cell.
Three features largely distinguish plant eukaryotic cells from animal cells in that
the plant cells have a cell wall, a large central vacuole, and chloroplasts. The cell wall
gives the plant cell strength and rigidity. The vacuole takes up most of the cell volume
and allows contact with gases. The chloroplasts are sites in which chlorophyll uses light
energy (hν) to synthesize carbohydrates as shown by the following reaction for the
photosynthetic generation of glucose sugar:
6CO 2 + 6H 2 O (light energy, hν) → C 6 H 12 O 6 (glucose) + 6O 2 (9.3.1.)Shown by the above reaction, photosynthesis was responsible for the greatest
changes that the biosphere has ever caused in the atmosphere and geosphere. This
occurred with the evolution of cyanobacteria (once thought to be algae and called
“blue-green algae”) about 3 billion years ago, the first organisms capable of carrying
out photosynthesis and producing oxygen, which for them was a waste product. This
raised the oxygen content of the atmosphere from virtually zero to the current value of
21% (by volume of dry air). The result was conversion of the atmosphere to an oxidizing
medium. Vast deposits of solid iron minerals now used for iron ore were formed when
atmospheric oxygen reacted with dissolved Fe in the oceans,
4Fe^2 + + O 2 + 4H 2 O → 2Fe 2 O 3 + 8H+ (9.3.2)to produce solid iron oxide. Part of the oxygen generated by photosynthesis dissolved
in water, where it was available for the development of organisms that used oxygen to
metabolize organic matter. Whereas Earth’s surface had been a most inhospitable place
for the existence of life, the oxygen released by photosynthesis enabled the formation
of the ultraviolet radiation-filtering layer of ozone (O 3 ) in the stratosphere that made life
possible outside the protective confines of water. Thus life became possible on Earth’s