Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 45

Cells are not equal in structure or function, however.
Cells are classified into two types, prokaryotic and eukary-
otic, based on structural differences. Prokaryotic cells, the
simpler of the two types, are representative of bacteria, such
as Esherichia coli ( E. coli ). They consist of a somewhat rigid
cell membrane, or plasmalemma, that surrounds the cell
cytoplasm both of which are contained within the cell wall.
The cell wall helps provide rigidity to the cell, especially
when the cellular components contain water and can exert
internal pressure on the inside of the cell wall. The cell wall
is composed of peptidoglycan, polysaccharides, and amino
acids. The genetic material that dictates the metabolism and
reproduction of the cell exists as a single strand of free-
floating deoxyribonucleic acid (DNA) and is not bounded
within the cytoplasm by a separate membrane.
In contrast, the eukaryotic cell is more complex and is
characteristic of higher life forms, such as algae, fungi,
plants, and animals. The generalized structure of a plant
cell is shown in Fig. 3.1 .Overall,aplantcellconsistsofthe
living cytoplasm, called protoplasm, that is contained
within a cell membrane (plasmalemma or cytoplasmic
membrane) similar to the prokaryotes. The cytoplasm is a
colloidal, gel-like material where most of the metabolism

of the cell occurs. In plants, the plasmalemma of each cell
is the interface between the exterior of the cell, called the
apoplast, which is composed of the continuous extracellu-
lar aqueous phase between cells, and the interior of the cell,
or symplast. The plasmalemma controls the passage of
various elements, organic compounds, and water into and
out of each cell.
Individual cells are not isolated from each other after
division and differentiation into tissues and various organs.
The cell walls permit the exchange of cytoplasmic
materials between adjacent cells through interconnections
called plasmadesmata. It is the presence of such
plasmadesmata that causes multicellular plants to be differ-
ent from single-celled bacteria, which tend to be like little
isolated factories that have to manufacture all the processes
of life and metabolism. Individual plant cells also can do
this to some extent, but generally act together as tissues that
are differentiated to perform separate tasks to benefit the
whole plant.
In plants, the cell protoplasm and plasmalemma are
surrounded by a tough cell wall for protection just like for
the prokaryotes. In the eukaryotic cell wall, peptidoglycan is
absent; instead, the cell wall is made of polysaccharides,
such as cellulose, which is a polymer of glucose molecules
(120 in all) and is arranged in thin sheets called microfibrils
that slide past each other to accommodate cell growth. The
cell wall has to be rigid to provide physical support but also
has to be porous and, therefore, semipermeable. As such,
water and other dissolved compounds can pass through, but
passage of smaller compounds or macromolecules essential
to life is excluded.
The ability of plants to synthesize cellulose was of pri-
mary importance in their transition from the oceans to land,
as its strength provided structural support that allowed the
cell cytoplasm inside to remain bathed in water. As cells
divide and elongate, the pressure exerted between adjacent
cells induces the production of pectin that causes the cells to
hold fast but remain pliable. Pectin, to those familiar with
the process of canning, is what binds jelly together. When
plants reach maturity, a secondary cell wall is made out of
cellulose and an additional compound called lignin, com-
posed of various aromatic compounds. When placed
together, cellulose and lignin are known commonly as
wood. Cellulose also is the principal component of paper,
which is most likely made of wood pulp.
The structure of the cytoplasmic membrane allows con-
trol of compound entry and exit. The inner part of this
membrane consists of hydrophobic compounds that resist
interaction with water, and the outer part consists of hydro-
philic compounds that readily interact with water. This
membrane is, therefore, very thin, only two molecules
thick (Fig. 3.2 ). This bilayer structure imparts a semiper-
meability to the cell to allow certain compounds, such as

Fig. 3.1 Simplified structure of a eukaryotic cell in various parts of a
plant, such as the leaf, bark, and root. The cytoplasmic membrane is
synonymous with the plasmalemma. Chloroplasts are not found in root
cells of most plants, except a few epiphytes like orchids. The impor-
tance of water is evident from the relative size of the vacuole, a water-
storage organelle.