Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 47

3.1.3 Photosynthesis, Chlorophyll, and ATP

well as all other parts of the plant. For example, the initial
cells in the shoots of a plant above ground can turn into roots
if placed below ground. Theophrastus observed the conse-
quence of this fact, without knowing what caused it, as stated
in Enquiry into Plants (Loeb Classical Library 1916)

Of all the organelles contained in the cytoplasm within the
plasmalemma inside the cell wall, the one unique to plant
cells are the plastids. Plastids may contain food, in the form
of starch, or pigments. Plastids also contain chloroplasts,
which form the center of the photosynthetic process because
they contain thylakoids, the membranes of which contain the
seminal substance of chlorophyll. Multiple layers of
thylakoids look much like a stack of coins and are called
grana, and these are surrounded by a liquid called stroma.
Chlorophyll was first isolated in 1817 by the French
chemists P.J. Pelletier and J.B. Caventou and named for its
green color from the Greek chloros , and phyllon , for leaf.
Chloroplasts are surrounded by an inner and outer mem-
brane. Chloroplasts contain DNA, RNA, and ribosomes
and can divide independently of the cell's nuclear division.
The ability of chloroplasts to undergo division separate from
the cell suggests that chloroplasts once were independent
cells, such as prokaryotic cyanobacteria, that became
engulfed by larger cells that perhaps could not perform
photosynthesis (Gray 1993; Clegg et al. 1994). Also, the
plastids move around in each cell independently in order to
maximize their exposure to changes in the position and
condition of light. The white color of most roots indicates
the absence of chloroplasts in the cytoplasm of these cells
(Fig. 3.1 ), but the cells do contain leucoplasts for storage of
starch. Some flowering plants such as orchids, however,
have chloroplasts in their root cells.
Chlorophyll is a cytochrome and is composed of proteins,
amino-acid building blocks that are conjugated with non-
amino-acid compounds. The presence of chlorophyll is the
fundamental criterion that separates those that have it, the
plant kingdom, from those that do not, the animal kingdom.
Bacteria, mold, yeast, and some fungi, however, are consid-
ered part of the plant kingdom even though they do not
possess chlorophyll.
Chloroplasts contain two forms of chlorophyll—the
blue-green chlorophyll a and the yellow-green chlorophyll
b pigments—that give the familiar color to most plants.
Typically, most plants have three times more chlorophyll a
than chlorophyll b . In general, a molecule of chlorophyll
consists of a magnesium atom (Mg) at the center of a
porphyrin ring (Fig. 3.3 ), similar to the structure of mamma-
lian hemoglobin, which contains an iron atom centered in a
porphyrin ring, suggesting that chlorophyll is simply a heme
that mutated to contain magnesium. The magnesium and
iron function as catalysts, or metalloenzymes, to increase
the probability a reaction occurs to completion. The porphy-
rin ring of chlorophyll is the center of light capture; the tail
of chlorophyll consists of a chain of phytol for linkage to the
lipid layers of the thylakoid membranes (Fig. 3.3 ). Many

A plant has the power of germination in all its parts, for it has
life in them all...the methods of generation of plants are these:
spontaneous, from a seed, a root, a piece torn off, a branch, a
twig, piece of wood cut up small, or from the trunk itself.

As we will see in Part II, the ability of plants to grow from
various pieces has implications for the kinds of plants and
methods of planting at sites where phytoremediation will be
used to remediate contaminated groundwater. Moreover, it
has a direct effect on the cost of remediation in that
phytoremediation is often less costly than other remedial
designs.
An important organelle in plant cells relative to plant and
groundwater interactions at contaminated sites is the vacu-
ole. A vacuole is a space within the plant cytoplasm
(Fig. 3.1 ). The origin of vacuoles in cells may be a conse-
quence of phagocytosis. In this manner, unlike prokaryotes
that must release digestive enzymes extracellularly to digest
food and then reabsorb the smaller particles back through
cell membranes, the presence of a vacuole enables
eukaryotes to process digestion within the cell itself, which
increases efficiency. The vacuole can account for most of the
volume of a cell and up to 90% of the total volume of some
mature cells. The presence of large vacuoles and cell walls
renders plants a much larger surface area than the same
volume of cytoplasm without these structures. Vacuoles
are bound to a membrane called the tonoplast, just like the
cytoplasmic membrane. The space of the vacuole can be
occupied by water, food in the form of starch, or pigments.
Vacuoles are often used by plants to isolate defensive toxic
compounds away from other parts of the cell. Whether or not
vacuoles can be used to store toxic compounds from
contaminated groundwater after uptake by a plant

is

unknown and is an area of promising research.
Groups of individual cells that perform similar functions
are called tissues. In plants, tissues include the meristematic
cells already discussed as well as epidermal tissue. The
parenchyma cells contain large vacuoles used to store vari-
ous compounds such as water, oil, crystals, and tannins.
These cells make up the pith of many plants. Collenchyma
tissues contain pectin to provide structural strength to plant
stems and petioles and contain chloroplasts and, therefore,
can conduct photosynthesis. Schlerenchyma tissues contain
lignin, unlike the collenchyma tissues. Other tissues that
have air pockets between adjacent cells are called aeren-
chyma tissues. As in mammals, each group of tissues
functions separately but usually to the benefit of the larger
organism.