Zea mays

CHAPTER 2
The plant cell
Introduction
Even in a small green plant there are millions of
cells; the vast majority of these are differentiated
and in the vascular plant often perform specialised
functions such as the transportation of water and
soluble nutrients in the xylem and phloem (1.11,
1.12). All differentiated cells originate from
actively dividing meristematic cells (2.1); these are
densely cytoplasmic (2.2) and are located in the
apical (1.18) and lateral meristems (1.4, 2.3) as
weJ! as other more localised regions (e.g. merisrcmoids).
Although most living cells arc uninucleate
(2.4) several or many nuclei may occur in
certain types (2.5), while sieve elements (1.11)
contain degraded protoplasrs (2.6) in which the
nucleus and most other organelles have scncsu:d.
During the differentiation of most sclerenchyma
and tracheary elements their protoplasts
also degenerate (2.7 co 2.9) and at maturity only
their thickened walls remain (1.11, 2.10). The
lumina of such dead cells (2.11) form a significant
part of the plant apopiast system 'while the walls
and intercellular spaces constitute the remainder.
The protoplasts of the living cells form the symplaSt
(2.11) and these protoplasrs arc in continuity
with each other via the plasmodesmata (2.ll to
2.13). Between 1,000 and 10,000 such protoplasmic
connections occur per cell, but it seems that
only relatively small molecules (with less than a
molecular weight of 1,000) are able to pass
through them.
The fine-structure of a partly differentiated cell
is illustrated diagramatically in 2.14. The external
primary wall is delimited from the protoplast by
the membranous plasmalemma. Several large
organelles are preseot including the nucleus,
several vacuoles and a number of chloroplasts.
Normally under the light microscope (LM), only
the wall and these larger organelles would be
apparent (2.1, 2.15). However, with rhe greater
resolution of the transmission eleerron microscope
(TEM), mitochondria, endoplasmic reticulum,
dictyosomes, ribosomes, microtubules and plasmodesmata
are also distinguishable (2.14). Most
of these organelles are membrane-bounded (2.4,
2.8,2.12 to 2.14, 2.16).
In this Chapter only the fine-structural features
of plant cells arc considered, whereas the histological
Structure of differentiated cells and tissues
is discussed in Chapter 3.
26
Cell membranes
Substances located exterior to the plasmalemma,
or in the cytosol surrounding membrane-bounded
organelles (2.4,2.14), cannot mix freely with the
materials localised internally because these membranes
arc semi-permeable. Membranes consist of
a lipid bilayer (2.16), with the interspersed proteins
and complexes forming the molecular
pumps, enzymes and other structural components.
Some proteins are large and project onto the surface
of the membrane (2.16 to 2.18). Differing
types of organelles normally remain discrete within
the cell since their membtanes vary somewhat
in individual structure.
The plasmalemma (2.12, 2.13, 2.16) and the
membranes of mature dictyosome cisternae and
vesicles (2.8, 2.19) are generally the thickest
membranes of the cell and measure about 10nm
wide. When viewed in transverse section (in
chemically fixed material) membranes usually
show a tripartite appearance (2.19, 2.20); but in
freeze-fractured specimens the plasmalemma
(2.17,2.18) and other membranes (2.21, 2.22)
show numerous particles which probably
reptesent protein complexes (2.16).
The plasmalemma adjacent to the plant cell
wall sometimes reveals hexagonal arrays of
parricles which are possibly the sites of cellulose
microfibrillar synthesis (2.16). Likewise in the
yeast Saccharomyces, chitin microfibrils in the
wall apparently link with particles in the
plasmalemma (2.18).
Nucleus
The genetic material of the cell is primarily located
in the nucleus (2.1, 2.14, 2.23). The non-dividing
nucleus is bounded by an envelope composed of
the outer and inner membranes (2.2, 2.14, 2.22).
These are separated by a perinuclear space ca. 20
nm wide, but are confluent at the margins of the
abundant nucleus pores (2.14, 2.21, 2.22). These
pores are ca. 70 nm wide but are apparently partly
occluded by a complex fibrillar-particulate network.
In the meristematic cell (2.4,2.23) the
nucleus may occupy a half or more of the volume
of the protoplast but this ratio rapidly decreases as
the cell increases in size, with the individual small
vacuoles (2.4) expanding and fusing to form a
large central vacuole (2.24). The nucleus somc-