The Molecular Biology of Plant Cells "d0e164"

Introduction

The term cell, as first used by Robert Hooke in 1665 signified an apparently empty space or lumen, surrounded by walls. We now know, of course, that the space is far from empty, and that rigid cell walls as seen by Hooke in thin slices of cork, are not ubiquitous in multicellular organisms. Indeed, the wall became to be regarded as the definitive structure of the cell, and when in the 1830s, the zoologist Schwann was able to recognise structures in cartilage resembling plant parenchymatous cell walls, the concept of the cell as the basic biological unit common to all organisms was born. Definitions have changed considerably in the subsequent century and a half, and, in particular, the cell wall is now seen in its proper perspective as being a structure, albeit of great importance, but restricted to plants and existing only outside the true cell. Nevertheless, the general concept of the cell as the basic minimlum unit of life remains.

Since all organisms need to perform a number of essential functions merely in order to survive, both as individuals and as species, it should not be surprising to find a basic unity between the cells of all organisms. Each cell, at least in the early stages of its development, possesses the capacity to synthesize complex substances from simple ones, to liberate and transform the potential chemical energy of highly reduced compounds, to react to internal and external stimuli, to control the influx and efflux of materials across the limiting cell membranes and to regulate its activities in relation to the information contained in its individual store, or stores, of hereditary genetic material. Evolution has solved the problems posed by these requirements in more or less identical ways in all organisms, and thus the basic processes, activities, and structures of each individual plant cell are similar, not only to other plant cells, but also to all other eucaryotic cells. This book concentrates on the unifying features of plant cells and relates them to present knowledge and general theories of molecular biology. It should not be forgotten, however, that cells are characterised as much by their diversity as their unity. A wide range of different cell types with varying specialized functions are necessary for the life of the higher green plant; however, the origin of cell heterogeneity is a topic outside the scope of this present book.

The basic structures of an undifferentiated plant cell can be seen in Fig. 1.1. The cell proper is delimited by the plasma membrane (or plasmalemma ) which is of unit membrane construction (chapters 2 and 8). Outside the plasma membrane, and thus actually extra-cellular, is the cell wall (chapter 1). The cell wall is normally closely appressed to the plasma membrane and in meristematic cells is thin and relatively weak. During differentiation various specialized

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wall structures develop; depending on the function of the mature cell, the walls may become relatively massive and extremely strong through the deposition of rigid, highly cross-linked polymeric substances. Adjacent protoplasts (i.e. the cells proper) are connected across the cell walls by narrow cytoplasmic channels, bounded by the plasma membrane, known as plasmodesmata (PD).

Within the cell a number of separate compartments, and interconnecting compartments, delimited by membranes, may be recognised (chapter 8). Vacuoles (V) are prominent, apparently empty spaces, spherical and numerous in the meristematic cell but irregular, very large, and coalescent in the mature expanded cell. Vacuoles serve as intracellular dust-bins—repositories for unwanted and often toxic byproducts of metabolism—and may also have functions similar to the lysosomes of animal cells. They are bounded by a single membrane known as the tonoplast (chapters 2 and 8).

The nucleus (N) (chapter 9), a major compartment in most cells, comprises a nuclear envelope possessing many large nuclear pores (NP) and nucleoplasm, the ground substance in which the hereditary material, chromatin, and the nucleolus (NU) lie. The nucleus is the principal site of the hereditary material of the cell, although both plastids and mitochondria also contain DNA. The material outside the nuclear envelope is commonly known as cytoplasm.

Ramifying throughout the cytoplasm, and occasionally connected to the outer membrane of the nuclear envelope, the cisternae of the endoplasmic reticulum act to integrate the biosynthetic functions of the cell (chapter 8). The endoplasmic reticulum is generally classified into two types: rough endoplasmic reticulum (RER), which has ribosomes attached to its outer face (chapter 10); and smooth endoplasmic reticulum (SER) which is not involved in protein synthesis. The endoplasmic reticulum may also, on occasion, be seen to be associated with stacks of vesicles (VE) known collectively as dictyosomes (D) or Golgi bodies. The endoplasmic reticulum and the dictyosomes are responsible for the formation and secretion of cellular membranes.

Three other membrane-bound compartments remain, each concerned with an aspect of energy or intermediary metabolism. Plastids (P), undifferentiated in meristematic cells and present only as proplastids, represent a general class of organelle in which the chloroplast is the characteristic member (chapters 3 and 4). Mitochondria (M) are smaller, but also bounded by a double membrane, and similarly involved in energy metabolism (chapter 5). As mentioned above, both mitochondria and plastids contain their own stores of hereditary material (chapter 11. The final compartments, in contrast, are bound by only a single membrane and do not contain hereditary material; these are known as microbodies (MB) and often contain dense, granular, or even crystalline contents (chapter 6). Within the cytoplasm just inside the plasma membrane lie long narrow cylinders known as microtubules (MT); microtubules function in a number of processes in which orientation of cellular components is important (chapter 7). Finally, plant cells contain many fine fibrils, known as

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microfilaments, which appear to be contractile in function and to be composed of a material similar to actin, one of the contractile components of muscle.

The 'typical' plant cell does not exist, of course, and the meristematic cell shown in Fig. 1.1 has only been chosen since it possesses all the essential characteristics of plant cells. Many of the cellular components are only present in very simple forms in meristematic cells, however, and the subsequent chapters in Section I necessarily involve a consideration of a variety of more specialized cell types.

Introduction

Further Reading