Conceptual Overview

Plant cells are composed of cell walls (discussed in a separate unit of study), vacuoles which are primarily water sacs that contain storage and waste products of metabolism and which provide turgor pressure for the cell, and living components constituting the protoplasm which are largely membranous structures. It is this last category that is considered in this unit of study. Protoplasmic organization includes nuclear structure and the cytoplasmic contents of the cell. Of all structures, membranes are the most basic to the organization of cells and represent the limiting boundary of the living components as well as unique functional bodies called organelles. Organelles are usually thought of as membrane-bound bodies that have a specific function and identifiable structural organization. The exceptions are ribosomes which are organelles not limited by a membrane.

With the development of the transmission electron microscope in the mid-twentieth century, plant biologists were able to visualize for the first time the organization of cells at a level much finer than that seen with the aid of the light microscope. Hence, the terms ultrastructure and fine structure of cells came about in reference to the detailed structure shown with the electron microscope. The information learned from the use of the electron microscope in plant anatomy has revealed that cells are composed of membrane-bound organelles associated with specific functions. Here, we consider the major structural entities and their functions. In recent years a variety of techniques have been developed which enable investigators to identify elemental composition, localize functional (usually enzyme) activities, and quantify cellular structure. These techniques extend our observations beyond merely descriptive ones. Such techniques will be covered in the last unit of study on "Analytical Plant Anatomy."

Both from high-resolution electron microscopy and from experimental techniques, it is known that most typical membranes consist of a bilayer of phospholipids. These are organized with the polar (hydrophilic) heads facing to the outside, and the hydrophobic tails oriented towards each other and the inner region of the membrane. Protein molecules are embedded in various regions of the membrane in a mosaic pattern and they may be intrinsic if they span the entire lipid bilayer, or extrinsic (also called peripheral proteins) if they are bound to only the external surface of the membrane. The membrane bilayer provides an interface between the external environment of the cell's protoplast and the aqueous contents of the protoplast. It forms a framework to bind molecules such as proteins. Some of the intrinsic proteins may also have oligosaccharides (short-chain carbohydrates) attached at the surface which serve as recognition sites for the binding of other molecules. In many cases, the binding is due to a negative charge on the surface of these molecular chains. The membrane is a heterogeneous fluid and its component molecules may move in the plane of membrane. This general plan of the cell membrane (specifically, the plasma membrane) was designated as the fluid mosaic model of membrane structure. Biological membranes are selectively permeable implying that some substances cross the membrane more easily and rapidly than others. Membranes can fuse one with another, and they also can grow by adding new molecules. They can form vesicles which segregate certain products and which are moved to different sites in the cytoplasm.

Plant cells have an internal architectural framework that is designated the cytoskeleton. It is a changing, dynamic system within the cell and is composed of filamentous elements called microtubules and microfilaments. Microtubules are polymers of the globular protein tubulin, which occurs as dimers of alpha- and beta-tubulin subunits. The subunits appear to be packed in a spiral manner with a hollow core. In cross-section the tubule diameter is approximately 25 nm (250 ┼) and typically shows 13 subunits. Microtubules are primarily known for their role in the movement of chromosomes during nuclear division and in the formation of the cell plate during cytokinesis, but in the interphase cells they are usually confined to the cell periphery. Such cortical microtubules are involved in orienting and depositing cellulose of plant cell walls, and some give shape to the cell prior to the deposition of the cell wall. Microfilaments are 5 to 7 nm (50 - 70 ┼) diameter structures composed of the protein actin which is polymerized into long strands. They are involved in the generation of cytoplasmic streaming and the independent movement of organelles and vesicles.

The nucleus is a dominant feature of eukaryotic cells and is characterized by a double-membrane envelope which is perforated by many pores. The space between the two membranes is called the perinuclear space. The nuclear pores function to selectively regulate the movement of molecules (including RNA and proteins) into and out of the nucleus. Inside the nucleus, chromosomal material (chromatin) may occur in two forms: euchromatin, that is in an active and diffused form, and heterochromatin in a condensed form which is largely inactive and not undergoing transcription. The nucleolus is the site of rRNA synthesis and pre-ribosome elaboration. It is composed of fibrillar,granular and (occasionally) vacuolar components. Ribosomes are small (20 nm or 200 ┼) spherical granules in the cytoplasm involved in protein synthesis. Plastids are organelles characteristic only of plant cells. Three main types of plastids are recognized: chloroplasts (plastids accumulating chlorophyll), chromoplasts (plastids accumulating carotenoids), and leucoplasts (colorless plastids).

Other membranous structures such as endoplasmic reticulum, mitochondria, the Golgi apparatus and microbodies are discussed in more detail in the sub-units of this unit of study.