Introduction to Plant Cell Components

3.3.3 Periderm 3.3.4 Collenchyma 3.3.5 Sclerenchyma 3.4 Summary

General References

3.1 Introduction to Plant Cell Components

Essential to the identification and classification of a plant for recognition of value as evidence, one must know the parts of a plant and features that can aid in identification. This chapter focuses on the internal composition of a plant cell and how plant cells are organized into organs to serve specific functions essential to plant life. Plants and animals do not appear similar when comparing whole organisms; however, at the cellular level they are remarkably alike. The commonality of animal and plant cells argues for a common ancient ancestral cell type that has, over the course of evolutionary time, evolved into the more specialized animal and plant cells that we recognize.

The cellular structure of a plant cell as observed under light microscopy contains a cell wall, a nucleus, a plasma membrane, a central vacuole, cyto-plasm, plastids and specialized plastid types, and often starch grains and crystals (see Figure 3.1). Cellular organelles can be observed easily with the use of inexpensive stains and light microscopy, or with transmission and scanning electron microscope techniques.

Figure 3.1 A schematic representation of a single plant cell is presented here.

Plant cells consist of their component parts, including the cell wall, cell mem-brane, nucleus, mitochondria, chloroplasts, ribosomes, and other important cellular substructures.

cell wall (external) and cell membrane (internal)

nucleus nucleolus

mitochondrian

chloroplast vacuole

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3.1.1 The Cell Wall

Cell walls provide support for the plant and offer protection to the cell.

Cellulose fibers and jelly-like pectin comprise the primary plant cell wall.

Cell walls do not isolate cells entirely from one another. Some exchange between cells can occur through specialized channels called plasmodesmata that allow for cytoplasmic continuity between cells. Cell walls naturally inhibit growth and cell division; therefore, cells that are designed for contin-ued growth have relatively thin plant cell walls. Some plant cells have more customized cell walls that become lignified and reinforced to assist in trans-port, protective, or support functions. These specialized cell walls are called secondary cell walls.

It is possible to “spectrotype” cell walls using Fourier transform infrared spectroscopy (FTIR). Mutations in genes that encode the cellular machinery involved in the synthesis, assembly, and disassembly of the cell wall have provided plants with defined alterations in wall composition and architec-ture. Cell wall biogenesis can be affected at the following stages:

• Substrate synthesis

• Polymer synthesis

• Secretion of Golgi-derived materials to the cell surface

• Assembly of polymers into a characteristic architecture

• Architectural remodeling during growth and differentiation

• Turnover and recycling of materials after their function has been completed

The discriminate analysis of FTIR as a robust method to identify cell walls in different plant species has been developed based on analysis of cell wall mutations in model systems of maize and arabidopsis. Mutants for which a specific defect has been deduced are valuable resources to better define the spectral deviations observed by FTIR between cell walls and to correlate these deviations with the chemical and physical alterations in architecture and composition responsible for them. For example, mutations in cellulose syn-theses provide distinctive spectra that are reduced in wave numbers charac-teristic of cellulose. When these infrared mutant “spectrotypes” are compared with those of standard polysaccharides and artificial polysaccharide matrices, they comprise a rudimentary library to characterize additional mutants. A comparable library could be useful for characterizing different plant species.

3.1.2 The Nucleus

The cell nucleus is oval and contains deoxyribonucleic acid (DNA) molecules that are the heritable information for regulating cell activity. The nuclear

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contents are separated from the rest of the cell by the nuclear envelope. Within the nuclear envelope is the nucleoplasm, which is the gel-like substance that houses the chromosomes (DNA-protein complexes). Eukaryotic plant cells have DNA and ribonucleic acid (RNA) molecules within a nucleus; prokaryotic cells lack a membrane-bound nucleus and their DNA and RNA is free-floating within the cytoplasm. Within plant cell nuclei, often multiple nucleoli are present. These nucleoli are comprised of RNA and proteins and are precursors of ribosomes. Ribosomes are often attached to the outer surface of the outer nuclear membrane.

3.1.3 Ribosomes

Ribosomes are small particles that occur in many places throughout the cell, including the cytoplasm, on the outside of the endoplasmic reticulum mem-branes, and in the nucleus, the mitochondria, and the chloroplasts. Ribosomes are the sites for protein synthesis and consist primarily of RNA and histone protein. They function for the assembly of amino acids into polypeptides.

3.1.4 The Plasma Membrane

The plasma membrane is a substance forming the outer boundary of the plant cell and lies internal to the cell wall. The cell membrane plays an important role in regulating the chemical composition of the cell. Other membranous structures occur in the cytoplasm, including the vacuole, the spherosome (lipid bodies), microbodies (sites for glycollic acid oxidation), the Golgi apparatus (involved in secretion), and the nucleus. In addition, the endo-plasmic reticulum (ER), a complex system of two unit membranes with a narrow space between them, is thought to be involved in intracellular trans-port and perhaps wall formation and some types of secretions (e.g., nectar).

3.1.5 The Vacuole

A vacuole is a watery compartment surrounded by a membrane called the tonoplast. The vacuole could contain many different substances, including, but not limited to, tannins, protein bodies, sugars, organic acids, flavenoid pigments, calcium oxalate, and phosphatides. Vacuoles serve a function in regulating water and solute content, and in such processes as osmoregulation and digestion, as well as for storage.

3.1.6 The Cytoplasm

The cytoplasm is fluid and can move (cytoplasmic streaming); however, its gel-like structure can serve to support other organelles located outside the nucleus. The major component of the cytoplasm is water (85 to 90%), and

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it is a viscous substance that is transparent in visible light. The cytoplasm is contained within the plasma membrane but is delimited further from other organelle-bound structures like vacuoles.

3.1.7 The Plastids

Numerous types of plastids exist: chloroplasts, leucoplasts, and chromoplasts.

All plastids originate from proplastids located in egg cells and young, dividing (meristematic) cells. Chloroplasts are the sites of photosynthesis for conversion of light energy into carbohydrates. Chloroplasts are green due to the chloro-phyll pigment contained within them. Leucoplasts store starch (amyloplasts), fats (elaioplasts), and proteins (proteinoplasts), and are typically found in tissues not exposed to light (e.g., roots). Chromoplasts are found in petals, fruits, and some roots and store carotenoid pigments. Due to the coloration of the pigments, these plant tissues will appear yellow, red, or orange in color.

3.1.8 Ergastic Substances

Waste products and reserved molecules produced by cells include the follow-ing: starch, crystals, proteins, silica bodies, and tannins. Starch is a long-chain carbohydrate that appears as grains. Starch grains are formed in chloroplasts and are then broken down and reformed in amyloplasts. Starch from plants can have nutritional and economic value, and much of the starch we use in commercial products or as food items are from cereals (wheat, maize, and rice), potato, tapioca, sago, and arrowroot. Proteins can be amorphous or crystalline, and are common in the endosperm and embryo of seeds. Crystals are formed by the excess deposition of inorganic molecules, and they can take on a variety of forms: prisms (rectangular or pyramids), druses (spheroid aggregates), raphides (elongated with sharp tips), styloids (elongated with blades on end), and crystal sand (tiny spherical, in masses). Most crystals form within the vacuoles for storage, and their appearance can be helpful in the classification of different species.

3.1.9 Spherosomes

Spherosomes are small, round lipid bodies found within the cytoplasm that are diagnostically opaque after fixation with osmium tetroxide under scan-ning electron microscopy. Their role is somewhat unclear, but it has been speculated that these oil-filled vesicles may be generated from the endo-plasmic reticulum and become detached and free-floating in the cytoplasm.

Other oils and fats are present in the vacuole as reserve lipids and are very common in seeds and fruits. Waxes are mostly found on the surface of leaves, stems, and fruits and serve to protect the plant.

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3.1.10 Microtubules

Microtubules are hollow, long, straight structures composed of protein sub-units and are located in the periphery of the cytoplasm, closest to the cell wall. They are also found in the mitotic and meiotic spindles and in another cell division-related structure, the phragmoplast, which appears in telophase.

Since microtubules are associated with cell division and near cell walls where growth is still occurring, one theory is that microtubules provide direction for developing microfibrils.

3.1.11 Mitochondria

Mitochondria are involved in cellular respiration and can be seen under a light microscope with the Janus Green B stain. They can look lobed, elongated, or spherical when cells are sectioned. These organelles have two unit-membranes:

the internal membrane forms many folds into the matrix (primarily composed of protein), and the outer membrane surrounds the inner membranes. The mitochondria are involved in energy conversion and contain enzymes used for the Krebs cycle.