Plant Cells 1 Chapter THE TERM CELL IS DERIVED from the Latin cella, meaning storeroom or chamber. It was first used in biology in 1665 by the English botanist Robert Hooke to describe the individual units of the honeycomb-like structure he observed in cork under a compound microscope. The “cells” Hooke observed were actually the empty lumens of dead cells surrounded by cell walls, but the term is an apt one because cells are the basic building blocks that define plant structure. This book will emphasize the physiological and biochemical func- tions of plants, but it is important to recognize that these functions depend on structures, whether the process is gas exchange in the leaf, water conduction in the xylem, photosynthesis in the chloroplast, or ion transport across the plasma membrane. At every level, structure and function represent different frames of reference of a biological unity. This chapter provides an overview of the basic anatomy of plants, from the organ level down to the ultrastructure of cellular organelles. In subsequent chapters we will treat these structures in greater detail from the perspective of their physiological functions in the plant life cycle. PLANT LIFE: UNIFYING PRINCIPLES The spectacular diversity of plant size and form is familiar to everyone. Plants range in size from less than 1 cm tall to greater than 100 m. Plant morphology, or shape, is also surprisingly diverse. At first glance, the tiny plant duckweed (Lemna) seems to have little in common with a giant saguaro cactus or a redwood tree. Yet regardless of their specific adaptations, all plants carry out fundamentally similar processes and are based on the same architectural plan. We can summarize the major design elements of plants as follows: • As Earth’s primary producers, green plants are the ultimate solar collectors. They harvest the energy of sunlight by converting light energy to chemical energy, which they store in bonds formed when they synthesize carbohydrates from carbon dioxide and water. • Other than certain reproductive cells, plants are non- motile. As a substitute for motility, they have evolved the ability to grow toward essential resources, such as light, water, and mineral nutrients, throughout their life span. • Terrestrial plants are structurally reinforced to sup- port their mass as they grow toward sunlight against the pull of gravity. • Terrestrial plants lose water continuously by evapo- ration and have evolved mechanisms for avoiding desiccation. • Terrestrial plants have mechanisms for moving water and minerals from the soil to the sites of photosyn- thesis and growth, as well as mechanisms for moving the products of photosynthesis to nonphotosynthetic organs and tissues. OVERVIEW OF PLANT STRUCTURE Despite their apparent diversity, all seed plants (see Web Topic 1.1) have the same basic body plan (Figure 1.1). The vegetative body is composed of three organs: leaf, stem, and root. The primary function of a leaf is photosynthesis, that of the stem is support, and that of the root is anchorage and absorption of water and minerals. Leaves are attached to the stem at nodes, and the region of the stem between two nodes is termed the internode. The stem together with its leaves is commonly referred to as the shoot. There are two categories of seed plants: gymnosperms (from the Greek for “naked seed”) and angiosperms (based on the Greek for “vessel seed,” or seeds contained in a ves- sel). Gymnosperms are the less advanced type; about 700 species are known. The largest group of gymnosperms is the conifers (“cone-bearers”), which include such commercially important forest trees as pine, fir, spruce, and redwood. Angiosperms, the more advanced type of seed plant, first became abundant during the Cretaceous period, about 100 million years ago. Today, they dominate the landscape, easily outcompeting the gymnosperms. About 250,000 species are known, but many more remain to be character- ized. The major innovation of the angiosperms is the flower; hence they are referred to as flowering plants (see Web Topic 1.2). Plant Cells Are Surrounded by Rigid Cell Walls A fundamental difference between plants and animals is that each plant cell is surrounded by a rigid cell wall. In animals, embryonic cells can migrate from one location to another, resulting in the development of tissues and organs containing cells that originated in different parts of the organism. In plants, such cell migrations are prevented because each walled cell and its neighbor are cemented together by a middle lamella. As a consequence, plant development, unlike animal development, depends solely on patterns of cell division and cell enlargement. Plant cells have two types of walls: primary and sec- ondary (Figure 1.2). Primary cell walls are typically thin (less than 1 µm) and are characteristic of young, growing cells. Secondary cell walls are thicker and stronger than primary walls and are deposited when most cell enlarge- ment has ended. Secondary cell walls owe their strength and toughness to lignin, a brittle, gluelike material (see Chapter 13). The evolution of lignified secondary cell walls provided plants with the structural reinforcement necessary to grow vertically above the soil and to colonize the land. Bryophytes, which lack lignified cell walls, are unable to grow more than a few centimeters above the ground. New Cells Are Produced by Dividing Tissues Called Meristems Plant growth is concentrated in localized regions of cell division called meristems. Nearly all nuclear divisions (mitosis) and cell divisions (cytokinesis) occur in these meristematic regions. In a young plant, the most active meristems are called apical meristems; they are located at the tips of the stem and the root (see Figure 1.1). At the nodes, axillary buds contain the apical meristems for branch shoots. Lateral roots arise from the pericycle, an internal meristematic tissue (see Figure 1.1C). Proximal to (i.e., next to) and overlapping the meristematic regions are zones of cell elongation in which cells increase dramatically in length and width. Cells usually differentiate into spe- cialized types after they elongate. The phase of plant development that gives rise to new organs and to the basic plant form is called primary growth. Primary growth results from the activity of apical meristems, in which cell division is followed by progres- sive cell enlargement, typically elongation. After elonga- tion in a given region is complete, secondary growth may occur. Secondary growth involves two lateral meristems: the vascular cambium (plural cambia) and the cork cam- bium. The vascular cambium gives rise to secondary xylem (wood) and secondary phloem. The cork cambium pro- duces the periderm, consisting mainly of cork cells. Three Major Tissue Systems Make Up the Plant Body Three major tissue systems are found in all plant organs: dermal tissue, ground tissue, and vascular tissue. These tis- 2 Chapter 1 FIGURE 1.1 Schematic representation of the body of a typi- cal dicot. Cross sections of (A) the leaf, (B) the stem, and (C) the root are also shown. Inserts show longitudinal sections of a shoot tip and a root tip from flax (Linum usitatissi- mum), showing the apical meristems. (Photos © J. Robert Waaland/Biological Photo Service.) ▲ Upper epidermis (dermal tissue) Cuticle Cuticle Palisade parenchyma (ground tissue) Xylem Phloem Phloem Vascular cambium Ground tissues Lower epidermis (dermal tissue) Spongy mesophyll (ground tissue) Guard cell Stomata Lower epidermis Epidermis (dermal tissue) Cortex Pith Xylem Vascular tissues Vascular tissues Leaf primordia Shoot apex and apical meristem Axillary bud with meristem Leaf Node Internode Vascular tissue Soil line Lateral root Taproot Root hairs Root apex with apical meristem Root cap (A) Leaf (B) Stem Mesophyll Bundle sheath parenchyma Root hair (dermal tissue) Epidermis (dermal tissue) Cortex Pericycle (internal meristem) Endodermis Ground tissues Phloem Xylem Vascular tissues (C) Root Vascular cambium Middle lamellaPrimary wall Simple pit Primary wall Secondary wall Plasma membrane FIGURE 1.2 Schematic representation of primary and secondary cell walls and their relationship to the rest of the cell. (A) Dermal tissue: epidermal cells (C) Ground tissue: collenchyma cells (D) Ground tissue: sclerenchyma cells (B) Ground tissue: parenchyma cells Primary cell wall Middle lamella Primary cell wall Nucleus Sclereids Fibers Simple pits Vessel elements End wall perforation (E) Vascular tisssue: xylem and phloem Secondary walls Bordered pits Primary walls Tracheids Sieve plate Sieve areas Sieve plate Sieve tube element (angiosperms) Companion cell Nucleus Sieve cell (gymnosperms) Xylem Phloem sues are illustrated and briefly chacterized in Figure 1.3. For further details and characterizations of these plant tis- sues, see Web Topic 1.3. THE PLANT CELL Plants are multicellular organisms composed of millions of cells with specialized functions. At maturity, such special- ized cells may differ greatly from one another in their struc- tures. However, all plant cells have the same basic eukary- otic organization: They contain a nucleus, a cytoplasm, and subcellular organelles, and they are enclosed in a mem- brane that defines their boundaries (Figure 1.4). Certain structures, including the nucleus, can be lost during cell maturation, but all plant cells begin with a similar comple- ment of organelles. Plant Cells 5 FIGURE 1.3 (A) The outer epidermis (dermal tissue) of a leaf of welwischia mirabilis (120×). Diagrammatic representa- tions of three types of ground tissue: (B) parenchyma, (C) collenchyma, (D) sclerenchyma cells, and (E) conducting cells of the xylem and phloem. (A © Meckes/Ottawa/Photo Researchers, Inc.) Chromatin Nuclear envelope Nucleolus Nucleus Vacuole Tonoplast Rough endoplasmic reticulum Ribosomes Smooth endoplasmic reticulum Golgi body Chloroplast Mitochondrion Peroxisome Middle lamella Primary cell wall Plasma membrane Cell wall Intercellular air space Primary cell wall Compound middle lamella FIGURE 1.4 Diagrammatic representation of a plant cell. Various intracellular com- partments are defined by their respective membranes, such as the tonoplast, the nuclear envelope, and the membranes of the other organelles. The two adjacent pri- mary walls, along with the middle lamella, form a composite structure called the compound middle lamella. ▲ An additional characteristic feature of plant cells is that they are surrounded by a cellulosic cell wall. The following sections provide an overview of the membranes and organelles of plant cells. The structure and function of the cell wall will be treated in detail in Chapter 15. Biological Membranes Are Phospholipid Bilayers That Contain Proteins All cells are enclosed in a membrane that serves as their outer boundary, separating the cytoplasm from the exter- nal environment. This plasma membrane (also called plas- malemma) allows the cell to take up and retain certain sub- stances while excluding others. Various transport proteins embedded in the plasma membrane are responsible for this selective traffic of solutes across the membrane. The accu- mulation of ions or molecules in the cytosol through the action of transport proteins consumes metabolic energy. Membranes also delimit the boundaries of the specialized internal organelles of the cell and regulate the fluxes of ions and metabolites into and out of these compartments. According to the fluid-mosaic model, all biological membranes have the same basic molecular organization. They consist of a double layer (bilayer) of either phospho- lipids or, in the case of chloroplasts, glycosylglycerides, in which proteins are embedded (Figure 1.5A and B). In most membranes, proteins make up about half of the mem- brane’s mass. However, the composition of the lipid com- ponents and the properties of the proteins vary from mem- brane to membrane, conferring on each membrane its unique functional characteristics. Phospholipids. Phospholipids are a class of lipids in which two fatty acids are covalently linked to glycerol, which is covalently linked to a phosphate group. Also attached to this phosphate group is a variable component, called the head group, such as serine, choline, glycerol, or inositol (Figure 1.5C). In contrast to the fatty acids, the head groups are highly polar; consequently, phospholipid mol- ecules display both hydrophilic and hydrophobic proper- ties (i.e., they are amphipathic). The nonpolar hydrocarbon chains of the fatty acids form a region that is exclusively hydrophobic—that is, that excludes water. Plastid membranes are unique in that their lipid com- ponent consists almost entirely of glycosylglycerides rather than phospholipids. In glycosylglycerides, the polar head group consists of galactose, digalactose, or sulfated galactose, without a phosphate group (see Web Topic 1.4). The fatty acid chains of phospholipids and glycosyl- glycerides are variable in length, but they usually consist of 14 to 24 carbons. One of the fatty acids is typically satu- rated (i.e., it contains no double bonds); the other fatty acid chain usually has one or more cis double bonds (i.e., it is unsaturated). The presence of cis double bonds creates a kink in the chain that prevents tight packing of the phospholipids in the bilayer. As a result, the fluidity of the membrane is increased. The fluidity of the membrane, in turn, plays a critical role in many membrane functions. Membrane flu- idity is also strongly influenced by temperature. Because plants generally cannot regulate their body temperatures, they are often faced with the problem of maintaining mem- brane fluidity under conditions of low temperature, which tends to decrease membrane fluidity. Thus, plant phos- pholipids have a high percentage of unsaturated fatty acids, such as oleic acid (one double bond), linoleic acid (two double bonds) and α-linolenic acid (three double bonds), which increase the fluidity of their membranes. Proteins. The proteins associated with the lipid bilayer are of three types: integral, peripheral, and anchored. Inte- gral proteins are embedded in the lipid bilayer. Most inte- gral proteins span the entire width of the phospholipid bilayer, so one part of the protein interacts with the outside of the cell, another part interacts with the hydrophobic core of the membrane, and a third part interacts with the inte- rior of the cell, the cytosol. Proteins that serve as ion chan- nels (see Chapter 6) are always integral membrane pro- teins, as are certain receptors that participate in signal transduction pathways (see Chapter 14). Some receptor-like proteins on the outer surface of the plasma membrane rec- ognize and bind tightly to cell wall consituents, effectively cross-linking the membrane to the cell wall. Peripheral proteins are bound to the membrane surface by noncovalent bonds, such as ionic bonds or hydrogen bonds, and can be dissociated from the membrane with high salt solutions or chaotropic agents, which break ionic and hydrogen bonds, respectively. Peripheral proteins serve a variety of functions in the cell. For example, some are involved in interactions between the plasma membrane and components of the cytoskeleton, such as microtubules and actin microfilaments, which are discussed later in this chapter. Anchored proteins are bound to the membrane surface via lipid molecules, to which they are covalently attached. These lipids include fatty acids (myristic acid and palmitic acid), prenyl groups derived from the isoprenoid pathway (farnesyl and geranylgeranyl groups), and glycosylphos- phatidylinositol (GPI)-anchored proteins (Figure 1.6) (Buchanan et al. 2000). The Nucleus Contains Most of the Genetic Material of the Cell The nucleus (plural nuclei) is the organelle that contains the genetic information primarily responsible for regulating the metabolism, growth, and differentiation of the cell. Collec- tively, these genes and their intervening sequences are referred to as the nuclear genome. The size of the nuclear genome in plants is highly variable, ranging from about 1.2 × 10 8 base pairs for the diminutive dicot Arabidopsis thaliana to 1 × 10 11 base pairs for the lily Fritillaria assyriaca. The 6 Chapter 1 Plant Cells 7 H 3 C H 3 C N + H H H H H H H H H H H H H H H H H H H H C H C O O O O P C C C C C C C C C C C C O O O O H H H H C C H H H H H H H H C C C C C C H H H H C C H H C C H H H H H H H H H H H C C H H H H C C H H H H C C H H H H C C H H H H C C H H H H H C C P O – O O CH H 2 C O CH 2 CH 2 O C O CH 2 C O O CH H 2 C O CH 2 CH 2 O C O CH 2 C O O Cytoplasm Outside of cell Cell wall Plasma membrane (A) (C) (B) Hydrophobic region Hydrophilic region Hydrophilic region Carbohydrates Phospholipid bilayer Choline Phosphate Hydrophilic region Hydrophobic region Glycerol Phosphatidylcholine Phosphatidylcholine Galactosylglyceride Choline Galactose Adjoining primary walls 1 mm Plasma membranes Integral protein Peripheral protein FIGURE 1.5 (A) The plasma membrane, endoplasmic retic- ulum, and other endomembranes of plant cells consist of proteins embedded in a phospholipid bilayer. (B) This trans- mission electron micrograph shows plasma membranes in cells from the meristematic region of a root tip of cress (Lepidium sativum). The overall thickness of the plasma mem- brane, viewed as two dense lines and an intervening space, is 8 nm. (C) Chemical structures and space-filling models of typical phospholipids: phosphatidylcholine and galactosyl- glyceride. (B from Gunning and Steer 1996.) remainder of the genetic information of the cell is contained in the two semiautonomous organelles—the chloroplasts and mitochondria—which we will discuss a little later in this chapter. The nucleus is surrounded by a double membrane called the nuclear envelope (Figure 1.7A). The space between the two membranes of the nuclear envelope is called the perinuclear space, and the two membranes of the nuclear envelope join at sites called nuclear pores (Fig- ure 1.7B). The nuclear “pore” is actually an elaborate struc- ture composed of more than a hundred different proteins arranged octagonally to form a nuclear pore complex (Fig- ure 1.8). There can be very few to many thousands of nuclear pore complexes on an individual nuclear envelope. The central “plug” of the complex acts as an active (ATP- driven) transporter that facilitates the movement of macro- molecules and ribosomal subunits both into and out of the nucleus. (Active transport will be discussed in detail in Chapter 6.) A specific amino acid sequence called the nuclear localization signal is required for a protein to gain entry into the nucleus. The nucleus is the site of storage and replication of the chromosomes, composed of DNA and its associated pro- teins. Collectively, this DNA–protein complex is known as 8 Chapter 1 O C HN Gly C S CH 2 Cys C N CH 2 S C CH 3 NO C OH N CH 2 S C CH 3 NO C OH N HO OH O NH P P Myristic acid (C 14 ) Palmitic acid (C 16 ) Farnesyl (C 15 ) Ceramide Geranylgeranyl (C 20 ) Lipid bilayer Fatty acid–anchored proteins Prenyl lipid–anchored proteins Glycosylphosphatidylinositol (GPI)– anchored protein Ethanolamine Galactose Glucosamine Inositol Mannose OUTSIDE OF CELL CYTOPLASM Amide bond FIGURE 1.6 Different types of anchored membrane proteins that are attached to the membrane via fatty acids, prenyl groups, or phosphatidylinositol. (From Buchanan et al. 2000.) [...]... apparatus in plant cells Plant Physiol Biochem 32: 731–749 Esau, K (1960) Anatomy of Seed Plants Wiley, New York Esau, K (1977) Anatomy of Seed Plants, 2nd ed Wiley, New York Faye, L., Fitchette-Lainé, A C., Gomord, V., Chekkafi, A., Delaunay, A M., and Driouich, A (1992) Detection, biosynthesis and some functions of glycans N-linked to plant secreted proteins In Posttranslational Modifications in Plants... 213–242 Plant Cells Frederick, S E., Mangan, M E., Carey, J B., and Gruber, P J (1992) Intermediate filament antigens of 60 and 65 kDa in the nuclear matrix of plants: Their detection and localization Exp Cell Res 199: 213–222 Gunning, B E S., and Steer, M W (1996) Plant Cell Biology: Structure and Function of Plant Cells Jones and Bartlett, Boston Harwood, J L (1997) Plant lipid metabolism In Plant. .. their great diversity in form and size, all plants carry out similar physiological processes As primary producers, plants convert solar energy to chemical energy Being nonmotile, plants must grow toward light, and they must have efficient vascular systems for movement of water, mineral nutrients, and photosynthetic products throughout the plant body Green land plants must also have mechanisms for avoiding... D., Hashimoto, J., Hirt, H., Inze, D., Jacobs, T., Kouchi, H., Rouze, P., Sauter, M., et al (1996) Plant cyclins: A unified nomenclature for plant A-, B- and D-type cyclins based on sequence organization Plant Mol Biol 32: 1003–1018 Robards, A W., and Lucas, W J (1990) Plasmodesmata Annu Rev Plant Physiol Plant Mol Biol 41: 369–420 Tilney, L G., Cooke, T J., Connelly, P S., and Tilney, M S (1991) The... cell–cell communication Web Material Web Topics 1.1 The Plant Kingdom The major groups of the plant kingdom are surveyed and described 1.2 Flower Structure and the Angiosperm Life Cycle The steps in the reproductive style of angiosperms are discussed and illustrated 1.3 Plant Tissue Systems: Dermal, Ground, and Vascular A more detailed treatment of plant anatomy is given 1.4 The Structures of Chloroplast... generated by this water uptake provides the structural rigidity needed to keep herbaceous plants upright, since they lack the lignified support tissues of woody plants Like animal lysosomes, plant vacuoles contain hydrolytic enzymes, including proteases, ribonucleases, and glycosidases Unlike animal lysosomes, however, plant vacuoles do not participate in the turnover of macromolecules throughout the life... cell cycle progression (see Figure 1.26B) Cyclin inhibitors play an important role in regulating the cell cycle in animals, and probably in plants as well, although little is known about plant cyclin inhibitors Finally, as we will see later in the book, certain plant hormones are able to regulate the cell cycle by regulating the synthesis of key enzymes in the regulatory pathway PLASMODESMATA Plasmodesmata... mature plant cells In such cells, the cytoplasm is restricted to a thin layer surrounding the vacuole The vacuole contains water and dissolved inorganic ions, organic acids, sugars, enzymes, and a variety of secondary metabolites (see Chapter 13), which often play roles in plant defense Active solute accumulation provides the osmotic driving force for water uptake by the vacuole, which is required for plant. . .Plant Cells (A) 9 (B) Nuclear envelope Nucleolus Chromatin (A) Transmission electron micrograph of a plant cell, showing the nucleolus and the nuclear envelope (B) Freeze-etched preparation of nuclear pores from a cell of an onion root (A courtesy of R Evert; B courtesy of D Branton.) FIGURE 1.7 chromatin The linear length of all the DNA within any plant genome is usually millions... the production of the cell plate during cytokinesis in plants are described Chapter References Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., and Walter, P (2002) Molecular Biology of the Cell, 4th ed Garland, New York Buchanan, B B., Gruissem, W., and Jones, R L (eds.) (2000) Biochemistry and Molecular Biology of Plants Amer Soc Plant Physiologists, Rockville, MD Ding, B., Turgeon, R., . functions in the plant life cycle. PLANT LIFE: UNIFYING PRINCIPLES The spectacular diversity of plant size and form is familiar to everyone. Plants range in. flowering plants (see Web Topic 1.2). Plant Cells Are Surrounded by Rigid Cell Walls A fundamental difference between plants and animals is that each plant