Chapter 1 Plant Cells THE TERM CELLIS 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. 2 Chapter 1 • Other than certain reproductive cells, plants are non- FIGURE 1.1 Schematic representation of the body of a typi- (cid:2) cal dicot. Cross sections of (A) the leaf, (B) the stem, and (C) motile. As a substitute for motility, they have evolved the root are also shown. Inserts show longitudinal sections the ability to grow toward essential resources, such of a shoot tip and a root tip from flax (Linum usitatissi- as light, water, and mineral nutrients, throughout mum), showing the apical meristems. (Photos © J. Robert their life span. Waaland/Biological Photo Service.) • Terrestrial plants are structurally reinforced to sup- port their mass as they grow toward sunlight against the pull of gravity. unlike animal development, depends solely on patterns of • Terrestrial plants lose water continuously by evapo- cell division and cell enlargement. ration and have evolved mechanisms for avoiding Plant cells have two types of walls: primary and sec- desiccation. ondary (Figure 1.2). Primary cell wallsare typically thin • Terrestrial plants have mechanisms for moving water (less than 1 µm) and are characteristic of young, growing and minerals from the soil to the sites of photosyn- cells. Secondary cell wallsare thicker and stronger than thesis and growth, as well as mechanisms for moving primary walls and are deposited when most cell enlarge- the products of photosynthesis to nonphotosynthetic ment has ended. Secondary cell walls owe their strength organs and tissues. and toughness to lignin, a brittle, gluelike material (see Chapter 13). OVERVIEW OF PLANT STRUCTURE The evolution of lignified secondary cell walls provided plants with the structural reinforcement necessary to grow Despite their apparent diversity, all seed plants (seeWeb vertically above the soil and to colonize the land. Topic 1.1) have the same basic body plan (Figure 1.1).The Bryophytes, which lack lignified cell walls, are unable to vegetative body is composed of three organs: leaf, stem, grow more than a few centimeters above the ground. and root. The primary function of a leaf is photosynthesis, New Cells Are Produced by Dividing Tissues that of the stem is support, and that of the root is anchorage Called Meristems and absorption of water and minerals. Leaves are attached to the stem at nodes, and the region of the stem between Plant growth is concentrated in localized regions of cell two nodes is termed the internode. The stem together with division called meristems. Nearly all nuclear divisions its leaves is commonly referred to as the shoot. (mitosis) and cell divisions (cytokinesis) occur in these There are two categories of seed plants: gymnosperms meristematic regions. In a young plant, the most active (from the Greek for “naked seed”) and angiosperms (based meristems are called apical meristems; they are located at on the Greek for “vessel seed,” or seeds contained in a ves- the tips of the stem and the root (see Figure 1.1). At the sel). Gymnospermsare the less advanced type; about 700 nodes, axillary buds contain the apical meristems for species are known. The largest group of gymnosperms is the branch shoots. Lateral roots arise from the pericycle, an conifers (“cone-bearers”), which include such commercially internal meristematic tissue (see Figure 1.1C). Proximal to important forest trees as pine, fir, spruce, and redwood. (i.e., next to) and overlapping the meristematic regions are Angiosperms, the more advanced type of seed plant, zones of cell elongation in which cells increase dramatically first became abundant during the Cretaceous period, about in length and width. Cells usually differentiate into spe- 100 million years ago. Today, they dominate the landscape, cialized types after they elongate. easily outcompeting the gymnosperms. About 250,000 The phase of plant development that gives rise to new species are known, but many more remain to be character- organs and to the basic plant form is called primary ized. The major innovation of the angiosperms is the growth. Primary growth results from the activity of apical flower; hence they are referred to as flowering plants(see meristems, in which cell division is followed by progres- Web Topic 1.2). sive cell enlargement, typically elongation. After elonga- tion in a given region is complete, secondary growthmay Plant Cells Are Surrounded by Rigid Cell Walls occur. Secondary growth involves two lateral meristems: Afundamental difference between plants and animals is the vascular cambium (plural cambia) and the cork cam- that each plant cell is surrounded by a rigid cell wall. In bium. The vascular cambium gives rise to secondary xylem animals, embryonic cells can migrate from one location to (wood) and secondary phloem. The cork cambium pro- another, resulting in the development of tissues and organs duces the periderm, consisting mainly of cork cells. containing cells that originated in different parts of the Three Major Tissue Systems organism. Make Up the Plant Body In plants, such cell migrations are prevented because each walled cell and its neighbor are cemented together by Three major tissue systems are found in all plant organs: a middle lamella.As a consequence, plant development, dermal tissue, ground tissue, and vascular tissue. These tis- (A) Leaf Upper epidermis Leaf primordia (dermal tissue) Cuticle Shoot apex and Palisade apical meristem parenchyma (ground tissue) Bundle sheath Axillary bud parenchyma with meristem Xylem Vascular tissues Mesophyll Phloem Leaf Lower epidermis (dermal tissue) Node Guard cell Internode Stomata Spongy mesophyll (ground tissue) Lower epidermis Cuticle Vascular (B) Stem tissue Soil line Epidermis (dermal tissue) Cortex Ground Pith tissues Xylem Lateral Vascular root Phloem tissues Vascular Taproot cambium Root hairs (C) Root Epidermis Root apex with (dermal tissue) apical meristem Cortex Root cap Pericycle Ground (internal tissues meristem) Endodermis Phloem Vascular tissues Xylem Root hair (dermal tissue) Primary wall Middle lamella Simple pit Vascular cambium Primary wall Secondary wall FIGURE 1.2 Schematic representation of primary and secondary cell walls and their relationship to Plasma membrane the rest of the cell. (A) Dermal tissue: epidermal cells (B) Ground tissue: parenchyma cells Primary cell wall Middle lamella (C) Ground tissue: collenchyma cells (D) Ground tissue: sclerenchyma cells Primary cell wall Sclereids Nucleus Fibers (E) Vascular tisssue: xylem and phloem Bordered pits Simple Secondary pits Sieve plate walls Nucleus Companion cell Sieve areas Sieve plate Primary walls End wall perforation Tracheids Vessel elements Sieve cell Sieve tube element (gymnosperms) (angiosperms) Xylem Phloem Plant Cells 5 FIGURE 1.3 (A) The outer epidermis (dermal tissue) of a THE PLANT CELL (cid:2) leaf of welwischia mirabilis(120×). Diagrammatic representa- tions of three types of ground tissue: (B) parenchyma, (C) Plants are multicellular organisms composed of millions of collenchyma, (D) sclerenchyma cells, and (E) conducting cells with specialized functions. At maturity, such special- cells of the xylem and phloem. (A© Meckes/Ottawa/Photo ized cells may differ greatly from one another in their struc- Researchers, Inc.) 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 sues are illustrated and briefly chacterized in Figure 1.3. structures, including the nucleus, can be lost during cell For further details and characterizations of these plant tis- maturation, but all plant cells beginwith a similar comple- sues, seeWeb Topic 1.3. ment of organelles. Vacuole Tonoplast Nucleus Nuclear Peroxisome envelope Nucleolus Chromatin Ribosomes Rough Compound endoplasmic middle reticulum lamella Smooth endoplasmic Mitochondrion reticulum Primary cell wall Plasma membrane Middle lamella Cell wall Golgi body Primary cell wall Chloroplast Intercellular air space 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. 6 Chapter 1 An additional characteristic feature of plant cells is that the bilayer. As a result, the fluidity of the membrane is they are surrounded by a cellulosic cell wall. The following increased. The fluidity of the membrane, in turn, plays a sections provide an overview of the membranes and critical role in many membrane functions. Membrane flu- organelles of plant cells. The structure and function of the idity is also strongly influenced by temperature. Because cell wall will be treated in detail in Chapter 15. plants generally cannot regulate their body temperatures, they are often faced with the problem of maintaining mem- Biological Membranes Are Phospholipid Bilayers brane fluidity under conditions of low temperature, which That Contain Proteins tends to decrease membrane fluidity. Thus, plant phos- All cells are enclosed in a membrane that serves as their pholipids have a high percentage of unsaturated fatty outer boundary, separating the cytoplasm from the exter- acids, such as oleic acid (one double bond), linoleic acid nal environment. This plasma membrane(also called plas- (two double bonds) and α-linolenic acid (three double malemma) allows the cell to take up and retain certain sub- bonds), which increase the fluidity of their membranes. stances while excluding others. Various transport proteins embedded in the plasma membrane are responsible for this Proteins. The proteins associated with the lipid bilayer selective traffic of solutes across the membrane. The accu- are of three types: integral, peripheral, and anchored. Inte- mulation of ions or molecules in the cytosol through the gral proteinsare embedded in the lipid bilayer. Most inte- action of transport proteins consumes metabolic energy. gral proteins span the entire width of the phospholipid Membranes also delimit the boundaries of the specialized bilayer, so one part of the protein interacts with the outside internal organelles of the cell and regulate the fluxes of ions of the cell, another part interacts with the hydrophobic core and metabolites into and out of these compartments. of the membrane, and a third part interacts with the inte- According to the fluid-mosaic model, all biological rior of the cell, the cytosol. Proteins that serve as ion chan- membranes have the same basic molecular organization. nels (see Chapter 6) are always integral membrane pro- They consist of a double layer (bilayer) of either phospho- teins, as are certain receptors that participate in signal lipids or, in the case of chloroplasts, glycosylglycerides, in transduction pathways (see Chapter 14). Some receptor-like which proteins are embedded (Figure 1.5Aand B).In most proteins on the outer surface of the plasma membrane rec- membranes, proteins make up about half of the mem- ognize and bind tightly to cell wall consituents, effectively brane’s mass. However, the composition of the lipid com- cross-linking the membrane to the cell wall. ponents and the properties of the proteins vary from mem- Peripheral proteinsare bound to the membrane surface brane to membrane, conferring on each membrane its by noncovalent bonds, such as ionic bonds or hydrogen unique functional characteristics. bonds, and can be dissociated from the membrane with high salt solutions or chaotropic agents, which break ionic Phospholipids. Phospholipids are a class of lipids in and hydrogen bonds, respectively. Peripheral proteins which two fatty acids are covalently linked to glycerol, serve a variety of functions in the cell. For example, some which is covalently linked to a phosphate group. Also are involved in interactions between the plasma membrane attached to this phosphate group is a variable component, and components of the cytoskeleton, such as microtubules called the head group, such as serine, choline, glycerol, or and actin microfilaments, which are discussed later in this inositol (Figure 1.5C). In contrast to the fatty acids, the head chapter. groups are highly polar; consequently, phospholipid mol- Anchored proteinsare bound to the membrane surface ecules display both hydrophilic and hydrophobic proper- via lipid molecules, to which they are covalently attached. ties (i.e., they are amphipathic). The nonpolar hydrocarbon These lipids include fatty acids (myristic acid and palmitic chains of the fatty acids form a region that is exclusively acid), prenyl groups derived from the isoprenoid pathway hydrophobic—that is, that excludes water. (farnesyl and geranylgeranyl groups), and glycosylphos- Plastid membranes are unique in that their lipid com- phatidylinositol (GPI)-anchored proteins (Figure 1.6) ponent consists almost entirely of glycosylglycerides (Buchanan et al. 2000). rather than phospholipids. In glycosylglycerides, the polar The Nucleus Contains Most of the Genetic head group consists of galactose, digalactose, or sulfated galactose, without a phosphate group (see Web Topic 1.4). Material of the Cell The fatty acid chains of phospholipids and glycosyl- The nucleus(plural nuclei) is the organelle that contains the glycerides are variable in length, but they usually consist genetic information primarily responsible for regulating the of 14 to 24 carbons. One of the fatty acids is typically satu- metabolism, growth, and differentiation of the cell. Collec- rated (i.e., it contains no double bonds); the other fatty acid tively, these genes and their intervening sequences are chain usually has one or more cis double bonds (i.e., it is referred to as the nuclear genome. The size of the nuclear unsaturated). genome in plants is highly variable, ranging from about 1.2 The presence of cis double bonds creates a kink in the ×108base pairs for the diminutive dicot Arabidopsis thaliana chain that prevents tight packing of the phospholipids in to 1 ×1011base pairs for the lily Fritillaria assyriaca. The Plant Cells 7 (A) (C) HC 3 Choline N+ H H3C C C H C O Phosphate H O Hydrophilic H O P region O Cell wall H C H Glycerol Plasma H H C H C membrane O O O C O C H C C H H H H H C C H C H H C H H H H H Outside of cell Carbohydrates HH CC HH HH C CC HH H C H H C H Hydrophobic H C H H H Hydrophilic region HH CC HH H CC CHHH region C H Hydrophobic Pbhiloayspehrolipid HHH CC HHH HHCHCCHHCHH region C H H C H H C H H H H C Hydrophilic H H region Phosphatidylcholine Cytoplasm Integral Peripheral protein protein Choline (B) O –O P O Galactose Plasma membranes O O H2C CH CH2 H2C CH CH2 Adjoining O O O O primary walls C O C O C O C O CH CH CH CH 2 2 2 2 1 mm 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- Phosphatidylcholine Galactosylglyceride 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.) 8 Chapter 1 OUTSIDE OF CELL Glycosylphosphatidylinositol (GPI)– anchored protein Ethanolamine P Galactose Glucosamine Mannose Inositol P Lipid bilayer HO NH OH O Myristic acid (C ) Palmitic acid (C ) Farnesyl (C ) Geranylgeranyl (C ) Ceramide 14 16 15 20 C O S S S Amide bond HN CH2 CH2 CH2 Gly Cys H C C O CH3 H C C O CH3 C N O N O N C Fatty acid–anchored proteins N N Prenyl lipid–anchored proteins CYTOPLASM 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.) remainder of the genetic information of the cell is contained ure 1.8). There can be very few to many thousands of in the two semiautonomous organelles—the chloroplasts nuclear pore complexes on an individual nuclear envelope. and mitochondria—which we will discuss a little later in The central “plug” of the complex acts as an active (ATP- this chapter. driven) transporter that facilitates the movement of macro- The nucleus is surrounded by a double membrane molecules and ribosomal subunits both into and out of the called the nuclear envelope (Figure 1.7A). The space nucleus. (Active transport will be discussed in detail in between the two membranes of the nuclear envelope is Chapter 6.) A specific amino acid sequence called the called the perinuclear space, and the two membranes of nuclear localization signalis required for a protein to gain the nuclear envelope join at sites called nuclear pores(Fig- entry into the nucleus. ure 1.7B). The nuclear “pore” is actually an elaborate struc- The nucleus is the site of storage and replication of the ture composed of more than a hundred different proteins chromosomes, composed of DNAand its associated pro- arranged octagonally to form a nuclear pore complex (Fig- teins. Collectively, this DNA–protein complex is known as Plant Cells 9 (A) (B) Nuclear envelope Nucleolus Chromatin FIGURE 1.7 (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. (Acourtesy of R. Evert; B cour- tesy of D. Branton.) chromatin. The linear length of all the DNAwithin any nucleus, segments of the linear double helix of DNAare plant genome is usually millions of times greater than the coiled twice around a solid cylinder of eight histonepro- diameter of the nucleus in which it is found. To solve the tein molecules, forming a nucleosome. Nucleosomes are problem of packaging this chromosomal DNAwithin the arranged like beads on a string along the length of each chromosome. During mitosis, the chromatin condenses, first by coil- ing tightly into a 30 nm chromatin fiber, with six nucleo- somes per turn, followed by further folding and packing CYTOPLASM processes that depend on interactions between proteins Nuclear pore complex and nucleic acids (Figure 1.9). At interphase, two types of 120 nm Cytoplasmic ring chromatin are visible: heterochromatin and euchromatin. About 10% of the DNA consists of heterochromatin, a Cytoplasmic Outer nuclear filament highly compact and transcriptionally inactive form of chro- membrane matin. The rest of the DNAconsists of euchromatin,the dispersed, transcriptionally active form. Only about 10% of Spoke-ring assembly the euchromatin is transcriptionally active at any given time. The remainder exists in an intermediate state of con- densation, between heterochromatin and transcriptionally active euchromatin. Nuclei contain a densely granular region, called the nucleolus(plural nucleoli), that is the site of ribosome syn- thesis (see Figure 1.7A). The nucleolus includes portions of Inner nuclear one or more chromosomes where ribosomal RNA(rRNA) Nuclear ring membrane genes are clustered to form a structure called the nucleolar organizer. Typical cells have one or more nucleoli per Nuclear Central nucleus. Each 80S ribosome is made of a large and a small transporter basket subunit, and each subunit is a complex aggregate of rRNA and specific proteins. The two subunits exit the nucleus NUCLEOPLASM separately, through the nuclear pore, and then unite in the cytoplasm to form a complete ribosome (Figure 1.10A). FIGURE 1.8 Schematic model of the structure of the nuclear Ribosomesare the sites of protein synthesis. pore complex. Parallel rings composed of eight subunits each are arranged octagonally near the inner and outer Protein Synthesis Involves membranes of the nuclear envelope. Various proteins form Transcription and Translation the other structures, such as the nuclear ring, the spoke- ring assembly, the central transporter, the cytoplasmic fila- The complex process of protein synthesis starts with tran- ments, and the nuclear basket. scription—the synthesis of an RNApolymer bearing a base 10 Chapter 1 FIGURE 1.9 Packaging of DNAin a metaphase chromo- some. The DNAis first aggregated into nucleosomes and 2 nm then wound to form the 30 nm chromatin fibers. Further coiling leads to the condensed metaphase chromosome. DNA double helix (After Alberts et al. 2002.) Linker DNA Translationis the process whereby a specific protein is 11 nm synthesized from amino acids, according to the sequence information encoded by the mRNA. The ribosome travels Histones Nucleosome the entire length of the mRNAand serves as the site for the sequential bonding of amino acids as specified by the base Nucleosomes ( “ b eads on a string”) sequence of the mRNA(Figure 1.10B). The Endoplasmic Reticulum Is a Network of Internal Membranes Cells have an elaborate network of internal membranes called the endoplasmic reticulum(ER). The membranes of 30 nm the ER are typical lipid bilayers with interspersed integral and peripheral proteins. These membranes form flattened or tubular sacs known as cisternae(singular cisterna). Ultrastructural studies have shown that the ER is con- Nucleosome tinuous with the outer membrane of the nuclear envelope. 30 nm chromatin fiber There are two types of ER—smooth and rough (Figure 1.11)—and the two types are interconnected. Rough ER (RER) differs from smooth ER in that it is covered with ribosomes that are actively engaged in protein synthesis; in addition, rough ER tends to be lamellar (a flat sheet com- 300 nm posed of two unit membranes), while smooth ER tends to be tubular, although a gradation for each type can be Looped domains observed in almost any cell. The structural differences between the two forms of ER are accompanied by functional differences. Smooth ER functions as a major site of lipid synthesis and membrane 700 nm assembly. Rough ER is the site of synthesis of membrane proteins and proteins to be secreted outside the cell or into the vacuoles. Condensed chromatin Secretion of Proteins from Cells Begins with the Rough ER Chromatids Proteins destined for secretion cross the RER membrane and enter the lumen of the ER. This is the first step in the 1400 nm Highly condensed, duplicated FIGURE 1.10 (A) Basic steps in gene expression, including (cid:2) metaphase chromosome transcription, processing, export to the cytoplasm, and of a dividing cell translation. Proteins may be synthesized on free or bound ribosomes. Secretory proteins containing a hydrophobic signal sequence bind to the signal recognition particle (SRP) in the cytosol. The SRP–ribosome complex then moves to the endoplasmic reticulum, where it attaches to the SRP sequence that is complementary to a specific gene. The receptor. Translation proceeds, and the elongating polypep- RNAtranscript is processed to become messenger RNA tide is inserted into the lumen of the endoplasmic reticu- lum. The signal peptide is cleaved off, sugars are added, (mRNA), which moves from the nucleus to the cytoplasm. and the glycoprotein is transported via vesicles to the The mRNAin the cytoplasm attaches first to the small ribo- Golgi. (B) Amino acids are polymerized on the ribosome, somal subunit and then to the large subunit to initiate with the help of tRNA, to form the elongating polypeptide translation. chain.