Contents
Preface
Acknowledgments
Chapter 1 The plant cell
Protoplast
Cell walls
Plasma membrane/cytoplasmic membrane/plasmalemma
Plasmodesmata
Cellular organelles
Variation in cellular structure
Chapter 2 Plant meristems and tissues
Meristems
Tissues formed during primary growth
Ground tissues
Epidermal tissue
Vascular tissues
Chapter 3 Plant roots
The functions of roots
Types of root systems
The organization of root growth zones
Root tissues
Symbiotic nitrogen fixation
Chapter 4 Plant stems
The function of stems
The structure of stems
Internal tissues of the stem
Secondary or woody growth of stems
Chapter 5 Plant leaves and translocation
The function of leaves
The structure of leaves
The development of leaves
Environmental effects on leaf development
Translocation
Chapter 6 Reproduction in flowering plants
Flower structure
Inflorescences
Vegetative reproduction
Chapter 7 Plant nutrition
Soil components
Acidity and alkalinity
Soil nutrients
Macronutrients
Micronutrients
Chapter 8 Plant–water relations
Uptake of water
Movement of water in plants
Transpiration
Drought
Water movement in stems
Chapter 9 Macromolecules and enzyme activity
Chemical bonds
Macromolecules
Chapter 10 Photosynthesis
Light and photosynthesis
Chloroplasts
Photosynthetic pigments
The “light” reactions of photosynthesis
The “dark” reactions of photosynthesis
Adaptations to make photosynthesis more efficient
Chapter 11 Respiration
When and where does respiration occur?
Sources of energy for respiration
Cellular respiration
Chapter 12 Environmental regulation of plant development
Photoperiodism
Other phytochrome-mediated responses
Plant environmental responses requiring cold temperature
Chapter 13 Hormonal regulation of plant development
Auxins
Gibberellins
Cytokinins
Ethylene
Abscisic acid
Chapter 14 Secondary plant products
Types of secondary plant products
Glossary
References
Index
Edition first published 2009
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Library of Congress Cataloging-in-Publication Data
MacAdam, Jennifer W.
Structure and function of plants/Jennifer W. MacAdam. – 1st ed.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-8138-2718-6 (pbk.: alk. paper) 1. Plant anatomy–Textbooks. 2. Plant physiology–Textbooks. I. Title.
QK641.M33 2009
571.2–dc22
2008035480
Chapter 1
The plant cell
We appreciate plants for their beauty and usefulness, and on a different level, for the ability of plant species to adapt to an amazing diversity of climates and soils (two of many abiotic influences) as well as their ability to interact with microbes, animals, and other plants (biotic influences). The differences in characteristics such as stem, leaf, and flower structure that result from these and other adaptations were the original basis for classification of plants into different taxonomic groups. However, for all their differences in overall appearance (morphology), plants have the same basic structures at the cellular level, so we begin by looking at the cellular structures of plants. Figure 1.1 is a simplified illustration of a plant cell, and the structures labeled in Figure 1.1 are discussed in more detail in this and other chapters.
Protoplast
The protoplast is a collective term that includes the plasma membrane and the cellular objects it contains. It is filled with liquid, the cytosol, that bathes the cellular organelles including the nucleus. The protoplast includes all the “living” parts of the cell, so the cell wall to its outside is not included. The protoplast is composed of 60–75% proteins by dry weight.
Cytoplasm
The cytoplasm is the protoplast minus the nucleus. The nucleus directs the work that goes on in the cytoplasm.
Cytosol
The cytosol is the liquid portion (matrix) of the cytoplasm, which surrounds organelles and in which a number of proteins, salts (including nutrient ions), and sugars are dissolved. The cytosol has the thickened consistency of a gel. The cytosol of adjacent cells is continuous, by way of plasmodesmata.
Cell walls
The plant cell protoplast is enclosed by a fibrous wall that grows as the cell expands to its mature size, but which becomes cross-linked and eventually limits the growth of the cell, defining and supporting the cell and collectively providing support for stems and leaves. Some cells, like photosynthetic and storage cells, only have a thin primary cell wall, and other cells have both a primary wall and a thick, lignified and therefore rigid secondary cell wall, either to retain the cell’s shape against the tension of water movement through the plant, as in xylem cells, or to provide concentrated regions of support or protection as in fiber cells or sclerids. The trunk of a tree is made up of concentric layers of water-transporting (xylem) cells with secondary walls that serve both water-carrying and support functions.
Components of the cell wall
Cellulose
The fundamental component of cell walls is cellulose, which in turn is made up of long chains of glucose molecules, from thousands to tens of thousands of glucose units per molecule of cellulose. The chemical structure of glucose is illustrated in Figure 1.2, with each of the six carbon atoms (C) numbered. α- and β-Glucose differ in the orientation of the bonds at C-1. Starch and cellulose are both long chains of glucose, but starch is easily digested by monogastrics, like humans, while the bonds between glucose molecules in cellulose are most commonly broken by enzymes produced by microbes inhabiting the guts of ruminants, such as cattle and sheep (and termites). The difference in these chains of glucose is illustrated in Figure 1.3. Bonds in both starch and cellulose are between the 1- and 4-carbons of successive glucose molecules, but while in starch the orientation of each α-glucose molecule in the chain is the same, in cellulose every other β-glucose molecule is flipped on its horizontal axis.
Cellulose is the “fiber” in paper. Cellulose molecules are grouped together into microfibrils consisting of 50–60 cellulose molecules held together by hydrogen bonds, which are relatively loose bonds but effective in large numbers, as in cellulose (Figure 1.4). Cellulose is such a big molecule that it is synthesized at the plasma membrane rather than inside the protoplasm. Microfibrils are extruded into the extracellular matrix, like toothpaste from a tube (Figure 1.5). Other cell wall components are secreted into the cell wall by way of Golgi vesicles, and assemble around the cellulose microfibrils.
Hemicellulose
Hemicellulose also consists of chains of sugars, but the sugars are much more diverse than in cellulose, which contains only glucose. Hemicelluloses are highly branched because of the bonds that form among the sugars that make them up, and they form a network that coats the much larger cellulose microfibrils. Hemicelluloses adhere to cellulose by way of hydrogen bonds. Hemicellulose molecules coating individual cellulose microfibrils become cross-linked or bound together by covalent bonds, which limits cell wall expansion because the cellulose microfibrils can no longer slide past each other and allow the cell wall to grow. In Figure 1.6, the components of the cell wall are illustrated to show hemicelluloses forming cross-linkages between cellulose microfibrils.
Pectin
The middle lamella is the outermost layer of a plant cell and has a high concentration of pectins, which consist of uronic acids, the acidic (and therefore charged) forms of glucose and galactose, and other sugars. The middle lamella is the first boundary formed between what will become adjacent cells during cell division. In cell division, the genetic material of the cell is duplicated and the two groups of chromosomes move to opposite ends of the cell. In Figure 1.7, the middle lamella (yellow) is beginning to form as the boundary between the two new cells. The phragmoplast, a remnant of the organizing structure needed to divide the genetic material, which is shown as groups of white cylinders, is oriented between the daughter nuclei and the developing middle lamella.
After cell division, the primary cell wall forms to the inside of the middle lamella, and also has a relatively high content of pectin (up to 35%). The secondary cell wall, when present, then forms to the inside of the primary cell wall. The components of both walls are formed in the protoplast and secreted via the Golgi apparatus across the plasma membrane.
Extensin
A structural protein (in contrast to enzymes, which are soluble in the cytoplasm or the matrices of the cellular organelles), extensin, forms a network within the cell wall that can become cross-linked, like the hemicellulose network. Extensins make up, at most, about 10% of the cell wall, and were first identified in broadleaf plants (dicots), but proteins with similar functions are found in the grasses (monocots).
Secondary cell walls
In structural cells like fibers and in the water-carrying xylem cells, additional cell wall layers are laid down inside the primary cell wall after cell growth stops. These secondary cell walls have a higher cellulose content than primary walls, and may be distinctly layered. In Figure 1.8, which is an electron micrograph of fiber cells, the middle lamella (ML), primary cell wall (CW1), and distinct layers of the secondary cell wall (S1, S2, and S3) can be seen. The walls of these cells also become lignified, a process in which small lignin precursor molecules are secreted into the cell wall and assemble into large, unorganized molecules that displace water (see Chapter 14). The function of lignin is to waterproof xylem vessels and to make cell walls resistant to degradation by invading pathogens. Lignin also greatly increases the rigidity of the cell wall, and is therefore an important component of wood. However, lignin must be extracted for the production of paper, and greatly reduces the digestibility of the fiber cells in plants such as the grasses used as animal feed. Xylem cells, which are the water-carrying cells in roots and shoots, and fiber cells do not contain a protoplasm at maturity and therefore are nonliving cells.
Plasma membrane/cytoplasmic membrane/plasmalemma
The plasma membrane, cytoplasmic membrane, and plasmalemma are all accepted names for the selectively permeable membrane that encloses the living contents of the cell and controls the movement of materials into and out of the cell.
Membranes
Plant cell membranes are primarily made of lipids (fats and oils) and proteins. Membranes are usually described as consisting of a lipid bilayer because of the way the lipid molecules are arranged, but many proteins are embedded in this bilayer (Figure 1.9). In many cases, these proteins act as gateways for regulation of the contents of the cell.
Membrane lipids
The dominant lipids in the plasma membrane are phospholipids, which have a central glycerol molecule with a phosphate molecule attached at one end (the “head”) which is water-loving (hydrophilic) and a water-fearing (hydrophobic) “tail” composed of two fatty acids (making these phospholipids diglycerides). The fatty acids are partly unsaturated, making the lipid bilayer fluid, like oils (see Chapter 9).
Phospholipids spontaneously self-assemble into a bilayer in aqueous solutions like a plant cell. They turn their hydrophilic heads outward, some toward the cell wall that encloses the plasma membrane, and some toward the aqueous cell protoplasm, and turn their hydrophobic tails inward to form a double layer. Membranes are fluid—the molecules they contain can easily move past each other in the membrane—but they are also very stable. Membranes can exclude most charged molecules, like nutrient ions, which allows them to control movement of these nutrients into and out of the cell. Water and the gases oxygen and carbon dioxide, however, can cross the lipid bilayer relatively easily.
Other membranes, especially the internal membranes of the chloroplast, contain a large amount of glycolipids, where the head group contains one or two molecules of the sugar galactose instead of a phosphate, and sulfolipids, with a sulfate instead of a phosphate as part of the head group. In these cases, as for phospholipids, the heads are hydrophilic.
Membrane proteins
Proteins make up as much as 50% of the mass of cell membranes. The amino acid composition of proteins determines how the protein is incorporated into the lipid bilayer. If the protein spans the membrane from inside to out, it is an integral protein. If it is bound only to the inside or outside of the bilayer, then it is a peripheral protein. Proteins must have a region that is hydrophobic to be incorporated into the membrane. These membrane proteins can function in the selective transport of solutes across the membrane if they fully span the lipid bilayer (Figure 1.9), or they can act as enzymes like cellulose synthase, or they may form part of an electron transport chain, which are groupings of many different enzymes that are used in photosynthesis and respiration.
Plasmodesmata
The plasmodesmata are narrow channels between cells through which dissolved substances but not organelles can pass. Plasmodesmata form during cell division, and allow cell-to-cell communication and transport. One is termed a plasmodesma (Figure 1.10). Plasmodesmata are lined with extensions of the plasma membrane and have an inner structure, the desmotubule, which is continuous with endoplasmic reticulum of the two adjacent cells. Dissolved substances can pass between the plasma membrane and the desmotubule to move between cells. The cytoplasm of adjacent cells connected by plasmodesmata forms a continuous living network among cells called the symplast.
In contrast, the apoplast is the nonliving space outside the protoplast and includes the cell wall, the intercellular space, and xylem tissue through which water is transported. The larger spaces between cells in leaves and stems is usually filled with air, although cells are coated with a film of water; in roots, these spaces between cells can contain water being taken up by the plant.
Cellular organelles
Organelles are membrane-defined compartments inside the cell, each with specific functions. The following are major plant cellular organelles.
Nucleus
The nucleus is the location of the genetic material (DNA, deoxyribonucleic acid) contained in almost all cells; the sieve tubes of the phloem are one exception. The nucleus directs the synthesis of the majority of enzyme production and is therefore considered the control center of the cell, since enzymes perform the work (or metabolism) of cells. DNA is organized into chromosomes in plants, and genes are discrete regions of DNA within chromosomes. DNA is the template used to synthesize RNA (ribonucleic acid), which is termed “transcription.” RNA is exported from the nucleus to the cytoplasm, where it directs the synthesis of proteins, which is termed “translation.” These processes are discussed further in Chapter 9.
Vacuole
Defined by a membrane called the tonoplast, the vacuole is filled with water, and may comprise 80 or 90% of the volume of a mature plant cell. The vacuole enlarges during growth, and this enlargement occurs by water uptake. The vacuole contains dissolved salts, sugars, organic acids, enzymes, and may contain pigments. Vacuolar transport processes are illustrated in Figure 1.11. The energy of a phosphate bond from ATP (see Chapter 11) is used to pump hydrogen ions (H+ or protons) into the vacuole; the higher concentration of H+ in the vacuole reduces the pH of the vacuole compared to the cytosol (for a discussion of pH, see Chapter 7). These H+ can be exchanged for other positively charged ions such as calcium (Ca2+) and balanced by the uptake of negatively charged ions such as chloride (Cl−), nitrate (NO3−), or the organic acid malate.
Endoplasmic reticulum
The endoplasmic reticulum (ER) is a tubular network that is formed from and continuous with the nuclear envelope, and which fills much of the volume of the cytosol. In Figure 1.1, slices through the ER are indicated as ovals covered with red dots that represent ribosomes. Ribosomes may also occur free in the cytosol. The function of the ER is the synthesis of lipids and proteins that are either used to make cellular membranes or exported from the cell. The space enclosed by the membrane layers is called the lumen of the ER. The smooth ER, without ribosomes, is involved in lipid synthesis. The rough ER, with ribosomes, is the site of protein synthesis. Proteins that will leave the cell are made on the rough ER and passed into the lumen of the ER after synthesis. In the ER lumen, they are altered by posttranslational modifications such as the addition of sugars to form glycoproteins, which help determine the specific function and location of the protein. The modified proteins move through the lumen to the smooth ER and small, enclosed pieces of the smooth ER bud off to form transport vesicles, with the proteins inside. The transport vesicles move to the Golgi apparatus.
Golgi apparatus (formerly dictyosomes)
A Golgi apparatus consists of a stack of separate flattened sacs (cisternae) where products of the ER are processed further. Transport vesicles from the ER deliver their contents by fusing with a membrane of the Golgi apparatus. After processing, products of the Golgi are packaged in secretory vesicles that bud off for transport within the cell, or fuse with the plasma membrane to secrete their contents outside the cell (Figure 1.12). Some of the carbohydrates that make up the cell wall are also formed in the Golgi.
Mitochondria
Mitochondria are the site of respiration (Chapter 11), which releases the chemical energy stored in food (most commonly carbohydrates and fats) and transfers this energy to form ATP (adenosine triphosphate), a chemical compound that can be transported to other locations in the cell. The outer membrane of a mitochondrion is readily permeable, and the inner membrane, which has many invaginations called cristae, is very high in protein (70%), and is much more selective. Inside the inner membrane is a matrix that has a high concentration (40–50%) of dissolved protein (enzymes) where key steps in respiration occur (Figure 1.13).
Chloroplasts and other plastids
Chloroplasts are football-shaped organelles found primarily in mesophyll cells of leaves and stems and guard cells of the epidermis. These organelles are the site of photosynthesis, the metabolic process that synthesizes sugars in plants. Chloroplasts consist of three membranes: a readily permeable outer membrane, a selective inner membrane, and the thylakoid membrane system. The matrix that fills the space around the outer surface of the thylakoid membranes is called the stroma, and is protein-rich (Figure 1.14). The reactions of photosynthesis that produce sugar occur in the stroma. Chlorophyll is a pigment that is embedded in the thylakoid membranes, which enclose a space called (as with the ER) the lumen. The lumen is where water is oxidized for the generation of the O2 that is a by-product of photosynthesis, and where protons (hydrogen ions; H+) accumulate, subsequently driving ATP synthesis. Some thylakoid membranes occur in stacks called grana. Other plastids are the storage sites for starch (amyloplasts; common in roots; Figure 2.4) and for pigments (chromoplasts), such as those in carrot roots, tomato fruits, and the petals of yellow, orange, or red flowers.
Endosymbiosis
Both chloroplasts and mitochondria are thought to have originated as bacteria (prokaryotic cells that have no nucleus; Figure 1.15a) that invaded or were consumed by eukaryotic cells, which do have nuclei (Figure 1.15b), forming a symbiotic (mutually beneficial) relationship (Figure 1.15c). The invasion by the bacteria that became chloroplasts (Figure 1.15d) followed the invasion by the bacteria that became mitochondria (Figure 1.15e), creating plant cells (Figure 1.15f). Our understanding of endosymbiosis was developed over many years by Dr Lynn Margulis. A unique feature of mitochondria and chloroplasts among plant cell organelles is the presence of inner and outer membranes in both organelles. The outer membrane is thought to have formed from an invagination and eventual budding off of the plasma membrane as the cell tried to contain an invasion by the bacterium. In addition, both organelles contain plasmid (bacterial-type) DNA, and still use it to carry out their own protein synthesis. These invaders benefited from their new surroundings, and were thought to have been tolerated because their presence confers clear advantages to the cells that contain them. Therefore, the relationship is mutually beneficial or symbiotic.
Variation in cellular structure
Plant cells have many basic structures in common, but as can be seen in the vascular bundle in Figure 1.16, variation in these structures can result in enormous differences in cellular appearance and function. Most remarkably, these very different cells are connected to one another through plasmodesmata; they divide and expand together as plants develop and work together through the life of the plant.