Four features set apart plant cells from those of other organisms:
A
cellulose cell wall; many protists, some fungi, and most bacteria also have rigid walls, but made of different materials, e.g. Chitin strengthens fungal cell walls and peptidoglycan those of bacteria.
Plasmodesmata, strands of cytoplasm that protrude through pores in the cell walls and connect the protoplasts of adjacent cells; these are avenues of material transport in plants.
Figure 1
Plastids, organelles thought to have an endosymbiotic origin; they have an intricate internal structure of folded membranes that greatly increases the internal surface area on which chemical reactions occur;
chloroplasts contain
chlorophyll, and their
thylakoid membranes are the sites of
photosynthesis.
Vacuoles are fluid filled sacs present in plants, animals, and some protists; those of plants differ from others in size and function, i.e. a typical plant cell has a single, centrally located vacuole whose water-filled interior pushes the cytoplasm outward against the cell wall giving rigidity or turgidity to the cell.
Not all functional cells in plants are alive. Many cells live for only a short time then die, but their cell walls remain in place giving strength and rigidity to the plant body. The dead cells of
xylem tissue form effective conduits for water between the roots and the above ground tissues.
Plant cells vary greatly in shape and function, but in general each has a
cell wall surrounding a
protoplast, which is differentiated into a
nucleus and
cytoplasm. Suspended within the watery cytoplasm are the membranes, organelles, and particles in which the biochemical processes of life occur. The
nucleus contains most of the cellular DNA and is thus the center from which these cellular activities are directed. Cytoplasm is in more or less constant motion in living cells and the movement is termed
cyclosis or
cytoplasmic streaming. (
Protoplasm is a commonly used name for all the watery cellular matter. It incudes the
nucleoplasm of the nucleus plus the cytoplasm.)
The
cell wall is produced by the cytoplasm and deposited outside of the
plasma membrane—the outer boundary of the protoplast. The wall and plasma membrane regulate the kinds of materials that enter and leave the cell thus making it possible for the cell to maintain an identity different from its surroundings. The wall is not simply an inert substance providing strength and giving structure to plant cells as once was thought, but is active in several metabolic processes including
absorption and
secretion of substances, the
detection of bacterial and fungal pathogens, and even
regulation of growth and development.
Cellulose, the most common material of plant cell walls, is a polysaccharide composed of long chains of glucose molecules that assume crystalline properties in the cell wall. During wall formation, adjacent cellulose molecules link together forming bundles called
microfibrils. The microfibrils, in turn, twist together rope-like, producing strong cords called
macrofibrils. Hemicelluloses and pectins are deposited among the fibril network and chemically bind the whole together. The result is a
primary wall. The whole process of synthesis and assembly is coordinated by the plasma membrane.
Lignin, the second most common material of plant cell walls, adds additional strength to cells, such as in wood cells. Other substances are deposited in walls of tissues serving particular functions. Walls of cells in the outer layer of leaves often contain
cutin or
waxes, both of which effectively reduce the loss of water from the leaves. Walls of
cork cells in the bark of trees contain
suberin, another type of protective material.
Membranes:
Are the sites of many of the biochemical processes of life.
Are in constant flux, changing both their physical and chemical structure and composition as long as the cell is alive.
Partition the cell into compartments in which metabolic reactions take place independently of processes occurring in nearby compartments, thus permitting a variety of reactions to occur simultaneously within a single cell.
The basic structure of all biological membranes is the same: a double layer of
lipids in which
proteins are embedded. The ratio of proteins to lipids is 3:2. Some of the proteins extend through the lipid layer, each end of a molecule projecting out opposite sides. Others remain on or partially embedded in one or the other surface.
Most membranes are
selectively (differentially) permeable, permitting or preventing materials from leaving or entering the cell. When toxic substances destroy the selective permeability and the membrane becomes
freely permeable, materials leak unchecked in and out, resulting in death of the cell.
Although it is difficult to study the plasma membrane separately from the rest of the protoplast, it is known that the plasma membrane deciphers the biochemical signals that control cell growth and differentiation and that it organizes the formation of the cell wall.
Table
1 shows cell structures, their composition, and principal functions.
TABLE 1
Cell Structure and Function
Structure
Function
Cell wall
Outer layer of plant cells; produced by the cytoplasm; gives shape and rigidity to the cell; cellulose the basic constituent
Middle lamella
Intercellular layer (mostly pectin) between primary walls of adjacent cells; binds them together
Primary wall
First wall deposited by actively growing and dividing cells
Secondary wall
Deposited inside primary wall after cell has stopped growing; cellulose, lignin, other materials deposited in layers give strength to plant;
pits present—areas where no secondary wall is deposited and through which plasmodesmata extend
Plasmodesmata
Strands of cytoplasm that connect adjacent cells; are pathways for material movement
Protoplast
All of the material contained within the cell wall
Nucleus
Structure that contains the genetic information (DNA) in eukaryotic cells; controls cellular activities
Nuclear envelope
Pair of fused membranes around the nucleus; connected to the endoplasmic reticulum; contains pores through which the nucleoplasm and cytoplasm connect
Nucleoplasm
The fluid portion of the nucleus; also called the nuclear matrix
Chromatin
In non-dividing cells, threads of
deoxyribonucleic acid (DNA) plus associated proteins
(histones) that are attached to sites on the nuclear envelope; condenses into a compact mass when cells divide, forming
chromosomes that carry the
genes
Nucleolus
Plural:
nucleoli; one or more spherical structures that are the site(s) of
ribosomal ribonucleic acid (rrna) assembly from rrna genes; subunits of ribosomes also produced here
Cytoplasm
Living cellular material exclusive of the nucleus
Plasma membrane
Outer boundary of the cytoplasm; a lipid bilayer with embedded proteins;
differentially permeable and regulates movement of materials into and out of cells; coordinates synthesis of cell wall; recognizes and transmits internal and external chemical signals
Cytosol
Liquid portion of the cytoplasm in which cellular structures are suspended; also called the cytoplasmic matrix
Organelles
General name for cellular structures bounded by membranes and specialized to perform specific functions
Bounded by two membranes
Evidence that these organelles once were independent prokaryotes; they retain many of their former traits within the eukaryote cells
Plastids
Semiautonomous, contain DNA and ribosomes and reproduce by fission; have an elaborate internal structure; in algae and plants
Chloroplasts
Green (contain
chlorophyll) sites of photosynthesis in
thylakoid membranes, amino acid and fatty acid synthesis
Chromoplasts
Yellow, orange, red (contain
carotenoid pigments); attract pollinators to flowers and dispersers to colored seeds and fruits
Leucoplasts
Colorless site with no pigments;
amyloplasts synthesize starch; others synthesize oils and probably proteins
Mitochondria
Singular:
mitochondrion; sites of aerobic respiration and release of
adenosine triphosphate (ATP); similar tolike plastids in being semiautonomous and containing DNA and ribosomes; also reproduce by fission; inner membrane with many folds or
cristae
Bounded by one membrane
Phospholipid bilayer that regulates material
Peroxisomes
Also called microbodies; no internal membranes, DNA, or ribosomes, but are self-replicating; some important in photorespiration; glyoxisomes contain enzymes that convert fats to sucrose during seed germination; others associated with mitochondria
Vacuoles
Sac of liquid, the
cell sap, surrounded by a membrane, the
tonoplast; in mature cells may occupy 90 percent of the cell; gives
turgor (rigidity) to the cell; serves as temporary storage site for Calcium and other materials;
anthocyanin pigments in cell sap give color (reds and, blues) to leaves and flowers; some small vacuoles (like animal lysosomes) are sites of digestion; others store wastes
Endomembrane system
Phospholipid bilayer that regulates material; collective name for all the cell membranes except those of mitochondria, plastids, and peroxisomes; membranes originate in the ER
Endoplasmic reticulum (ER)
An extensive membranous system of flattened sacs (
cisternae) that extends throughout the cytoplasm as a communication and transport system;
rough ER is covered with ribosomes and delivers proteins;
smooth ER lacks ribosomes, synthesizes lipids; rough ER is cisternal, smooth ER is tubular
Golgi complex
Collection of
Golgi bodies (dictyosomes) that are stacks of flattened cisternae associated with secretion; some synthesize and export polysaccharides; others handle glycoproteins
Vesicles
Small sacs of secretory material pinched off from the cisternae; move from the Golgi complex to the plasma membrane (with the assistance of actin filaments) and liberate their contents outside of the cell; process is called
exocytosis—secretion of materials carried in vesicles from the cell
Cytoskeleton
Matrix of protein fibers that gives support and on which organelles, enzymes, macromolecules are attached; composed of two kinds of protein filaments with similar functions
Microtubules
Long hollow tubes composed of the protein
tubulin; in constant flux, breaking down and reforming; function in cell division, cellular movement, and movement of materials within the cell
Actin filaments
Long chains of the protein
actin; responsible for cytoplasmic streaming, movement of nucleus in cell division, organization of the ER, and other movements of cellular materials
Ribosomes
Sites of protein assembly in the cytoplasm or on the rough ER; are small (17–23 Φm) particles assembled from a
large and a
small subunit produced in the nucleolus; are half
ribosomal RNA (rrna) and half proteins in composition;
messenger RNA (mrna) brings code from a gene, attaches to rrna and initiates protein synthesis; at sites of active synthesis clusters of ribosomes are called
polysomes or
polyribosomes
Oil bodies
Spherical drops of lipids (especially triglycerides used to synthesize membranes) common in the cytoplasm of cells of seeds and fruits; used as energy and carbon sources for developing seedlings; not bound by a membrane; synthesized in plastids or in the ER
Flagella and cilia
Singular: flagellum and cilium; extensions of cytoplasm enclosed by the plasma membrane that project from the cell wall; made of two microtubules surrounded by nine others (a
9 + 2 structure); cilia same structure but shorter; are used as locomotor structures by algae and protists; the only flagellated cells in plants are the motile sperm of mosses, liverworts, ferns, cycads, and ginkgo
Themes of Plant Biology
Cells
