All animals have either internal or external skeletons or skeleton-like systems to support their tissues. Animal cells do not have cell walls; instead, the plasma membrane, called the cell membrane by most zoologists (animal scientists), forms the outer boundary of animal cells. Higher plant cells have walls that are thickened and rigid to varying degrees, with a framework of cellulose fibrils. Higher plant cells also have plasmodesmata connecting the protoplasts with each other through microscopic holes in the walls. Animal cells lack plasmodesmata since they have no walls. When higher plant cells divide, a cell plate is formed during the telophase of mitosis, but cell plates do not form in animal cells, which divide by pinching in two.
Other differences pertain to the presence or absence of certain organelles. Centrioles, for example, the tiny paired keg-shaped structures found just outside the nucleus, occur in all animal cells but are generally absent from higher plant cells. Plastids, common in plant cells, are not found in animal cells. Vacuoles, which are often large in plant cells, are either small or absent in animal cells.
1. All living organisms are composed of cells. Cells are modified according to the functions they perform; some live for a few days, while others live for many years.
2. The discovery of cells is associated with the development of the microscope. In 1665, Robert Hooke coined the word cells for boxlike compartments he saw in cork. Leeuwenhoek and Grew reported frequently during the next 50 years on the existence of cells in a variety of tissues.
3. In 1809, Lamarck concluded that all living tissue is composed of cells, and in 1824, Dutrochet reinforced Lamarck's conclusions. In 1833, Brown discovered that all cells contain a nucleus, and shortly thereafter, Schleiden saw a nucleolus within a nucleus. Schleiden and Schwann are credited with developing the cell theory in 1838 to 1839. The theory holds that all living organisms are composed of cells and that cells form a unifying structural basis of organization.
4. In 1858, Virchow contended that every cell comes from a preexisting cell and that there is no spontaneous generation of cells from dust. In 1862, Pasteur experimentally confirmed Virchow's contentions and later proved that fermentation involves activity of yeast cells. In 1897, Buchner found that yeast cells do not need to be alive for fermentation to occur. This led to the discovery of enzymes.
6. Electron microscopes have electromagnetic lenses and a beam of electrons within a vacuum that achieve magnification. Transmission electron microscopes magnify up to 200,000 or more times. Scanning electron microscopes, which can be used with opaque objects, usually magnify up to 10,000 times.
7. Scanning tunneling microscopes use a minute probe to scan surfaces at a width as narrow as that of two atoms.
9. Cells are minute, varying in diameter between 10 and 100 micrometers. They number into the billions in larger organisms, such as trees. Plant cells are bounded by walls that surround the living protoplast. The cytoplasm contains a souplike fluid called the cytosol and all cellular components between the plasma membrane and nucleus.
10. Apectic middle lamella is sandwiched between the primary cell walls of adjacent cells. The primary wall and also the secondary cell wall, often added inside the primary wall, are composed of cellulose polymers, with hemicelluloses and glycoproteins. Secondary cell walls contain lignin that strengthens the wall.
11. Living cells are in contact with one another via fine strands of cytoplasm called plasmodesmata, which often extend through minute holes in the walls.
12. A flexible plasma membrane, which is sandwich-like and often forms folds, constitutes the outer boundary of the cytoplasm. It regulates the substances that enter and leave the cell.
13. The nucleus is bounded by a nuclear envelope consisting of two membranes that are perforated by numerous pores. Within the nucleus are a fluid called nucleo-plasm, one or more spherical nucleoli, and thin strands of chromatin, which condense and become chromosomes when nuclei divide. Each species of organism has a specific number of chromosomes in each cell.
14. The endoplasmic reticulum is a system of flattened sacs and tubes associated with the storing and transporting of protein and other cell products. Granular particles called ribosomes, which function in protein synthesis, may line the outer surfaces of the endoplasmic reticulum. Ribosomes also occur independently in the cytoplasm.
15. Dictyosomes are structures that appear as stacks of sacs and function as collecting and packaging centers for the cell.
16. Plastids are larger green, orange, red, or colorless organelles. Green plastids, known as chloroplasts, contain enzymes that catalyze reactions of photosynthesis. These reactions take place in the membranes of structures that resemble stacks of coins, called thylakoids, as well as the surrounding matrix, called the stroma. Plastids develop from proplastids, which divide frequently, and also arise from the division of mature plastids.
18. One or more vacuoles may occupy 90% or more of the volume of a mature cell. Vacuoles are bounded by a vacuo-lar membrane (tonoplast) and contain a watery fluid called cell sap. Cell sap contains dissolved substances and sometimes water-soluble red or blue anthocyanin pigments.
19. The cytoskeleton, which is involved in the architecture of cells and internal movement, is composed of micro-tubules and microfilaments. Microfilaments may be responsible for cytoplasmic streaming.
20. Cells that are not dividing are in interphase, which is subdivided into three periods of intense activity that precede mitosis or division of the nucleus. Mitosis is usually accompanied by division of the rest of the cell and takes place in meristems.
21. Mitosis is arbitrarily divided into four phases: (1) prophase, in which the chromosomes and their two-stranded nature become apparent and the nuclear envelope
breaks down; (2) metaphase, in which the chromosomes become aligned at the equator of the cell; a spindle composed of spindle fibers is fully developed, with some spindle fibers being attached to the chromosomes at their centromeres; (3) anaphase, in which the sister chromatids of each chromosome (now called daughter chromosomes) separate lengthwise, with each group of daughter chromosomes migrating to opposite poles of the cell; and (4) telophase, in which each group of daughter chromosomes becomes surrounded by a nuclear envelope, thus becoming new nuclei, and a wall dividing the daughter nuclei forms, creating two daughter cells.
22. Animal cells differ from those of higher plants in not having a wall, plastids, or large vacuoles. Also, they have keg-shaped centrioles in pairs just outside the nucleus and pinch in two instead of forming a cell plate when they divide.
1. What cellular structures can be observed with the aid of light microscopy and electron microscopy?
2. Why are cells so small, and how is this small size beneficial for transport of substances within and between cells?
3. What is the function of the plant cell wall?
4. What is the difference between protoplasm and cytoplasm?
5. What is the function of a cell nucleus? How does it perform its function?
6. What are plasmodesmata? What is their importance to living plant cells?
7. Describe the major parts and functions of a chloroplast.
8. In a typical complete cell cycle, how long, proportionately, does mitosis take?
9. What cellular structures are responsible for division of cytoplasm, and how does this occur?
10. What are the differences and similarities between plant and animal cells?
1. Would you consider any one type of cell more useful than another? Why?
2. After you have completed your introductory plant science course, do you believe you would be able to determine the function of each of a cell's organelles in a laboratory? Explain.
Visit our web page at www.mhhe.com/botany for interesting case studies, practice quizzes, current articles, and animations within the Online Learning Center to help you understand the material in this chapter. You'll also find active links to these topics:
Microscopy Cell Theory
Prokaryotes and Eukaryotes Cell Structure of Plants Nucleus
Mitosis in Plant Cells
Chromosomes and Their Structure
The Cell Cycle
Alberts, B., et al. 1994. Molecular biology of the cell, 3d ed. New York: Garland.
Argyroudi-Akoyungolo, J. H. (Ed.). 1992. Regulation of chloroplast biogenesis. New York: Plenum. Becker, W. L. et al. 2000. The world of the cell, 4th ed. San
Francisco: Benjamin Cummings. Bidlack, J. E., et al. 1992. Molecular structure and component integration of secondary cell walls in plants. Proceedings of the Oklahoma Academy of Science 72: 51-56. Buvat, R. 1989. Ontogeny, cell differentiation and structure of vascular plants. New York: Springer-Verlag. Cross, P. C., and K. L. Mercer. 1995. Cell and tissue ultrastructure:
A unique perspective, 2d ed. New York: W. H. Freeman. Darnell, J. E., and H. Lodish. 1995. Molecular cell biology. New
York: W. H. Freeman. Loewy, A. G., et al. 1991. Cell structure and function, 3d ed.
Philadelphia, PA: Saunders. Murray, A., and T. Hunt. 1993. The cell cycle: An introduction.
New York: Oxford University Press. Ohki, S. (Ed.). 1992. Cell and model membrane interactions. New York: Plenum.
Sadava, D. E. 1993. Cell biology: Organelle structure. Boston,
MA: Jones & Bartlett. Sussex, I., et al. (Eds.). 1985. Plant cell interactions. New York:
Cold Spring Harbor. Wolfe, S. L. 1993. Molecular and cell biology. Belmont, CA: Wadsworth.
Stern-Jansky-Bidlack: Introductory Plant Biology, Ninth Edition
© The McGraw-H Companies, 2003
A heliconia (Heliconia sp.), native to the New World tropics. There are about 200 known species of these striking plants, many of which are becoming increasingly popular as ornamentals in tropical and subtropical areas.
Meristematic Tissues Apical Meristems Lateral Meristems Intercalary Meristems
Tissues Produced by Meristems Simple Tissues Complex Tissues Summary
A discussion of meristems (apical meristems, vascular cambium, cork cambium, intercalary meristems) and non-meristematic tissues (parenchyma, collenchyma, sclerenchyma, secretory tissues, xylem, phloem, epidermis, periderm) forms the body of this chapter.
Some Learning Goals
1. Know the meristems present in plants and where they are found.
2. Learn the conducting tissues of plants and the function of each cell component.
3. Learn tissues of plants that are neither meristematic nor function in conduction at maturity.
once was privileged to have a new house constructed
I for me on a vacant lot. I followed the stages of construction with considerable interest. First, a foundation was laid; then, trucks arrived with various building materials, and construction of a frame began. This was followed by the installation of plumbing, electrical wiring, windows, heating and air-conditioning units, vents, and various other devices. Finally, waterproof walls and a roof were added, and, upon occupation, food and other materials were stored in their appropriate niches.
In a sense, the growth of a plant from a seed is something like the construction of a house. Using raw materials from the soil and a superb manufacturing process, each plant develops a framework, "plumbing," a waterproof covering that includes "windows," vents, means of waste disposal, and food-storage areas. Even a form of air-conditioning, which enables plants to survive and thrive in the hottest summer sun, is included in each mature plant package. The building components of the framework, plumbing, and related features of plants form the body of this chapter.
There are many interesting modifications of higher plants discussed in the three chapters that follow this one, but, regardless of the outer form, most plants have three or four major groups of organs—roots, stems, leaves, and in some instances, flowers. Each of these organs is composed of tissues, which are defined as "groups of cells performing a similar function." Any plant organ may be composed of several different tissues; each tissue is classified according to its structure, origin, or function.
Three basic tissue patterns occur in roots and stems (see woody dicots, herbaceous dicots and monocots, discussed in Chapters 5 and 6). The following are major kinds of tissues found in higher plants. The specific types of cells associated with each tissue, as well as illustrations of them, are included in the discussions that follow the classification.
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