Download Lab3 celldivision tissues

Lab Exercise 3: Cell Division and Basic Plant Tissues. At the cellular level, the growth and development of plants involves three processes: cell division (an increase in the number of cells in the plant body), cell expansion (an increase in the size of newly formed cells), and cell differentiation (the formation of functional, mature tissues). Cell division and expansion result in an increase in size of the plant body, while differentiation leads to the development of the unique structural and functional features of plant tissues and organs. In the previous lab, you looked at basic features of single cells and their contents. Here, you will examine how cells are organized into the tissues and tissue systems that are the basis of plant form and function. After completing this exercise, you should be able to: ❀ Explain the three different processes involved in plant growth, and the importance of each process. ❀ List the four stages of mitosis in the correct order. Explain the distinction between mitosis and cytokinesis. ❀ Know the two specific cell types generally found in epidermal tissue, and the function of each. ❀ Identify four stages of mitosis (prophase, metaphase, anaphase, telophase) in sections or photographs of plant cells undergoing division. ❀ Know the three tissue systems of the plant primary body (dermal, ground, vascular), their functions, and arrangement in the plant. ❀ Understand the characteristics of, and differences between, parenchyma, collenchyma, and sclerenchyma cells and tissues, and identify each cell/tissue type in an image or microscope slide. ❀ Define or describe the meaning of : tissue, meristematic tissue, simple tissue, complex tissue, cuticle, guard cell, epidermis, ground tissue, dermal tissue, vascular tissue, apical meristem, xylem, phloem, sclereid, fiber, root cap, interphase, phragmoplast, spindle, chromatid, centromere, daughter cells. I. Cell Division: Mitosis and Cytokinesis In plants, cell division is generally restricted to specialized regions known as meristems (or meristematic tissues). Meristematic tissues are composed of undifferentiated cells that are capable of repeated cell divisions. The process of cell division requires two separate events: mitosis (division of the nucleus and distribution of genetic material) and cytokinesis (division and distribution of cytoplasmic contents to the daughter cells. These two events are usually linked, with cytokinesis following closely after mitosis. The result of mitotic cell division is the formation of two daughter cells, each containing the same genetic information as the parent cell. As plants grow, new cells are continuously produced in meristematic regions through this process of mitotic division. Almost all cells in the plant body are produced in this manner, and thus contain the same number of chromosomes. Biology 241, Spring 2015, Lab 3 1/12 For our convenience, mitotic cell division is divided into several phases. In the following exercise, you will observe the phases of mitotic division in the meristematic region of onion (Allium cepa) root tips, and learn to recognize each phase based on the events taking place in the cell. The Cell Cycle Interphase G1: cell growth S: DNA replication G2: prep for division Division Mitosis Prophase Metaphase Anaphase Telophase Cytokinesis Mitosis in Plant Cells 1) Obtain a prepared slide of a longitudinal section of an onion root tip. Refer to Atlas pp 9-­‐10 for a visual guide to the stages of the cell cycle. 2) First, examine the preparation at low or medium power. At the extreme tip of the root (the pointed end), you should see a region of relatively disorganized cells that form a protective covering of the tip. This is the root cap. We will revisit the root cap in a later lab on roots; for now, know that the root cap is not a good place to look for dividing cells. In the image above, the meristematic region is just to the left of the root cap. Describe the general shape of cells in the meristematic region. (1) Biology 241, Spring 2015, Lab 3 2/12 Describe the relative size of the nucleus in the cells of the meristematic region. How does this compare to the nucleus size in the onion epidermal cells we looked at previously, for instance? (2) Note: The diploid chromosome number for Allium cepa is 16. Each cell, therefore, has 16 chromosomes. But keep in mind that you are looking at a thin section of a three dimensional structure. Parts of cells and chomosomes, even entire chromosomes, have been sliced away from the cells you are looking at. It is only by chance that a cell may be sectioned in the correct plane for the charactistic features of mitosis phases to be seen. If you have trouble finding a particular phase, switch to one of the other sections on your slide, or try a different slide entirely. 3) Switch to high power and search the preparation for cells in various stages of the cell cycle. Interphase: This is not a stage of mitosis, but is the other (longer) part of the cell cycle. Most cells in the root tip are in this phase. A cell in interphase is characterized by its relatively large and intact nucleus. The nuclear membrane should be intact, and its location should be discernable with our microscopes. Smaller, darkly stained nucleoli are also clearly visible. The chromosomes are not visible as distinct structures, but are so thin and elongated in this stage that they appear only as granular material (chromatin) within the nucleus. You can draw an interphase cell in the space below. Label the nuclear membrane, nucleolus, chromatin, and, of course, the cell wall. Prophase: This is the first stage of division. The chromosomes condense during early prophase, and become visible as distinct structures. The nuclear membrane also begins to deteriorate during early prophase. By late prophase, all nucleoli will have disappeared almost entirely, and the nuclear membrane will be gone. Individual chromosomes will be visible, and in a particularly lucky section can be seen as being composed of two parallel threads called chromatids, held together by a centromere (which cannot be seen directly in our slides, usually). This paired condition of chromosomes is a result of Biology 241, Spring 2015, Lab 3 3/12 replication during S phase of Interphase. Each daughter cell resulting from this division cycle will receive one chromatid from each replicated chromosome. Label a chromosome and the nucleolus in this image of a cell in late prophase: Metaphase: This stage is easily identified by the arrangement of chromatids along the equatorial plane of the cell. All nucleoli and the nuclear membrane have vanished. Thin, threadlike spindle fibers may be visible. Spindle fibers may be difficult to see; try manipulating light intensity to bring out this detail. Label the chromosomes, spindle fibers, and cell wall in this image of a cell in metaphase. Anaphase: This stage of division is characterized by the separation of the chromatids. Once separated, each chromatid is known as a chromosome. The two separated chromosomes are pulled toward opposite poles of the cell. In an ideal anaphase cell, two distinct clusters of chromosomes can be seen moving pole-­‐ward. Draw an anaphase cell in the space below, labeling a chromosome, and the polar region it is heading toward. Biology 241, Spring 2015, Lab 3 4/12 Telophase: This stage is marked by the end of poleward migration of the chromosomes, and the beginning cytokinesis. It is essentially the reverse of prophase: the nuclear membrane reforms, nucleoli reappear, and the spindle fibers and chromosomes vanish from view. By the end of telophase, two distinct nuclei have formed. Cytokinesis occurs as the cell plate (phragmoplast) forms on a plane between the two daughter nuclei. The cell plate will eventually develop into the cell wall separating the daughter cells. Try to observe a few mid-­‐ to late-­‐telophase cells. Does the phragmoplast appear to initiate near the middle of the cell and grow outward, or initiate at the periphery and grow inward? (3) Daughter cells: The two cells that result from this division process are known as daughter cells. How can you recognize such newly formed cells? (4) Biology 241, Spring 2015, Lab 3 5/12 II. Plant Tissues The new cells formed in meristematic regions are all similar in appearance and function. Some of these cells must necessarily remain meristematic and continue to divide as long as the plant continues to grow. Most cells, however, must mature, differentiating in concert with other cells to form functional tissues and organs. A tissue may be composed of one or more types of differentiated cells, and usually performs a specific function. The basic cell and tissue types (parenchyma, collenchyma, sclerenchyma) are distinguished by cell wall characteristics. The tissues of most plants can be classified in the following manner, based on their function, position, and origins: I. Meristematic tissue. -­‐ undifferentiated cells, capable of division. II. Primary tissue systems (continuous throughout plant). a. Dermal (surface) tissues i. Epidermis ii. Periderm (a secondary dermal tissue) b. Ground tissues. i. Parenchyma (thin 1° wall only) ii. Collenchyma (irregularly-­‐thickened 1° wall) iii. Sclerenchyma (2° wall present, often lignified) c. Conducting (vascular) tissues. i. Xylem (water conducting tissue) ii. Phloem (sugar conducting tissue) Simple tissues are those that are composed of only a single type of cell: parenchyma, collenchyma, or sclerenchyma. Cells of these tissues are generally mature and non-­‐meristematic; parenchyma cells are the least-­‐
differentiated, and most capable of resuming division. Complex tissues contain two or more types of cells (i.e., cells that have matured along different pathways of differentiation). Xylem and phloem, for instance, are complex tissues, each with two or more types of cells contributing to tissue structure and function. In the following exercises you will look at examples of dermal and ground tissues; xylem and phloem will be covered in the next lab session. Meristematic tissue. There are two principle meristems found in plants: apical meristems and lateral meristems. Apical meristems are located at the tips of stems and roots. The activity of these meristems result in an increase in the length of stems and roots. This is known as primary growth, and the resulting tissues are called primary tissues. Lateral meristems are found near the periphery of stems and roots, and are responsible for increasing the diameter of these organs. This is Biology 241, Spring 2015, Lab 3 6/12 known as secondary growth, and involves the production of secondary tissues. The cells of meristematic tissues tend to be small, regularly-­‐shaped, and thin-­‐walled. Dermal (surface) tissue. Epidermis: The epidermis is the dermal (surface) tissue of young plants, leaves, and herbaceous plants that lack active lateral meristems or 2° growth. It is often only a single cell layer thick. (see Atlas figs. 9.91-­‐9.93 for comparison.) Periderm is a secondary dermal tissue that replaces the epidermis in plants that undergo significant increase in diameter during growth. We’ll look at periderm in the next lab. Observation of dermal tissue (epidermis): 1) Obtain a leaf from the plant provided for observing epidermis. Note the smooth/waxy texture of the leaf surface. The layer of waxy material that covers the epidermis is called the cuticle. 2) Gently break the leaf and pull the pieces apart slowly, trying to peel a portion of the epidermis from the underlying tissue. Make a wet mount of the epidermis. 3) Observe the section at low, medium, and high power to observe the two types of differentiated cells: guard cells and regular epidermal cells. Is the epidermis a simple or complex tissue? 4) Guard cells always occur in pairs, side by side. How do the guard cells differ in shape from the surrounding epidermal cells? How do the contents of guard cells differ from those of the surrounding cells? 5) The small space between the guard cells is known as a stoma (pl. = stomata). Note the lack of air spaces between the other epidermal cells. 6) What are the functions of the two types of cells in the epidermis? (5) 7) Draw a pair of guard cells and the regular epidermal cells that surround them. Label the guard cells, regular epidermal cell, and the stoma. Biology 241, Spring 2015, Lab 3 7/12 Simple ground tissues Collenchyma: Collenchyma is a simple tissue composed of elongated cells with tapered ends. The cell wall is irregularly thickened. Collenchyma tissues confer flexible mechanical support to the plant body; they can bend to a certain extent without breaking. Collenchyma cells remain alive at functional maturity. Observe collenchyma in the celery (Apium graveolens) petiole. 1) Make a wet mount of a very thin cross section of a celery stalk. Take your tissue from the outer edge of the stalk. 2) Using low or medium power, locate the clusters of light grayish cells that lie immediately within the epidermis, in the ridges along the outer portion of the section. These are strands of collenchyma tissue, and comprise the “celery strings.” 3) If available/desired, add a drop of neutral red dye at the edge of your coverslip, and allow it to flow into the section. You can apply a paper towel to the opposite side of the coverslip to help draw the dye into the section faster. 4) Using high power, observe the irregular thickenings of cell walls in this tissue. See Atlas fig 1.13 for comparison. 5) Keep this mount for the observation of parenchyma tissue, described below: Parenchyma: Parenchyma is the most common simple tissue, and can be found in most roots, stems, leaves, flowers, and other plant parts. Parenchyma cells are generally characterized by thin, unmodified cell walls, and are living at functional maturity. This tissue type makes up the bulk of a herbaceous (non-­‐woody) plant body. Observe parenchyma in the celery (Apium graveolens) petiole. 1) In your celery wet-­‐mount, examine the larger cells near the center of the petiole. 2) Notice the thin cell walls, the generally isodiametric shape of the cells, and the intercellular spaces (air spaces where three or four cells are in contact). These are general characteristics of parenchyma tissues and cells. Can you think of any other instances of parenchyma cells/tissue that you have observed (in previous labs)? (6) Biology 241, Spring 2015, Lab 3 8/12 Sclerenchyma: sclerenchyma cells have thick secondary cell walls and are dead at functional maturity; these features differentiate sclerenchyma from parenchyma and collenchyma. In addition to being thick, the sclerenchyma cell wall is often impregnated with lignin, which hardens and solidifies the wall and makes it impermeable to water. Lignified cell walls stain red in many of the prepared slides, making them easy to identify. In contrast to collenchyma, the walls of sclerenchyma are more regularly and evenly thickened. There are two basic types of sclerenchyma cells: fibers (thin, elongated cells often grouped in bundles or strands) and sclereids (not elongated, variable in shape, may occur singly or in groups). Observation of sclereids in pear (Pyrus communis): 1) Prepare a wet mount of a small amount of the fleshy part of a pear fruit. The gritty particles that you feel when you eat a pear are clusters of sclereids (often referred to as “stone cells”). 2) Examine the preparation at low or medium power, noticing that the clusters of sclereids vary greatly in size. 3) Now take the slide off the stage and place it on the lab bench. Using a pencil eraser or piece of cork, apply gentle pressure to the coverslip to mash the tissue. This will separate the sclereids and allow observation of individual cells. Be careful not to crush the coverslip! 4) Try to locate two sclereids that are still in contact with each other, and examine at high power. 5) Notice the cell cavity, or lumen, and the numerous pit canals that radiate through the cell wall from the lumen. The lumen is where the cytoplasm and nucleus once existed. The pit canals mark places where strands of cytoplasm connected adjacent cells. In living plants, those strands are known as plasmodesmata. Remember: these sclereids are dead cells; all that can be seen are the tubes where the plasmodesmata once were. 6) Draw two sclereids; show and label the pit canals and lumen. Biology 241, Spring 2015, Lab 3 9/12 Fibers: Fibers are elongated sclerenchyma cells with thick walls and usually small lumens. They are often found associated with vascular tissue, where they may form a protective cap or sheath for the conducting cells. Strands of fibers are often also found distributed throughout the ground tissue in stems, where they confer structural support. See Atlas p. 136, figs 9.32, 9.34, 9.35, to orient yourself. Other details of stem tissues will be covered in the next lab; for now, concentrate on observing the fibers. Observation of fibers: 1) Obtain a prepared slide of alfalfa (Medicago sp.) or sunflower (Helianthus sp.) stem cross section. Examine at low power. The clusters of cells that form a ring around the periphery of the stem are vascular bundles. 2) Toward the outside of each vascular bundle is a mass of sclerenchyma fibers. Examine these with high power and note the thick cell walls and small central lumens. 3) To appreciate the elongated nature of fibers, examine the Helianthus stem long section slides. Locate the strands of narrow, thick-­‐walled cells running vertically through the section. Make sure you observe the fibers, the red-­‐stained cells toward the stem surface, rather than the vascular tissue (more toward the center of the stem; we’ll look at that next time). 4) Examine Figs. 1.21, 1.23, 1.24 for additional examples of sclerenchyma. The image below represents a cross section of a plant stem. What tissues can you identify? Label them. Biology 241, Spring 2015, Lab 3 10/12