- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7150) Unit Title: PLANT CELLS ____________________________________________________________________________ (CLF7151) Topic: PLANT CELLS Time Taught in Year(s) 3 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (B-1) - In photos of plant cells, locate and name (with correct spelling) the major cell components. (B-1) - Describe the role of each cell component. (B-2,C-2) - Describe the functions of plant meristematic tissues - cambium and shoot and root meristems. (B-3) - Sketch an animal and plant cell and contrast the two. (See Basic Core CLF224 "Plant and Animal Cells" for lesson plans on this topic if review is necessary.) Special Materials and Equipment: Compound microscope, green onions or sedum leaves. Razor blades, glass slides and coverslips. References: Hartman, H., Flocker, W., and Kofranek, A. PLANT SCIENCE, GROWTH DEVELOPMENT AND UTILIZATION OF CULTIVATED PLANTS. Evaluation: Quiz by instructor TOPIC PRESENTATION: PLANT CELLS A. The structural unit of plants, as well as all other living organisms, is the cell. A plant consists of many living cells as well as the walls of dead cells. B. Cells vary greatly in size and shape. 1. The smallest cells must be measured in micrometers (1/1000 of a millimeter). 2. Some fiber cells are several millimeters long. 3. Think of a cell having length, width, and depth. a. Rather than being in the shape of a block or a box, it has many sides. b. The average cell has about 16 sides. c. Cells can be spherical, polyhedral, ameboidal, or cylindrical in shape. C. Plant cells have many parts or components. 1. The cell wall protects the cell. a. It provides an external structure. b. It may act as a strong support. c. It is nonliving and permeable to almost all solutes and solvents. d. Cellulose is an elastic, but strong material that gives growing walls most of their strength. 1) Many cellulose polymers combine to form microfibrils, and many microfibrils occur in each wall. 2) The microfibrils have the same effect as steel or nylon cords in automobile tires. e. Other polysaccharides, including pectins, hold the microfibrils in place. These polymers serve the same role as rubber in a tire. They keep the wall flexible and make it somewhat elastic, while holding the wall together. f. Some cells add a rigid material called lignin to their walls after they stop growing. 1) Lignin is a rigid polymer that fills the spaces between other wall materials. It occurs in conducting cells of the xylem, and in fiber cells. 2) Wood consists of the lignified walls of dead cells (a tissue called secondary xylem). 2. The plasma membrane, or plasmalemma, lies just inside the cell wall. a. The membrane is selectively permeable, allowing only selected molecules to enter or leave the cell. b. The electron microscope shows that the membrane has a definite structure. c. The plasma membrane can form pockets that pinch off inside the cell, forming water-bags called vesicles that may carry useful materials brought in from outside the cell. 3. The cell contains organized structures called organelles, which are embedded in a fluid called the cytosol. 4. The cytosol is about 85-90% water. The rest of its material consists of inorganic and organic substances, which are either dissolved or colloidal. (Colloidal materials have water bound to their surface, but they remain as solid particles.) 5. There are several kinds of organelles, some of which are systems of related units. a. The nucleus is a dense, usually spherical body located within the cytoplasm. 1) It is the most conspicuous feature of a stained cell, because of the close affinity that the nuclear material has for many dyes. 2) It is the control center of the cell because it contains the chromosomes which hold the genetic material responsible for all inherited characteristics. 3) It also contains the nucleolus, a smaller body, which aids in the synthesis of protein. 4) A nuclear envelope encloses the nucleus. a) It is a double membrane, and b) it controls the movement of materials into and out of the nucleus. b. Biologists often refer to all the cell's contents outside the nucleus as cytoplasm. c. Mitrochondria are the cells power plants. 1) They appear as small, dense granules under the light microscope, but they have a very complex internal structure. 2) The energy for the various cellular activities is produced here. 3) They produce energy-rich compounds such as adenosine triphosphate (ATP). d. Vacuoles are cavities that are lined with membranes located within the cytoplasm. 1) They are filled with a variety substance known as cell sap. 2) The cell sap may contain many dissolved materials including salts, pigments, and various organic metabolic constituents. 3) In actively dividing cells the many vacuoles are small. 4) In mature cells they may coalesce into one large vacuole. 5) Many complex materials may be found in the cytoplasm and vacuoles. 6) Among these materials are included crystals, starch grains, oil droplets silica, resin, gums and alkaloids. 7) Many of these are reserve or waste products of the cell. e. Chloroplasts are the complete structural and functional unit of photosynthesis. 1) Twenty to one hundred chloroplasts are found in each cell of a green leaf. f. The endomembrane system consists of many vesicles and flattened, membrane-lined sacs that lie between the other organelles in the cytosol. 1) The endomembrane system manufactures many compounds, and packages them in specialized membranes, making vesicles that have various uses. 2) Secretion vesicles contain enzymes that make cell wall polymers. They add material to the wall. 3) Other vesicles contain enzymes that break down outmoded cell components and toxic byproducts of metabolism. g. The cytoskeletal system consists of many elements that hold other organelles in place and guide such processes as cytoplasmic streaming, wall formation, and cell division. 1) The elements of this system are protein molecules that can combine to form stiff, rod-like microtubules or flexible, rope-like microfilaments. 2) Both microtubules and microfilaments can combine with still other proteins to generate the motions needed in cell division and other processes. D. Cells with related functions occur in groupings called tissues. 1. Every plant organ has four main tissues. Organs differ in the organization of the tissues, and the details of their structure. 2. Meristematic tissue is the most fundamental tissue. a. A meristem is a mass of cells that continue to divide without becoming specialized. b. Meristems provide cells for building organs, and they produce hormonal signals that guide development. c. Meristematic cells are typically small and thin-walled. d. Meristems are classified by their locations in the plant. 1) Apical meristems occur at the tips (apices; singular apex) of roots, stems, and leaves. 2) Lateral meristems occur along the flanks of roots and many stems and young leaves. a) In roots, a lateral meristem called the pericycle initiates new branch roots. b) Woody plants have lateral meristems called cambiums that add to the girth of the organ, making wood and bark. 3. Epidermal tissue consists of one or two layers of cells that form a protective covering over the whole plant. 4. Vascular tissue stiffens the plant and carries material between organs. a. There are two kinds of vascular tissue. b. Xylem is vascular tissue that carries water and minerals through the plant. Its chief conducting units are chains of dead cells, called vessels. c. Phloem is vascular tissue that carries foods (as a solution in water) through the plant. Its conducting units are chains of living cells called sieve tubes. d. The two kinds of vascular tissue occur side by side, making up units called vascular bundles in young stems, and a vascular cylinder in roots and woody stems. e. In addition to conducting cells, vascular tissue contains fiber cells that add strength to the tissue. 5. Ground tissue builds and stores food, adds bulk to organs, and provides packing or insulation for vascular tissue. a. Most cells of ground tissue are rounded and thin-walled, with large vacuoles (parenchyma cells). b. Some parenchyma cells specialize in food and water storage, but those near the surface of stems and leaves may develop green chloroplasts. Ground tissue conducts most of the plant's photosynthesis. E. Meristems produce the cells and developmental signals that lead to tissues and organs. 1. Meristems are regions of the plant where cells divide repeatedly. a. Between cell divisions, the cells of the meristem grow enough to regain their original size. b. Zones of growth and differentiation occur beside each meristem. 1) Here, cells expand with little or no division. 2) In differentiation, cells acquire special traits that equip them for particular tasks in the mature plant organ. a) Cells at the surface of organs differentiate into plate-like epidermal cells with thick, waxy outer walls that resist damage and dehydration. b) Some of the cells in the interior of organs differentiate into long, tapered, thick-walled fiber cells that add strength to organs. c) Other cells within the organ differentiate into rounded, thin-walled parenchyma cells with large vacuoles, which store foods and add bulk to the organ. d) Some parenchyma cells form green chloroplast for photosynthesis. e) Other cells differentiate into water-conducting units that are part of xylem vessels. They become elongated, their side walls thicken and harden with lignin. Finally they lose their end walls and die, so the vessel is a dead, wooden pipe. f) Still other cells differentiate into thin-walled, elongated food-conducting units that are part of sieve tubes of the phloem. These cells lose their nucleus as they mature. 3) To choose their path of differentiate, cells depend on information from their own genes as well as influences from the environment. a) When cells divide, each cell gets a full set of genes, with all the information needed to build a whole plant. b) Each cell reads and uses only a fraction of the information stored in the genes. c) Signals that choose which genes to read can include chemicals produced by the cell itself, as well as pressures and chemicals from neighboring cells, and environmental factors including temperature, light and gravity. 2. Hormones are growth-controlling substances made in minute quantities (as low as one part per billion). a. Plants produce at least five kinds of hormones: auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Each is a different chemical substance. b. Meristems emit hormones, which travel through the plant to other meristems and growth zones, influencing cell growth, division, and differentiation. c. Auxins are made chiefly in very young leaves and fruits. They travel through parenchyma cells by an active process that carries the auxin away from the tip of the organ (polar transport). d. Ethylene is made chiefly in stressed or injured organs, and in ripening fruits. It diffuses between cells and, being a gas, escapes easily into the air. e. Cytokinins are made chiefly in root tips. They travel in xylem to shoots. f. Gibberellins are made chiefly in growing fruits, but also in shoot tips. They move in vascular tissue. g. Abscisic acid is made in developing fruits, and in water- stressed leaves. It moves through the vascular tissues. 3. Scientists and chemical companies produce artificial compounds with auxin-like activity. These artificial compounds are called growth regulators. 4. Agriculturalists use hormones and growth regulators in various ways. a. Auxin-like growth regulators such as 2,4-D, IBA, and NAA are used to thin apple crops, to retard early fruit drop in apple orchards, to induce rooting in stem cuttings, and to kill broad- leafed weeds. b. Ethylene, and compounds that release ethylene (ethephon) are used to induce fruit drop and ripening. This gives a more uniform harvest time and quality. c. In grapes, gibberellins are used to thin the clusters and enhance berry growth. __________________________________________________________ ACTIVITY: 1. Take a small piece of Elodea leaf and place this piece into a drop of water place on a clean slide. Make sure the piece lies flat and will be only one layer thick. Place a clean cover slip on top, lowering the cover slip from one edge in order to exclude air bubbles. The green chloroplasts should be apparent. 2. Study the cells under low and high power. Identify the cell walls, cytoplasm, nucleus and nucleolus. 3. Draw one of the more visible cells and label its parts. __________________________________________________________ 7/20/90 sg #%&C