I. PLANT CELL, CELL WALL Bot 404--Fall 2004

Transcription

1 I. PLANT CELL, CELL WALL Bot 404--Fall 2004 A. Review of General Anatomy 1. Major organs are stem, leaf, root. Flower is usually interpreted as a modified shoot, so sepals, petals, stamens and carpels are its organs, comparable to the leaf. However, some parts of flowers (ovule, fruit) become so different that the term organ is a bit vague. 2. These organs are composed of cells that are aggregated into various tissues. a. Tissue = an aggregation of cells which is visibly distinctive and which usually has a common major function. If the tissue is composed of a single type of cell, it is a simple tissue; if two or more cell types are involved, it is a complex tissue. b. Three main types of tissue are: Tissue Simple Complex Dermal epidermis periderm Vascular phloem xylem Fundamental Parenchyma Collenchyma Sclerenchyma Refer to table at front of laboratory exercise course pack: summary of cell types. B. Cells (should be mostly review) 1. Cells in General a. Why Multicellularity? Some organisms are acellular (really, one-celled), e.g., there are marine algae that are tubular and multinucleate; examples in fungi, etc.; the embryophytes are all multicellular. -control osmotic relations -division of labor -localize damage -structural support b. Cells are Connected -in multicellular organisms, cells must be connected, that is, the cytoplasm of one cell must be connected to the cytoplasm of its neighboring cells so that transport and communication can occur. This is accomplished in various ways. -we will come back to how plant cells are connected

2 c. Common Features of Eukaryotic Cells -plasmalemma (plasma membrane) to isolate cell from environment -nucleus containing genetic material, also the nucleolus (sometimes more than one, source of precursors for ribosomes) -mitochondria (with a few exceptions among single-celled internally parasitic protozoa) for aerobic respiration -endoplasmic reticulum and ribosomes for protein synthesis -microbodies for metabolism of certain pathways; called glyoxysomes in plants -dictyosomes (coll. Golgi apparatus) for packaging and distribution of various molecules, especially glycoproteins and glycolipids including cell wall products in plants -cytoskeleton that supports shape of cell and anchors organelles (scaffolding) 2. Plant Cells a. Some Definitions -protoplast = the organized living unit of a cell excluding the cell wall -cytoplasm = the least differentiated part of the protoplast that encloses all other components of the protoplast (our definition); alternatively, this includes all of the protoplasm of the cell excluding the nucleus and vacuole(s) -symplast = all interconnected living cells in the plant -apoplast = all interconnected dead cells, cell walls and intercellular spaces in the plant b. Structures Characteristic of Plant Cells -plastids the class of double membrane-bound organelle characteristic of plants (chloroplasts, amyloplasts and chromoplasts among others) that often contain pigments or have a storage function; contain their own genetic material, about 150 Kb on average although this varies widely; derived from endosymbiotic anaerobic, photosynthetic bacteria; plastids are usually inherited maternally (through the egg); young, undifferentiated plastids are called proplastids (very small, usually in meristematic regions and some internal tissues), and subsequent development depends on what kind of cell they are located in but differentiation is reversible. -vacuoles a membrane-bound compartment that contains water and a variety of often dissolved substances including reserve compounds and waste products; usually one large vacuole is present, the central vacuole, and it occupies a large volume of the cell (up to 90%); in meristematic cells can see several small vacuoles (collectively the vacuome); the tonoplast (surrounding membrane) is differentially permeable and functions in osmotic regulation, especially maintenance of turgor; other functions of vacuole include biochemical recycling of materials (e.g., storage and digestion) -phragmoplast the particular manner of cell division (cytokinesis) that is

3 found in many green algae (including charophytes) and land plants; a system of perpendicular microtubules that develop from the spindle during cytokinesis, and if a cell plate (M Fig. 2.19) is present as in charophytes and land plants it forms within the phragmoplast; in animals generally see furrowing, in some green algae may see furrowing with or without associated microtubules, or in the chlorophycean green algae a phycoplast (system of parallel microtubules) forms -plasmodesma (pl. ta) a canal through the walls of two cells connecting their protoplasts; the canal is lined by plasmalemma, a central desmotubule connects two ER cisternae located at the two opposite openings of the canal, and cytoplasm fills the space between the plasma lemma and the desmotubule -cell wall well developed rigid cellulosic wall outside of the plasmalemma is characteristic of plants C. Cell Wall 1. Function -provide mechanical support and protection -restricts size and shape of mature cell (allows turgor pressure to occur) -affects absorption, transpiration, translocation, secretion 2. Wall structure i. Constituents (1º and 2º) -the main constituent is cellulose (polysaccharide of glucose units, therefore a carbohydrate); forms the framework; most abundant organic compound on the planet -other polysaccharides (e.g., hemicelluloses, pectic substances); lignin (provides rigidity, chemically complex but is a polymer); fatty compounds (e.g., cutin, suberin, waxes) on protective surfaces; wide variety of other substances both organic and inorganic including enzymes (and other proteins) -cellulose molecules are linear, ribbonlike chains of glucose; linked together to form microfibrils; these are grouped to form macrofibrils (E Fig. 4.1); forms architecture of the wall -the matrix among the cellulose fibrils is filled by the other constituents ii. Layers -wall is produced in increments of growth, giving rise to distinctive layers -basically two major layers are recognized: the primary wall and the secondary wall (this may be subdivided) -the first layer to be deposited is the primary wall; often thin (esp. in cells having secondary walls and metabolically active cells) but can be thick; primarily cellulosic or cellulosic and pectic, and generally lacks lignin but can become lignified with age; laid down immediately outside of the plasmalemma

4 -the walls of two adjacent cells are essentially glued together by a layer of largely pectic material; this is the middle lamella; generally thin but can be thicker at the junctures of cells; may become lignified with age -if additional layers are deposited, they become the secondary wall; cellulosic but also generally are lignified; often subdivided into three layers, the S1, S2 and S3, recognized by differential orientations of microfibrils; laid down sequentially just outside the plasmalemma -DIAGRAM sequence of layers from outside to inside (assuming all present including a living protoplast) 3. Wall formation and growth i. Cell Plate -in charophytes and land plants, a partition (cell plate) forms within the phragmoplast during cytokinesis (M Fig. 2.19; E Fig. 4.11) -initiates through fusion of vesicles, at this stage does not extend to the mother cell walls -grows from ends toward the mother cell wall and plasmalemma on each side; eventually hooks up to the mother cell wall and plasmalemma -additional wall material is deposited from both sides to increase thickness ii. Growth -each new daughter cell lays down a complete new primary wall all around the cell -walls grow in thickness and in surface area; this latter occurs only in cells that are still increasing in size; secondary wall laid down only after growth ceases 4. Cell Connections i. Plasmodesmata -can be scattered in walls of even thickness, or can be grouped in 1º pit fields ii. Primary Pit-Fields (M Fig. 2.20) -depression in a primary wall traversed by plasmodesmata (primary wall thicker where no pit fields) -primary wall is continuous over the pit-field membrane -primary pit-fields of adjacent cells are usually lined up with each other -DIAGRAM iii. Pits -deposition of secondary wall occurs over the entire surface except where there are primary pit-fields; these cavities are pits -pits represent discontinuities in the secondary wall but there is not a oneto-one correspondence between primary pit-fields and pits because several pits can form over one primary pit-field or a pit may not form over a pit-field -two opposing pits together are a pit-pair; their cavities are separated by a pit membrane (two primary walls and a middle lamella in between)

5 -if the cavity is about the same diameter from top to bottom, it is described as a simple pit (or simple pit-pair) -if the secondary wall overarches the pit cavity, this is described as a bordered pit (or bordered pit-pair); we will go into more detail when we get to xylem -DIAGRAM simple and bordered pit-pairs