Lecture Notes for Friday, September 4: BTNY 1210, Fall 1998

Handout today - Plant Growth II Lab Exercise for the week of Sept. 7

Outline

Cell structure as seen with the Electron Microscope: plasmodesmata, secondary cell walls

Prokaryotic cell structure

Cell reproduction: cell cycle, mitosis

Continuation of Eukaryotic Cell Structure as seen in the Electron Microscope

Plasmodesmata are cytoplasmic channels about 50 nm in diameter that connect adjacent plant cells through the adjoining cell walls. Such structures are necessary if individual cells are to communicate to coordinate activities throughout the plant. Animals cells communicate through connections made directly between the plasma membranes of adjoining cells. Plasmodesmata are lined by plasma membrane and contain a modified tube of ER that is continuous between adjacent cells. See Figs. 3-38 and 3-39. Plasmodesmata are formed between cells during cell division, but may also be formed later in non-dividing cells. Plasmodesmata are involved in the transport of small molecules between cells, although larger molecules and even virus particles can move through plasmodesmata (see Chapt 4, pp. 87-89, if you are interested in more information). Plasmodesmata tend to be produced in clusters where the primary wall is thinner -- see Fig. 3-9.

Secondary Cell Wall Formation To review, primary cell walls are found in all plant cells, contain cellulose microfibrils and matrix polysaccharides, and are stretchable and permeable to water and anything dissolved in water. Secondary cell walls are formed later by the protoplast and are therefore necessarily formed to the inside of the primary cell wall. They are thicker and stronger than primary cell walls. They are only produced by some cells, primarily those that function in support and water transport, cells that are typically dead at maturity (the remaining cell wall is all that is needed for their function). Secondary cell walls contain more cellulose than primary cell walls, some matrix polysaccharides and the very important polymer lignin. Secondary cell walls are not stretchable because lignin permanently binds cellulose microfibrils together; therefore cell growth and division stop once the secondary cell wall is produced. Secondary cell walls are also impermeable except where special pores called pits remain in the secondary wall. Examine Fig. 3-33 and notice that pits tend to occur opposite one another in adjacent cells; the reason is that pits tend to be formed next to clusters of plasmodesmata in the primary cell walls. Why might that be? Pits allow water and other materials to pass from cell to cell despite the impermeable secondary cell wall.

PROKARYOTIC CELL ST|RUCTURE

The eukaryotic type of cell structure we have just learned about is found in all animals, all plants, all fungi, all "true" algae and all protozoa. All these cells are basically alike, but there are differences. The most notable differences are those between animal cells and plant cells. Animal cells lack cell walls, they lack plastids and they lack large vacuoles. The absence of large vacuoles relates to the absence of "reaching" growth in animals because animals do not need to produce lots of surface area as plants do.

Prokaryotic cells have DNA but it is not surrounded by a nuclear envelope and therefore prokaryotes are said to lack a true nucleus (prokaryote = "before a nucleus"). The DNA-containing area in the prokaryotic cell is called the nucleoid. Prokaryotic cells also lack organelles and thus have few, if any, internal membranes. Prokaryotes also lack a cytoskeleton (which is now thought to be the original defining feature of eukaryotic cells, the feature that distinguished early eukaryotic cells from the other cells of its time and contributed to the success of this evolutionary lineage). The prokaryotic type of cell structure is found in bacteria, cyanobacteria (which are an advanced type of bacteria and were once called "blue green algae") and archaea (which are an entirely different lineage of organisms, more closely related to eukaryotes than to bacteria). Examine Fig. 3-2 noticing the cell wall, plasma membrane (all organisms must have a plasma membrane), the nucleoid and the granular cytoplasm full of ribosomes. Examine Fig. 3-3 which illustrates cyanobacteria. Notice that, in addition to the cell wall, plasma membrane, nucleoid and ribosomes, the cytoplasm is full of thylakoid membranes. Cyanobacteria are photosynthetic, autotrophic organisms, even though they lack a chloroplast (i.e. the thylakoid membranes are not surrounded by double membrane).

CELL REPRODUCTION Chapter 8

Cell reproduction is the series of events in which one cell grows and divides to form 2 cells. This is an important process since all cells come from preexisting cells. The cell cycle encompasses the events from one division to the next division. I put a diagram on the screen in which a small cell grows to a large cell during what is called interphase which lasts several hours to several days). Then the nucleus divides (mitosis) and then the cytoplasm divides (cytokinesis). "Cell division" is a more general term including both mitosis and cytokinesis and takes 1-3 hours.

To oversimplify the fates of the two "daughter cells", one cell starts to repeat the cycle (i.e. remains meristematic) while the other cell stops dividing and begins to specialize or differentiate into a particular kind of cell. As examples I mentioned the basal layer of cells in our epidermis (using a figure from another book) that is always dividing and producing the dead, specialized, keratin-containing cells that are eventually sloughed off the outermost layer of our skin (movement from the basal layer to the surface takes 3-4 weeks). I also diagrammed the shoot tip showing how cells left behind by the apical meristem differentiate and elongate (by vacuole enlargement), pushing the apical meristem up into the air.

For daughter cells to be equivalent to the original cell, all components have to double in number and be distributed equally between daughter cells. Cytosol and organelles are partitioned roughly equally during cytokinesis. The process must be more exact for the genetic material. First, an exact copy has to be made; DNA replication takes place during the S (synthesis) phase of interphase. Second, the two copies have to be distributed, one copy to each daughter cell in a process called mitosis.

Genetic information is contained in the structure of the DNA molecule which is a blueprint for making and running a cell. DNA is a long thread-like molecule that is wrapped around a specialized protein ("packaging material"). The combined DNA and protein makes a long thread called a chromosome.

This long thread (along with all the other chromosomes in the nucleus) is replicated during the S phase of the cell cycle to form two sister chromatids still joined at one point along the chromosome by a centromere. The problem is how to get one complete set of long threads (chromosomes) into one daughter cell and the other set into the other daughter cell.

MITOSIS There is obviously a mechanical problem in manipulating these long threads. To put this into a scale we are more familiar with, imagine a human cell, 30 micrometers in diameter enlarged to 12 inches in diameter. It's nucleus (~10 micrometers) would now be 4 inches in diameter. If you take the DNA in all 46 chromosomes in a single human nucleus, and place it end to end, its actual length is about10 ft. On our expanded scale, it would be 20 miles. Wrapped around the packing protein, its total length is reduced to about 3 miles. Imagine the 46 pieces of the smallest thread you can imagine, with a total length of 3 miles, undergoing replication in a 4 inch sphere. And then things really get complicated when one copy of each thread has to be distributed to each of the daughter 4 inch spheres.

The solution is MITOSIS - i.e. condense the long threads into short compact units which are much easier to manipulate and then stretch them out again after they have been separated. During mitosis they are visible in the light microscope, especially if stained, and it is possible to see the two sister chromatids still attached at the centromere. So the process of mitosis is simply one of coiling and shortening the chromosomes (prophase), organizing and lining them up (metaphase), separating sister chromatids, each of which now becomes a chromosome in its own right (anaphase) and finally uncoiling and lengthening the chromsomes to resume their long thread form (telophase). See Fig. 8-5 and 8-6 and be able to recognize each phase of mitosis.