|Lab 9: Autotrophic Protists - The Eukaryotic Algae||
(D) Cell Walls Composition and Construction
Like the higher plants, many algae have cellulose as part of their cell wall. The cell wall is constructed of microfibrils of cellulose with the microfibrils intertwined and criss-crossed in layers, like a sheet of plywood. Cell walls are difficult to observe in algae as they are clear and rather thin compared to cell walls in plants. Stains are used to distinguish cell walls and a few prepared slides may be available for demonstration on the front desk.
In addition to cellulosic cell walls, algae have a number of other compounds used in cell wall construction. Euglena and Chlamydomonas have proteinaceous materials in their cell walls. The cell wall material in Euglena is arranged in a spiral pattern around the cell and is called a pellicle. This flexible wall allows it to move freely in the bottom mud of ponds and lakes.
Another compound found in algal cell walls is silica. Diatoms are the primary algae exhibiting this type of construction and their cell wall is called a frustule. The cell wall is quite resistant to decay and in some parts of the world (such as Lompoc, California), large deposits of marine diatoms are mined for diatomaceous earth. This material is used as a ingredient in many commercial products such as polishes, paint removers, detergents, and insulation. It is also used as a filtering agent for pool filters and beer clarification. Obtain some diatomaceous earth and examine it under your compound microscope. A frustule is composed of two halves that fit together much like a petri dish. Much of the material will be broken, but you should be able to observe the ornate patterns of the cell walls. Unlike the pennate (or elongate) cells you observed above, most of the planktonic marine diatoms are circular and are called centric diatoms. The ornamentation is used as a taxonomic tool in the identification of the diatoms. The cell wall in living diatoms is often obscured by the protoplasm and the cells must be cleared in nitric acid before identification.
(2) Calcium carbonate
Calcium carbonate is found in the cell walls of many groups of algae, including Chrysophyta, Chlorophyta, and especially Rhodophyta. Calcium carbonate is often a by-product of photosynthesis and the crystals are deposited in the cell wall matrix. In the seaweeds, the presence of calcium carbonate is thought to have evolved as a defense against herbivores.
(a) Articulate vs. Crustose
Examine the calcareous algae available in the laboratory. Dried preserved specimens will be available, and fresh examples may be. In the red algae, the calcareous members are divided into two groups; those which are erect and jointed are called articulate, and those which grow flat over the surface of rocks are called crustose. Examine both types under your dissecting microscope. You should be able to see small pores in the surface of the crustose algae, which are the location of the reproductive structures. In the articulate algae notice the filaments at the joints of each segment that hold the segments together. Place a small amount of 10% HCl (if available) on the articulate alga and wait for 5 minutes (be careful with HCl, it is an acid and can cause skin irritation. If you get any in your eyes, wash out immediately). The calcium carbonate should dissolve and you will be able to see the underlying filamentous structure of the alga.
Many algae also have polysaccharides in their cell walls that are important commercial products, notably algin, carrageenan, and agar. You have probably come in contact with one of these products today if you brushed your teeth, put on make-up, ate some ice cream, drank chocolate milk, or had a cheese product. These chemicals are used as stabilizers in a variety of industrial and food related products and have become common in our diet and household products. Algin comes from kelp and its production is a major industry in California, especially in southern California between Santa Barbara and San Diego. Carrageenan and agar come from red seaweeds. You can extract agar from local seaweeds, but it is not of high quality in terms of gelling strength.
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