Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section Two:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9

Biology: Flowering Plant Reproduction: Part Four Pollen Green algae, the ancestors of plants, lived in water and released their gametes directly into the water. The gametes swam around until they met and fertilization occurred. Land plants had to develop a different means of reproducing. Spores (for non-seed plants like ferns) and pollen are part of that new strategy. The term pollen comes from the Greek palynos for “dust” or “pollen.” Pollen grains contain the male gametophyte (or microgametophyte) phase of the plant. Microspore mother cells located along the inner edge of the anther sacs (the microsporangia) undergo meiosis. After meiosis, a pollen tetrad is produced. Some plants release their pollen as tetrads, while the majority of plants release single pollen grains. The tetrad of four haploid cells is located inside an anther sac (microsporangium) of Lilium. Mature lily pollen. Mature 2-cell stage of a pollen grain. Note the thick sculptured exine around the pollen grain of Lilium. The outer part of the pollen, the exine, is practically indestructible. It protects the two (or three) cells that make up the male gametophyte inside the pollen grain. The tube cell (or tube nucleus) develops into the pollen tube. The germ cell divides by mitosis to produce two sperm cells. The sperm are not released into the air. Instead, they travel through the protected pollen tube to reach the egg and accomplish fertilization. The germ cell may divide before or after the pollen is transferred to the female part of the flower being pollinated. A layer of polysaccharides and other materials known as the pollenkit covers the exine. You might be familiar with a product known as bee pollen that is sold in health food stores as a quick energy boost. The material in the pollenkit provides this energy to us. There are some insects that harvest pollen for this energy. In doing so, they also transport pollen from flower to flower. Pollen is familiar to most of us as a cause of allergies. However, not all pollen causes allergies. What is it about pollen that causes allergies in some people? Almost every cell has surface markers, special identifying proteins on the cell surface. For some of us, our immune system recognizes pollen as a harmful substance. Antibodies are produced in response to pollen surface proteins. When the immune system overreacts to the surface markers, an allergic reaction occurs. Usually allergies are caused by pollen produced in large quantities by wind-pollinated plants, such as ragweed, grasses, and oaks. Allergies Allergies result from immune system hypersensitivity to weak antigens that do not cause an immune response in most people. Allergens, substances that cause allergies, include dust, molds, pollen, cat dander, certain foods, and some medicines (such as penicillin). Up to 10 percent of the U.S. population suffers from at least one allergy, and between 26 and 33 percent of the population has some genetic susceptibility to allergies.  After exposure to an allergen, some people make IgE antibodies as well as B and T memory cells. Subsequent exposure to the same allergen causes a massive secondary immune response that releases plenty of IgE antibodies. These bind to mast cells found usually in connective tissues surrounding blood vessels. Mast cells then release histamine, which starts the inflammatory response (causing watery eyes and drippy nose). In some individuals the histamine release causes life-threatening anaphylaxis or anaphylactic shock.  Allergies are treated with oral medications, such as Claritin and Allegra (to name two of the more popular ones). In extreme cases, immunotherapy (allergy shots) is used to desensitize the patient so their immune system no longer has such a strong secondary reaction. Some common plants that cause allergies. Grasses Trees  Weeds Timothy Walnut Ragweed Orchard Pecan Redroot Kentucky Bluegrass Oak Sagebrush Sweet Vernal Box Elder  English Plantain Bermuda  Ash Pigweed Johnson Mountain Cedar Careless Bahia Elm Lamb's Quarters Redtop Sycamore Spiny Amaranth Alder   Hickory  Russian Thistle (tumbleweed)   Mulberry Burning Bush   Look into allergenic pollen in your area. Who makes pollen counts? What is the pollen count for this week? Now go on to the next part.

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