Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

organic deposits that, under pressure, became oil fields.

 

Ciliophora

 

Members of the phylum Ciliophora move by means of cilia. The organisms are all heterotrophic and have specialized organelles in their cytoplasm. For example, they have two nuclei: a large macronucleus and a number of smaller micronuclei. The micronuclei carry the genetic information of the cell.

 

The ciliate Paramecium typifies the phylum Ciliophora. This organism has a slipper-shaped body with a covering called a pellicle. Defensive organelles called trichocysts are present in the pellicle. The organism reproduces by mitosis and by an elaborate form of sexual behavior called conjugation, which occurs when two Paramecium join to one another in the oral region and exchange nuclear material. The cilia of Paramecium provide a precise form of motion not provided by flagella or pseudopodia. The cilia can propel the Paramecium either forward or backward and move it in a spiral manner.

 

Sporozoa

 

Members of the phylum Sporozoa are exclusively parasites. They are so named because some members produce sporelike bodies. Often they have an amoeboid body form, but they are not related to the Sarcodina.

Sporozoans are parasitic organisms with complex life cycles involving several stages. One of the best-known members of the group is the Plasmodium species, which are the agents of malaria. The organisms spend portions of their life cycle within mosquitoes. After being injected into the human bloodstream by the mosquito, the parasites invade the red blood cells, undergo numerous changes, and emerge from the red blood cells, destroying them. The infected human experiences a malaria attack soon after.

 

Another important member of the Sporozoa group is Toxoplasma gondii, which causes toxoplasmosis, a disease of the white blood cells. Toxoplasmosis is normally not a dangerous disease, but pregnant women can pass it to their fetuses, where it can cause tissue damage. Also, people who are immunocompromised are susceptible to severe cases of toxoplasmosis.

 

Two other important pathogens of the Sporozoa group are Cryptosporidium coccidi and Pneumocystis carinii. Both of these organisms cause severe opportunistic disease in immunocompromised patients. The first causes a severe intestinal diarrhea that is accompanied by the loss of substantial volumes of water; the second causes a type of pneumonia.

 

Slime Molds

 

Although slime molds have properties that resemble fungi, many scientists classify the organisms with the protists because of their protozoalike qualities. Slime molds may be plasmodial slime molds, which consist of a single, flat, very large cell with many nuclei; or they may be “cellular” slime molds, amoebalike cells that live independently and unite with other cellular slime molds to form a single, large, flat cell with many nuclei.

 

Though scientists are uncertain how slime molds evolved, the organisms show a hint of cellular cooperation that is characteristic of more complex, multicellular organisms. Cellular slime molds normally move about like amoebas. The organisms exist as a mass of cytoplasm with diploid nuclei. This mass is a plasmodium. Cytoplasmic streaming can be seen within the plasmodium. Slime molds are excellent research tools because they are large and easy to cultivate.

 

As long as a food supply is adequate and other environmental conditions are optimal, a slime mold grows indefinitely in its plasmodial stage. However, if conditions become harsh, the plasmodium can join with other plasmodia to form a large, sluglike mass. This giant plasmodium can transform itself into a spore-bearing structure similar to a fungus. A stalk arises from the plasmodium, and nuclei within the stalk cells divide by meiosis. Knobs called sporangia develop at the end of the stalk and fill with haploid spores. The spores are shed and removed by the wind when the stalk dries. Eventually, the spores germinate into flagellated swarm cells. Swarm cells function as gametes and fuse to form a diploid zygote, which divides by mitosis and forms a new plasmodium. The plasmodium represents the new generation of slime mold.

Chapter 17: Fungi

 Fungi 

 

Fungi, together with bacteria, are the major decomposers of organic matter on Earth. Most fungi are saprobes; that is, they digest nonliving organic matter, such as wood, fallen leaves, and dead animals. However, some fungi are parasites that attack living things and cause disease. Fungi cause many agricultural diseases, as well as several human diseases.

 

A unique physical structure and the method by which they obtain nutrients distinguish fungi from the other kingdoms. Fungi secrete enzymes into the environment and break down organic matter, and then absorb the small nutrient particles through their cell membranes. This process is called extracellular digestion.

 

Except for unicellular yeast, fungi are multicellular organisms, and the body of a fungus consists primarily of cells joined in filaments. Each microscopic filament of a fungus is a hypha (the plural is hyphae). Hyphae may form a huge tangled interwoven network called a mycelium (the plural is mycelia). A mycelium is a visible structure. Fungi cells are unique because they possess a polysaccharide called chitin. In some cases, the cell walls also contain cellulose. Fungi live in environments that are generally acidic, and they prefer carbohydrate-rich foods.

 

Reproduction in fungi can be sexual or asexual. In some cases, filaments break from the main mycelium and grow into new individuals. Alternately, a fungus may produce spores by an asexual or sexual process. The spores disperse, germinate, divide, and produce genetically identical fungi. The hyphae and spores are usually haploid (containing one set of chromosomes), and the haploid phase dominates the life cycle of a fungus. Spores can withstand extreme dryness and cold to produce a new fungus when conditions permit.

 

Fungi can also reproduce by a sexual process, which results in a short-lived diploid cell (with two sets of chromosomes) that soon produces haploid cells through meiosis. The spores develop into cells that divide by mitosis to form a new hypha, and then a new mycelium.

 

In the most accepted classification scheme, fungi are placed in four major divisions: Zygomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes.

 

Ascomycetes

 

Members of the division Ascomycetes are diverse. They range from unicellular yeasts to powdery mildews, cottony molds, and large, complex, “cup” fungi. In the latter, the hyphae are tightly packed and form a cup-shaped structure.

 

During sexual reproduction, the ascomycetes form a sac called an ascus. The ascus forms where sexually opposite hyphae have fused. Within the sac, a number of ascospores form, and each ascospore can reproduce the entire fungus.

 

Within the Ascomycetes division is the yeast Saccharomyces. This organism is used in fermentation processes and baking. The producer of penicillin, Penicillium, is also in this class, as is Aspergillus, a producer of citric acid, soy sauce, and vinegar. The chestnut blight and Dutch elm diseases are caused by ascomycetes.

 

Basidiomycetes

 

Members of the division Basidiomycetes are known as club fungi. They include the common mushroom, the shelf fungi, puffballs, and other fleshy fungi. Sexual spores called basidiospores are formed on clublike structures called basidia (the singular is basidium).

 

Perhaps the most familiar member of this division is  an edible mushroom. The mycelium forms below ground. After the hyphae have fused, a mushroom cap emerges. Basidia form on the cap’s underside along the gills, and basidiospores form on the basidia. Some mushrooms are edible while others are poisonous, but they are similar in their form and shape.

 

Basidiomycetes also cause agricultural diseases, including rust and smut diseases. These diseases affect corn, blackberries, and grains such as wheat, oats, and rye.

 

Deuteromycetes

 

Those fungi that lack a known sexual reproduction cycle are in the division Deuteromycetes. As far as we know, these fungi reproduce only by an asexual process. (When such an organism’s sexual stage is discovered, the fungus is usually reclassified.)

 

Many human pathogens are currently in this division. These organisms reproduce by fragmenting, with hyphae segments commonly blown about in dust by air currents and the wind. A familiar deuteromycete is the athlete’s-foot fungus, which can be picked up from fragments on towels and shower-room floors.

 

Fungal Disease

 

Several fungi cause diseases in humans, some of them serious. For example, a disease of the lungs and spinal cord is caused by a fungus called Cryptococcus neoformans. This disease is serious in AIDS patients, and it is often spread in dust by air currents.

 

Another human pathogen is Candida albicans. This organism causes disease of the oral cavity (thrush), as well as yeast disease of the reproductive tract. Normally the disease is mild, but in people infected with HIV, for example, it can be serious.

 

Other human fungal diseases include ringworm and athlete’s foot. Each is caused by fungi of various genera, and each is characterized by blisterlike regions on the skin or in the webs of toes or fingers. Fungal diseases of the lung tissues include histoplasmosis, blastomycosis, and coccidioidomycosis.

 

Lichens

 

Lichens are associations between fungi and cyanobacteria (formerly called blue-green algae). The cyanobacteria are the photosynthetic elements in the association, which is an example of mutualism (a living arrangement where both partners benefit). The photosynthetic organisms provide nutrients for themselves and the fungus, while the fungus provides protection, water, and minerals for the photosynthetic metabolism.

 

Lichens can exist in harsh habitats with extreme climatic conditions. They are found on mountaintops, on rock faces in the desert, and on tree bark. Commonly, lichens set the stage for the growth of more complex plants. Lichens can recover from freezing and drying to resume normal metabolic activities. Their growth is so slow that some patches of lichens may be thousands of years old.

 

Zygomycetes

 

The Zygomycetes division of fungi includes mostly terrestrial fungi. Because the hyphae have no cross walls between the cells, they are said to be coenocytic. Sexual reproduction in these organisms occurs when sexually opposite hyphae fuse and form spores called zygospores.

 

A common member is the bread mold Rhizopus stolonifer. This fungus forms a white or gray mycelium on bread. The sporangia containing the asexual spores can be seen extending into the air. One species of Rhizopus is used to ferment rice to make sake, and another species is used in the production of cortisone.

Chapter 18: Plants: Diversity and Reproduction

  Introduction to Plants

 

Plants are multicellular eukaryotic organisms with the ability to produce their own food by the process of photosynthesis. (They are autotrophs.) Algae have historically been included with the plants, but they are now classified with the protists. The modern definition of plants includes organisms that live primarily on land (and sometimes in water), excluding algae that live primarily in water.

 

Another distinguishing characteristic of plants is their type of chlorophyll. Chlorophyll is used to absorb energy from the sun during the process of photosynthesis. Plants have chlorophyll a and chlorophyll b, while many species of algae do not have chlorophyll b. Many evolutionary biologists believe that the green algae gave rise to the land plants.

 

Plants occur in two major groups: nonvascular plants