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

2 found in the human stomach.

Activities of bacteria

 

Bacteria play many beneficial roles in the environment. For example, some species of bacteria live on the roots of pod-bearing plants (legumes) and “fix” nitrogen from the air into organic compounds that are then available to plants. The plants use the nitrogen compounds to make amino acids and proteins, providing them to the animals that consume them. Other bacteria are responsible for the decay that occurs in landfills and other debris in the environment. These bacteria recycle the essential elements in the organic matter.

 

In the food industry, bacteria are used to prepare products such as cheeses, fermented dairy products, sauerkraut, and pickles. In other industries, bacteria are used to produce antibiotics, chemicals, dyes, numerous vitamins and enzymes, and a number of insecticides. Today, they are used in genetic engineering to synthesize certain pharmaceutical products that cannot be produced otherwise (see Chapter 11).

 

Cyanobacteria 

 

Cyanobacteria are photosynthetic bacteria formerly known as blue-green algae. Most are found in the soil and in freshwater and saltwater environments. The majority of species are unicellular, but some may remain linked and form filaments.

 

Cyanobacteria, which are autotrophic, serve as important fixers of nitrogen in food chains. In addition, cyanobacteria, a key component of the plankton found in the oceans and seas, produce a major share of the oxygen present in the atmosphere, while also serving as food for fish. Some species of cyanobacteria coexist with fungi to form lichens.

 

Cyanobacteria have played an important role in the development of Earth. Scientists believe that they were the first photosynthetic organisms to occur on Earth’s surface. Beginning about 2.7 billion years ago, the oxygen produced by cyanobacteria enriched Earth’s atmosphere and converted it to its modern form. This conversion made possible all life forms that use oxygen for cellular respiration.

 

In the human intestine, bacteria play an important role in our digestive health. Some species synthesize several vitamins not widely obtained in food, especially vitamin K. Bacteria also often break down certain foods that otherwise escape digestion in the body. Signals from gut microbes spur capillary development in the small intestine, increasing absorption of nutrients into the bloodstream.

 

Unfortunately, many bacteria are pathogenic; that is, they cause human disease. Such diseases as cholera, tuberculosis, gonorrhea, syphilis, scarlet fever, food poisoning, Lyme disease, plague, tetanus, typhoid fever, and most pneumonias are due to bacteria. In many cases, the bacteria produce powerful toxins that interfere with normal body functions and bring about disease. Botulism (food poisoning) and tetanus toxins are examples. In other cases, bacteria grow aggressively in the tissues (for example, tuberculosis and typhoid fever), destroying them and thereby causing disease.

 

Viruses

 

Technically, viruses are not members of any domain of life. They are considered here because, like bacteria, they are microscopic and can cause human diseases. Viruses are acellular particles that lack the properties of living things but have the ability to replicate inside living cells. They have no energy metabolism, they do not grow, they produce no waste products, they do not respond to stimuli, and they do not reproduce independently. In the view of biologists, they are not alive.

 

Viruses consist of a central core of either DNA or RNA surrounded by a coating of protein. The core of the virus that contains the genes is the genome, while the protein coating is the capsid. Viruses have characteristic shapes. Certain viruses have the shape of an icosahedron, a 20-sided figure made up of equilateral triangles. Other viruses have the shape of a helix, a coil-like structure. The viruses that cause herpes simplex, infectious mononucleosis, and chickenpox are icosahedral (solid with 20 faces). The viruses that cause rabies, measles, and influenza are helical (spiral-shaped).

 

Viruses reproduce only within living cells. They attach to the plasma membrane of the host cell and release their nucleic acid into the cytoplasm of the cell. The capsid may remain outside the cell, or it may be digested by the host cell within the cytoplasm. In the host cytoplasm, the DNA or RNA of the viral genome encodes the proteins that act as enzymes for the synthesis of new viruses. The enzymes use amino acids in the cell for protein synthesis and nucleotides from the host DNA for nucleic acid synthesis. The viruses obtain cellular ATP and use cellular ribosomes for additional viral synthesis. After some minutes or hours, the new viral capsids and genomes combine to form new viruses.

 

Once formed, the viruses may escape the host cell when the host cell disintegrates. Alternately, the new viruses may force their way through the plasma membrane of the cell and assume a portion of the plasma membrane as a viral envelope. In either process, the cell is often destroyed and hundreds of new viruses are produced.

 

Viruses can cause a number of human diseases, including measles, mumps, chickenpox, AIDS, influenza, hepatitis, polio, and encephalitis. Protection from these diseases can be rendered by using vaccines composed of weak or inactive viruses. A viral vaccine induces the immune system to produce antibodies, which provide long-term protection against a viral disease.

Chapter 16: Protista

  Algae

 

The term algae refers to a large number of photosynthetic organisms that are multi- or unicellular eukaryotes and are not classified with plants. The organisms are plantlike, however, because they contain chloroplasts with chlorophyll. Most algae can be found in the oceans, but freshwater forms are also abundant.

 

The algae are subdivided into several divisions (rather than phyla, like the protozoa): Rhodophyta, Pyrrophyta, Chrysophyta, Phaeophyta, and Chlorophyta. The divisions are based in part on the types of pigments and colors they have.

 

Rhodophyta

 

Rhodophyta is the division of red algae. These organisms are almost exclusively marine types. Most are unicellular, but some multicellular forms grow anchored to rocks below the level of the low tide. Some are large enough to be seaweeds. Red algae carry on photosynthesis using chlorophyll a. The red pigments are very similar to those in many species of cyanobacteria. A derivative of red algae called agar is commonly used in bacteriological media in the laboratory.

 

Pyrrophyta

 

Members of the Pyrrophyta are dinoflagellates. Dinoflagellates are unicellular organisms that are usually surrounded by thick plates that give them an armored appearance. Two flagella move the organism. Many dinoflagellates are luminescent. When affected by sudden movements, they give off light. When optimal conditions exist in the oceans, the dinoflagellates reproduce at explosive rates. Their red pigments cause the water to turn the color of blood. This condition is known as red tide.

 

Chrysophyta

 

Members of the division Chrysophyta are golden algae, most of which are diatoms. Diatoms have cell walls or shells composed of two overlapping halves impregnated with silica. In the oceans, the diatoms carry on photosynthesis. They serve as an important source of food in the oceanic food chains. Diatomaceous earth, a light-colored porous rock composed of the shells of diatoms, is made into a commercial product called diatomite. Diatomite is used as a filler, as an absorbent, and as a filtering agent.

 

Phaeophyta

 

Members of the division Phaeophyta are the brown algae. These organisms, which are multicellular, are found almost exclusively in saltwater, where they are known as rock weeds and kelp. Despite their great size, the tissue organization in these algae is quite simple compared with plants. Often they are used as fertilizers and sources of iodine.

 

Chlorophyta

 

Members of the division Chlorophyta are green algae, some of which are flagellated. The common laboratory specimen Spirogyra is classified here, as are other flagellated species. Many colonial forms are also classified as Chlorophyta. The colonial forms may represent the first evidence of multicellularity in evolution, and possess features that are intermediate between those of single-celled eukaryotes and those of more complex multicellular organisms. Ulva, the common sea lettuce, is classified here.

 

Oomycetes

 

Oomycetes are called water molds, a reference to the fact that most species are aquatic. They are not, however, members of kingdom Fungi. During sexual reproduction, the members form clusters of egglike bodies at the tips of their hyphae. Nearby hyphae grow toward the bodies and fuse with them. Nuclear fusions lead to the formation of sexual spores called oospores that germinate to produce new hyphae.

 

In the sexual process of reproduction, oomycetes form a unique cell called a zoospore. The zoospore has flagella and is able to move like an animal cell. Certain oomycetes cause downy mildew of grapes, white rust of cabbage, and the late blight of potatoes. Aquatic oomycetes infect fish in aquaria and nature.

 

Protozoa

 

Further analysis of kingdom Protista has suggested the need for restructuring phylogenic classification. Genetic and morphological research has led to subdividing the Protista kingdom into numerous separate kingdoms, each with its own lineage of protist. In truth, the classification of the protists remains in flux. This chapter provides descriptions of representative members of the Protista group and their roles in our lives and the environment, using the still-accepted kingdom and phyla nomenclature.

 

Members of the kingdom Protista are a highly varied group of organisms, all of which are eukaryotic. In addition, protists are unicellular or, in some cases, colonial. Many species are autotrophs, creating their own food, while others are heterotrophs, feeding on organic matter. Many species are nonmotile, but the majority of protists are able to move by various means. Many protists have contractile vacuoles, which help them to remove excessive amounts of water from their cytoplasm. The kingdom Protista includes the protozoa, slime molds, and algae.

 

Protozoa can be divided into four phyla based on their locomotion: Mastigophora, Sarcodina, Ciliophora, and Sporozoa.

 

Mastigophora

 

Members of the phylum Mastigophora move about by using one or more whiplike flagella. The genus Euglena contains flagellated species. Members are freshwater protists with typical eukaryotic properties, including two flagella, reproduction by mitosis, and flexible nutritional requirements. Euglena species also possess chlorophyll within chloroplasts. This pigment allows the organisms to synthesize organic compounds in the presence of sunlight. When no sunlight is available, the organism feeds on dead organic matter in the surrounding environment. Thus, the organism is autotrophic and heterotrophic. Some biologists consider Euglena to be the basic stock of evolution for both animals and plants.

 

Certain species of Mastigophora are zooflagellates, while some are phytoflagellates. The zooflagellates live within the bodies of animals and are typified by the wood-digesting flagellates in the intestines of termites. Among the pathogenic zooflagellates are those that cause sleeping sickness, trichomoniasis, and giardiasis. The phytoflagellates have photosynthetic abilities and are often discussed with algae in textbooks.

 

Some species of Mastigophora organize themselves into colonies. Members of the genus Volvox are typical colonial forms. The cell colonies are not differentiated into tissues or organs, but the colonies show how a preliminary step in evolutionary development might have occurred.

 

Sarcodina

 

Members of the phylum Sarcodina are the amoebas and their relatives. Amoebas consist of a single cell without a definite shape. They feed on small organisms and particles of organic matter, and they engulf the particles by phagocytosis. Extensions of the cytoplasm called pseudopodia (the singular is pseudopodium) assist phagocytosis and motion in the organisms.

 

Amoebas are found in most lakes, ponds, and other bodies of freshwater. They move by a creeping form of locomotion called amoeboid motion. One amoeba called Entamoeba histolytica causes a type of dysentery in humans.

 

Two interesting amoebas are the foraminiferans and the radiolarians. Both are marine amoebas that secrete shells. Their shells have been identified as markers for oil deposits because both were present in the ocean communities that became the