Chapter 7 - Viruses, Bacteria, Protists, and Fungi

Section 1 - Viruses


  A virus is a tiny, nonliving particle that enters and then reproduces inside a living cell. Biologists consider viruses to be nonliving because viruses are not cells and do not have the characteristics of organisms. The only way in which viruses are like organisms is that they can multiply.

  Although viruses can multiply, they do so differently than organisms. Viruses can multiply only when they are inside a living cell. The organism that a virus enters and multiplies inside is called a host. A host is an organism that provides a source of energy for a virus or another organism. Organisms that live on or in a host and cause it harm are called parasites. Most viruses are like parasites because they destroy their host cells.

  Viruses are smaller than cells and vary in shape and size. Viruses can be round, or shaped like rods, bricks, threads, or bullets. Some viruses, such as bacteriophages, have complex, robot-like shapes. A bacteriophage is a virus that infects bacteria.

  All viruses have two basic parts: a protein coat that protects the virus and an inner core made of genetic material. Some viruses are surrounded by an additional membrane envelope. Each virus contains unique proteins on its outer surface. The shape of these proteins allows the virus to attach to, or lock onto, only certain host cells.

  After a virus attaches to a host cell, it enters the cell. Once inside a cell, a virus's genetic material takes over many of the cell's functions. It instructs the cell to produce the virus's proteins and genetic material. These proteins and genetic material then assemble into new viruses.

  An active virus immediately takes over the cell's functions, and the cell quickly begins to produce the virus's proteins and genetic material. These parts are assembled into new viruses. When it is full of new viruses, the host cell bursts open and releases the new viruses.

  When a hidden virus enters a host cell, the virus's genetic material becomes part of the cell's genetic material. The virus's genetic material may stay inactive for a long time. Then, the virus's genetic material suddenly becomes active and takes over the cell's functions and replicates. Once the host cell is full of new viruses, it bursts open to release them.

  Viral diseases can be spread in various ways. There are currently no cures for viral diseases. Resting, drinking plenty of fluids, and eating wellbalanced meals may be all you can do while you recover from a viral disease. Vaccines also help prevent the spread of viral diseases. A vaccine is a substance introduced into the body to stimulate the production of chemicals that destroy specific disease-causing viruses and organisms.




Section 2 - Bacteria


  Bacteria are single-celled organisms.Bacteria are prokaryotes. The genetic material in their cells is not contained in a nucleus. Bacterial cells have one of three basic shapes: spherical, rodlike, or spiral.

  Most bacterial cells are surrounded by a rigid cell wall that helps to protect the cell. Inside the cell wall is the cell membrane that controls what materials pass into and out of the cell. Inside the cytoplasm are ribosomes and the cell's genetic material. Some bacteria have flagella. A flagellum is a long, whiplike structure that extends from the cell membrane and out through the cell wall. A flagellum helps a cell to move.

  All bacteria need certain things to survive. Bacteria must have a source of food and a way of breaking down the food to release its energy. Some bacteria are autotrophs and make their own food. Others are heterotrophs that obtain food by consuming autotrophs or other heterotrophs. Like all organisms, bacteria need a constant supply of energy. This energy comes from breaking down food in the process of respiration.

  When bacteria have plenty of food, the right temperature, and other suitable conditions, they thrive and reproduce frequently. Bacteria reproduce by binary fission, a process in which one cell divides to form two identical cells. Binary fission is a form of asexual reproduction. Asexual reproduction is a reproductive process that involves only one parent and produces offspring that are identical to the parent. Some bacteria perform a simple form of sexual reproduction called conjugation. Sexual reproduction involves two parents who combine their genetic material to produce a new organism that differs from both parents. During conjugation, one bacterium transfers some of its genetic material into another. After the transfer the cells separate.

  Many bacteria can survive harsh conditions by forming endospores. An endospore is a small, rounded, thick-walled, resting cell that forms inside a bacterial cell.

  Some bacteria cause diseases and other harmful conditions. However, most bacteria are either harmless or helpful to people. Bacteria are involved in oxygen and food production, environmental recycling and cleanup, and health maintainance and medicine production. Helpful bacteria produce foods such as cheese and pickles. However, some bacteria cause food to spoil. One method to slow down food spoilage is pasteurization, where food is heated to a temperature that is high enough to kill most harmful bacteria without changing the taste of the food. Heterotrophic bacteria in the soil break down materials for reuse. These bacteria are decomposers- organisms that break down large chemicals in dead organisms into small chemicals.




Section 3 - Protists


  The protist kingdom is very diverse. All protists are eukaryotes that cannot be classified as animals, plants, or fungi. All live in moist surroundings. Most are unicellular, but some are multicellular. Some are heterotrophs, some are autotrophs, and some are both. Protists can be divided into three categories: animal-like, plantlike, and funguslike protists.

  Like animals, animal-like protists are heterotrophs, and most are able to move from place to place to obtain food. Animal-like protists are also called protozoans. Protozoans can be divided into four types: sarcodines, ciliates, flagellates, and those that are parasites. Sarcodines move and feed by using pseudopods. Pseudopods are temporary bulges of the cell. Pseudopods form when cytoplasm flows toward one location and the rest of the organism follows. Protozoans that live in fresh water, such as amoebas, have a contractile vacuole,which collects the extra water and expels it from the cell. Ciliates have structures called cilia, which are hairlike projections that move with a wavelike motion. Flagellates move using whiplike flagella. Some flagellates live inside the bodies of other organisms in a state of symbiosis. Symbiosis is a close relationship between two species in which at least one of the species benefits. Sometimes, flagellates harm their hosts. In other cases, their relationship is one of mutualism,in which both partners benefit. Protozoans that are parasites feed on their hosts' cells and body fluids.

  Plantlike protists are called algae. Like plants, algae are autotrophs. Algae can exist in a variety of colors because they contain many types of pigments. Plantlike protists include diatoms, dinoflagellates, euglenoids, red algae, green algae, and brown algae. Diatoms have beautiful, glasslike cell walls. Dinoflagellates are covered by stiff plates and move using two flagella. Euglenoids can be heterotrophs when sunlight is not available. Red algae and brown algae live in the oceans. Green algae live in fresh water, salt water, and moist places on land.

  Like fungi, funguslike protists are heterotrophs, have cell walls, and use spores to reproduce. Spores are tiny cells that are able to grow into new organisms. All funguslike protists are able to move at some point in their lives. The three types of funguslike protists are slime molds, water molds, and downy mildews. Slime molds live in moist soil and on decaying plants. Water molds and downy mildews grow as tiny threads in water or moist places.




Section 4 - Fungi


  Most fungi share several important characteristics: Fungi are eukaryotes that have cell walls, are heterotrophs that feed by absorbing their food, and use spores to reproduce. Fungi also need moist, warm places in which to grow. They vary in size from unicellular yeasts to multicellular mushrooms.

  Hyphae (singular hypha) are branching, threadlike tubes that make up the bodies of multicellular fungi. What a fungus looks like depends on the arrangement of its hyphae.

  Fungi are heterotrophs, but they do not take food into their bodies in the way that animals do. First, the fungus grows hyphae into a food source. Then digestive chemicals ooze from the hyphae into the food. The digestive chemicals break down the food into small substances that can be absorbed by the hyphae. Some fungi feed on the remains of dead organisms. Others are parasites that break down the chemicals in living organisms.

  Fungi usually reproduce by making spores. The lightweight spores are surrounded by a protective covering and can be carried easily through the air or water to new sites. Fungi produce spores in reproductive structures called fruiting bodies.Unicellular yeasts use a form of asexual reproduction called budding. In budding, a small cell grows from the body of a large, well-fed cell. Asexual reproduction results in fungi that are genetically identical to the parent. Fungi may reproduce sexually, especially when conditions become less favorable. This occurs when the hyphae of two fungi grow together and new genetic material is exchanged. In time, a new reproductive structure grows from the joined hyphae and produces spores. These spores develop into fungi genetically different from either parent. Three major groups of fungi include sac fungi, club fungi, and zygote fungi. The groups are named for the appearance of their reproductive structures.

  Fungi play important roles as decomposers and recyclers on Earth. Many fungi provide foods for people. Some fungi cause disease while others fight disease. Still other fungi live in symbiosis with other organisms.Fungi break down the chemicals in dead organisms. This returns nutrients to the soil. Yeasts are important in the preparation of foods such as bread. People also eat some types of fungi, such as mushrooms. Many fungi cause disease in crops and in humans. Others, such as Penicillium, make useful substances that kill bacteria. The hyphae of some fungi grow among the roots of plants. The hyphae help the plant absorb more water and nutrients from the soil. In return, the fungus feeds on extra food the plant makes. A lichen consists of a fungus living in a mutualistic relationship with either algae or autotrophic bacteria



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