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Selasa, 27 Januari 2009

How Safe is the Air Indoors?

What is in the air in your home, where you work, or in public buildings? There may be bioaerosols — airborne biological contaminants. Aerobiological health hazards affect everyone on a daily basis and include allergens, mold spores, bacteria, and viruses that cause infectious diseases. How can these hazards be controlled indoors?

Aspergillus fumigatus is a fungus whose spores are common inhalation pollutants that pose a health hazard. Photo: Centers for Disease Control and Prevention.

Aerobiological engineering is a field of study that combines elements of engineering and microbiology that focus on reducing the risk of airborne disease by controlling the aerobiology of our indoor environments. It offers some solutions to the hazards of bioaerosols:

  • Existing technologies can collectively control these bioaerosols if we retrofit old buildings or specifically design new buildings to control airborne microbes.
  • By re-engineering our buildings on city-wide scales, the population can be broadly protected and potentially immunized against epidemics.
  • Developing standards for indoor environments and educating the public are critical steps to transforming our disease-prone society.



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Aerobiological health threats

Contagious diseases are the most dangerous and costly threats posed to building occupants today, including influenza, SARS, tuberculosis, pneumonia, and meningitis. Emerging pathogens such as avian flu and the resurgence of old diseases like plague, scarlet fever, whooping cough, and measles highlight the increasing vulnerability of populations to epidemic disease.
Evolving drug resistance, especially among hospital-acquired infections, complicates treatment of microbial agents, and physicians see their once abundant arsenal of antibiotics shrinking faster than new miracle drugs can be developed. Vaccines, once thought to be magic bullets, seem insufficient by themselves to combat airborne pathogens that can be transmitted freely in our unprotected buildings and have the potential to spread globally and cause pandemics.
Relatively mundane health threats like mold, dander, and allergens burden homes, schools, and offices, while the threat of bioterrorism leaves our buildings vulnerable to manmade epidemics that could decimate cities. Such formidable challenges can be placed into a manageable context if we recognize that protecting our buildings against the most common microbes simultaneously protects against the most dangerous threats as well.
Air- and surface-cleaning technologies

The technologies needed to create healthy buildings already exist, but they are not implemented widely enough to interdict epidemics. Optimized combinations of filtration and ultraviolet germicidal irradiation (UVGI) can be used to remove airborne microbes with high efficiencies. Combining and optimizing these technologies is the most cost effective means of disinfecting indoor air.
  • Filtration removes airborne particles including mold spores, many bacteria, and allergens.
  • UVGI eliminates many harmful bacteria and viruses.

Existing buildings can be retrofitted with air disinfection systems, but the most economic long-term solution is to construct new buildings that maintain aerobiological cleanliness by design. Air circulation is often poor in older buildings, and there are limits to what retrofitted air-cleaning systems can do for them. New buildings can be built in which the airflow is more evenly distributed and in which effectiveness of air cleaning can be maximized.2 A variety of other technologies, including photocatalytic oxidation (PCO), ozone, pulsed light, and antimicrobial materials, are also available options for air and surface biocontamination problems.

Criteria for rating healthy buildings

Modern air disinfection systems can achieve high levels of air cleaning, but limited budgets often require us to ask exactly how much air cleaning is needed to protect health. This question ultimately hinges on how buildings rate:

  • aerobiologically — the indoor levels of airborne microbes
  • epidemiologically — the infection risk of the building

Airborne levels of microbes

Indoor air contains a great variety of bioaerosols, most of which are relatively harmless to healthy humans. The concentration of airborne microbes in indoor environments, treated collectively without regard to species, provides a reasonable indication of overall aerobiological air quality. Levels of bacteria and fungi vary by season, with lows in winter, and increase with occupancy, as people are the primary source of contagious pathogens. Airborne levels are measured in terms of colony-forming units (cfu) of bacteria or fungi per cubic meter. Some hospital operating rooms are designed to maintain levels as low as 10 cfu/m3, although this level often proves difficult to achieve. Levels in homes and offices need not be this low, making solutions there less cost-prohibitive.

Infection risk

The infection risk (IR) of any building might be estimated by collecting data on infection rates and symptoms or through methods of risk analysis.3 Another approach is to estimate the risk using computer models of building airflow to calculate daily doses of inhaled contaminants. Airborne levels can be easily, if not always accurately, assessed with air samplers. The IR to an occupant in a particular building can be evaluated from epidemiological data. The IR can also be inversely viewed as the percentage of occupants protected from infection, a parameter called the building protection factor (BPF).

The BPF is the complement of the IR— a low IR implies a high BPF— and it can be used to rate and compare buildings under a common design basis. The BPF is primarily a function of the volume, airflow, outside air fraction, and removal efficiency of the air disinfection system. Being an intrinsic property of the building, it applies generically to all microbial species.4
Buildings differ according to their operating parameters. A completely unprotected building may have a BPF of 0% to 1%, whereas a building that maximizes protection of occupants may have a BPF of up to 99%. BPF can be considerably improved in existing buildings through the addition of air cleaning or other ventilation system improvements.

At least four general categories of buildings have been suggested:

  1. Problem buildings foster aerobiological problems or act as amplifiers. Their airborne levels may exceed 10,000 cfu/m3. IR can approach 99% or more and BPF 1% or less.
  2. Normal buildings have average airborne levels, about 500 to 5000 cfu/m3. Typically, IR is about 50% to 75% and BPF about 25% to 50%.
  3. Healthy buildings promote good air quality and health or are above average. Airborne levels are 100 to 1000 cfu/m3. Typically, IR is less than 50% and BPF 50% or higher.
  4. Immune buildings are designed to actively prevent airborne disease transmission. Airborne levels are as low as 10 cfu/m3. IR is less than 10% and BPF 90% or higher.

Disease-free buildings

Buildings concentrate allergens due mainly to the protective effects of shade, warmth, substrate materials, and moisture. For the same basic reasons, they act as vectors (carriers) for contagious airborne diseases. Humans have been building enclosed habitats for perhaps half a million years, and in this course of time airborne pathogens evolved the ability to survive indoors just long enough to transmit to new hosts. They have adapted to our enclosed habitats so completely that they cannot survive outdoors. This evolutionary process accelerated when man began husbanding animals, from which almost all human pathogens seem to have jumped species. The evolutionary process continues today as emerging pathogens adapt to indoor transmission, and the number of new disease species has increased exponentially over time, in concert with the size and density of the human population.

By designing our habitats strictly for human comfort, we have unwittingly fostered the adaptation and proliferation of dangerous pathogens. It is only by re-engineering our buildings to eliminate, rather than foster, airborne disease transmission that we can reverse this evolutionary trend. By immunizing enough buildings against disease, it is theoretically possible to develop herd immunity in a community or city. The percentage of buildings that would need to be immunized to block an airborne epidemic is similar to the percentage of a population vaccinated to achieve herd immunity, and depending on the contagiousness of the species, this may be as low as 30%.

In addition to air disinfection and improved delivery of clean air, there are other factors that can aid in the development of healthy buildings. Rugs, carpets, furniture, draperies, and the like can absorb mold spores and regenerate new ones if they become wet. Material selectivity can be one beneficial approach, and other alternatives include the use of self-disinfecting materials, pressurization, and isolation of zones within buildings, including the provision of buffer zones between the inside and outdoor air and the creation of clean inner zones safe from airborne health threats.

Regulating healthy buildings

Implementing changes to building construction on a vast enough scale to control epidemics would require governmental programs. As yet there are virtually no existing standards or laws regarding the aerobiological healthiness of buildings. It is curious to note that airborne chemical contaminants are regulated in many states while airborne pathogens, which cause far more fatalities, are not.

The key to regulation is the development of aerobiological air quality standards. Several organizations and government agencies are involved in the control of disease epidemics, including the Centers for Disease Control and Prevention (CDC), National Institute of Occupational Safety and Health (NIOSH), and World Health Organization (WHO), but none of them is responsible for regulating the living environments in which these diseases are transmitted.

The task of improving air quality in homes, schools, and offices has mostly fallen to independent professional societies. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has had a long and active interest in air quality, healthy buildings, and green buildings, and it is currently developing new programs in these directions. The International Ultraviolet Association (IUVA) is currently drafting a set of guidelines to assist in the design, development, implementation, and testing of UVGI and other air- and surface-cleaning systems.

Aerobiologically green buildings

Green building design is a field geared toward constructing sustainable indoor environments without damaging the environment. Green is clean, as they say, and healthy. The concept of human health is intrinsic to both this field and to aerobiological engineering, and common ground can be found through the exploration of aerobiologically green buildings that implement sustainable technologies for air and surface cleaning.

An example of where these fields overlap is the selection of building materials and furnishings that are both ecofriendly and less likely to contribute to health problems. Solar exposure can provide benefits, since sunlight can destroy mold spores, bacteria, and viruses. Radiant floor heating is an energy-efficient alternative to covering floors with carpets, as are dedicated outside air systems that efficiently control humidity. Although forced air is generally considered a necessity for air cleaning, buildings can also be naturally ventilated using wind energy.

Hygienic protocols

Engineering may go a long way toward the control of airborne diseases, but it may not be sufficient to eradicate them if other transmission routes remain unattenuated. Direct contact may be the dominant route of infection for many pathogens considered airborne, and engineering alone cannot control unhygienic human behavior. People must be educated to protect themselves, and for this purpose we need to define a set of protocols for human hygiene. These might include hand-washing procedures, quarantining contagious individuals, and other commonsense practices that can be taught in elementary school.

Many office workers today are so motivated they come to work during the contagious phase of their infection, placing other workers at risk. Economic losses from lost work and diminished productivity can be staggering. Working at home and in-office quarantine are two options for employers.

What can the individual do?

The most important thing individuals can do to protect themselves against airborne disease is to become educated about sources and transmission routes of airborne pathogens. Proximity to a contagious individual for as little as one hour can cause a secondary infection. Families with children must be especially careful since the youngest children tend to bring home diseases from schools, which are then transmitted to the rest of the family. Frequent hand washing and isolating sick children in bedrooms is one approach to protecting the rest of the family.

In regard to allergens, the home environment can be improved in some simple ways even without air cleaning. Old rugs and carpets that absorb spores can be cleaned, removed, or replaced with alternatives such as linoleum or other growth-resistant materials, and the amount of sunlight entering a home can be increased in various ways.

Misconceptions about disease must be dispelled. For example:

  • The myth that colds and flus come from outdoor air has persisted since the ancient world and is kept alive every time children are told to “bundle up or you’ll catch a cold.”
  • Another popular misconception is that some disease is beneficial, or that disease makes you stronger, but such fuzzy ideas are not grounded in science. Acquired immunity from pathogenic disease is always specific, never providing any general protection, and is often temporary at best. It is true our bodies are filled with friendly bacteria that were once parasites, but if selection for antibiotic resistance is allowed to continue, millions could become victims of unnecessary plagues.

Conclusion

Humanity stepped beyond the hardships of living in the elements by building habitats, but modern human culture and technology have created new contingencies and unexpected problems. It is well within human capabilities to redesign buildings and cities to be resistant to epidemic airborne disease, which is arguably the most serious threat we face today. Human health is a global concern, and achieving it begins with education, redesign of living environments, and large-scale implementation of aerobiological standards. The ultimate goal of these efforts must be disease eradication; all other remedies are merely triage and half-measures that fail to deal directly with the environments that are the root of the airborne disease problem today.

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Newly Discovered Protein Kills Anthrax Bacteria By Exploding Their Cell Walls

Not all biological weapons are created equal. They are separated into categories A through C, category A biological agents being the scariest: They are easy to spread, kill effectively and call for special actions by the pubic health system. One of these worrisome organisms is anthrax, which has already received its fair share of media attention. But work in Vince Fischetti’s laboratory at Rockefeller University suggests that a newly discovered protein could be used to fight anthrax infections and even decontaminate areas in which anthrax spores have been released.


“Anthrax is the most efficient biowarfare agent. Its spores are stable and easy to produce, and once someone inhales them, there is only a 48-hour window when antibiotics can be used,” says Fischetti. “We’ve found a new protein that could both potentially expand that treatment window and be used as a large-scale decontaminant of anthrax spores.” Because anthrax spores are resistant to most of the chemicals that emergency workers rely on to sterilize contaminated areas, a solution based on the protein would be a powerful tool for cleaning up after an anthrax attack.

A bacillus bacterium, a close relative of anthrax, begins to explode after being treated with PlyPH. The PlyPH protein, discovered by Rockefeller scientists, offers several advantages over existing anthrax treatments. (Image courtesy of Rockefeller University)



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All bacteria, anthrax included, have natural predators called bacteriophage. Just as viruses infect people, bacteriophage infect bacteria, reproduce, and then kill their host cell by bursting out to find their next target. The bacteriophage use special proteins, called lysins, to bore holes in the bacteria, causing them to literally explode. Fischetti and colleagues identified one of these lysins, called PlyG, in 2004, and showed that it could be used to help treat animals and humans infected by anthrax. Now, they have identified a second lysin, which they have named PlyPH, with special properties that make it not only a good therapeutic agent, but also useful for large-scale decontamination of areas like buildings and military equipment.


The new protein has several advantages. Most lysins, including PlyG, are only active in a very specific pH range of six to seven, so that they work very effectively in our bloodstream, but may not useful in many environmental conditions. “PlyPH works in an extremely wide pH range, from as low as four to as high as eight,” says Fischetti. “I don’t know of any other lytic enzyme that has such a broad range of activity.”

In addition, PlyPH, like PlyG, is highly specific in terms of the types of bacteria it affects. When Fischetti and colleagues added PlyPH to different bacterial species, only the anthrax bacteria were killed. This is a great benefit over antibiotics, which kill many different kinds of bacteria, including many helpful species. Because it is so specific, the chances of anthrax becoming resistant to PlyPH, as it is to many of the antibiotics currently available to treat it, are extremely low.


“We have never seen bacterial resistance to a lysin,” says Fischetti. “PlyPH and PlyG are probably the most specific lysins we, or anyone, has ever identified — they only kill anthrax and its very close relatives. This feature, and the wide pH range offered by PlyPH, is why we think it could be used as an environmental decontaminant.”


Fischetti hopes to combine PlyPH with a non-toxic aqueous substance developed by a group in California that will germinate any anthrax spores it comes in contact with. As the spores germinate, the PlyPH protein will kill them, usually in a matter of minutes. The combined solution could be used in buildings, on transportation equipment, on clothing, even on skin, providing a safe, easy way to fight the spread of anthrax in the event of a mass release.
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Cilia and Flagella

Cilia and flagella are motile cellular appendages found in most microorganisms and animals, but not in higher plants. In multicellular organisms, cilia function to move a cell or group of cells or to help transport fluid or materials past them. The respiratory tract in humans is lined with cilia that keep inhaled dust, smog, and potentially harmful microorganisms from entering the lungs. Among other tasks, cilia also generate water currents to carry food and oxygen past the gills of clams and transport food through the digestive systems of snails. Flagella are found primarily on gametes, but create the water currents necessary for respiration and circulation in sponges and coelenterates as well. For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms. Protozoans belonging to the phylum Ciliophora are covered with cilia, while flagella are a characteristic of the protozoan group Mastigophora.










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In eukaryotic cells, cilia and flagella contain the motor protein dynein and microtubules, which are composed of linear polymers of globular proteins called tubulin. The core of each of the structures is termed the axoneme and contains two central microtubules that are surrounded by an outer ring of nine doublet microtubules. One full microtubule and one partial microtubule, the latter of which shares a tubule wall with the other microtubule, comprise each doublet microtubule (see Figure 1). Dynein molecules are located around the circumference of the axoneme at regular intervals along its length where they bridge the gaps between adjacent microtubule doublets.

A plasma membrane surrounds the entire axoneme complex, which is attached to the cell at a structure termed the basal body (also known as a kinetosome). Basal bodies maintain the basic outer ring structure of the axoneme, but each of the nine sets of circumferential filaments is composed of three microtubules, rather than a doublet of microtubules. Thus, the basal body is structurally identical to the centrioles that are found in the centrosome located near the nucleus of the cell. In some organisms, such as the unicellular Chlamydomonas, basal bodies are locationally and functionally altered into centrioles and their flagella resorbed before cell division.

Eukaryotic cilia and flagella are generally differentiated based on size and number: cilia are usually shorter and occur together in much greater numbers than flagella, which are often solitary. The structures also exhibit somewhat different types of motion, though in both cases movement is generated by the activation of dynein and the resultant bending of the axoneme. The movement of cilia is often described as whip-like, or compared to the breast stroke in swimming. Adjacent cilia move almost simultaneously (but not quite), so that in groups of cilia, wave-like patterns of motion occur. Flagella, however, exhibit a smooth, independent undulatory type of movement in eukaryotes. Prokaryotic flagella, which have a completely different structure built from the protein flagellin, move in a rotating fashion powered by the basal motor.

Defects in the cilia and flagella of human cells are associated with some notable medical problems. For example, a hereditary condition known as Kartagener's syndrome is caused by problems with the dynein arms that extend between the microtubules present in the axoneme, and is characterized by recurrent respiratory infections related to the inability of cilia in the respiratory tract to clear away bacteria or other materials. The disease also results in male sterility due to the inability of sperm cells to propel themselves via flagella. Damage to respiratory cilia may also be acquired rather than inherited and is most commonly linked to smoking cigarettes. Bronchitis, for instance, is often triggered by a build-up of mucus and tar in the lungs that cannot be properly removed due to smoking-related impairment of cilia.

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Peroxisomes

Microbodies are a diverse group of organelles that are found in the cytoplasm of almost all cells, roughly spherical, and bound by a single membrane. There are several types of microbodies, including lysosomes, but peroxisomes are the most common. All eukaryotes are comprised of one or more cells that contain peroxisomes. The organelles were first discovered by the Belgian scientist Christian de Duve, who also discovered lysosomes.



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Peroxisomes contain a variety of enzymes, which primarily function together to rid the cell of toxic substances, and in particular, hydrogen peroxide (a common byproduct of cellular metabolism). These organelles contain enzymes that convert the hydrogen peroxide to water, rendering the potentially toxic substance safe for release back into the cell. Some types of peroxisomes, such as those in liver cells, detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to molecules of oxygen (a process termed oxidation). Others are more important for their ability to initiate the production of phospholipids, which are typically used in the formation of membranes.


In order to carry out their activities, peroxisomes use significant amounts of oxygen. This characteristic of the organelles would have been extremely important millions of years ago, before cells contained mitochondria, when the Earth's atmosphere first began to amass large amounts of oxygen due to the actions of photosynthetic bacteria. Peroxisomes would have been primarily responsible at that time for detoxifying cells by decreasing their levels of oxygen, which was then poisonous to most forms of life. The organelles would have provided the cellular benefit of carrying out a number of advantageous reactions as well. Later, when mitochondria eventually evolved, peroxisomes became less important (in some ways) to the cell since mitochondria also utilize oxygen to carry out many of the same reactions, but with the additional benefit of generating energy in the form of adenosine triphosphate (ATP) at the same time.



Peroxisomes are similar in appearance to lysosomes, another type of microbody, but the two have very different origins. Lysosomes are generally formed in the Golgi complex, whereas peroxisomes self-replicate. Unlike self-replicating mitochondria, however, peroxisomes do not have their own internal DNA molecules. Consequently, the organelles must import the proteins they need to make copies of themselves from the surrounding cytosol. The importation process of peroxisomes is not yet well understood, but it appears to be heavily dependent upon peroxisomal targeting signals composed of specific amino acid sequences. These signals are thought to interact with receptor proteins present in the cytosol and docking proteins present in the peroxisomal membrane. As more and more proteins are imported into lumen of a peroxisome or are inserted into its membrane, the organelle gets larger and eventually reaches a point where fission takes place, resulting in two daughter peroxisomes. Illustrated in Figure 2 is a fluorescence digital image of an African water mongoose skin fibroblast cell stained with fluorescent probes targeting the nucleus (red), actin cytoskeletal network (blue), and peroxisomes (green).
Since the early 1980s, a number of metabolic disorders have been discovered to be caused by molecular defects in peroxisomes. Two major categories have been described so far. The first category consists of disorders of peroxisome biogenesis in which the organelle fails to develop normally, causing defects in numerous peroxisomal proteins. The second category involves defects of single peroxisomal enzymes. Studies indicate that approximately one in every 20,000 people has some type of a peroxisomal disorder. The most serious of these disorders is Zellweger syndrome, which is characterized by an absence or reduced number of peroxisomes in the cells. Present in patients at birth (congenital), Zellweger syndrome has no cure or effective treatment and usually causes death within the first year of life.
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Lysosomes

The main function of these microscopic organelles is to serve as digestion compartments for cellular materials that have exceeded their lifetime or are otherwise no longer useful. In this regard, the lysosomes recycle the cell's organic material in a process known as autophagy. Lysosomes break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds, which are then transferred back into the cytoplasm as new cell-building materials. To accomplish the tasks associated with digestion, the lysosomes utilize about 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum and modified in the Golgi apparatus. Lysosomes are often budded from the membrane of the Golgi apparatus, but in some cases they develop gradually from late endosomes, which are vesicles that carry materials brought into the cell by a process known as endocytosis.



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Like other microbodies, lysosomes are spherical organelles contained by a single layer membrane, though their size and shape varies to some extent. This membrane protects the rest of the cell from the harsh digestive enzymes contained in the lysosomes, which would otherwise cause significant damage. The cell is further safeguarded from exposure to the biochemical catalysts present in lysosomes by their dependency on an acidic environment. With an average pH of about 4.8, the lysosomal matrix is favorable for enzymatic activity, but the neutral environment of the cytosol renders most of the digestive enzymes inoperative, so even if a lysosome is ruptured, the cell as a whole may remain uninjured. The acidity of the lysosome is maintained with the help of hydrogen ion pumps, and the organelle avoids self-digestion by glucosylation of inner membrane proteins to prevent their degradation.
The discovery of lysosomes involved the use of a centrifuge to separate the various components of cells. In the mid-twentieth century, the Belgian scientist Christian Ren� de Duve was investigating carbohydrate metabolism of liver cells and observed that that the cells released an enzyme called acid phosphatase in larger amounts when they received proportionally greater damage in the centrifuge. To explain this phenomenon, de Duve suggested that the digestive enzyme was encased in some sort of membrane-bound organelle within the cell, which he dubbed the lysosome. After estimating the probable size of the lysosome, he was able to identify the organelle in images produced with an electron microscope.
Lysosomes are found in all animal cells, but are most numerous in disease-fighting cells, such as white blood cells. This is because white blood cells must digest more material than most other types of cells in their quest to battle bacteria, viruses, and other foreign intruders. Several human diseases are caused by lysosome enzyme disorders that interfere with cellular digestion. Tay-Sachs disease, for example, is caused by a genetic defect that prevents the formation of an essential enzyme that breaks down complex lipids called gangliosides. An accumulation of these lipids damages the nervous system, causes mental retardation, and death in early childhood. Also, arthritis inflammation and pain are related to the escape of lysosome enzymes.
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The Cytoskeleton

What is the cytoskeleton?



The cytoskeleton is a network of fibers throughout the cell's cytoplasm that helps the cell maintain its shape and gives support to the cell.


A variety of cellular organelles are held in place by the cytoskeleton.


Fibroblast cells. Fluorescent light micrograph of two fibroblast cells, showing their nuclei (purple) and cytoskeleton. The cytoskeleton is made up of microtubules of the protein tubulin (yellow) and filaments of the protein actin (blue). The cytoskeleton supports the cell's structure, allows the cell to move and assists in the transport of organelles and vesicles within the cell. Fibroblasts are cells forming connective tissue, and are responsible for secreting connective tissue proteins such as collagen.









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What are some distinguishing characteristics?


The cytoskeleton is composed of at least three different types of fibers: microtubules, microfilaments and intermediate filaments.
These types are distinguished by their size with microtubules being the thickest and microfilaments being the thinnest.
  • Microtubules are hollow rods functioning primarily to help support and shape the cell and as "routes" along which organelles can move. Microtubules are typically found in all eukaryotic cells.
  • Microfilaments or actin filaments are solid rods and are active in muscle contraction. Microfilaments are particularly prevalent in muscle cells but similar to microtubules, they are also typically found in all eukaryotic cells.
  • Intermediate filaments can be abundant in many cells and provide support for microfilaments and microtubules by holding them in place.In addition to providing support for the cell, the cytoskeleton is also involved in cellular motility and in moving vesicles within a cell, as well as assisting in the formation of food vacuoles in the cell.
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The Golgi Apparatus

The Golgi apparatus (GA), also called Golgi body or Golgi complex and found universally in both plant and animal cells, is typically comprised of a series of five to eight cup-shaped, membrane-covered sacs called cisternae that look something like a stack of deflated balloons. In some unicellular flagellates, however, as many as 60 cisternae may combine to make up the Golgi apparatus. Similarly, the number of Golgi bodies in a cell varies according to its function. Animal cells generally contain between ten and twenty Golgi stacks per cell, which are linked into a single complex by tubular connections between cisternae. This complex is usually located close to the cell nucleus.





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Due to its relatively large size, the Golgi apparatus was one of the first organelles ever observed. In 1897, an Italian physician named Camillo Golgi, who was investigating the nervous system by using a new staining technique he developed (and which is still sometimes used today; known as Golgi staining or Golgi impregnation), observed in a sample under his light microscope a cellular structure that he termed the internal reticular apparatus. Soon after he publicly announced his discovery in 1898, the structure was named after him, becoming universally known as the Golgi apparatus. Yet, many scientists did not believe that what Golgi observed was a real organelle present in the cell and instead argued that the apparent body was a visual distortion caused by staining. The invention of the electron microscope in the twentieth century finally confirmed that the Golgi apparatus is a cellular organelle.


The Golgi apparatus is often considered the distribution and shipping department for the cell's chemical products. It modifies proteins and lipids (fats) that have been built in the endoplasmic reticulum and prepares them for export outside of the cell or for transport to other locations in the cell. Proteins and lipids built in the smooth and rough endoplasmic reticulum bud off in tiny bubble-like vesicles that move through the cytoplasm until they reach the Golgi complex. The vesicles fuse with the Golgi membranes and release their internally stored molecules into the organelle. Once inside, the compounds are further processed by the Golgi apparatus, which adds molecules or chops tiny pieces off the ends. When completed, the product is extruded from the GA in a vesicle and directed to its final destination inside or outside the cell. The exported products are secretions of proteins or glycoproteins that are part of the cell's function in the organism. Other products are returned to the endoplasmic reticulum or may undergo maturation to become lysosomes.

The modifications to molecules that take place in the Golgi apparatus occur in an orderly fashion. Each Golgi stack has two distinct ends, or faces. The cis face of a Golgi stack is the end of the organelle where substances enter from the endoplasmic reticulum for processing, while the trans face is where they exit in the form of smaller detached vesicles. Consequently, the cis face is found near the endoplasmic reticulum, from whence most of the material it receives comes, and the trans face is positioned near the plasma membrane of the cell, to where many of the substances it modifies are shipped. The chemical make-up of each face is different and the enzymes contained in the lumens (inner open spaces) of the cisternae between the faces are distinctive. Illustrated in Figure 2 is a fluorescence digital image taken through a microscope of the Golgi apparatus (pseudocolored red) in a typical animal cell. Note the close proximity of the Golgi membranes to the cell nucleus.
Proteins, carbohydrates, phospholipids, and other molecules formed in the endoplasmic reticulum are transported to the Golgi apparatus to be biochemically modified during their transition from the cis to the trans poles of the complex. Enzymes present in the Golgi lumen modify the carbohydrate (or sugar) portion of glycoproteins by adding or subtracting individual sugar monomers. In addition, the Golgi apparatus manufactures a variety of macromolecules on its own, including a variety of polysaccharides. The Golgi complex in plant cells produces pectins and other polysaccharides specifically needed by for plant structure and metabolism. The products exported by the Golgi apparatus through the trans face eventually fuse with the plasma membrane of the cell. Among the most important duties of the Golgi apparatus is to sort the wide variety of macromolecules produced by the cell and target them for distribution to their proper location. Specialized molecular identification labels or tags, such as phosphate groups, are added by the Golgi enzymes to aid in this sorting effort.
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The Endoplasmic Reticulum

What is the endoplasmic reticulum?


The endoplasmic reticulum (ER) is a network of flattened sacs and branching tubules that extends throughout the cytoplasm in plant and animal cells. These sacs and tubules are all interconnected by a single continuous membrane so that the organelle has only one large, highly convoluted and complexly arranged lumen (internal space). Usually referred to as the endoplasmic reticulum cisternal space, the lumen of the organelle often takes up more than 10 percent of the total volume of a cell. The endoplasmic reticulum membrane allows molecules to be selectively transferred between the lumen and the cytoplasm, and since it is connected to the double-layered nuclear envelope, it further provides a pipeline between the nucleus and the cytoplasm.






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The endoplasmic reticulum manufactures, processes, and transports a wide variety of biochemical compounds for use inside and outside of the cell. Consequently, many of the proteins found in the cisternal space of the endoplasmic reticulum lumen are there only transiently as they pass on their way to other locations. Other proteins, however, are targeted to constantly remain in the lumen and are known as endoplasmic reticulum resident proteins. These special proteins, which are necessary for the endoplasmic reticulum to carry out its normal functions, contain a specialized retention signal consisting of a specific sequence of amino acids that enables them to be retained by the organelle. An example of an important endoplasmic reticulum resident protein is the chaperone protein known as BiP (formally: the chaperone immunoglobulin-binding protein), which identifies other proteins that have been improperly built or processed and keeps them from being sent to their final destinations.


There are two basic kinds of endoplasmic reticulum morphologies: rough and smooth. The surface of rough endoplasmic reticulum is covered with ribosomes, giving it a bumpy appearance when viewed through the microscope. This type of endoplasmic reticulum is involved mainly with the production and processing of proteins that will be exported, or secreted, from the cell. The ribosomes assemble amino acids into protein units, which are transported into the rough endoplasmic reticulum for further processing. These proteins may be either transmembrane proteins, which become embedded in the membrane of the endoplasmic reticulum, or water-soluble proteins, which are able to pass completely through the membrane into the lumen. Those that reach the inside of the endoplasmic reticulum are folded into the correct three-dimensional conformation, as a flattened cardboard box might be opened up and folded into its proper shape in order to become a useful container. Chemicals, such as carbohydrates or sugars, are added, then the endoplasmic reticulum either transports the completed proteins to areas of the cell where they are needed, or they are sent to the Golgi apparatus for further processing and modification.

Most proteins exported from the endoplasmic reticulum exit the organelle in vesicles budded from the smooth portion, which has a more even appearance than rough endoplasmic reticulum when viewed through the electron microscope because of the lack of ribosomes. The smooth endoplasmic reticulum in most cells is much less extensive than the rough endoplasmic reticulum and is sometimes alternatively termed transitional. Smooth endoplasmic reticulum is chiefly involved, however, with the production of lipids (fats), building blocks for carbohydrate metabolism, and the detoxification of drugs and poisons. Therefore, in some specialized cells, such as those that are occupied chiefly in lipid and carbohydrate metabolism (brain and muscle) or detoxification (liver), the smooth endoplasmic reticulum is much more extensive and is crucial to cellular function. Smooth endoplasmic reticulum also plays a role in various cellular activities through its storage of calcium and involvement in calcium metabolism. In muscle cells, smooth endoplasmic reticulum releases calcium to trigger muscle contractions. Presented in Figure 2 is a fluorescence digital image taken through the microscope of the endoplasmic reticulum network in a bovine (cow) pulmonary artery endothelial cell grown in culture.

It is the rough endoplasmic reticulum that is directly continuous with the nuclear envelope (as illustrated in Figure 1), which is also studded with ribosomes, and the two organelles are thought to have evolved simultaneously in ancient cells. Due to their physical membranous connection, the lumen of the endoplasmic reticulum and the space between the layers of the nuclear envelope comprise a single compartment. Accordingly, the nucleus has direct access to proteins (many of which are produced by the ribosomes upon its surface) and other materials present in the endoplasmic reticulum lumen, so that transport vesicles are not needed to obtain them. The close association between the endoplasmic reticulum and the nucleus also enables the organelles to share information in a very efficient manner. For instance, if the endoplasmic reticulum begins to undergo functional problems and unfolded proteins accumulate within the organelle, which can be extremely hazardous to the cell, the organelle quickly sends a signal to the nucleus (as well as to the cytoplasm). The nucleus responds by slowing ribosomal translation through a several-step process, thereby giving the endoplasmic reticulum extra time to catch up on its protein folding, thus maintaining cellular health.

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Ribosomes

In Journey into the Cell, we looked at the structure of the two major types of cells: prokaryotic and eukaryotic cells. Now we turn our attention to the protein assemblers of a eukaryotic cell, the ribosomes.

Ribosomes are organelles that consist of RNA an proteins. They are responsible for assembling the proteins of the cell. Depending on the protein production level of a particular cell, ribosomes may number in the millions.

Distinguishing Characteristics:

Ribosomes are typically composed of two subunits: a large subunit and a small subunit. Ribosomal subunits are synthesized by the nucleolus. These two units join together when the ribosome attaches to messenger RNA to produce a protein in the cytoplasm.


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Location in the Cell:

There are two places that ribosomes usually exist in the cell: suspended in the cytosol and bound to the endoplasmic reticulum. These ribosomes are called free ribosomes and bound ribosomes respectively. In both cases, the ribosomes usually form aggregates called polysomes.

Free ribosomes usually make proteins that will function in the cytosol, while bound ribosomes usually make proteins that are exported or included in the cell's membranes. Interestingly enough, free ribosomes and bound ribosomes are interchangeable and the cell can change their numbers according to metabolic needs.

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Journey Into The Cell: Mitochondria

In Journey into the Cell, we looked at the structure of the two major types of cells: prokaryotic (pro-) and eukaryotic (eu-) cells. Now we turn our attention to the "power houses" of a eukaryotic cell, the mitochondria.

What are mitochondria?

Mitochondria are the cell's power producers. They convert energy into forms that are usable by the cell. They are the sites of cellular respiration which ultimately generates fuel for the cell's activities.

Mitochondria Structural Features


Mitochondrion


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What are their distinguishing characteristics?

Mitochondria are bounded by a double membrane. Each of these membranes is a phospholipid bilayer with embedded proteins. The outermost membrane is smooth while the inner membrane has many folds. These folds are called cristae. The folds enhance the "productivity" of cellular respiration by increasing the available surface area.


Muscle Cell Mitochondria, Copyright Dennis Kunkel

The double membranes divide the mitochondrion into two distinct parts: the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow part between the two membranes while the mitochondrial matrix is the part enclosed by the innermost membrane. Several of the steps in cellular respiration occur in the matrix due to its high concentration of enzymes.

Mitochondrion with matrix

Mitochondria are semiautonomous (semi- auto-) in that they can divide and grow to make more of themselves. They also have their own DNA and ribosomes.
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Senin, 26 Januari 2009

Journey into the Cell: The Nucleus

The Nucleus:

In Journey into the Cell, we looked at the structure of the two major types of cells: prokaryotic and eukaryotic cells. Now we turn our attention to the "nerve center" of a eukaryotic cell, the nucleus.

The nucleus is a membrane bound structure that contains the cell's hereditary information and controls the cell's growth and reproduction.
It is commonly the most prominent organelle in the cell.

Nucleus

Nucleus with Nuclear Pores

Liver Cell Nucleus with Dark Nucleolus
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Distinguishing Characteristics:
The nucleus is bounded by a double membrane called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm.

The envelope helps to maintain the shape of the nucleus and assists in regulating the flow of molecules into and out of the nucleus through nuclear pores.

Chromosomes are also located in the nucleus.
When a cell is "resting" i.e. not dividing, the chromosomes are organized into long entangled structures called chromatin and not into individual chromosomes as we typically think of them.

The Nucleolus:
The nucleus also contains the nucleolus which helps to synthesize ribosomes.
The nucleolus contains nucleolar organizers which are parts of chromosomes with the genes for ribosome synthesis on them. Copious amounts of RNA and proteins can be found in the nucleolus as well.

The nucleus controls the synthesis of proteins in the cytoplasm through the use of messenger RNA. Messenger RNA is produced in the nucleolus of the cell and travels to the cytoplasm through the pores of the nuclear envelope.
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Reproduction in Animals: Sexual Reproduction

Individual organisms come and go, but, to a certain extent, organisms "transcend" time through reproducing offspring.


Now let's take a look at sexual reproduction.


Human Ovum(Egg) and SpermCopyright Dennis Kunkel



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Sexual Reproduction


In sexual reproduction, two individuals produce offspring that have genetic characteristics from both parents. Sexual reproduction introduces new gene combinations in a population.

Gametes

In animals, sexual reproduction encompasses the fusion of two distinct gametes to form a zygote. Gametes are produced by a type of cell division called meiosis.

The gametes are haploid (containing only one set of chromosomes) while the zygote is diploid (containing two sets of chromosomes).

In most cases, the male gamete, called the spermatozoan, is relatively motile and usually has a flagellum. On the other hand, the female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete.

Types of Fertilization

There are two mechanisms by which fertilization can take place.

The first is external (the eggs are fertilized outside of the body); the second is internal (the eggs are fertilized within the female reproductive tract).

Patterns and Cycles

Reproduction is not a continuous activity and is subject to certain patterns and cycles. Oftentimes these patterns and cycles may be linked to environmental conditions which allow organisms to reproduce effectively.

For example, many animals have estrous cycles that occur during certain parts of the year so that offspring can typically be born under favorable conditions.

Likewise, these cycles and patterns can be controlled by hormonal cues as well as other seasonal cues like rainfall.

All of these cycles and patterns allow organisms to manage the relative expenditure of energy for reproduction and maximize the chances of survival for the resulting offspring.

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Biology Study Tips from Mickey'07....:)

Studying for biology can seem overwhelming, but it doesn't have to be. If you follow a few simple steps, studying for biology will be less stressful and more enjoyable. I've compiled a list of several helpful biology study tips for biology students. Whether you're in middle school, high school, or college, these tips are bound to produce results!!...:)

Biology Study Tips

Bio-Study Tip 1
Always read the lecture material before the classroom lecture. I know, I know--you don't have time, but believe me, it makes an immense difference.
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Bio-Study Tip 2


Biology, like most sciences, is hands-on. Most of us learn best when we are actively participating in a "topic." So make sure to pay attention in lab sessions and actually perform the experiments. Remember, you won't be graded on your lab partner's ability to perform an experiment, but your own.
Bio-Study Tip 3
Sit in the front of the class. Simple, yet effective. College students, pay close attention. You'll need recommendations one day, so make sure your professor knows you by name and you aren't 1 face in 400.
Bio-Study Tip 4
Compare notes with a friend. Since much of biology tends to be abstract, have a "note buddy." Each day after class compare notes with your buddy and fill in any gaps. Two heads are better than one!
Bio-Study Tip 5
Use the "lull" period between classes to immediately review the biology notes you have just taken.
Bio-Study Tip 6
Don't cram! As a rule, you should start studying for biology exams a minimum of two weeks prior to the exam.
Bio-Study Tip 7
This tip is very important -- stay awake in class. I've observed too many people snoozing (even snoring!) in the middle of class. Osmosis may work for water absorption, but it won't work when it comes time for biology exams.
Bio-Study Tip 8
Find some useful resources to help you when you study after class. Here are a few resources that I would suggest to help make learning biology interesting and fun...:)
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Endangered Species

What are Endangered Species?

Rare, endangered, or threatened plants and animals are elements of our natural heritage that are declining rapidly or are on the verge of vanishing. They are plants and animals that exist in small numbers that may be lost forever if we do not take quick action to stop their decline. If we cherish these species, like we do other rare and beautiful objects, these living organisms become treasures of the highest magnitude.
Why Preserve Endangered Plants and Animals?
Preservation of plants and animals is important, not only because many of these species are beautiful, or can provide economic benefits for us in the future, but because they already provide us many valuable services. These organisms clean air, regulate our weather and water conditions, provide control for crop pests and diseases, and offer a vast genetic "library" from which we can withdraw many useful items.
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Extinction of a species could potentially mean the loss of a cure for cancer, a new antibiotic drug, or a disease-resistant strain of wheat. Each living plant or animal may have values yet undiscovered. Scientists estimate there are thirty to forty million species on earth. Many of these species are represented by dozens of genetically distinct populations. We know very little about most species; less than two million are even described. Oftentimes, we do not even know when a plant or animal becomes extinct. Game animals and a few insects are watched and studied. Other species need attention too. Perhaps in them may be found a cure for the common cold or a new organism that will prevent millions of dollars of loss to farmers in their constant fight against crop diseases.

There are many examples of a species' value to society. An antibiotic was discovered in the soils of the threatened New Jersey Pine Barrens Natural Area. A species of perennial corn was found in Mexico; it is resistant to several diseases of corn. An insect was discovered that when frightened produces an excellent insect-repelling chemical.

Why Have Species Become Endangered?
  • Habitat Loss
Loss of habitat or the "native home" of a plant or animal is usually the most important cause of endangerment. Nearly all plants and animals require food, water, and shelter to survive, just as humans do. Humans are highly adaptable, however, and can produce or gather a wide variety of foods, store water, and create their own shelter from raw material or carry it on their backs in the form of clothing or tents. Other organisms cannot.
Some plants and animals are highly specialized in their habitat requirements. A specialized animal in North Dakota is the piping plover, a small shorebird which nests only on bare sand or gravel on islands of rivers or shorelines of alkali lakes. Such animals are much more likely to become endangered through habitat loss than a generalist like the mourning dove, which nests successfully on the ground or in trees in the country or city.
Some animals are dependent on more than one habitat type and need a variety of habitats near each other to survive. For example, many waterfowl depend on upland habitats for nest sites, and nearby wetlands for food supplies for themselves and their broods.
It must be emphasized that habitat does not have to be completely eliminated to lose its usefulness to an organism. For example, the removal of dead trees from a forest may leave the forest relatively intact, but eliminate certain woodpeckers that depend on dead trees for nest cavities.
The most serious habitat loss totally changes the habitat and renders it unfit for most of its original resident organisms. In some areas, the greatest changes come from plowing native grasslands, draining wetlands, and constructing flood-control reservoirs.
  • Exploitation
Direct exploitation of many animals and some plants took place before conservation laws were enacted. In some places, exploitation was usually for human food or furs. Some animals, such as Audubon's sheep, were hunted to extinction. Others such as the grizzly bear, maintain remnant populations elsewhere.
  • Disturbance
The frequent presence of man and his machines may cause some animals to abandon an area, even if the habitat is not harmed. Some large raptors, like the golden eagle, fall into this category. Disturbance during the critical nesting period is especially harmful. Disturbance combined with exploitation is even worse.
What Are The Solutions?

Habitat protection is the key to protecting our rare, threatened, and endangered species. A species cannot survive without a home. Our first priority in protecting a species is to ensure its habitat remains intact.

Habitat protection can be done in a variety of ways. Before we can protect a plant's or animal's habitat, we need to know where this habitat is found. The first step, then is to identify where these vanishing species are found. This is being accomplished today by state and federal agencies and conservation organizations.

Second to identification is planning for protection and management. How can the species and its habitat be best protected, and once protected, how can we make sure the species continues healthy in its protected home? Each species and habitat is different and must be planned on a case-by-case basis. A few protection and management efforts have proven effective for several species, however.

Legislation was passed to protect the most endangered species in the United States. These special species cannot be destroyed nor can their habitat be eliminated. They are marked in the endangered species list by an *. Several federal and state agencies are beginning to manage threatened and endangered species on public lands. Recognition of private landowners who have voluntarily agreed to protect rare plants and animals is underway. All these efforts need to continue and be expanded to keep our natural heritage alive.

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The African House Snake

Red phase House Snake from Botswana


hatching albino "Zululand" House Snakes


hatchling "Zululand" House Snakes


red phase male

cinnamon phase female about to swallow a mouse

L. inornatus


Lamprophis aurorae

L. fuliginosus


L. aurorae



Striped House Snake


Albino House Snake



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normal and albino "Zululand" House Snakes


juvenile brown phase African House Snake



juvenile green phase African House Snake

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Insect Biology and Ecology

Insects are the dominant life-form on earth. Millions may exist in a single acre of land. About one million species have been described, and there may be as many as ten times that many yet to be identified. Of all creatures on earth, insects are the main consumers of plants. They also play a major role in the breakdown of plant and animal material and constitute a major food source for many other animals.

Insects are extraordinarily adaptable creatures, having evolved to live successfully in most environments on earth, including deserts and the Antarctic. The only place where insects are not commonly found is the oceans. If they are not physically equipped to live in a stressful environment, insects have adopted behaviors to avoid such stresses. Insects possess an amazing diversity in size, form, and behavior.

It is believed that insects are so successful because they have a protective shell or exoskeleton, they are small, and they can fly. Their small size and ability to fly permits escape from enemies and dispersal to new environments. Because they are small they require only small amounts of food and can exist in very small niches or spaces. In addition, insects can produce large numbers of offspring relatively quickly. Insect populations also possess considerable genetic diversity and a great potential for adaptation to different or changing environments. This makes them an especially formidable pest of crops, able to adapt to new plant varieties as they are developed or rapidly becoming resistant to insecticides.

Insects are directly beneficial to humans by producing honey, silk, wax, and other products. Indirectly, they are important as pollinators of crops, natural enemies of pests, scavengers, and food for other creatures. At the same time, insects are major pests of humans and domesticated animals because they destroy crops and vector diseases. In reality, less than one percent of insect species are pests, and only a few hundred of these are consistently a problem. In the context of agriculture, an insect is a pest if its presence or damage results in an economically important loss.

The adage "know your enemy" is especially appropriate when it comes to insect pests. The more we know about their biology and behavior, including their natural enemies, the more likely we will be able to manage them effectively.



Left: Hippodamia glacialis, a predator of aphids. J.Ogrodnick
Center: Cotesia congregata, a parasitoid of caterpillars. K.Kester
Right: The larvae of Sphenoptera jugoslavica feed on the roots of the plant pest diffuse knapweed. R.Richard
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Insect Anatomy


Insects and closely related organisms have a lightweight, but strong exterior skeleton (exoskeleton) or integument. Their muscles and organs are on the inside. This multi-layered exoskeleton protects the insect from the environment and natural enemies. The exoskeleton also has many sense organs for detecting light, pressure, sound, temperature, wind, and odor. Sense organs may be located almost anywhere on the insect body, not just on the head.

Insects have three body regions: head, thorax, and abdomen. The head functions mainly for food and sensory intake and information processing. Insect mouthparts have evolved for chewing (beetles, caterpillars), piercing-sucking (aphids, bugs), sponging (flies), siphoning (moths), rasping-sucking (thrips), cutting-sponging (biting flies), and chewing-lapping (wasps). The thorax provides structural support for the legs (three pairs) and, if present, for one or two pairs of wings. The legs may be adapted for running, grasping, digging, or swimming. The abdomen functions in digestion and reproduction.

The internal anatomy of insects is characterized by an open circulatory system, a multitude of breathing tubes, and a three-chambered digestive system. With the exception of a heart and an aorta, there are few blood vessels; insect blood simply flows around inside the body cavity. Air enters the insect through a few openings (spiracles) in the exoskeleton, and makes its way to all areas of need by way of branching tubes, which permeate the body. The insect digestive system is long and tube-like, often divided into three sections, each with a different function. The insect nervous system transports and processes information received from the sense organs (sight, smell, taste, hearing, and touch). The brain, located in the head, processes information, but some information is also processed at nerve centers elsewhere in the body.

Knowledge about the structure and function of the insect exoskeleton has proven critical in developing insecticide formulations that are able to penetrate this multi-layered protective covering. Studies of insect communication have led to the discovery of chemical compounds used by insects to locate each other or host plants, and many of these have now been identified and produced synthetically. For example, pheromones are very specific compounds released by insects to attract others of the same species, such as for mating. Synthetic pheromones are now widely used to bait insect traps for detecting the presence of a pest, to determine its abundance, or for control. Control may involve the use of many traps to "trap out" the pest or the pheromones can be dispersed throughout the crop to "confuse" insects, making it more difficult for them to find a mate.

As simple as it may seem, knowing what type of mouthparts an insect has can be very important in deciding on a management tactic. For example, insects with chewing mouthparts can be selectively controlled by some insecticides that are applied directly to plant surfaces and are only effective if ingested; contact alone will not result in death of the insect. Consequently, natural enemies that feed on other insects, but not the crop plant, will not be harmed.

Since insects obtain oxygen through their spiracles, plugging these openings causes death. That is how insecticidal oils control insects. Components of the microbial insecticide Bacillus thuringiensis enter the digestive system and break down the gut lining. Knowledge of the nervous system of insects has led to the development of several types of insecticides designed to disrupt normal nerve function. Some of these are effective simply by contacting the insect.

Insect Reproduction

Most species of insects have males and females that mate and reproduce sexually. In some cases, males are rare or present only at certain times of the year. In the absence of males, females of some species may still reproduce. This is common, particularly among aphids. In many species of wasps, unfertilized eggs become males while fertilized eggs become females. In a few species, females produce only females.

A single embryo typically develops within each egg, except in the case of polyembryony, where hundreds of embryos may develop per egg. Insects may reproduce by laying eggs or, in some species, the eggs may hatch within the female which shortly thereafter deposits young. In another strategy common to aphids, the eggs hatch within the female and the immatures remain within the female for some time before birth.

Insect Growth and Development (Metamorphosis)


Insects typically pass through four distinct life stages: egg, larva or nymph, pupa, and adult. Eggs are laid singly or in masses, in or on plant tissue or another insect. The embryo within the egg develops, and eventually a larva or nymph emerges from the egg. There are generally several larval or nymphal stages (instars), each progressively larger and requiring a molt, or shed of the outer skin, between each stage. Most weight gain (sometimes > 90%) occurs during the last one or two instars. In general, neither eggs, pupae, nor adults grow in size; all growth occurs during the larval or nymphal stages.

The two types of metamorphosis typical of insect pests and natural enemies are gradual (egg > nymph > adult) and complete (egg > larva > pupa > adult). In gradual metamorphosis, the nymphal stages resemble the adult except that they lack wings and the nymphs may be colored differently than the adults. Nymphs and adults usually occupy similar habitats and have similar hosts. Gradual metamorphosis is typical of true bugs and grasshoppers; complete metamorphosis is typical of beetles, flies, moths, and wasps. The immatures of these latter species do not resemble the adults, may occupy different habitats, and feed on different hosts. Some moth and wasp larvae weave a silken shell (cocoon) to protect the pupal stage; in flies, the last larval skin becomes a puparium that protects the pupal stage.

Insects are cold-blooded, so that the rate at which they develop is mostly dependent on the temperature of their environment. Cooler temperatures result in slowed growth; higher temperatures speed up the growth process. If a season is hot, more generations may occur than during a cool season.

A better understanding of how insects grow and develop has contributed greatly to their management. For example, knowledge of the hormonal control of insect metamorphosis led to the development of a new class of insecticides called insect growth regulators (IGR). The insect growth regulators are very selective in the insects they affect. Based on information about insect growth rates relative to temperature, computer models can be used to predict when insects will be most abundant during the growing season and, consequently, when crops are most at risk.
Insect Classification and Identification

It is necessary to classify insects so that we can organize what we know about them and determine their relationships with other insects. For example, all members of a particular species will feed on similar foods, have similar developmental characteristics, and exist in similar environments. Most often, insect species are classified based on similarities in appearance (morphology). The flies, for example, can be distinguished and classified separately from all other winged insects because they have only one pair of wings. The hierarchy used to classify the diamondback moth, a worldwide pest of crucifers, is as follows:
  • Phylum - Arthropoda
  • Class - Insecta
  • Order - Lepidoptera
  • Family - Plutellidae
  • Genus - Plutella
  • species - Plutella xylostella

This universal method is used to prevent confusion among geographic regions of the world. Consequently, Plutella xylostella refers to the same insect species in the United States as it does in Asia or anywhere else in the world. Common names, however, can vary from one location to another.


Ecology is the study of the interrelationships between organisms and their environment. An insect's environment may be described by physical factors such as temperature, wind, humidity, light, and biological factors such as other members of the species, food sources, natural enemies, and competitors (organisms using the same space or food source). An understanding or at least an appreciation of these physical and biological (ecological) factors and how they relate to insect diversity, activity (timing of insect appearance or phenology), and abundance is critical for successful pest management.

Some insect species have a single generation per season (univoltine), while others may have several (multivoltine). The striped cucumber beetle, for example, overwinters as an adult, emerges in the spring, and lays eggs near the roots of young cucurbit plants. The eggs hatch, producing larvae that emerge as adults later in the summer. These adults overwinter to start the cycle again the next year. In contrast, egg parasitoids like Trichogramma overwinter as immatures within the egg of their host. During the summer they may have several generations.
Insects adapt to many types of environmental conditions during their seasonal cycle. To survive the harsh winters, cucumber beetles enter a dormant state. While in this dormant state, metabolic activity is minimal and no reproduction or growth occurs. Dormancy can also occur at other times of the year when conditions may be stressful for the insect.

It is often better to consider insects as populations rather than individuals, especially within the context of an agroecosystem. Populations have attributes such as density (number per unit area), age distribution (proportion in each life stage), and birth and death rates. Understanding the attributes of a pest population is important for good management. Knowing the age distribution of a pest population may indicate the potential for crop damage. For example, if most of the striped cucumber beetles are immatures, direct damage to the above ground portions of the plant is unlikely. Similarly, if the density of a pest is known and can be related to the potential for damage, an action may be required to protect the crop. Information about death rates due to natural enemies can be very important. Natural enemies do nothing but reduce pest populations and understanding and quantifying their impact is important to effective pest management. This is all the more reason to conserve their numbers.
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