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In this lesson, we will review the basics of the life sciences. Specifically, we are going to cover how we organize and categorize life. We will be able to identify all the major groups of living things and describe how they are related to each other.
Although it may not always seem clear, taxonomy is designed to facilitate understanding by organizing the vast array of living things into meaningful, and hopefully manageable, groups. This is no small task and the job is far from complete as many species remain unidentified.
As discoveries are made, different systems of classifications develop. Depending on the system used, there are five or more kingdoms of living organisms with seven levels of classification and often several sublevels within each. At each level, organisms get more similar until an individual species is reached. Here’s one way that will probably look familiar:
Using the five kingdom system, these classifications include Animal, Plant, Fungi, Protista, and Monera. Additional kingdoms arose when some systems split the Monerans into the true bacteria and the blue-green or cyanobacteria. Other systems split Protista into the photosynthetic and the mobile Protists.
Biologist Carl Woese proposed two new ways of organizing life—either by six kingdoms or three domains.
The system of taxonomy and classification used today is based in part on the work of Carolus Linnaeus. Linnaeus’s system uses the genus and species name of an organism as its scientific name. These names are in Latin or are Latinized because that language is non-evolving and at the time was well known to most scientists. The purpose of this naming system is to avoid confusion that can occur when using more common names. For example, the common names puma, mountain lion, and cougar are all used to describe Felis concolor, a large predator native to the United States.
In taxonomy, living things are categorized at various levels and are classified by numerous physical and genetic traits as well as fossil evidence. Biologists often disagree, however, about whether something is a subspecies or a separate species. Classification is essential in determining where new species fit and understanding evolutionary relationships among living organisms. These types of decisions are important especially in areas such as conservation where endangered or threatened status may depend on whether an organism represents a separate species.
We’ll discuss the fossil evidence for evolution in a later section, but based on phylogenetic relationships from the fossil record, the following diagram illustrates how the kingdoms of living organisms are related.
This NASA image shows the division among bacteria, archaea, and eucarya.
The first organisms were single-celled prokaryotes that arose almost four billion years ago. Protists arose when oxygen began to build up in the atmosphere around 2.5 billion years ago. This group has both animal- and plantlike members giving rise to the fungi, plants, and animals.
The use of DNA analysis in taxonomy is most useful in
B is correct and represents the greatest use of DNA evidence. Though it can reveal new species, DNA has also shown separate species to actually be the same genetically, thus clarifying relationships among species.
The major life processes, such as respiration, obtaining energy, excreting waste, and reproduction, all occur at a cellular level. As animals, we take in oxygen and excrete carbon dioxide because our cells need to do this to survive. Hence, cells are known as the basic unit of life as they are capable of the complex biochemical processes that enable organisms to maintain themselves, grow, develop, and reproduce. Homeostasis is the maintenance of status quo, or proper levels of energy, water, waste, temperature, hormones, nutrients, etc., in order to function optimally.
The organelles within cells perform these many functions, allowing each cell to act as a unit and the body as a whole to maintain homeostasis. Click here for an overview of the different parts of an animal cell.
The organelles in both plant and animal cells share the same functions, though plant cells have specific organelles used in photosynthesis and in support that animal cells lack. Cell walls are rigid outer layers comprised of cellulose and lignin, which provide a rigid support for the cell and plant.
Here’s an obvious question: Since cells are capable of all living processes, why are there so many complex organisms? Why aren’t we just giant cells? The main reason is that as cells grow, the volume increases faster than the surface membrane, and the membrane is unable to supply food or remove wastes sufficiently to support the cell; thus cells cooperate in order for larger organisms to develop.
Tissues are groups of cells of any given type (muscle, nerve, etc.) that form the organs of living plants and animals. Organs have specific functions yet generally work in conjunction with other organs to comprise body systems such as the cardiovascular system, the nervous system, the digestive system, the immune system, etc. Very simple organisms may not have as complex a structure or may have more primitive systems to carry out life processes.
Click here for a general review of the body’s structure.
Plants have the following characteristics:
Plants are divided into two groups: nonvascular and vascular. Nonvascular plants, which include mosses and liverworts, have simple conducting tissue that does not differentiate into roots, stems, and leaves. Vascular plants have tissue that is differentiated into roots, stems, and leaves to facilitate the movement of water and other materials throughout the body of the plant.
Vascular plants are divided into seedless plants and seed plants. Seedless live close to water. Over time, as plants developed further away from water sources, they developed seeds. Seeds permit an embryo to survive long periods of time in unfavorable conditions and to disperse an embryo from its parent plant.
Stems vary in size and shape from one plant species to another but all stems have two basic functions: holding leaves up in the sunlight and conducting various substances between roots and leaves. In addition, some stems store water and nutrients.
The leaves of a plant are the world’s most important manufacturers of food. Most leaves have a basic structure of a large, flattened surface called the blade that is attached to the stem. Leaves are covered by a layer of tough epidermal cells and a waxy cuticle. Most leaf tissue is composed of specialized cells called mesophyll. This tissue contains chloroplasts and performs most of the plant’s photosynthesis. Some plants have modified leaves specialized for protection, water conservation, climbing, and reproduction. Remember that most plant parts are basically modified leaves.
Many single-celled organisms use only anaerobic respiration, where carbohydrates such as glucose are split apart and a small amount of ATP is released. Larger organisms, however, require more massive inputs of ATP and depend on aerobic respiration and hence oxygen. Let’s look at the process in detail and see where energy is put into and received from these three steps.
In the cytoplasm, glucose molecules are broken apart using the energy from two ATP molecules in the process called glycolysis. Glycolysis releases four ATP for a net of two molecules.
In the second stage, the glucose molecule has been broken apart and has formed two pyruvate and two NADH molecules. In the mitochondria of the cell, the Krebs Cycle converts the pyruvate into two ATP and six NADH while releasing carbon dioxide. All of the NADH then is used in the third stage, the electron transport stage.
The electron transport chain utilizes the oxygen we breathe to act as an electron acceptor and drive the production of thirty-two ATP molecules.
Here’s the important part: For a total investment of two ATP initially along with a supply of oxygen and removal of carbon dioxide, our cells can produce a total of thirty-six ATP molecules to operate the various processes inside the cell.
Plant cells also use aerobic respiration to produce ATP. Unlike animals, plants can utilize energy from the sun to drive photosynthesis and produce ATP as well as glucose molecules, which are then used in respiration to drive cell processes. It is by this pathway that energy from the sun is made available to living organisms.
12H2O + 6CO2 → 6O2 + C6H12O2 + 6H2O
Thus for every six molecules of CO2 and twelve of H2O taken in by the plant, a molecule of glucose can be produced. This process removes carbon dioxide (CO2) from the air, releases oxygen, and is what has made almost all forms of life besides very simple single-celled organisms possible. There are two major sets of reactions in photosynthesis: light dependent and light independent.
Light-trapping pigments such as chlorophyll exist in the chloroplasts. Light energy is used to release electrons from the chlorophyll molecule. These electrons will pass through at least one electron transport chain (passed from molecule to molecule).
Aerobic respiration occurs mainly in
The correct answer is D. Remember photosynthetic organisms can make their own food, but they still must use this food for aerobic respiration to drive cell processes.
Cell division is essential for body growth, repair, and maintenance, as well as reproduction. The most critical aspect of cell division is the copying of the genetic information in the nucleus to ensure each new cell has the genetic information necessary to operate.
Mitosis is the copying and division of the genetic material in the nucleus and is followed by cell division; it occurs in the somatic (body) cells.
Meiosis is a specialized form of mitosis, which occurs in the reproductive tissues only. The basic steps are the same but with an extra division. In human beings (we have twenty-three paired chromosomes), a gamete (reproductive or sex cell) will have only twenty-three total chromosomes. Fertilization will restore the original pair by combining an equal number of chromosomes from each parent.
Click here to see an image of mitosis.
The process of meiosis is essentially the same as mitosis except that the other major phases are repeated resulting in two divisions of the cell.
Which of the following statements is true of mitosis?
Answer C is correct. Mitosis only occurs in somatic, or body, cells. Choice A is incorrect, as anaphase is the second-to-last stage. Choice B is incorrect—it’s actually meiosis that contains one more step than mitosis. Choice D describes aerobic respiration and photosynthesis, not mitosis.
Although we do get sick, our bodies fight a continuous battle against various infections and diseases from many sources. The two most common include bacteria and viruses.
Viruses are capable of penetrating living cells in order to use the cell’s biochemical resources to reproduce themselves.
Fungi also can invade as can some protists such as giardia.
Our bodies mount several types of responses to infection. Nonspecific defenses are those that respond no matter which type of infection we face, whereas specific defenses recognize specific invading entities.
Our biggest nonspecific defense is our biggest organ: our skin. Intact skin is one of the most important and initial defense mechanisms; it allows almost no bacteria to pass through it. Excretions from exocrine glands in the skin provide chemical defenses against invading organisms. Ciliated mucous membranes act to literally sweep airborne invaders from the respiratory tract. The gastrointestinal system has digestive acids capable of destroying many types of pathogens that enter the stomach with food. The intestinal tract also contains necessary bacteria that make it more difficult for invading bacteria to obtain food and survive.
If you were to cut your skin and allow pathogens to bypass the external defenses, several internal defenses are ready. For example:
These are white blood cells that can ingest and kill foreign cells or pathogens in the body through phagocytosis. By recognizing antigens on invading cells as foreign, phagocytes and other immune response cells attack these cells while ignoring body cells.
When these immune cells are unable to tell the difference between native and foreign cells, autoimmune diseases, such as rheumatoid arthritis, can occur. This cell recognition and attack mechanism must be suppressed when organ transplants are made.
These circulate in the bloodstream and help kill invading pathogens by promoting lysis, or disintegration, of the plasma membrane or by attracting phagocytes to a site of injury or invasion. The inflammatory response is the result of the many actions the body takes to battle an invasion.
When the nonspecific defenses are not enough to win out over an infection, additional white blood cells (B-cells, T-cells, and natural killer cells) may be called upon. These cells are the foundation of the body’s immune system and can launch very targeted attacks on invading organisms, as well as develop immunity from future infections.
When the specific immune response is required, two major types occur:
Lymphocytes called B cells are produced in the antibodies for a specific type of antigen. When a given B cell recognizes an invader with the antigens capable of binding to the B cell’s antibodies, it will bind and will essentially “tag” the invading cell for destruction by means such as phagocytosis or lysis.
T cells called “killer T cells” develop in the bone marrow and move eventually into the thymus gland to produce more of their own kind. These cells, along with what are known as NK or natural killer cells, directly attack and kill cells that are already infected—mostly by viruses. These cells must be controlled when organs are donated.
Helper T cells turn on the immune-system response and promote the production of both B cells and killer T cells. Suppressor T cells are controller lymphocytes, which slow down or prevent immune responses. Memory cells are portions of the B and T cell populations produced when a specific invader is recognized. These are essentially reserve troops programmed to attack a specific invader but not used in the initial attack.
AIDS, or Acquired Immune Deficiency Syndrome, is so difficult to combat because it is a virus that invades the very T cells which would normally be used to combat it. Hence a person with HIV can have an extremely compromised immune response and can be very susceptible to other types of infections or cancer, which most often prove fatal.
Cancer cells are cells that have lost control over cell division and that may eventually kill surrounding tissues. These cells can be attacked by killer T cells but often are not recognized by the body’s immune system. The cancer cells may not have surface markers that are different than normal body cells.
AIDS is such a difficult disease to treat because
The answer is C. Since the HIV virus attacks T cells, it is occupying the cells that should be attacking the disease, thus compromising the whole immune response.
Here’s a review of mitosis: