10.2: Introduction - Biology

10.2: Introduction - Biology

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Quantifying the bacterial level in the sample can be done in several ways. Later in the course you will use a statistical technique on water samples called the Most Probably Number (MPN).

  1. Direct microscopic counts of cells can be done, visually, and by various types of electronic particle counters.
  2. Viable bacterial counts can be determined by the Standard Plate Count (SPC) method. A sample is diluted several times (serial dilution) and then plated over the agar surface of a petri plate. After incubation the number of colonies on the plate can be counted and are directly related to the number of bacteria in original sample, based on the magnitude of the dilutions. Each colony arises from a single bacterium, or group of bacterium depending on the typical arrangement (tetrad, staph, diplo, etc.) and is referred to as a Colony Forming Unit (CFU).
  3. Indirect counts of a sample can be performed by using a spectrophotometer and measuring the turbidity (aka absorbance or Optical Density) of the sample. This is called the Turbidimetric method. A standard curve must be created using the known count of a sample, and then counts of various dilutions can be extrapolated.

There are advantages and disadvantages to each method. Direct counts sample small amounts of a larger sample and one can’t distinguish between live cells and dead cells. One could however, see a variety of cell types. Likewise, the Turbidimetric method would also count live and dead cells, though one could not distinguish different types of cells. In addition, the Optical Density (OD) of a culture may not always be linear especially at high cell density (twice the number of cells may not cause twice the turbidity). The Standard Plate Count does represent live cells, and sometimes a variety of colony types indicate the composition of the original sample. The SPC would not account for cells that might not grow well on the type of media being used, or cells that might be inhibited by other bacteria or other environmental factors. This is not a problem in a sample of known bacteria.


A human, like every sexually reproducing organism, begins life as a fertilized egg (embryo) or zygote. In our species, billions of cell divisions subsequently must occur in a controlled manner in order to produce a complex, multicellular human comprising trillions of cells. Thus, the original single-celled zygote is literally the ancestor of all cells in the body. However, once a human is fully grown, cell reproduction is still necessary to repair and regenerate tissues, and sometimes to increase our size! In fact, all multicellular organisms use cell division for growth and the maintenance and repair of cells and tissues. Cell division is closely regulated, and the occasional failure of this regulation can have life-threatening consequences. Single-celled organisms may also use cell division as their method of reproduction.

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    Production of Vaccines, Antibiotics, and Hormones

    Traditional vaccination strategies use weakened or inactive forms of microorganisms or viruses to stimulate the immune system. Modern techniques use specific genes of microorganisms cloned into vectors and mass-produced in bacteria to make large quantities of specific substances to stimulate the immune system. The substance is then used as a vaccine. In some cases, such as the H1N1 flu vaccine, genes cloned from the virus have been used to combat the constantly changing strains of this virus.

    Antibiotics kill bacteria and are naturally produced by microorganisms such as fungi penicillin is perhaps the most well-known example. Antibiotics are produced on a large scale by cultivating and manipulating fungal cells. The fungal cells have typically been genetically modified to improve the yields of the antibiotic compound.

    Recombinant DNA technology was used to produce large-scale quantities of the human hormone insulin in E. coli as early as 1978. Previously, it was only possible to treat diabetes with pig insulin, which caused allergic reactions in many humans because of differences in the insulin molecule. In addition, human growth hormone (HGH) is used to treat growth disorders in children. The HGH gene was cloned from a cDNA (complementary DNA) library and inserted into E. coli cells by cloning it into a bacterial vector.

    Stressors in Our Everyday Lives

    The stressors for Seyle’s rats included electric shock and exposure to cold. Although these are probably not on your top-10 list of most common stressors, the stress that you experience in your everyday life can also be taxing. Thomas Holmes and Richard Rahe (1967) developed a measure of some everyday life events that might lead to stress, and you can assess your own likely stress level by completing the measure in Table 10.2 “The Holmes and Rahe Stress Scale”. You might want to pay particular attention to this score, because it can predict the likelihood that you will get sick. Rahe and colleagues (1970) asked 2,500 members of the military to complete the rating scale and then assessed the health records of the soldiers over the following 6 months. The results were clear: The higher the scale score, the more likely the soldier was to end up in the hospital.

    Table 10.2 The Holmes and Rahe Stress Scale

    Life event Score
    Death of spouse 100
    Divorce 73
    Marital separation from mate 65
    Detention in jail, other institution 63
    Death of a close family member 63
    Major personal injury or illness 53
    Marriage 50
    Fired from work 47
    Marital reconciliation 45
    Retirement 45
    Major change in the health or behavior of a family member 44
    Pregnancy 40
    Sexual difficulties 39
    Gaining a new family member (e.g., through birth, adoption, oldster moving, etc.) 39
    Major business readjustment (e.g., merger, reorganization, bankruptcy) 39
    Major change in financial status 38
    Death of close friend 37
    Change to different line of work 36
    Major change in the number of arguments with spouse 35
    Taking out a mortgage or loan for a major purchase 31
    Foreclosure on a mortgage or loan 30
    Major change in responsibilities at work 29
    Son or daughter leaving home (e.g., marriage, attending college) 29
    Trouble with in-laws 29
    Outstanding personal achievement 28
    Spouse beginning or ceasing to work outside the home 26
    Beginning or ceasing formal schooling 26
    Major change in living conditions 25
    Revision of personal habits (dress, manners, associations, etc.) 24
    Trouble with boss 23
    Major change in working hours or conditions 20
    Change in residence 20
    Change to a new school 20
    Major change in usual type and/or amount of recreation 19
    Major change in church activities (a lot more or less than usual) 19
    Major change in social activities (clubs, dancing, movies, visiting) 18
    Taking out a mortgage or loan for a lesser purchase (e.g., for a car, television , freezer, etc.) 17
    Major change in sleeping habits 16
    Major change in the number of family get-togethers 15
    Major change in eating habits 15
    Vacation 13
    Christmas season 12
    Minor violations of the law (e.g., traffic tickets, etc.) 11
    Total ______

    You can calculate your score on this scale by adding the total points across each of the events that you have experienced over the past year. Then use Table 10.3 “Interpretation of Holmes and Rahe Stress Scale” to determine your likelihood of getting ill.

    Table 10.3 Interpretation of Holmes and Rahe Stress Scale

    Number of life-change units Chance of developing a stress-related illness (%)
    Less than 150 30
    150–299 50
    More than 300 80

    Although some of the items on the Holmes and Rahe scale are more major, you can see that even minor stressors add to the total score. Our everyday interactions with the environment that are essentially negative, known as daily hassles , can also create stress as well as poorer health outcomes (Hutchinson & Williams, 2007). Events that may seem rather trivial altogether, such as misplacing our keys, having to reboot our computer because it has frozen, being late for an assignment, or getting cut off by another car in rush-hour traffic, can produce stress (Fiksenbaum, Greenglass, & Eaton, 2006). Glaser (1985) found that medical students who were tested during, rather than several weeks before, their school examination periods showed lower immune system functioning. Other research has found that even more minor stressors, such as having to do math problems during an experimental session, can compromise the immune system (Cacioppo et al., 1998).

    10.4.3 Electron Carriers

    In living systems, a small class of compounds functions as electron shuttles: They bind and carry high-energy electrons between compounds in pathways. The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (by accepting electrons) or oxidized (by losing electrons). Nicotinamide Adenine Dinucleotide (NAD + ) (Figure 10.16) is derived from vitamin B3 (niacin). NAD + is the oxidized form of the molecule NADH is the reduced form of the molecule after it has accepted two electrons and a proton (the equivalent of a hydrogen atom plus an extra electron).

    NAD + can accept electrons from an organic molecule according to the general equation:

    Watch the video: - HL Inheritance (September 2022).


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