half-life

Examples of half-life in the following topics:

• Carbon Dating and Estimating Fossil Age

  • Paleontology seeks to map out how life evolved across geologic time.
  • This rate is represented by the half-life, which is the time it takes for half of a sample to decay.
  • The half-life of carbon-14 is 5,730 years, so carbon dating is only relevant for dating fossils less than 60,000 years old.
  • After one half-life has elapsed, one half of the atoms of the Lead-212 nuclide will have decayed into a "daughter" nuclide or decay product.
  • In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain, eventually ending with the formation of a stable (nonradioactive) daughter nuclide in this case Lead-208; each step in such a chain is characterized by a distinct half-life.

• Lipid-Derived, Amino Acid-Derived, and Peptide Hormones

  • For example, cortisol has a half-life of 60 to 90 minutes, whereas epinephrine, an amino acid derived-hormone, has a half-life of approximately one minute.

• Signaling Molecules

  • NO has a very short half-life; therefore, it only functions over short distances.

• Life Cycles of Sexually Reproducing Organisms

  • Fertilization and meiosis alternate in sexual life cycles.
  • The process of meiosis, the division of the contents of the nucleus that divides the chromosomes among gametes, reduces the chromosome number by half, while fertilization, the joining of two haploid gametes, restores the diploid condition.
  • In the diploid-dominant life cycle, the multicellular diploid stage is the most obvious life stage, as occurs with most animals, including humans.
  • There is no multicellular haploid life stage.
  • Within haploid-dominant life cycles, the multicellular haploid stage is the most obvious life stage.

• Gametogenesis (Spermatogenesis and Oogenesis)

  • During meiosis, two cell divisions separate the paired chromosomes in the nucleus and then separate the chromatids that were made during an earlier stage of the cell's life cycle, resulting in gametes that each contain half the number of chromosomes as the parent.
  • If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.
  • A primary oocyte begins the first meiotic division, but then arrests until later in life when it will finish this division in a developing follicle.

• Theories of Life History

  • Modern theories of life history incorporate life and survivorship factors with ecological concepts associated with r- and K-selection theories.
  • While reproductive strategies play a key role in life histories, they do not account for important factors such as limited resources and competition.
  • By the second half of the twentieth century, the concept of K- and r-selected species was used extensively and successfully to study populations.
  • This includes the way they obtain resources and care for their young, as well as length of life and survivorship factors.
  • New demographic-based models of life history evolution have been developed which incorporate many ecological concepts included in r- and K-selection theory, as well as population age structure and mortality factors.

• Genes as the Unit of Heredity

  • For all forms of life, continuity of the species depends upon the genetic code being passed from parent to offspring.
  • A child will inherit half of its genes (one of each of its 23 pairs) from its mother and the other half from its father.

• Protists as Human Pathogens

  • Members of the genus Plasmodium must colonize both a mosquito and a vertebrate to complete their life cycle.
  • In 2010, it was estimated that malaria caused between one and one-half million deaths, mostly in African children.
  • During the course of malaria, P. falciparum can infect and destroy more than one-half of a human's circulating blood cells, leading to severe anemia.

• Advantages and Disadvantages of Sexual Reproduction

  • This is one of the important unanswered questions in biology and has been the focus of much research beginning in the latter half of the twentieth century.
  • The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell.
  • There are three main categories of sexual life cycles: diploid-dominant, demonstrated by most animals; haploid-dominant, demonstrated by all fungi and some algae; and the alternation of generations, demonstrated by plants and some algae.

• Agnathans: Jawless Fishes

  • Jawless fishes or agnathans are craniates that represent an ancient vertebrate lineage that arose over one half-billion years ago.
  • Many species have a parasitic stage of their life cycle during which they are ectoparasites of fishes .