Study Notes: Studying Life

1 Studying Life

  • Opening context: Over one-third of the world’s amphibian species are threatened with extinction; Tyrone Hayes studies atrazine, a common agricultural pesticide, and its effects on breeding and reproduction. Opening Question: Could atrazine in the environment affect species other than amphibians?

  • This chapter introduces biology as the science of living things and sets up key concepts used throughout the course.

1.1 What Is Biology?

  • Definition: Biology is the scientific study of living things (organisms).

  • Deep time origin: Living things are descended from a common origin of life that occurred

    • as early as ext{approximately }4 imes 10^9 ext{ years ago} (

    • and the Earth formed around 4.6 imes 10^9 ext{ to } 4.5 imes 10^9 ext{ years ago}).

  • Diversity of life is illustrated (Figure 1.1) with a range of organisms from microorganisms to plants, showing that life comes in many forms and sizes (e.g., Sulfolobus, Escherichia coli, Coronosphaera mediterranea, Passiflora quadrangularis, Phallus indusiatus).

  • Key characteristics shared by living organisms:

    • Made of a common set of chemical components: carbohydrates, fatty acids, nucleic acids, amino acids.

    • Most are composed of cells enclosed by plasma membranes.

    • Convert molecules from their environment into new biological molecules (metabolism).

    • Extract energy from the environment and use it to perform biological work.

    • Contain genetic information that uses a universal code to specify proteins.

    • Share similarities among a fundamental set of genes and replicate this genetic information when reproducing.

    • Exist in populations that evolve through changes in frequencies of genetic variants over time (population-level evolution).

    • Self-regulate their internal environment, maintaining conditions that allow survival (homeostasis).

  • Earth’s history and life’s timeline: Life emerged long after Earth formed; life took hundreds of millions of years to arise after planetary formation. The history is sometimes pictured as a 30-day month to illustrate the long timescale of life’s evolution (Figure 1.2).

  • Visuals and structure (Figure references):

    • Figure 1.2 outlines a timeline of major events, including origin of life, origin of photosynthesis, origin of eukaryotic cells, multicellularity, land plants, insects, fishes, amphibians, reptiles, birds, mammals, and humans.

  • Cellular structure and units:

    • Cells are the building blocks of life (Figure 1.3). Key components include:

    • Cell membrane enclosing the cell.

    • Membrane around the nucleus.

    • Mitochondria (membrane-bound organelles involved in energy production).

  • Early cellular life and domains:

    • For about 2 imes 10^9 ext{ years}, life consisted of single-celled prokaryotes (two main groups emerged early): Bacteria and Archaea.

    • Some prokaryotes formed close, interdependent relationships and merged to form eukaryotes, which have internal membranes enclosing organelles (including the nucleus).

  • The rise of oxygen and photosynthesis:

    • About 2.5 imes 10^9 ext{ years ago}, photosynthesis began to transform sunlight into chemical energy, becoming the energy basis for most life on Earth and enabling growth of aerobic (oxygen-using) organisms.

    • Early photosynthetic cells resembled cyanobacteria; atmospheric O₂ began to accumulate as these organisms proliferated.

    • The accumulation of O₂ led to environmental and biological changes, including the formation of an ozone (O₃) layer that absorbs UV radiation.

    • By around 5 imes 10^8 ext{ years ago}, enough ozone existed to permit life to leave the aquatic environment and colonize land.

  • Genome and genes:

    • Genome: the sum total of all the DNA in a cell.

    • DNA consists of repeating subunits called nucleotides.

    • Gene: a specific segment of DNA that contains information for making a protein.

    • (Figure 1.5 illustrates DNA, a nucleotide, a gene, and the flow to a protein.)

  • Evolution as the unifying principle:

    • Charles Darwin provided evidence for evolution by natural selection: differential survival and reproduction among individuals in a population can account for much of life's diversity.

    • Natural selection leads to adaptations—structural, physiological, or behavioral traits that enhance survival and reproduction in a given environment.

    • (Figure 1.6 shows adaptations to the environment across multiple species.)

  • Binomial nomenclature and evolutionary relationships:

    • Each species has a distinct scientific name in binomial format: Genus species (e.g., Homo sapiens).

    • The genus groups species that share a recent common ancestor.

    • Our understanding of evolutionary relationships has been enhanced by molecular techniques, including genome sequencing, leading to the use of phylogenetic trees to illustrate evolutionary histories.

  • Tree of life and domains (Figure 1.7): three domains of life:

    • Bacteria (prokaryotes), Archaea (prokaryotes), Eukarya (eukaryotes).

  • Unicellular to multicellular life:

    • For most of Earth's history, life was unicellular.

    • Multicellular Eukarya (plants, animals, fungi) evolved from protists (unicellular microbial eukaryotes).

  • Biological hierarchy and organization:

    • Cells differentiate to form tissues; tissues form organs; organs form organ systems within multicellular organisms.

    • Figure 1.8 shows the levels of organization from atoms and small molecules up to organ systems.

  • Cellular work and homeostasis:

    • Examples of cellular work include:

    • Movement of molecules or entire organisms.

    • Synthesis of new complex molecules from smaller units.

    • Electrical work of information processing in nervous systems.

    • Homeostasis: organisms regulate their internal environment, maintaining a narrow range of conditions that support survival.

1.2 How Do Biologists Investigate Life?

  • Hypothesis–prediction approach (five steps):
    1) Making observations
    2) Asking questions
    3) Forming hypotheses (tentative explanations)
    4) Making predictions based on the hypotheses
    5) Testing the predictions

  • Controlled experiments:

    • Manipulate one or more factors (variables) and compare results between an experimental group and a control group.

    • One variable is manipulated at a time to isolate effects.

    • Independent variable: the variable deliberately changed by the experimenter.

    • Dependent variable: the measured response.

  • Statistics and significance:

    • Statistical methods assess whether observed differences between groups are significant.

    • Experiments often begin with a null hypothesis: there are no differences between groups.

  • Science vs. religion in testing:

    • Religious or spiritual explanations are not testable by scientific methods and are not part of science; science does not claim religious beliefs are true or false, but they are not testable by the same methods.

1.3 Why Does Biology Matter?

  • Modern agriculture depends on biology:

    • Knowledge of plant biology has increased food production and supported larger human populations.

    • New crop varieties are developed to resist pests or tolerate drought (Green Revolution, Figure 1.13).

  • Medicine and medical practice:

    • Biology underpins medical knowledge about how organisms work, why problems arise, and why diseases occur.

    • Genetic variations can contribute to certain diseases.

    • Visuals: Figure 1.14 shows medical applications of biology and potential health improvements.

  • Public policy and ethics:

    • Decoding and manipulating genomes raises policy and ethical questions.

    • Biologists advise government agencies on issues such as overfishing (e.g., bluefin tuna) (Figure 1.15).

  • Ecology and global change:

    • Biology helps us understand ecosystems and the consequences of human activities.

    • Increasing atmospheric CO₂ contributes to climate warming, influencing extinction risk and disease spread.

    • Warmer world visuals (Figure 1.16) illustrate observed changes in glaciers over time.

  • Discovering life on Earth:

    • Additional figures (e.g., Figure 1.17) summarize ongoing discoveries and the expanding tree of life.

  • Opening question answer (as of the conclusion):

    • Replication of results is essential in science; many researchers have examined atrazine’s feminizing effects across amphibian and vertebrate species.

    • Molecular mechanisms of atrazine’s effects are similar across amphibians, fish, reptiles, and human cell cultures, indicating broader relevance beyond amphibians.

  • Atrazine in context:

    • Atrazine is an agricultural pesticide used widely; investigations have explored its effects across multiple taxa, reinforcing the need for replication and cross-species studies to understand potential risks.

  • Summary takeaway:

    • Biology connects molecular details to organisms, populations, ecosystems, and human society, offering insights with practical implications for health, agriculture, policy, and ethics.