Understanding the Characteristics of Life

Learning Objectives

• Learn the characteristics of life.
• Learn the characteristics of humans.
• Understand life's organization and diversity.
• List the steps of the scientific method of inquiry.

Section 1: The Characteristics of Life

• Living and nonliving things share common characteristics, such as being composed of atoms, which are the smallest units of nature's fundamental substances.
• Living things have distinctive features:
  • Taking in and using energy and materials
  • Sensing and responding to specific changes in their environment.
  • Reproducing and growing.
  • Consisting of one or more cells.
  • Maintaining homeostasis (dynamic balance).
  • Populations capable of evolving.
• Autotrophs (plants, some bacteria, and some protists) obtain energy from sunlight and create their own food through photosynthesis. Their materials include water, carbon dioxide, nitrogen, and phosphorus.
• Heterotrophs (animals, fungi, some bacteria, and some protists) obtain energy from eating/decomposing other organisms, ultimately from the sun. Their materials come from other organisms.

Section 2: Our Place in the Natural World

• Humans have evolved over time, part of biological evolution—the change in organisms through the generations.
• Humans are mammals belonging to the animal kingdom, one of the four kingdoms of life in the domain Eukarya.
• Humans share characteristics with their closest primate relatives.
• Humans also have distinctive features: increased dexterity, a large brain, analytical skills, sophisticated communication, and social behaviors.

Section 3: Life's Organization

• Life is organized on many levels:
  • Atoms and molecules: nonliving materials from which nature is built.
  • Cells organized into increasingly complex levels: tissues, organs, and organ systems.
  • Organisms form populations, which in turn form communities, ecosystems, and finally the biosphere.
• Organisms are connected through the flow of energy and cycling of materials.
  • Energy flows from the sun, trapped by plants (producers).
  • Animals (consumers) feed on the stored energy in plants.
  • Bacteria and fungi (decomposers) break down biological molecules of other organisms to recycle materials.
  • All organisms rely on each other for energy and raw materials.

Section 4: Science Is a Way of Learning about the Natural World

• Science is an approach to gathering knowledge, and biology uses a methodical search to reveal the secrets of nature.
• Explanations are sought using the scientific method.
  • Observe an aspect of the natural world, ask a question.
  • Develop hypotheses (educated guesses) using all known information.
  • Predict outcomes if the hypothesis is valid.
  • Test the hypothesis using experiments, models, and observations.
  • Repeat tests for consistency; analyze and report objectively on the tests and conclusions.
• Experiments are major scientific tools involving controlled conditions and control groups used to identify side effects. Samples should be representative of the broader population.

Section 5: Critical Thinking in Science and Life

• Critical thinking is an objective evaluation of information, based on evidence and credible sources.
• Consider the source, question credentials and motives, evaluate the content for cause and correlation.
• Separate facts from opinions and rely on reliable sources, not hearsay.
• Look for checkable facts and avoid signs of obvious bias, such as paid testimonials.

Section 6: Science in Perspective

• A scientific theory explains many observations through relationships between hypotheses that form a broad explanation of phenomena.
• Theories are subject to revision based on tests and repetitions.
• Scientists must be prepared to change their minds in light of new evidence.
• Science is limited to testable questions; subjective questions do not easily lend themselves to scientific analysis or experiments.
• Controversial uses of scientific knowledge require responsible use by all of society.

General Observations, Hypotheses, and Studies on Smoking and Lung Cancer

• An initial observation: Many smokers experience serious health problems, including lung cancer, high blood pressure, heart attacks, and emphysema.
• A testable hypothesis: People who smoke are more likely to develop lung cancer than nonsmokers.
• Observational study plan: Randomly sample 100,000 smokers and 100,000 nonsmokers. Track cases of lung cancer over many years.
• Observational study results: In the sample, 60 smokers developed lung cancer compared to only 2 nonsmokers.
• Rejection of the null hypothesis (that there's no difference). The evidence supports the hypothesis.
• Experimental study plan: Randomly divide 200,000 mice into two groups; expose one to cigarette smoke and not the other; monitor incidence of lung cancer.
• Experimental study results:
  • Significant increase in lung cancer in the smoking group.
  • A very limited incidence of lung cancer in the non-smoking group.
• Cigarette companies initially rejected the evidence or emphasized limitations of early studies, but accumulated scientific evidence from various studies eventually led them to accept that smoking causes lung cancer.

Test your knowledge of biology with questions on autotrophs, heterotrophs, DNA significance, and responses to stimuli. Explore concepts like homeostasis, cell theory, and environmental interaction. Perfect for biology students.