What Is Life?

Characteristics of Living Things

Biology is the study of life, and living organisms share several fundamental characteristics that distinguish them from non-living things.

• Organized Complexity: Living things are composed of cells, the basic unit of life. In multicellular organisms, cells form tissues, organs, and organ systems, making the characteristics of life more apparent.

• Acquire & Use Energy: Organisms exchange matter and energy with their environment. Plants perform photosynthesis to make food. Maintaining a stable internal environment is called homeostasis.

• Sense & Respond to Stimuli: Living things can detect and respond to changes in their environment. For example, plants grow toward light, and animals respond to stimuli such as temperature or light.

• Growth: Living organisms grow, such as a seed developing into a mature plant.

• Reproduction & Use of DNA: Offspring inherit instructions from parents through DNA (deoxyribonucleic acid).

• Evolution: Populations of living things change over generations. Mutations create variations in DNA, and natural selection allows individuals with helpful traits to survive and reproduce more.

Example: Bacteria are living organisms because they are made of cells and carry out all characteristics of life, including using energy, responding to stimuli, growing, reproducing, and containing DNA.

Application: Sweating during hot weather is an example of homeostasis, as the body works to maintain a stable internal temperature.

What Is Evolution?

Mechanisms of Evolution

Evolution is the process by which populations change over generations, driven by differences among individuals, heredity, and natural selection.

• Mutations: Changes in DNA that can create new traits. Some mutations are beneficial.

• Adaptation: An inherited trait that helps an organism survive or reproduce.

• Natural Selection: Individuals with traits that help them survive and reproduce are more likely to pass those traits to the next generation.

• Extinction: If a species cannot adapt to environmental changes, it may become extinct.

Example: Antibiotic-resistant bacteria become more common because mutations that confer resistance allow those bacteria to survive and reproduce.

Levels of Biological Organization

Hierarchy from Smallest to Largest

Biological organization is structured from the smallest components to the largest systems.

• Atom

• Molecule

• Organelle

• Cell

• Tissue

• Organ

• Organ system

• Organism

• Population

• Community

• Ecosystem

Classification of Life

Organisms are classified into domains based on cell type and other characteristics.

• Bacteria & Archaea: Prokaryotic cells (lack a nucleus).

• Eukarya: Eukaryotic cells (have a nucleus); only Eukarya include multicellular organisms.

The Scientific Method

Steps of the Scientific Method

The scientific method is a systematic approach to investigating questions and testing hypotheses.

1. Observation

2. Question

3. Hypothesis

4. Prediction

5. Experiment

6. Conclusion

• Controls: Factors kept constant during an experiment.

• Variables: Factors that can be changed.

• Independent variable: The factor manipulated by the researcher.

• Dependent variable: The factor measured or observed.

• Control group: The group that does not receive the experimental treatment and is used for comparison.

Scientific Theories and Reasoning

Nature of Scientific Theories

A scientific theory is a well-supported and well-tested explanation for a phenomenon. Theories must be testable.

• Inductive reasoning: Making generalizations from specific observations.

• Deductive reasoning: Making predictions or hypotheses based on general principles or theories.

Case Study: Green Film on Tiger Stadium Steps

Applying Biological Concepts

• Is the film alive? It may be alive if it is made of cells and carries out life processes such as using energy, growing, responding to stimuli, reproducing, and containing DNA.

• How might it obtain energy? Through photosynthesis or by obtaining nutrients from its environment.

• How might natural selection act? Individuals with traits that help them survive on the stadium steps would be more likely to survive and reproduce.

• Designing an experiment: Test how different environmental conditions affect the growth of the green film by varying light exposure and keeping other factors constant.

Biology in Everyday Life

Applications and Examples

• Homeostasis: Sweating helps maintain a stable internal body temperature.

• Energy metabolism: The body converts food (e.g., a smoothie) into usable energy.

• Sensing and responding to stimuli: A tiger blinks in response to a camera flash.

• Plant response: A plant bends toward sunlight, demonstrating response to a stimulus.

• Viruses: Viruses are not considered alive because they cannot independently reproduce or carry out metabolism.

Evolution in the Real World

• Mutations and natural selection: New color variations in crawfish arise from mutations; traits that help survival become more common through natural selection.

• Pesticide resistance: Mosquitoes become resistant to pesticides through natural selection.

• Extinction: Species unable to adapt to environmental changes may go extinct.

Scientific Method in Everyday Life

• Independent variable: Study location (library vs. dorm).

• Dependent variable: Exam score.

• Control: Keep study time the same for both groups.

• Prediction: The step in the scientific method where expected outcomes are stated.

• Controlled factors: Age, caffeine tolerance, and activity level should be controlled in experiments to ensure valid results.

Classification & Cells

• Bacteria: Prokaryotic cells; lack a nucleus.

• Mushroom: Domain Eukarya, Kingdom Fungi.

• Scientific name: Consists of genus and species.

Energy, Homeostasis, and the Environment

• Homeostasis failure: Overheating during exercise is dangerous because the body cannot maintain a stable internal temperature.

• Photosynthesis analogy: A solar-powered calculator resembles photosynthesis in living organisms.

• Material exchange: Organisms must exchange matter and energy with their environment to obtain materials and remove waste.

Critical Thinking

Biology's Impact on Daily Life

• Informed decisions: Biology helps people understand health, food, disease, and environmental issues, enabling informed choices.

• Evolution in medicine: Antibiotic resistance is an example of evolution affecting daily life.

• Scientific method: Understanding the scientific method helps evaluate evidence and make informed decisions.

Summary Table: Domains and Cell Types

Key Terms and Definitions

• Cell: The basic unit of life.

• Homeostasis: The maintenance of a stable internal environment.

• DNA: Deoxyribonucleic acid, the molecule that carries genetic instructions.

• Mutation: A change in DNA sequence.

• Natural selection: The process by which individuals with advantageous traits reproduce more successfully.

• Adaptation: An inherited trait that increases an organism's chance of survival.

• Prokaryotic cell: A cell without a nucleus.

• Eukaryotic cell: A cell with a nucleus.

• Scientific method: A systematic approach to investigation.

• Scientific theory: A well-supported explanation for a phenomenon.

Sample Equation: Photosynthesis

The process by which plants convert sunlight into chemical energy:

Additional info: This equation summarizes the conversion of carbon dioxide and water into glucose and oxygen using light energy.