Biology: The Study of Life

Introduction

Biology is the scientific study of life and living organisms. This chapter introduces the foundational concepts that define life, the scientific theories that underpin biological science, and the methods used to investigate biological questions.

What Does It Mean to Say That Something is Alive?

Five Fundamental Characteristics of Life

• Cells: All living organisms are composed of one or more membrane-bound cells, which are the basic units of life.

• Replication: All organisms are capable of reproduction, ensuring the continuation of their species.

• Information: Organisms process hereditary information encoded in genes and respond to environmental information.

• Energy: All organisms acquire and use energy to stay alive.

• Evolution: Populations of organisms are continually evolving.

Example: Bacteria, plants, and animals all share these characteristics, distinguishing them from non-living matter.

Theories in Biology

Definition and Importance

• Theory: An explanation for a broad class of phenomena or observations, supported by a wide body of evidence. In science, a theory is not a guess but a well-substantiated explanation.

• Key Biological Theories:

  • Cell Theory: All organisms are made of cells; all cells come from preexisting cells.

  • Theory of Evolution by Natural Selection: Explains how organisms are related and how they change over time.

  • Chromosome Theory of Inheritance: Describes how hereditary information is transmitted from one generation to the next.

Life is Cellular and Replicates through Cell Division

Discovery of Cells

• Robert Hooke (1665): First observed cells using a microscope (30x magnification).

• Anton van Leeuwenhoek: Improved microscope (300x), observed single-celled organisms (“animalcules”).

• Cell Theory: All organisms consist of cells; all cells arise from preexisting cells.

Levels of Organization: Cell → Tissue → Organ → Organ System → Organism

Testing Cell Theory: Spontaneous Generation vs. All-Cells-From-Cells

• Spontaneous Generation Hypothesis: Life arises spontaneously from non-living matter.

• All-Cells-From-Cells Hypothesis: Cells arise only from preexisting cells.

• Louis Pasteur’s Experiment: Demonstrated that cells do not arise by spontaneous generation, but from other cells.

Conclusion: Cell theory is foundational to biology, supporting the idea of a common lineage among all living organisms.

Life Processes Information and Requires Energy

Genetic Information and the Central Dogma

• Chromosome Theory of Inheritance: Genes, located on chromosomes, encode hereditary information.

• DNA Structure: DNA is a double helix composed of four nucleotides (A, T, C, G). The sequence of these bases encodes genetic information.

• Base Pairing: A pairs with T, C pairs with G. This allows DNA to be copied accurately.

• Central Dogma: Describes the flow of genetic information:

• Proteins: Carry out most cellular functions, including catalyzing reactions and providing structure.

• Mutations: Changes in DNA sequence can alter proteins, leading to heritable variation and diversity.

Energy in Living Systems: Organisms must acquire energy (often in the form of ATP) and building blocks for growth and reproduction. The method of energy acquisition is central to the diversity of life.

Example: Plants use sunlight to produce sugars, which are then used to generate ATP or stored for later use.

Life Evolves

Evolution and Natural Selection

• Evolution: Change in the characteristics of a population over time; species are related by common ancestry.

• Population: Group of individuals of the same species living in the same area at the same time.

• Natural Selection: The process by which individuals with heritable traits that increase fitness are more likely to survive and reproduce.

• Fitness: The ability of an individual to produce surviving offspring.

• Adaptation: A trait that increases an individual’s fitness in a particular environment.

Example: Galápagos finches with small, pointed beaks had higher fitness when small, soft seeds were abundant, leading to an increase in this trait in the population.

The Tree of Life Depicts Evolutionary History

Phylogeny and Classification

• Tree of Life: A diagram depicting the evolutionary relationships among species, with a single ancestral species at its base.

• Phylogeny: The actual genealogical relationships among all organisms, often inferred from genetic data.

• Domains of Life: Three major groups—Bacteria, Archaea (both prokaryotes), and Eukarya (eukaryotes).

• Taxonomy: The science of naming and classifying organisms. Major taxonomic ranks: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

• Scientific Names: Binomial nomenclature (Genus species), always italicized (e.g., Homo sapiens).

Doing Biology: The Scientific Method

Hypothesis Testing and Experimental Design

• Hypothesis: A testable statement explaining an observation.

• Prediction: A measurable or observable result expected if the hypothesis is correct.

• Experiment: A controlled test to determine the effect of a single variable.

• Control Group: A group that does not receive the experimental treatment, used for comparison.

• Replication: Repeating experiments to ensure reliability of results.

Case Study: Why Do Giraffes Have Long Necks?

• Food Competition Hypothesis: Long necks evolved to reach food unavailable to other mammals. Prediction: Giraffes feed high in trees.

• Sexual Competition Hypothesis: Long necks evolved because longer-necked males win more fights and father more offspring. Data support this hypothesis over the food competition hypothesis.

Case Study: How Do Ants Navigate?

• Pedometer Hypothesis: Ants track the number of steps and stride length to navigate back to their nest.

• Experimental Groups: Ants with shortened, typical, or lengthened legs were tested for their ability to return to the nest.

• Results: Shortened-leg ants stopped short, typical ants returned accurately, and lengthened-leg ants overshot the nest, supporting the pedometer hypothesis.

Characteristics of Good Experimental Design

• Include a control group.

• Keep experimental conditions constant.

• Use large sample sizes.

• Repeat experiments for reliability.

Summary Table: Major Characteristics of Life