Biology: The Study of Life

Chapter Overview

This chapter introduces the foundational concepts of biology, focusing on the characteristics of life, the scientific process, and the major unifying theories that structure biological understanding.

Key Themes in Biology

What Does It Mean to Be Alive?

All living organisms share five fundamental characteristics that distinguish them from non-living matter.

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

• Replication: All organisms are capable of reproduction, producing offspring similar to themselves.

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

• Energy: All organisms acquire and use energy to maintain internal order and sustain life processes.

• Evolution: Populations of organisms are continually evolving, leading to diversity and adaptation.

Example: Bacteria, plants, and animals all meet these criteria, while non-living things like rocks do not.

Defining Life: Key Characteristics

• Not all characteristics commonly associated with life are universal. For example, conscious thought is not a requirement for life.

The Nature of Scientific Theories

What is a Theory?

A theory in science is an explanation for a broad class of phenomena or observations that is supported by a wide body of evidence. This differs from the everyday use of the word, which often means speculation or guess.

Major Theories in Biology

• Cell Theory: Addresses what organisms are made of and where they come from.

• 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): Used a 30x microscope to observe small compartments in cork, which he called "cells."

• Anton van Leeuwenhoek: Improved the microscope to 300x magnification and observed single-celled organisms, which he termed "animalcules."

• By the 1800s, it was established that all organisms are composed of cells.

Cell Theory

• All organisms are made up of cells.

• All cells come from preexisting cells.

Example: Multicellular organisms like humans develop from a single cell (the zygote) through repeated cell division.

Levels of Biological Organization

Biological systems are organized in a hierarchy, from molecules and cells up to the biosphere.

• Molecule → Organelle → Cell → Tissue → Organ → Organ System → Organism → Population → Community → Ecosystem → Biosphere

The Scientific Method in Biology

Key Terms

• Theory: Broad explanation supported by evidence.

• Hypothesis: A testable statement explaining an observation.

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

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

Testing Hypotheses: Spontaneous Generation vs. All-Cells-From-Cells

• All-cells-from-cells hypothesis: Cells arise only from preexisting cells.

• Spontaneous generation hypothesis: Life can arise spontaneously from non-living matter under certain conditions.

• Louis Pasteur's Experiment: Used swan-necked flasks to show that cells do not arise spontaneously, supporting the all-cells-from-cells hypothesis.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

• Hereditary information is encoded in genes, which are located on chromosomes.

• Chromosomes are composed of deoxyribonucleic acid (DNA).

Structure of DNA

• DNA is a double helix composed of two strands made up of four types of nucleotides: A (adenine), T (thymine), C (cytosine), and G (guanine).

• Base pairing: A pairs with T, and C pairs with G.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a cell:

• DNA is transcribed into RNA.

• RNA is translated into protein.

Equation:

Role of RNA and Proteins

• RNA: Carries out specialized functions; messenger RNA (mRNA) is read to make proteins.

• Proteins: Perform crucial cellular tasks, including structural support and catalyzing chemical reactions.

Genetic Variation and Evolution

• DNA is copied accurately, but mistakes (mutations) can occur.

• Mutations can lead to changes in proteins and heritable variation, which is the basis for evolution.

Energy and Metabolism

• Organisms require energy for chemical reactions and to build cellular components.

• Energy is often stored and transferred in the form of adenosine triphosphate (ATP).

• Organisms acquire energy in different ways, such as photosynthesis (plants) or consuming other organisms (animals).

Evolution: The Unifying Theme of Biology

What is Evolution?

• Evolution is the change in the characteristics of a population over time.

• Species are related by common ancestry and can change through descent with modification.

Natural Selection

• Natural selection explains how evolution occurs.

• Conditions for natural selection:

  • Individuals in a population vary in heritable traits.

  • Certain traits increase reproductive success in a given environment.

• Natural selection acts on individuals, but evolutionary change occurs in populations.

• Speciation occurs when populations diverge to form new species.

Fitness and Adaptation

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

• Adaptation: A trait that increases fitness in a particular environment.

• Example: Galapagos 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 and Classification

Phylogeny and the Tree of Life

• The tree of life depicts evolutionary relationships among organisms, with a single ancestral species at its base.

• Phylogeny: The actual genealogical relationships among all organisms.

• Genetic variation is analyzed by comparing DNA or RNA sequences; fewer differences indicate closer relationships.

Major Domains of Life

• Three major domains:

  • Bacteria (prokaryotes, lack nucleus)

  • Archaea (prokaryotes, lack nucleus)

  • Eukarya (eukaryotes, have nucleus)

Classification and Nomenclature

• Taxonomy: The effort to name and classify organisms.

• Taxon: A named group of organisms.

• Domain: The most inclusive taxonomic category.

• Binomial Nomenclature: Each species is given a two-part scientific name (genus and species), e.g., Homo sapiens.

Doing Biology: The Scientific Process

The Nature of Science

• Science involves asking testable questions and collecting data through observation and experimentation.

• Scientists formulate hypotheses and seek evidence to support or refute them.

Hypothesis Testing

• State the hypothesis and list its predictions.

• Design an observational or experimental study to test the predictions.

Case Study: Why Do Giraffes Have Long Necks?

• Food Competition Hypothesis: Long necks evolved to reach food high in trees.

• Predictions: Neck length is variable and heritable; giraffes feed high in trees.

• Evidence did not support this hypothesis; giraffes do not usually feed high in trees.

• Sexual Competition Hypothesis: Long necks evolved because longer-necked males win more fights and father more offspring. Supported by data.

Experimental Design: How Do Ants Navigate?

• Experiments allow testing of a single, well-defined factor.

• Pedometer Hypothesis: Ants track distance from the nest by counting steps and knowing stride length.

• Experimental groups: Shortened legs, typical legs, lengthened legs.

• Results: Shortened ants stopped short, typical ants returned to nest, lengthened ants overshot nest. Supports pedometer hypothesis.

Characteristics of Good Experimental Design

• Include a control group.

• Keep experimental conditions constant.

• Repeat tests and use large sample sizes.

Additional info: The chapter emphasizes the importance of evidence-based reasoning and the iterative nature of scientific inquiry in biology.