Chapter 1: Biology – The Study of Life

Biology: The Study of Life

Introduction

Biology is the scientific study of life and living organisms. This chapter introduces the fundamental characteristics that define life, the foundational theories of biology, and the processes that drive biological diversity and evolution.

1.1 What Does It Mean to Say That Something is Alive?

Five Fundamental Characteristics of Life

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

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

• Information: Organisms process hereditary information encoded in genes and respond to information from their environment.

• Energy: All organisms acquire and utilize energy to maintain life processes.

• Evolution: Populations of organisms are continually evolving, adapting to their environments over time.

Theories in Biology

Definition of Theory

• A theory is an explanation for a broad class of phenomena or observations, supported by a wide body of evidence.

• This scientific meaning differs from the everyday use of "theory" as a mere guess or speculation.

Three Foundational Theories

• Cell Theory: Addresses what organisms are made of and their origins.

• 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.

1.2 Life is Cellular and Replicates through Cell Division

Discovery of Cells

• Robert Hooke (1665): Used a microscope to observe small compartments in cork, naming them "cells."

• Anton van Leeuwenhoek: Improved microscope technology and observed single-celled organisms, which he called "animalcules."

• By the 1800s, biologists recognized that all organisms consist of cells.

Cell Theory

• Cells are highly organized compartments separated from their environment by a membrane.

• All organisms are made up of cells, and all cells come from preexisting cells.

Cell Theory vs. Spontaneous Generation

• All-cells-from-cells hypothesis: Cells are produced only when pre-existing cells grow and divide.

• Spontaneous generation hypothesis: The belief that organisms could arise spontaneously under certain conditions.

Louis Pasteur’s Experiment

• Tested whether cells arise from preexisting cells or by spontaneous generation using nutrient broth in flasks.

• Only flasks exposed to air (and thus to preexisting cells) developed new cells, supporting the all-cells-from-cells hypothesis.

Cell Division and Chemical Evolution

• Cells must replicate for life to persist.

• All cells in multicellular organisms are connected by common lineage.

• Evidence suggests life arose from non-life through chemical evolution early in Earth's history.

1.3 Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

• Proposed by Sutton and Boveri: Hereditary information is encoded in genes located on chromosomes.

• Chromosomes are composed of deoxyribonucleic acid (DNA).

• Genes are segments of DNA that code for cell products.

Structure of DNA

• James Watson and Francis Crick (1950s) proposed that DNA is a double-stranded helix.

The Central Dogma

• Describes the flow of genetic information in cells:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• DNA codes for ribonucleic acid (RNA), which codes for proteins.

Genetic Variation and Heredity

• DNA is copied with high accuracy, but mistakes (mutations) can occur.

• Changes in DNA sequence may alter proteins, leading to heritable variations and diversity of life.

Energy and Nutritional Needs

• Chemical reactions in cells require energy.

• Organisms need:

  • Chemical energy in the form of adenosine triphosphate (ATP).

  • Molecules that serve as building blocks for DNA, RNA, proteins, etc.

• The way organisms acquire energy is central to the diversification of life.

1.4 Life Evolves

Evolution and Common Ancestry

• Evolution: Change in the characteristics of a population over time.

• Darwin and Wallace proposed that species are related by common ancestry and can be modified across generations ("descent with modification").

Natural Selection

• Explains how evolution occurs.

• Two conditions for natural selection:

  1. Individuals vary in heritable characteristics.

  2. Certain heritable traits increase reproductive success in specific environments.

• 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 an individual's fitness in a particular environment.

• Example: On the Galápagos Islands, finches with small, pointed beaks had higher fitness when small, soft seeds were abundant. This adaptation increased their population frequency.

1.5 The Tree of Life Depicts Evolutionary History

Phylogeny and the Tree of Life

• The tree of life is a family tree depicting evolutionary relationships among species, with a single ancestral species at its base.

• Phylogeny: The actual genealogical relationships among all organisms.

Analyzing Genetic Variation

• Biologists compare DNA and RNA sequences to analyze genetic variation and infer evolutionary relationships.

• Fewer sequence differences indicate closer relationships.

• Example:

Organism	DNA Sequence
Land plant	A-T-A-T-C-G-A-G
Green algae	A-T-A-T-G-G-A-G
Brown algae	A-A-A-T-G-G-A-C

• Green algae is more closely related to land plants than brown algae.

Major Groups of Life

• The tree of life reveals three major groups:

  • Eukaryotes (have a nucleus): Eukarya

  • Prokaryotes (lack a nucleus): Bacteria and Archaea

Additional info: Phylogenetic trees are constructed using genetic data and help scientists classify organisms and understand evolutionary history.