Chapter 1: Biology – The Study of Life (Mini-Textbook Study Notes)

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 major theories that form the foundation of biological science, and the evolutionary relationships among all living things.

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 organisms are capable of reproduction, ensuring the continuation of their species.

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

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

• Evolution: Populations of organisms are continually evolving, leading to the diversity of life observed today.

Theories in Biology

Definition of Theory

• A theory is an explanation for a very general class of phenomena or observations that is supported by a wide body of evidence.

• This differs from the everyday use of the word "theory," which often means speculation or guess.

Three Foundational Theories

• Cell Theory: What are organisms made of? Where do organisms come from?

• Theory of Evolution by Natural Selection: How are organisms related to one another?

• Chromosome Theory of Inheritance: How is hereditary information 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 (30x) to observe small compartments in cork, which he called "cells."

• Anton van Leeuwenhoek: Improved the microscope (300x) and observed single-celled organisms, which he called "animalcules."

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

Cell Theory

• All organisms are made up of cells.

• All cells come from preexisting cells ("all-cells-from-cells").

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

Spontaneous Generation vs. Cell Theory

• Spontaneous Generation Hypothesis: The belief that organisms could arise spontaneously under certain conditions.

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

Louis Pasteur’s Experiment

• Tested whether cells arise spontaneously or from pre-existing cells using nutrient broth in two types of flasks (straight-necked and swan-necked).

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

Cell Division and Chemical Evolution

• Cells must replicate for life to exist.

• All cells in a multicellular organism are descended from preexisting cells, sharing a common lineage.

• Evidence suggests that 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, which are located on chromosomes.

• By the 1950s, it was established that chromosomes are made of deoxyribonucleic acid (DNA).

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

Structure of DNA

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

• DNA consists of a backbone and base pairs (A, T, C, G).

The Central Dogma of Molecular Biology

• Describes the flow of information in cells:

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

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

Genetic Information and Variation

• DNA is copied to pass genetic information from cell to cell or from parent to offspring.

• Copying DNA is highly accurate, but mistakes (mutations) can occur.

• DNA sequence changes may lead to changes in proteins, resulting in heritable variations that underlie the diversity of life.

Energy and Nutritional Needs

• Chemical reactions inside cells require energy.

• Organisms have two fundamental nutritional needs:

  1. Acquiring chemical energy in the form of adenosine triphosphate (ATP).

  2. Obtaining molecules that can be used as building blocks to make DNA, RNA, proteins, etc.

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

1.4 Life Evolves

Evolution and Natural Selection

• Evolution: Change in the characteristics of a population over time; species are related and can change through time.

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

• Darwin and Wallace proposed that species are related by common ancestry and that characteristics can be modified from generation to generation ("descent with modification").

Natural Selection

• Explains how evolution occurs.

• Two conditions for natural selection:

  1. Individuals must vary in heritable characteristics.

  2. Certain versions of these traits help individuals reproduce more than others in a particular 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 the fitness of an individual 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 survival and reproduction, leading to an increase in the population of finches with this trait.

1.5 The Tree of Life Depicts Evolutionary History

Phylogeny and the Tree of Life

• The tree of life is a family tree of organisms that describes genealogical relationships among species, with a single ancestral species at its base.

• Phylogeny: The actual genealogical relationships among all organisms.

Analyzing Genetic Variation

• Biologists analyze genetic variation by comparing RNA and DNA sequences from different organisms.

• Fewer sequence differences between two species indicate a closer evolutionary relationship.

• Example: DNA sequences show that green algae are more closely related to land plants than to brown algae.

Phylogenetic Tree

• A phylogenetic tree visually represents the evolutionary relationships between species.

• Branches that share a recent common ancestor represent closely related species.

• Branches that do not share recent common ancestors represent more distantly related species.

• The tree of life is estimated from genetic data.

Major Groups of Life

• The tree of life indicates three major groups of organisms:

  • Eukaryotes (have a nucleus): Eukarya

  • Prokaryotes (lack a nucleus): Bacteria and Archaea