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

Biology: The Study of Life

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

Biology is the scientific study of life. All living organisms share five fundamental characteristics that distinguish them from non-living matter:

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

• Replication: All organisms have the capacity to reproduce, 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 their internal order and support life processes.

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

Theories in Biology

A theory in science is an explanation for a broad class of phenomena or observations that is supported by a substantial body of evidence. This differs from the everyday use of the word, which often means speculation or a guess. Three foundational theories form the framework for modern biological science:

• 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.

Life is Cellular and Replicates through Cell Division

Development of Cell Theory

Advancements in microscopy by Robert Hooke (30x magnification) and Anton van Leeuwenhoek (300x magnification) in the 17th century allowed scientists to observe cells for the first time. Hooke coined the term "cells" after observing small compartments in cork, while van Leeuwenhoek observed single-celled organisms he called "animalcules." By the 1800s, it was established that all organisms consist of cells.

• Cells: Highly organized compartments separated from their environment by a membrane barrier.

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

Cell Theory vs. Spontaneous Generation

Cell theory challenged the idea of spontaneous generation, which posited that organisms could arise spontaneously under certain conditions. The alternative, "all-cells-from-cells" hypothesis, proposed that cells are produced only when pre-existing cells grow and divide.

• Louis Pasteur’s Experiment: Pasteur tested whether cells arise spontaneously or from other cells using nutrient broth in swan-necked flasks. His results supported the all-cells-from-cells hypothesis.

Cell Division and the Origin of Life

For life to persist, cells must replicate through cell division. According to cell theory, all cells in a multicellular organism are descended from preexisting cells, sharing a common lineage. Modern evidence suggests that life originated from non-life through chemical evolution early in Earth's history.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

The chromosomal theory of inheritance, proposed by Sutton and Boveri, states that hereditary information is encoded in genes, which are located on chromosomes. In the 1950s, it was discovered that chromosomes are composed of deoxyribonucleic acid (DNA), the hereditary material. Genes are segments of DNA that code for cell products.

The Central Dogma of Molecular Biology

James Watson and Francis Crick proposed that DNA is a double-stranded helix. The central dogma describes the flow of genetic information in cells:

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

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

Genetic Variation and Heredity

DNA is copied with high accuracy to pass genetic information from cell to cell or from parent to offspring. However, mistakes (mutations) can occur, leading to changes in proteins and, consequently, heritable variations that drive the diversity of life.

Energy and Metabolism

Chemical reactions within cells require energy. Organisms have two fundamental nutritional needs:

• Acquiring chemical energy, often in the form of adenosine triphosphate (ATP).

• Obtaining molecules that serve as building blocks for DNA, RNA, proteins, and other cellular components.

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

Life Evolves

Evolution and Natural Selection

Evolution is the change in the characteristics of a population over time. Charles Darwin and Alfred Russel Wallace proposed that:

• Species are related by common ancestry.

• Species can be modified from generation to generation (descent with modification).

A population is a group of individuals of the same species living in the same area at the same time.

Mechanism of Natural Selection

Natural selection explains how evolution occurs. Two conditions must be met for natural selection:

• Individuals in a population vary in heritable characteristics.

• Certain heritable traits help individuals survive and reproduce more successfully in a particular environment.

Over time, these advantageous traits become more common in the population. 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. Individuals with higher fitness leave more descendants.

• 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 survival and reproduction, leading to an increase in the frequency of small, pointed beaks in the population.

The Tree of Life Depicts Evolutionary History

Phylogeny and the Tree of Life

The tree of life is a family tree that depicts the evolutionary history and genealogical relationships among all organisms, with a single ancestral species at its base. Phylogeny refers to the actual evolutionary relationships among organisms.

Analyzing Genetic Variation

Biologists analyze genetic variation by comparing DNA and RNA sequences among different organisms. Fewer sequence differences indicate a closer evolutionary relationship.

• Example: DNA sequences of land plants and green algae are more similar to each other than to brown algae, indicating a closer relationship.

Interpreting the Phylogenetic Tree

The phylogenetic tree shows relationships between species. Branches that share a recent common ancestor represent closely related species, while those that do not are more distantly related. Genetic data is used to estimate the tree of life.

Major Groups of Life

The tree of life reveals three major groups of organisms:

• Eukaryotes: Organisms with a nucleus (Domain Eukarya).

• Bacteria: Prokaryotes lacking a nucleus.

• Archaea: Prokaryotes lacking a nucleus, but distinct from bacteria.