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 evolve over time, adapting to their environments through changes in heritable traits.

Theories in Biology

A theory in science is a well-supported explanation for a broad range of phenomena, distinct from the everyday use of the word as a mere guess. Three foundational theories underpin modern biology:

• Cell Theory: Addresses the composition and origin of organisms.

• Theory of Evolution by Natural Selection: Explains the relationships and diversity among organisms.

• Chromosome Theory of Inheritance: Describes how hereditary information is transmitted across generations.

Life is Cellular and Replicates through Cell Division

Discovery of Cells

The development of microscopy in the 17th century allowed scientists to observe cells for the first time:

• Robert Hooke (1665): Used a 30x microscope to observe and name "cells" in cork tissue.

• Anton van Leeuwenhoek: Improved magnification to 300x and observed single-celled organisms ("animalcules").

By the 1800s, it was established that all organisms are made of cells, forming the basis of cell theory.

Principles of Cell Theory

• All living things are composed of cells.

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

• All cells arise from preexisting cells, not by spontaneous generation.

Testing Spontaneous Generation

Louis Pasteur's experiments provided evidence against spontaneous generation:

• He used two flasks with nutrient broth—one with a swan neck open to air and one without.

• Only the flask exposed to airborne particles developed life, supporting the hypothesis that cells arise from preexisting cells.

Cell Division and Common Lineage

Cell division is essential for life. All cells in a multicellular organism descend from a common ancestor cell, supporting the concept of a shared lineage. Modern evidence suggests that life originated from non-life through chemical evolution.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

Proposed by Sutton and Boveri, this theory states that genetic information is encoded in genes located on chromosomes. In the 1950s, it was discovered that chromosomes are composed of deoxyribonucleic acid (DNA), which serves as the hereditary material. Genes are segments of DNA that code for cellular products.

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 proteins.

Proteins determine the structure and function of cells.

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

Genetic Variation and Heredity

• DNA is copied with high accuracy during cell division.

• Mutations (changes in DNA sequence) can alter proteins and lead to heritable variation, which is the basis for biological diversity.

Energy and Metabolism

All cellular chemical reactions require energy. Organisms must:

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

• Obtain molecules needed to build DNA, RNA, proteins, and other cellular components.

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

Life Evolves

Evolution and Natural Selection

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

• Species are related by common ancestry.

• Species can change over generations ("descent with modification").

Mechanism of Natural Selection

Natural selection explains how evolution occurs. Two conditions are required:

• Individuals in a population vary in heritable traits.

• Certain traits increase reproductive success in a given environment.

Over time, advantageous traits become more common, leading to evolutionary change. 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 led to an increase in the frequency of small, pointed beaks in the population.

Analyzing Genetic Variation

Biologists compare DNA and RNA sequences among organisms to determine evolutionary relationships. Fewer sequence differences indicate a closer relationship.

Example: DNA sequence comparison shows that green algae are more closely related to land plants than to brown algae.

Major Domains of Life

The tree of life reveals three major groups (domains):

• Eukarya: Organisms with a nucleus (eukaryotes).

• Bacteria: Prokaryotes lacking a nucleus.

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