Chapter 1: Biology – The Study of Life

Biology: The Study of Life

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

Biologists define life by identifying five fundamental characteristics shared by all living organisms. These characteristics distinguish living things from non-living matter.

• Cells: All organisms are composed of membrane-bound cells, which are highly organized compartments separated from their environment by a membrane barrier.

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

• Information: All organisms process hereditary information encoded in genes as well as information from their environment.

• Energy: All organisms acquire and use energy to stay alive, supporting cellular processes and growth.

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

Theories in Biology

A theory in science 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, which often means speculation or guess.

• 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.

1.2 Life is Cellular and Replicates through Cell Division

Discovery of Cells

• Robert Hooke (1665): Used a microscope with 30x magnification to observe small compartments in cork tissue, which he termed "cells."

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

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

Cell Theory and Spontaneous Generation

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

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

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

Louis Pasteur’s Experiment

• Hypothesis: Cells arise from cells and do not arise by spontaneous generation.

• Experiment: Pasteur used two glass flasks with nutrient broth, one with a straight neck and one with a swan neck open to the air. Only the straight-neck flask showed cell growth, supporting the all-cells-from-cells hypothesis.

Life Replicates through Cell Division

• Cells must replicate for life to exist.

• All cells in multicellular organisms descend from preexisting cells, sharing a common lineage.

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

1.3 Life Processes Information and Requires Energy

Chromosomal Theory of Inheritance

• Proposed by Sutton and Boveri, stating that hereditary or genetic information is encoded in genes located on chromosomes.

• By the 1950s, it was established that chromosomes are molecules 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.

The Central Dogma

• 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 one organism to its 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: Claimed 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 offspring. Individuals with higher fitness produce more 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 depicts evolutionary history, describing genealogical relationships among species with a single ancestral species at its base.

• Phylogeny: The actual genealogical relationships among all organisms.

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

The Phylogenetic Tree of Life

• Phylogenetic trees are used to show relationships between species.

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

• Branches without a recent common ancestor 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