Biology: The Study of Life

Defining Life

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

• Cells: All organisms are made up 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, as well as information from their environment.

• Energy: All organisms acquire and use energy to stay alive and carry out cellular processes.

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

Theories in Biology

Scientific Theories and Hypotheses

A theory in science is an explanation for a broad class of phenomena or observations, supported by a wide body of evidence. This differs from the everyday use of the word, which often means speculation or guess. In biology, three major theories form the framework for modern science:

• 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.

Life is Cellular and Replicates through Cell Division

Discovery of Cells

The cell is the fundamental unit of life. The discovery of cells was made possible by the invention of the microscope:

• In 1665, Robert Hooke used a microscope to observe small compartments in cork tissue, which he called "cells."

• Anton van Leeuwenhoek improved the microscope and observed single-celled organisms, which he called "animalcules."

By the 1800s, it was established that all organisms consist of cells, which are highly organized compartments separated from their environment by a membrane barrier.

Cell Theory and Spontaneous Generation

The cell theory states that all organisms are made up of cells and all cells come from preexisting cells. This challenged the idea of spontaneous generation, which held that organisms could arise spontaneously under certain conditions.

• Louis Pasteur's experiment demonstrated that cells arise only from preexisting cells, not by spontaneous generation.

Cell division is essential for life, as all cells in a multicellular organism have descended from preexisting cells, sharing a common lineage.

Life Processes Information and Requires Energy

Genetic Information and the Chromosome Theory of Inheritance

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

Structure of DNA

DNA is a double helix composed of four building blocks (A, T, C, G). The sequence of these bases encodes the information needed for an organism's growth and reproduction. The two strands are joined by base pairing (A with T, C with G), which allows DNA to be copied accurately.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information in cells: DNA codes for RNA, which codes for proteins. Messenger RNA (mRNA) is read to make proteins, which are crucial for cellular structure and function.

Genetic Variation and Heredity

DNA is copied to pass genetic information from cell to cell or from parent to offspring. Mistakes in DNA replication can lead to changes in proteins, resulting in heritable variations that underlie the diversity of life.

Energy and Metabolism

All chemical reactions in cells require energy. Organisms have two fundamental nutritional needs:

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

• Obtaining molecules used as building blocks for DNA, RNA, proteins, etc.

How organisms acquire energy is central to the diversification of life. For example, plants and some bacteria can produce sugar using sunlight, which is then used to make ATP or stored in energy-rich molecules.

Life Evolves

Evolution and Natural Selection

Evolution is the change in characteristics of a population over time. Charles Darwin and Alfred Russel 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 must be met:

• Individuals must vary in heritable characteristics.

• Certain heritable traits help individuals reproduce more successfully 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 is the ability of an individual to produce surviving offspring. Adaptation is a trait that increases fitness in a particular environment. For example, finches on the Galápagos Islands with small, pointed beaks had higher fitness when small, soft seeds were abundant, leading to an increase in this trait in the population.

The Tree of Life and Phylogeny

Depicting Evolutionary History

The tree of life is a family tree of organisms, describing genealogical relationships among species with a single ancestral species at its base. Phylogeny refers to the actual genealogical relationships among all organisms. Biologists analyze genetic variation by comparing DNA and RNA sequences; fewer sequence differences indicate a closer relationship.

Phylogenetic Trees

A phylogenetic tree shows the relationships between species. Branches that share a recent common ancestor represent closely related species. The tree of life, estimated from genetic data, indicates three major groups:

• Eukarya: Eukaryotes (have a nucleus)

• Bacteria: Prokaryotes (lack a nucleus)

• Archaea: Prokaryotes (lack a nucleus)

Taxonomy and Classification

Taxonomic Systems

Taxonomy is the effort to name and classify organisms. A taxon is a named group. The highest taxonomic level is the domain (Bacteria, Archaea, Eukarya). Within domains, a phylum is a major lineage.

Carolus Linnaeus established the modern classification system, giving each organism a unique two-part scientific name (genus and species). Scientific names are always italicized, with the genus capitalized and the species not capitalized (e.g., Homo sapiens).

Doing Biology: The Nature of Science

Scientific Method and Hypothesis Testing

Science involves asking questions that can be answered by collecting data. The process includes:

• Formulating hypotheses

• Finding evidence that supports or conflicts with those hypotheses

Hypothesis testing is a two-step process:

1. State the hypothesis as precisely as possible and list its predictions.

2. Design an observational or experimental study capable of testing those predictions.

Examples of Hypothesis Testing

• Giraffe Neck Length: The food competition hypothesis predicted that giraffes evolved long necks to reach food high in trees. However, data did not support this, and an alternative hypothesis (sexual competition) was proposed and supported by evidence.

• Ant Navigation: The pedometer hypothesis suggested that ants use stride length and step number to navigate. Experimental manipulation of ant leg length supported this hypothesis, as ants with altered stride lengths misjudged the distance back to their nest.

Experimental Design

Good experimental design includes:

• Control groups to check for other factors

• Constant experimental conditions

• Repeating tests

• Large sample sizes