Biology: The Study of Life

What Does It Mean to Say That Something is Alive?

Biology is the scientific study of life and living organisms. All living things 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 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 life processes.

• 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 broad 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 a guess or speculation.

• 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: Explains how hereditary information is transmitted from one generation to the next.

Life is Cellular and Replicates through Cell Division

Discovery of Cells

The cell theory is foundational to biology. In the 1660s, Robert Hooke and Anton van Leeuwenhoek were the first to observe cells using microscopes. Hooke observed compartments in cork tissue, which he called "cells," while van Leeuwenhoek observed single-celled organisms, which he termed "animalcules." Cells are highly organized compartments separated from their environment by a membrane barrier.

• All organisms are made up of cells.

• All cells come from preexisting cells.

Cell Theory vs. Spontaneous Generation

The cell theory challenged the idea of spontaneous generation, which posited that organisms could arise spontaneously under certain conditions. Louis Pasteur's experiments demonstrated that cells arise only from preexisting cells, not by spontaneous generation.

Cell Division and Common Lineage

Cells must replicate for life to exist. In multicellular organisms, all cells descend from preexisting cells, sharing a common lineage. Modern evidence suggests that life arose from non-life through chemical evolution early in Earth's history.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

Hereditary or genetic information is encoded in genes, which are units located on chromosomes. In the 1950s, it was established that chromosomes are composed 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, with each strand made up of four building blocks: adenine (A), thymine (T), cytosine (C), and guanine (G). The sequence of these bases encodes the information necessary for an organism's growth and reproduction. The two strands are joined by specific base pairing: A pairs with T, and C pairs with G. This structure allows DNA to be accurately copied and preserves genetic information.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information in cells: DNA codes for ribonucleic acid (RNA), which codes for proteins. Messenger RNA (mRNA) is transcribed from DNA and then translated to produce proteins, which are crucial for cellular structure and function.

• Proteins determine the physical traits of organisms.

• Changes in DNA sequence can lead to changes in proteins, resulting in heritable variation and diversity of life.

Energy and Nutritional Needs

All chemical reactions in cells require energy. Organisms must acquire chemical energy (often in the form of adenosine triphosphate, ATP) and 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. For example, plants and some bacteria can produce sugars using sunlight, which they use to make ATP or store as energy-rich molecules.

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 and that characteristics of species can be modified from generation to generation (descent with modification).

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

• Natural Selection: Explains how evolution occurs. Two conditions must be met:

  1. Individuals must vary in heritable characteristics.

  2. Certain heritable traits must 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: The ability of an individual to produce surviving offspring. Individuals with higher fitness leave more 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 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 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 tree: Used to show relationships between species. Branches that share a recent common ancestor represent closely related species.

Major Groups of Organisms

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

• Prokaryotes: Organisms without a nucleus (Domains: Bacteria and Archaea).

Taxonomy and Classification

Taxonomy is the effort to name and classify organisms. The highest taxonomic level is the domain, which includes Bacteria, Archaea, and Eukarya. Each organism is given a unique two-part scientific name (genus and species), following rules established by Carolus Linnaeus. 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 and finding evidence that supports or refutes them. 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.

Example: Why Do Giraffes Have Long Necks?

• Food Competition Hypothesis: Long necks evolved to reach food unavailable to other mammals. Predictions: neck length is variable and heritable; giraffes feed high in trees. However, data showed giraffes do not usually feed high in trees.

• Sexual Competition Hypothesis: Long necks evolved because longer-necked males win more fights and father more offspring. Data support this hypothesis.

Experimental Design: How Do Ants Navigate?

Experiments allow researchers to test the effect of a single, well-defined factor. For example, Wittlinger and colleagues tested the "pedometer hypothesis"—that ants track the number of steps and stride length to navigate back to their nest. By manipulating ant leg length, they found that stride length and step number affect navigation, supporting the hypothesis.

• Control group: Essential for good experimental design to rule out other factors.

• Repetition and sample size: Important for reliable results.

Additional info: These notes provide a comprehensive overview of the foundational concepts in biology, including the characteristics of life, cell theory, genetic information, evolution, taxonomy, and the scientific method. They are suitable for exam preparation and as an introduction to further topics in General Biology.