Biology: The Study of Life

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

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

• Cells: All organisms are composed 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: Organisms process hereditary information encoded in genes and respond to environmental signals.

• Energy: Organisms acquire and use energy to maintain life processes.

• Evolution: Populations of organisms evolve over time, adapting to their environments.

Theories in Biology

A theory in science is an explanation for a broad class of phenomena, supported by extensive evidence. Three major theories form the framework for modern biological science:

• Cell Theory: All organisms are made of cells; all cells come from preexisting cells.

• 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 theory was developed after the invention of microscopes. Robert Hooke observed cork tissue and termed the compartments "cells." Anton van Leeuwenhoek observed single-celled organisms, which he called "animalcules."

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

• Cell Theory: All organisms consist of cells; all cells come from preexisting cells.

Cell Theory and Spontaneous Generation

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.

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

• Spontaneous generation hypothesis: Organisms arise spontaneously.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

Hereditary information is encoded in genes located on chromosomes. DNA is the hereditary material, and genes are segments of DNA that code for cell products.

Structure of DNA

DNA is a double helix composed of four building blocks: adenine (A), thymine (T), cytosine (C), and guanine (G). The sequence of these bases encodes genetic information.

• Base Pairing: A pairs with T, C pairs with G. This pairing allows DNA to be copied accurately.

The Central Dogma

The central dogma describes the flow of genetic information in cells: DNA codes for RNA, which codes for proteins. Proteins determine physical traits and carry out essential cellular functions.

• Messenger RNA (mRNA): Carries genetic information from DNA to the ribosome for protein synthesis.

• Proteins: Form structural components and promote chemical reactions.

Genetic Variation and Heredity

DNA is copied to pass genetic information from cell to cell or from one organism to its offspring. Changes in DNA sequence can lead to heritable variations, which underlie the diversity of life.

Energy and Metabolism

Chemical reactions inside cells require energy. Organisms must acquire chemical energy (often in the form of ATP) and molecules used as building blocks for DNA, RNA, proteins, etc. How organisms acquire energy is central to the diversification of life.

• Plants and bacteria: Can produce sugar using energy from sunlight, then use sugar to make ATP or store it in energy-rich molecules.

Life Evolves

Evolution and Natural Selection

Evolution is the change in characteristics of a population over time. Darwin and Wallace proposed that species are related by common ancestry and can be modified from generation to generation (descent with modification).

• Population: 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: individuals vary in heritable characteristics, and certain traits increase reproductive success in a particular environment.

• Fitness: Ability of an individual to produce offspring.

• Adaptation: Trait that increases fitness in a particular environment.

Example of Natural Selection

Finches on the Galápagos Islands with small, pointed beaks had higher fitness when small, soft seeds were abundant. This adaptation increased their ability to thrive, and the trait became more common in the population.

The Tree of Life Depicts Evolutionary History

Phylogeny and Genetic Variation

The tree of life depicts evolutionary history and genealogical relationships among species. Biologists analyze genetic variation by comparing DNA and RNA sequences; fewer sequence differences indicate closer relationships.

• Phylogenetic tree: Used to show relationships between species. Branches sharing a recent common ancestor represent closely related species.

Major Groups of Organisms

The tree of life indicates three major groups:

• Eukaryotes: Have a nucleus (Eukarya).

• Bacteria: Prokaryotes lacking a nucleus.

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

Taxonomy and Classification

Linnaeus’ System and Nomenclature

Taxonomy is the effort to name and classify organisms. Each organism is given a unique two-part scientific name (genus and species). Scientific names are always italicized; genus names are capitalized, species names are not (e.g., Homo sapiens).

Doing Biology: The Nature of Science

Scientific Method and Hypothesis Testing

Science involves formulating hypotheses and finding evidence that supports or conflicts with them. Hypothesis testing is a two-step process: state the hypothesis and predictions, then design a study to test them.

Experimental Design

Experiments allow researchers to test the effect of a single factor. Good experimental design includes a control group, constant conditions, repetition, and a large sample size.

Example: Giraffe Neck Length

The food competition hypothesis suggested giraffes evolved long necks to reach high food. However, data did not support this. The sexual competition hypothesis, which posited that longer-necked males win more fights and father more offspring, was supported by data.

Example: Ant Navigation

Wittlinger and colleagues tested the pedometer hypothesis in desert ants. Manipulating leg length affected the distance ants traveled, supporting the hypothesis that ants use stride length and number to navigate.

Additional info: These notes expand on brief points with academic context, definitions, and examples to provide a comprehensive overview suitable for General Biology students.