Biology: The Study of Life

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 membrane-bound cells, which are highly organized compartments separated from their environment.

• 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 information.

• Energy: All organisms acquire and use energy to sustain life processes.

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

Theories in Biology

A theory is an explanation for a broad class of phenomena, supported by a wide body of evidence. In biology, three major theories form the framework for modern science:

• Cell Theory: All organisms are made of cells, and 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 originated from observations by Robert Hooke and Anton van Leeuwenhoek, who used microscopes to observe cells and single-celled organisms. This led to the understanding that:

• All organisms consist of cells.

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

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

Cell Theory and Spontaneous Generation

Cell theory challenged the idea of spontaneous generation, which posited that organisms could arise spontaneously. Louis Pasteur's experiment demonstrated that cells arise only from preexisting cells, supporting the cell theory.

Life Replicates through Cell Division

Cell division is essential for life. All cells in multicellular organisms descend from preexisting cells, sharing a common lineage. Evidence suggests that life originated from non-life through chemical evolution.

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-stranded helix composed of four building blocks: A, T, C, and G. The sequence of these bases encodes genetic information. The two strands are joined by base pairing (A with T, C with G), allowing DNA to be copied accurately.

The Central Dogma

The central dogma describes the flow of genetic information in cells:

• DNA codes for RNA

• RNA codes for proteins

Proteins carry out essential cellular functions, including structural roles and catalyzing chemical reactions.

Genetic Variation and Heredity

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

Energy and Metabolism

Cells require energy for chemical reactions. Organisms acquire energy in the form of ATP and molecules used as building blocks for DNA, RNA, and proteins. The method of energy acquisition is central to the diversification of life.

• Plants and bacteria can produce sugar using sunlight, which is used to make ATP or stored in energy-rich molecules.

• Other organisms absorb molecules from their environment as food.

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

• Natural Selection: Occurs when individuals vary in heritable traits, and certain traits increase reproductive success in a particular environment.

• Speciation: The process by which 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 with small, pointed beaks had higher fitness when small, soft seeds were abundant, leading to an increase in this adaptation in the population.

The Tree of Life Depicts Evolutionary History

Phylogeny and Genetic Variation

The tree of life depicts evolutionary history, showing genealogical relationships among species. Phylogeny is determined by comparing DNA and RNA sequences; fewer sequence variations indicate closer relationships.

• Phylogenetic trees 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: Organisms with a nucleus (Eukarya).

• Bacteria: Prokaryotes lacking a nucleus.

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

Taxonomy and Classification

Linnaeus’ System and Scientific Names

Taxonomy is the effort to name and classify organisms. Linnaeus established a system where each organism is given a unique two-part scientific name (genus and species). Scientific names are italicized, with genus capitalized and species not capitalized (e.g., Homo sapiens).

Doing Biology: The Nature of Science

Scientific Method and Hypothesis Testing

Science involves formulating hypotheses and testing them through observation and experimentation. A hypothesis is a testable statement explaining an observation, and experiments allow researchers to test the effect of a single factor.

• Hypothesis testing involves stating a hypothesis and predictions, then designing a study to test them.

• A null hypothesis specifies what should be observed if the tested hypothesis is incorrect.

Case Study: Giraffe Neck Length

The food competition hypothesis suggested giraffes evolved long necks to reach high food. However, data showed giraffes do not usually feed high in trees, refuting this hypothesis. The sexual competition hypothesis, supported by data, proposed that longer-necked males win more fights and father more offspring.

Experimental Design: How Do Ants Navigate?

Experiments are powerful tools for testing hypotheses. Wittlinger and colleagues tested the pedometer hypothesis, which posited that ants use stride length and step number to navigate back to their nest. Ants were manipulated into three groups: shortened, typical, and lengthened legs. Results showed that stride length and step number affect navigation, supporting the pedometer hypothesis.

Characteristics of Good Experimental Design

• Include a control group to check for other influencing factors.

• Keep experimental conditions constant.

• Repeat tests and use large sample sizes for reliability.

Additional info: These notes expand brief points into full academic explanations, providing context and examples for each concept. Images included are directly relevant to the explanation of giraffe neck length and ant navigation experiments.