Biology: The Study of Life – Chapter 1 Study Notes

Biology: The Study of Life

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

Biologists define life by five fundamental characteristics that all living organisms share. Understanding these properties is essential for distinguishing living things from non-living matter.

• Cells: All organisms are composed of membrane-bound cells, which serve as 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 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 time.

Example: Bacteria, plants, and animals all meet these criteria, demonstrating the universality of life's properties.

Theories in Biology

Scientific theories provide broad explanations for natural phenomena, supported by substantial evidence. In biology, three major theories form the foundation of the discipline:

• Cell Theory: Addresses the composition and origin of organisms, stating that all organisms are made of cells and all cells come from preexisting cells.

• Theory of Evolution by Natural Selection: Explains the relationships among organisms and how species change over time.

• Chromosome Theory of Inheritance: Describes how hereditary information is transmitted from one generation to the next via genes located on chromosomes.

Definition: A theory in science is an explanation for a general class of phenomena, supported by a wide body of evidence, and differs from the everyday use of the word as a mere guess.

1.2 Life is Cellular and Replicates through Cell Division

Discovery of Cells

The development of microscopes enabled scientists to observe cells, leading to the formulation of cell theory.

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

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

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

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

Cell Theory and Spontaneous Generation

Cell theory challenged the idea of spontaneous generation, which posited that organisms could arise from non-living matter under certain conditions.

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

• Spontaneous generation hypothesis: Organisms could arise spontaneously from non-living material.

Louis Pasteur’s Experiment

Pasteur’s experiment provided evidence against spontaneous generation and supported cell theory.

• Two glass flasks containing nutrient broth were used: one with a straight neck and one with a swan neck open to the air.

• Only the flask exposed to airborne cells developed new cells, confirming that cells arise from preexisting cells.

Conclusion: The all-cells-from-cells hypothesis was correct.

Life Replicates through Cell Division

For life to persist, cells must replicate. All cells in multicellular organisms descend from preexisting cells, sharing a common lineage. Modern evidence suggests that life originated from non-life through chemical evolution.

• Chemical evolution: The process by which life arose from non-living chemical substances on early Earth.

1.3 Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

Hereditary information is encoded in genes, which are located on chromosomes. Chromosomes are composed of deoxyribonucleic acid (DNA), the hereditary material.

• Genes: Segments of DNA that code for cell products.

• DNA: Double-stranded helix structure proposed by Watson and Crick.

The Central Dogma

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

• DNA codes for ribonucleic acid (RNA), which codes for proteins.

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Proteins determine the physical traits of organisms.

Genetic Information and Variation

DNA is copied with high accuracy during cell division, passing genetic information to offspring. Mistakes in DNA replication can lead to changes in proteins, resulting in heritable variations that contribute to the diversity of life.

• Heritable variation: Genetic differences that can be passed to offspring.

• Diversity of life: The result of accumulated genetic variations over generations.

Energy and Nutritional Needs

Cells require energy to carry out chemical reactions. Organisms must acquire:

• Chemical energy in the form of adenosine triphosphate (ATP).

• Building blocks for synthesizing DNA, RNA, proteins, and other cellular components.

How organisms acquire energy is central to the diversification of life.

1.4 Life Evolves

Evolution and Natural Selection

Evolution is the change in characteristics of populations over time. Darwin and Wallace proposed that species are related by common ancestry and that characteristics can be modified across generations, a process called "descent with modification."

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

Natural Selection

Natural selection is the mechanism by which evolution occurs. Two conditions are required:

• Individuals must vary in heritable characteristics.

• Certain versions of these traits must help individuals reproduce more successfully in their 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 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 ability to thrive, and the trait became more common in the population.

1.5 The Tree of Life Depicts Evolutionary History

Phylogeny and the Tree of Life

The tree of life is a family tree that describes the 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 among organisms. Fewer sequence differences indicate a closer relationship.

• Example: Green algae DNA is more similar to land plant DNA than to brown algae DNA, indicating a closer evolutionary relationship.

The Phylogenetic Tree of Life

A phylogenetic tree visually represents the evolutionary relationships between species. Branches sharing a recent common ancestor represent closely related species, while those without a recent common ancestor are more distantly related. The tree of life is estimated from genetic data.

Major Groups of Organisms

The tree of life indicates three major groups:

• Eukaryotes (have a nucleus): Eukarya

• Prokaryotes (lack a nucleus): Bacteria and Archaea

Additional info: Eukaryotes include animals, plants, fungi, and protists, while prokaryotes are unicellular organisms without a membrane-bound nucleus.