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. These criteria distinguish living entities from non-living matter.

• Cells: All organisms are composed of membrane-bound cells, which serve as highly organized compartments separated from their environment.

• Replication: All organisms possess the ability to reproduce, ensuring the continuation of their species.

• Information: Organisms process hereditary information encoded in genes and respond to environmental signals.

• Energy: All organisms acquire and utilize 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 these characteristics.

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 arise 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 chromosomes.

Definition: In science, a theory is not a guess but a well-supported explanation based on evidence.

1.2 Life is Cellular and Replicates through Cell Division

Discovery of Cells

The cell theory emerged from the work of early microscopists:

• Robert Hooke (1665): Used a microscope to observe small compartments in cork, naming them "cells."

• Anton van Leeuwenhoek: Improved magnification and observed single-celled organisms, termed "animalcules."

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

Example: Cork tissue (dead cells) and pond water (living single-celled organisms) were among the first observations.

Cell Theory and Spontaneous Generation

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

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

• Spontaneous generation hypothesis: Organisms could arise spontaneously under certain conditions.

Louis Pasteur's Experiment: Pasteur demonstrated that cells arise only from preexisting cells, not spontaneously, using nutrient broth in swan-necked flasks.

Life Replicates through Cell Division

For life to persist, cells must replicate. Cell theory predicts that 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.

1.3 Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

Genetic information is encoded in genes, which are located on chromosomes. Chromosomes are composed of deoxyribonucleic acid (DNA), the hereditary material. Genes are segments of DNA that code for cellular products.

Structure of DNA

James Watson and Francis Crick discovered that DNA is a double-stranded helix, with a backbone and paired bases (A, T, C, G).

The Central Dogma

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

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

Equation:

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

Proteins determine the physical traits of organisms.

Genetic Variation and Heritability

DNA is copied with high accuracy during cell division. Mistakes in DNA sequence (mutations) can lead to changes in proteins, resulting in heritable variations that contribute to the diversity of life.

Energy and Nutritional Needs

Cells require energy to drive chemical reactions. Organisms must:

• Acquire chemical energy, often in the form of adenosine triphosphate (ATP).

• Obtain molecules that serve as building blocks for DNA, RNA, proteins, and other cellular components.

The method by which 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.

• 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.

Mechanism of Natural Selection

Natural selection explains how evolution occurs. Two conditions are required:

• Individuals must vary in heritable characteristics.

• Certain versions of these traits must enhance reproductive success in specific environments.

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 contribute more to the next generation.

• 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 led to an increase in the frequency of small, pointed beaks 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.

Analyzing Genetic Variation

Biologists analyze genetic variation by comparing RNA and DNA sequences among organisms. Fewer sequence differences indicate a closer evolutionary relationship.

Organism	DNA Sequence
Land plant	A-T-A-T-C-G-A-G
Green algae	A-T-A-T-G-G-A-G
Brown algae	A-A-A-T-G-G-A-C

Example: Green algae is more closely related to land plants than brown algae, based on DNA sequence similarity.

Interpreting the Tree of Life

The tree of life reveals three major groups of organisms:

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

• Prokaryotes: Organisms without a nucleus, divided into two domains:

  • Bacteria

  • Archaea

Additional info: The classification of life into three domains is based on genetic and structural differences, with phylogenetic trees constructed from genetic data.