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.

• Replication: All organisms are capable of reproduction, ensuring the continuation of their species.

• Information: 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 characteristics.

Theories in Biology

Scientific theories provide broad explanations for natural phenomena, supported by extensive 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: In science, a theory is not a guess, but a well-supported explanation for a broad set of observations.

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 30x microscope to observe small compartments in cork, naming them "cells."

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

• Cell Theory: All organisms are made up of cells, and all cells arise from preexisting cells.

Example: Multicellular organisms like humans develop from a single cell (zygote) through repeated cell division.

Cell Theory vs. Spontaneous Generation

Cell theory challenged the idea of spontaneous generation, which posited that life 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.

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

1.3 Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

Genetic information is encoded in genes, which are located on chromosomes. This theory explains how traits are inherited.

• Chromosomes: Molecules of deoxyribonucleic acid (DNA) that carry genetic information.

• Genes: Segments of DNA that code for specific cell 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 of Molecular Biology

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 Variation and Heredity

DNA is copied with high accuracy during cell division, but mutations can occur, leading to genetic variation.

• Mutations: Changes in DNA sequence that may alter proteins and result in heritable variation.

• Diversity of Life: Heritable variations are the basis for the diversity observed among living organisms.

Energy and Nutritional Needs

All cellular chemical reactions require energy. Organisms must acquire:

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

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

Example: Plants acquire energy via photosynthesis, while animals obtain energy by consuming other organisms.

1.4 Life Evolves

Evolution and Natural Selection

Evolution is the change in the characteristics of populations over time. Darwin and Wallace proposed that species are related by common ancestry and change through "descent with modification."

• Population: Group of individuals of the same species living in the same area.

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

  1. Individuals vary in heritable characteristics.

  2. Certain traits increase reproductive success in specific environments.

• Speciation: Occurs when populations diverge to form new species.

Fitness and Adaptation

Natural selection favors individuals with traits that increase their fitness, defined as the ability to produce surviving offspring.

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

• Example: Galápagos finches with small, pointed beaks had higher fitness when small, soft seeds were abundant, leading to an increase in this trait 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: The actual genealogical relationships among all organisms.

• Genetic Variation Analysis: Biologists compare DNA and RNA sequences to determine evolutionary relationships. Fewer sequence differences indicate closer relationships.

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.

Major Groups of Life

The tree of life indicates three major groups of organisms:

• Eukaryotes: Organisms with a nucleus (Eukarya).

• Prokaryotes: Organisms without a nucleus, divided into Bacteria and Archaea.

Additional info: The classification of life into these three domains is based on genetic and structural differences.