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 have the capacity 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 e;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;volving, adapting to their environments over time.

Example: Bacteria, plants, and animals all meet these criteria, though they differ greatly in complexity.

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.

• Theory of Evolution by Natural Selection: Explains relationships and changes among organisms over time.

• Chromosome Theory of Inheritance: Describes how hereditary information is transmitted across generations.

Definition: A scientific theory is not a mere guess; it is a well-supported explanation for a general class of phenomena.

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 and name "cells" in cork tissue.

• Anton van Leeuwenhoek: Improved magnification to 300x, observing single-celled organisms ("animalcules").

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

Cell Theory and 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 only when pre-existing cells grow and divide.

• Spontaneous generation hypothesis: Organisms could arise spontaneously.

Louis Pasteur’s Experiment

Pasteur’s experiment provided evidence against spontaneous generation.

• Two flasks with nutrient broth: one with a straight neck, one with a swan neck open to air.

• Only the straight-neck flask showed cell growth, supporting the all-cells-from-cells hypothesis.

Conclusion: Cells arise from pre-existing cells, not spontaneously.

Life Replicates through Cell Division

Cell division is essential for life, as all cells in multicellular organisms descend from pre-existing cells, sharing a common lineage. Modern evidence suggests life originated from non-life via chemical evolution.

1.3 Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

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

• Genes: 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 RNA, which codes for proteins.

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

Proteins determine the physical traits of organisms.

Genetic Variation and Heritability

DNA is copied with high accuracy during cell division. Mistakes (mutations) can alter protein production, leading to heritable variations and diversity among organisms.

• Heritable variation: Changes in DNA sequence that can be passed to offspring.

Energy and Nutritional Needs

Cells require energy for chemical reactions and two main nutritional needs:

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

• Molecules for building cellular components (DNA, RNA, proteins).

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 can be modified across generations (descent with modification).

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

Natural Selection

Natural selection is the process by which certain heritable traits increase an individual's reproductive success, leading to evolutionary change in populations.

• Individuals must vary in heritable characteristics.

• Certain traits confer higher reproductive success in specific environments.

Over time, these traits become more common, and populations may diverge to form new species (speciation).

Fitness and Adaptation

• Fitness: The ability of an individual to produce surviving offspring.

• Adaptation: A trait that increases fitness in a particular environment.

Example: On the Galápagos Islands, 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 refers to the actual evolutionary relationships among organisms.

Analyzing Genetic Variation

Biologists compare RNA and DNA sequences to analyze genetic variation and infer 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.

Major Groups in the Tree of Life

The tree of life reveals three major groups of organisms:

• Eukaryotes (have a nucleus): Eukarya

• Prokaryotes (lack a nucleus): Bacteria and Archaea

Additional info: Eukaryotic cells are generally larger and more complex than prokaryotic cells.