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?

Biology is the scientific study of life, and 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 the basic units of life.

• 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 signals.

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

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

Theories in Biology

A theory in science is an explanation for a broad class of phenomena, supported by extensive evidence. This differs from the everyday use of "theory" as mere speculation.

• Cell Theory: Addresses what organisms are made of and their origins.

• 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 across generations.

Life is Cellular and Replicates through Cell Division

Discovery of Cells

The cell theory was developed through the work of early microscopists:

• Robert Hooke (1665): Used a microscope to observe small compartments, 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.

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: Suggested organisms could arise spontaneously.

Louis Pasteur’s Experiment

Pasteur tested whether cells arise from pre-existing cells or by spontaneous generation using nutrient broth in two flasks, one with a swan neck open to air. His results supported the all-cells-from-cells hypothesis.

Cell Division and Lineage

Cells must replicate for life to persist. According to cell theory, all cells in multicellular organisms descend from pre-existing cells, sharing a common lineage. Evidence suggests life originated from non-life via chemical evolution.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

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

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

• DNA is copied to pass genetic information from cell to cell or from parent to offspring.

Structure of DNA

James Watson and Francis Crick proposed that DNA is a double-stranded helix, which allows for accurate copying and transmission of genetic information.

The Central Dogma

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

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

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

Genetic Variation and Diversity

DNA sequence changes (mutations) can alter proteins, leading to heritable variations that drive the diversity of life.

• Outward appearance is determined by the proteins produced.

• Heritable variations are the basis for evolution.

Energy and Nutritional Needs

Chemical reactions within cells require energy. Organisms must:

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

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

How organisms acquire energy is central to their diversification.

Life Evolves

Evolution and Natural Selection

Evolution is the change in characteristics of a population over time. Darwin and Wallace proposed:

• Species are related by common ancestry.

• Species characteristics can be modified across generations (descent with modification).

Population and Natural Selection

A population is a group of individuals of the same species living in the same area. Natural selection explains how evolution occurs:

• Individuals vary in heritable characteristics.

• Certain traits increase reproductive success in specific environments.

Fitness and Adaptation

Fitness is the ability of an individual to produce offspring. Adaptation is a trait that increases fitness in a particular environment.

• Example: Finches on the Galápagos Islands with small, pointed beaks had higher fitness when small, soft seeds were abundant. This adaptation increased their population frequency.

The Tree of Life Depicts Evolutionary History

Phylogeny and Genetic Variation

The tree of life is a family tree depicting evolutionary relationships among organisms, with a single ancestral species at its base. Phylogeny refers to the actual genealogical relationships among all organisms.

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

• Fewer sequence differences indicate closer relationships.

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

Phylogenetic Tree of Life

A phylogenetic tree visually represents relationships between species:

• Branches sharing a recent common ancestor indicate closely related species.

• Branches without a recent common ancestor indicate more distant relationships.

Major Groups of Organisms

The tree of life reveals three major groups:

• Eukaryotes: Organisms with a nucleus (Eukarya).

• Bacteria: Prokaryotes lacking a nucleus.

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