What is Life

Properties of Life

Living organisms are distinguished from non-living entities by a set of fundamental properties. Understanding these properties is essential for classifying organisms and studying biology.

• Cellular Organization: All living things are composed of one or more cells, which are the basic units of life.

• Metabolism: The sum of all chemical reactions that occur within an organism, enabling it to obtain and use energy. Example: Cellular respiration and photosynthesis.

• Homeostasis: The ability to maintain a stable internal environment despite external changes. Example: Regulation of body temperature in mammals.

• Growth and Development: Living organisms grow and develop according to specific instructions coded in their DNA.

• Reproduction: The ability to produce new individuals, either sexually or asexually.

• Response to Stimuli: Organisms respond to environmental changes. Example: Plants bending toward light (phototropism).

• Evolution: Populations of organisms change over time through adaptation and natural selection.

• Emergent Properties: New properties that arise at each level of biological organization, not present in the preceding level.

Cell Theory and Biological Organization

The cell theory is a foundational concept in biology, stating that all living things are composed of cells, and all cells arise from pre-existing cells. Cells fit into a hierarchy of biological organization:

• Cells → Tissues → Organs → Organisms → Populations → Communities → Ecosystems

• Biotic factors: Living components (e.g., plants, animals).

• Abiotic factors: Non-living components (e.g., temperature, water).

Types of Cellular Organization

Cells can be organized in three main ways:

• Unicellular: Single-celled organisms (e.g., Escherichia coli).

• Colonial: Groups of identical cells living together (e.g., Volvox).

• Multicellular: Organisms with specialized cells (e.g., humans, plants).

Reproduction: Sexual vs. Asexual

Reproduction is essential for the continuation of species. There are two main types:

• Sexual Reproduction: Involves the fusion of gametes, increases genetic diversity. Example: Humans, flowering plants.

• Asexual Reproduction: Offspring arise from a single parent, genetically identical. Example: Bacteria (binary fission), hydra (budding).

Energy and Carbon Sources

Organisms are classified by their energy and carbon sources:

• Photoautotrophs: Use light as energy and CO2 as carbon source. Example: Plants, cyanobacteria.

• Chemoheterotrophs: Use organic molecules for both energy and carbon. Example: Animals, fungi.

• Mixotrophs: Can use both autotrophic and heterotrophic modes. Example: Euglena.

Metabolism

Metabolism refers to all chemical reactions in an organism. It includes:

• Anabolism: Building up molecules (e.g., protein synthesis).

• Catabolism: Breaking down molecules (e.g., glycolysis).

Homeostasis and Feedback Loops

Homeostasis is maintained by feedback loops:

• Negative Feedback: Counteracts changes (e.g., blood glucose regulation).

• Positive Feedback: Amplifies changes (e.g., blood clotting).

Responses in Plants and Animals

Organisms respond to stimuli in various ways:

• Morphological: Physical changes (e.g., thicker fur in winter).

• Behavioral: Actions (e.g., migration).

• Physiological: Internal processes (e.g., sweating).

• Biochemical: Changes in molecular pathways (e.g., hormone release).

Responses differ between plants (e.g., phototropism) and animals (e.g., reflexes).

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information:

• DNA → RNA → Protein

Proteins are produced via transcription and translation.

Organismal Hierarchy

From smallest to largest:

• Cell

• Tissue

• Organ

• Organism

• Population

• Community

• Ecosystem

Evolution and Adaptation

Evolution is the change in populations over time. Adaptations are traits that increase fitness. The central dogma links genetic changes to protein function and adaptation.

• Morphological Adaptation: Physical traits (e.g., beak shape).

• Behavioral Adaptation: Actions (e.g., nocturnal activity).

• Physiological Adaptation: Internal processes (e.g., antifreeze proteins in fish).

Acclimation is a short-term adjustment; adaptation is a long-term genetic change.

Fitness is the ability to survive and reproduce. Individuals with beneficial adaptations are more fit.

Emergent Properties

Emergent properties arise from the interaction of simpler elements, resulting in new characteristics at higher levels of organization.

Viruses

Cellular Nature of Viruses

Viruses are not cells. They lack cellular structures and metabolic machinery, distinguishing them from prokaryotic and eukaryotic cells.

• Prokaryotic Cells: No nucleus, simple structure (e.g., bacteria).

• Eukaryotic Cells: Nucleus, complex organelles (e.g., plants, animals).

• Viruses: Composed of genetic material (DNA or RNA) and a protein coat; require a host to replicate.

Properties of Life in Viruses

Viruses meet some properties of life only when associated with a host:

• Can reproduce and evolve, but only inside host cells.

• Lack metabolism and homeostasis independently.

Types of Viruses and Life Cycles

Viruses are classified by their genetic material and replication cycles:

• DNA Viruses: Use DNA as genetic material.

• RNA Viruses: Use RNA as genetic material.

• Lytic Cycle: Virus replicates and destroys host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome, replicates with cell.

Vaccination and Viral Evolution

Some viruses require one vaccination (e.g., measles), while others require annual vaccines (e.g., influenza) due to rapid mutation rates and antigenic variation.

Pattern and Process of Evolution

Modern Phylogeny vs. Historical Classification

Modern phylogeny uses molecular and morphological data to explain evolutionary relationships, differing from historical systems like Aristotle's and Linnaeus's, which relied on observable traits.

Pattern vs. Process of Evolution

• Pattern: The observable outcomes of evolution (e.g., diversity of life).

• Process: The mechanisms driving evolution (e.g., natural selection).

Evolutionary Mechanisms

• Natural Selection: Differential survival and reproduction.

• Gene Flow: Movement of genes between populations.

• Genetic Drift: Random changes in allele frequencies.

• Mutations: Changes in DNA sequence.

Phylogeny and the Tree of Life

Binomial Nomenclature

Scientific names use binomial nomenclature: Genus species (e.g., Homo sapiens), fitting into Linnaeus's hierarchy and modern phylogeny.

Phylogenetic Trees

Phylogenetic trees represent hypotheses about evolutionary relationships. Types include:

• Rooted: Shows common ancestor.

• Unrooted: No explicit ancestor.

• Cladogram: Shows branching order.

• Phylogram: Branch lengths indicate evolutionary change.

Building Parsimonious Trees

Shared characteristics are used to build the simplest (parsimonious) phylogenetic trees.

Phylogenetic Terminology

Extinct vs. Extant Taxa

• Extinct: No longer living.

• Extant: Currently living.

Interpreting Phylogenetic Trees

Phylogenetic trees allow interpretation of relationships and characteristics among taxa, including divergent (from common ancestor) and convergent (independent evolution) relationships.

Phylogenies are graphical hypotheses of relationships among organisms.