chapter 1.1 & 1.2
1.1 Characteristics of Life
Learning outcomes: distinguish among levels of biological organization; identify the basic characteristics of life.
Life on Earth is incredibly diverse and globally distributed. Estimates suggest roughly species, not counting bacteria (which are historically hard to identify). Size range spans from tiny bacteria to giants like 300‑foot sequoias or ~ in mass for large whales.
The science of biology is the study of life. Biologists may study life from many perspectives. Figure 1.1 (referenced) shows major groups of living organisms.
Examples of diversity in form and nutrition:
Bacteria: widely distributed, microscopic, simple structure.
Paramecium: an example of a microscopic protist (larger and more complex than bacteria).
Other organisms visible to the naked eye:
Fungi (e.g., molds) digest food externally.
Sunflowers: photosynthetic plants that make their own food.
Whales: aquatic animals that ingest food.
Basic characteristics that unify life (shared by all organisms):
Life is organized and obeys the same laws of chemistry and physics that govern nonliving matter.
Life is composed of chemical elements; but life is distinguished by how these elements are organized and function.
Emergent organization: life is organized in a hierarchical manner beyond the level of a single cell; new properties arise at higher levels due to interactions among parts.
Major idea: unity of life with enormous diversity; life follows the same fundamental principles across scales.
Levels of Biological Organization
Life is organized hierarchically (Figure 1.2 reference):
Atoms → small molecules → macromolecules → cell (the basic unit of life).
Some organisms are single-celled (e.g., bacteria); many organisms are multicellular.
Multicellular organization includes:
Cells may form tissues (e.g., nerve tissue, muscle tissue).
Tissues form organs (e.g., brain, leaf).
Organs form organ systems (e.g., nervous system: brain, spinal cord, nerves).
Organ systems form an organism (e.g., elephant).
Populations: members of similar organisms in a given area (e.g., herds of elephants, stands of trees in a savannah).
Species: populations capable of interbreeding, e.g., humans, zebras, trees in a region.
Community: interacting populations within a region.
Ecosystem: community plus the physical environment (water, land, climate).
Biosphere: all of Earth’s ecosystems.
Emergent properties and interconnectedness:
Each level builds on the previous level and becomes more complex.
Emergent properties arise from interactions among parts (e.g., a small molecule like CO2 cannot perform life functions, but the system of organs and organisms can).
Changes at one level (e.g., atmospheric CO2) can influence organs, organisms, and ecosystems.
Life Requires Materials and Energy
Living organisms require an external source of nutrients and energy to maintain organization and carry on life activities.
Metabolism: the sum of all chemical reactions in a cell; requires energy input and materials.
The sun is the ultimate energy source for nearly all life on Earth.
Photosynthesis: some organisms capture solar energy and convert it into chemical energy stored in organic molecules.
All life ultimately acquires energy by metabolizing nutrients produced by photosynthetic organisms.
Energy flow and chemical cycling in ecosystems:
Producers (e.g., grasses) convert solar energy and inorganic nutrients into organic nutrients via photosynthesis.
Chemical cycling: nutrients move through populations in a food chain; death and decomposition return inorganic nutrients to producers, continuing the cycle.
Energy flow: energy moves from the sun through the food chain as organisms feed on one another and is gradually dissipated as heat to the atmosphere.
Important consequence: energy does not recycle; solar energy and photosynthesis are essential for sustaining ecosystems.
Ecosystem characteristics shaped by energy flow and nutrient cycling:
Climate, rainfall, and energy availability influence which ecosystems and communities exist where.
Deserts occur where rain is minimal; forests require more rain.
The most biologically diverse ecosystems (e.g., tropical rainforests and coral reefs) occur where solar energy is abundant.
Example ecosystem: North American grasslands
Populations include rabbits, hawks, grasses, etc.
Food chains: grasses → rabbits → hawks, etc.
Homeostasis and Responsiveness
Homeostasis: living organisms maintain a state of biological balance; internal conditions must stay within the organism's tolerance range for life to continue.
Regulation is achieved by systems that monitor and adjust internal conditions; not all regulation requires conscious thought.
Examples of regulatory mechanisms: liver releasing stored sugar to keep blood glucose within normal limits when you forget lunch.
Some regulation is behavioral and managed by the nervous system (often not consciously controlled):
Lizard behavior example: bask in sun to raise temperature or seek shade to cool down.
Response to environmental and inter-organism interactions:
Organisms interact with their environment and with other organisms.
Even single-celled organisms can respond to stimuli (e.g., cilia beating or whiplike tails guiding movement toward/away from light or chemicals).
Multicellular organisms enable more complex responses (e.g., a vulture detecting a carcass from kilometers away; monarch butterflies migrate in response to seasonal cues).
Behavior defined: a suite of activities that help organisms maintain homeostasis and obtain energy, nutrients, shelter, and mates; includes communication, hunting, and defense strategies.
Growth, Development, Reproduction, and Genetic Inheritance
Reproduction and development: life arises from life; all organisms reproduce.
Asexual reproduction (e.g., bacteria and many single-celled organisms) often involves division (binary fission).
In most multicellular organisms, reproduction begins with the pairing of sperm and egg, followed by cell divisions and development to an immature stage that becomes an adult.
Offspring inherit genetic instructions from parents; genes are passed to the next generation.
Genetic material:
Genes are made of long DNA molecules.
DNA provides the blueprint for organization and metabolism.
All cells in a multicellular organism contain the same set of genes, but only a subset is active (turned on) in each cell type (gene expression).
Variation and mutations:
Not all individuals in a species are identical; mutations introduce inheritable variation in genetic information.
Mutations are not universally detrimental; they can explain visible differences such as eye and hair color, contributing to diversity within a group.
Adaptations:
Adaptations are modifications that help an organism function better in a specific environment.
Penguin adaptations example: aquatic lifestyle with an extra downy layer and a waterproof coat; blubber; modified wings for swimming; feet and tails used as rudders; behavioral adaptations such as sliding on the belly to conserve energy; egg-carrying strategies (eggs carried on feet and protected by skin pouch); huddling for warmth.
Evolutionary perspective:
Life on Earth responds to changing environments by developing new adaptations over long time scales.
Evolution encompasses how populations change across generations to become better suited to their environments.
Over long periods, advantageous traits can spread and diversify populations, potentially leading to new species.
Evolution is a unifying concept in biology and explains how life forms arose from a common ancestor.
Evolution, Taxonomy, and Systematics
Evolutionary framework:
Common descent with modification: lineages descend from a common ancestor, with changes accumulating over time.
Natural selection is the primary mechanism by which adaptations arise: environmental factors favor certain heritable traits over others.
Mutations fuel natural selection by increasing variation within a population.
Selective agents can be abiotic (e.g., altitude, climate) or biotic (e.g., predators, herbivores).
Example illustrating natural selection: a plant species normally produces smooth leaves, but a mutation leads to hairy leaves; deer prefer smooth leaves; hairy-leaved plants have higher survival and reproduction, gradually increasing prevalence of hairy leaves in the population.
Human adaptation as an evolutionary example:
Tibetans at high elevations exhibit adaptations to life at high altitude (elevations > or > ).
High hemoglobin levels can cause hypertension and clotting risks; natural selection favors individuals with lower hemoglobin under hypoxic stress.
The EPAS1 gene (also written as EPAS1) on chromosome 2 encodes a transcription factor that regulates genes involved in oxygen use and hemoglobin production.
Tibetan populations show alleles of EPAS1 that reduce hemoglobin production, enabling better adaptation to high altitude.
Timeline nuances: initial signals suggested about for population-level adaptation, but later genomic analyses indicate a deeper history, with a Denisovan-like allele (the EPAS1 region) dating around , possibly via interbreeding or ancestral introgression.
Questions to consider: what other environments might show evidence of human adaptation? what other traits (besides hemoglobin) might differ with high-altitude adaptation?
Biodiversity and classification:
Evolutionary processes generate biodiversity across long timescales.
The diversity of life is historically grouped into three large domains: .
An evolutionary tree traces ancestry back to a common ancestor, with a single ancestral lineage giving rise to diverse descendants.
An evolutionary tree functions like a family tree: one ancestral population may lead to multiple descendant groups, each adapted to different environments.
Taxonomy and systematics:
Taxonomy is the science of identifying and grouping organisms according to rules to reflect evolutionary relationships.
Systematics studies the evolutionary relationships among organisms; DNA-based data have reshaped many classifications.
Classic categories (from least to most inclusive) include:
Within the domain Eukarya, there has been refinement into several kingdoms: Protista (diverse group), Plantae (multicellular, photosynthetic), Fungi (decomposers), and Animalia (multicellular, ingestive heterotrophs).
Protists are a diverse set of organisms; some are photosynthetic, others must obtain food.
Plants (kingdom Plantae) are multicellular and photosynthetic.
Fungi (kingdom Fungi) include molds and mushrooms, important for decomposition.
Animals (kingdom Animalia) are multicellular and must ingest/consume their food.
Ongoing DNA-based analyses have led to the proposal of a six‑supergroup framework within Eukarya to better reflect evolutionary relationships. Example supergroups include: Excavata, Chromalveolata, Rhizaria, Archaeplastida, Amoebozoa, Opisthokonta.
Example connections within supergroups and representative organisms: Excavata (e.g., diplomonads, euglenozoans); Chromalveolata (dinoflagellates, ciliates, diatoms, golden/brown algae, water molds); Rhizaria (foraminiferans, radiolarians); Archaeplastida (red algae, green algae, plants); Amoebozoa (amoeboids, slime molds); Opisthokonta (fungi, animals).
Binomial nomenclature (scientific name): two-part name consisting of genus and species epithet; examples include for mistletoe; the genus may be abbreviated (e.g., ) when the species is known; if the species is not determined, it may be indicated as or with a similar generic abbreviation.
Latin serves as the universal basis for scientific names to avoid regional language confusion.
The first word (genus) is capitalized; the second word (species) is not; both are italicized in formal writing.
Note: some of the older classification schemes listed kingdoms differently; modern approaches increasingly rely on the three-domain system and, within Eukarya, the six supergroups to reflect phylogenetic relationships.
Check Your Progress 1.1 questions (conceptual review):
Distinguish between an ecosystem and a population within the levels of biological organization.
List the common characteristics of all living organisms.
Explain how adaptations relate to evolutionary change.
Check Your Progress 1.2 questions (taxonomy and evolution):
Explain how natural selection leads to new adaptations within a species.
List the levels of taxonomic classification from most inclusive to least inclusive.
Describe differences that distinguish the kingdoms within Domain Eukarya (Protista, Plantae, Fungi, Animalia).
Three domains recap:
Domain Bacteria and Domain Archaea: prokaryotes (lacking a membrane-bound nucleus); different cell wall compositions; archaea often inhabit extreme environments (e.g., hot, salty, acidic, oxygen-poor).
Domain Eukarya: eukaryotes with a nucleus, endoplasmic reticulum, mitochondria, chloroplasts, and other organelles. Not all eukaryotes have cell walls (e.g., animals).
Cell-wall differences: Bacteria typically have peptidoglycan; Archaea lack peptidoglycan but possess other polymers; Eukarya (plants, fungi) have walls with different compositions; animals lack cell walls.
A note on terminology and evolution:
The text emphasizes that three domains is a working framework, with ongoing refinements as DNA data yield new relationships.
The unity of life is grounded in shared cellular organization, genetic material (DNA), and core metabolic processes, even as diversity expands through evolution and adaptation.
Quick definitions to remember:
Metabolism: all chemical reactions in a cell.
Homeostasis: maintenance of internal stability within tolerable limits.
Emergent properties: new characteristics that arise when parts interact at a higher level.
Natural selection: differential reproductive success based on heritable traits influenced by the environment.
EPAS1: a transcription factor gene associated with high-altitude adaptation in Tibetans; influences hemoglobin production and oxygen usage.
Binomial nomenclature: two-part Latin name for species (genus + species epithet).
Protista/Plantae/Fungi/Animalia: traditional kingdoms within Eukarya, with Protista serving as a diverse, paraphyletic group in many schemes.
Connected themes for study include: how energy flow and nutrient cycling shape ecosystems; the interplay between genetics, variation, and evolution; how taxonomy organizes life to reflect evolutionary relationships; and the ongoing refinement of classification as new data emerge.