12.1 What are living things, and how are they classified?

living things are classified based on their physical characteristics, with organisms that are closely related to one another sharing a host of common traits.

A Working Definition of Life

unusual nature of some life-forms makes defining life a challenging task

• both living and nonliving things composed of same basic building block: atoms, which move continuously in and out of living and nonliving systems

  • free exchange of idential components between life and nonlife is one of the factors that complicates attempts at formally defining life

one of the simplest definitions of life: it consumes energy from its environment

• however, car engine could be classified as alive under this definition but car can’t self-replicate or reproduce

Key qualities in defining life:

• has to be able to self-replicate or reproduce

• water most likely needs to be part of a living organism bc living things need a solvent for biochemical reactions (ammonia or sulfuric acid can work)

• has to have some sort of a membrane to distinguish itself from its environment

• most living things tend to respond to stimuli or adapt to their environment

• life is carbon-based bc carbon is useful in making chemical compounds

NASA definition of life: “life is a self-sustained chemical system capable of undergoing Darwinian Evolution

• problematic bc it’d require observation of several successive generations over a considerable length of time to verify evolution in a life form

a good working definition of life should incorporate most of these ideas:

• living things can capture, store, and convert energy

• capable of reproduction

• can adapt to their environment

• they change through time

The Three Domains of Life

ALL living things belong to one of three domains or “superkingdoms” of life:

(1) bacteria: simple life-forms w/ cells that usually lack a nucleus

• ex. purple bacteria, green nonsulfur bacteria, cyanobacteria (blue-green algae)

(2) archaea: group of simple, microscopic bacteria-like creatures that includes methane producers and sulfur oxidizers that inhabit deep-sea vents and seeps; many prefer environments w/ extreme temps and/or pressures

(3) eukarya: includes complex organisms i.e.

• multicellular plants

• multicellular animals

• fungi

• protists = diverse array of mostly microscopic organisms that don’t fit into any other group

**main component of eukaryotes (DNA) is housed in a discrete nucleus and their cells contain structures that supply energy to build the cell and maintain its functions

Starting w/ a common ancestral community of primitive cells, life on earth can be visualized as branching out into:

(a) 3 domains of life and

(b) 6 kingdoms of organisms

what were the ancestors of these 3 domains of life?

• according to evolutionary theory - all organisms on earth share a common genetic heritage, each being the genealogical descendant of a single primitive species from the distant past = universal common ancestry

  • UCA proved to be valid in many statistical analyses using a set of commonly retained proteins found in a wide range of living organisms —> ancestors of 3 domains thought to have consisted of a community of early primitive cells, some of which apparently acquired new genetic material by engulfing microbial neighbors, taking genetic codes

• through symbiosis - groups of organisms helped each other coexist for their mutual benefit —> evolved into new organisms that contained merged genes

The Six Kingdoms of Organisms

• within 3 domains of life, a system of 5 kingdoms of organisms was first proposed by ecologist and biologist Robert H. Whittaker in 1969

• 1977: microbiologist and biophysicist Carl R. Woese and colleagues decided on 6 kingdoms based on biochemical differences

6 kingdoms of organisms that are most widely accepted:

(1) eubacteria

(2) archaebacteria

(3) plantae

(4) animalia

(5) fungi

(6) protista

**some biologists group eubacteria and archaebacteria together = bacteria kingdom and recognize additional divisions w/in protista i.e. protozoa and chromista

Kingdom Eubacteria

• includes some of simplest organisms

• single-celled organisms

• lack discrete nuclei and internal organelles present in ALL other organisms

• ex. heterotrophic bacteria and cyanobacteria

Kingdom Archaebacteria

• group of simple, microscopic, bacteria-like creatures

• includes methane producers and sulfur-oxidizers that inhabit deep-sea vents and seeps

• many prefer environments w/ extreme temps and/or pressures

• genetic analyses suggest these are some of the most ancient life-forms on earth

Kingdom Plantae

• multicelled plants

• all photosynthesize

• only a few species of true plants i.e. surf grass and eelgrass inhabit shallow coastal environments

• in ocean, photosynthetic marine algae occupy ecological niche of land plants

Kingdom Animalia

• multicelled animals

• range in complexity from simple sponges to complex vertebrates (animals w/ backbones i.e. humans)

Kingdom Fungi

• >>100,000 species of mold and lichens, though less than 0.5% are sea dwellers

• exist in specialized places throughout marine environment

• most commonly found in intertidal zone - live here symbiotically w/ cyanobacteria or green algae —> lichen

• other fungi remineralize organic matter and function primarily as decomposers in the marine ecosystem

Kingdom Protista

• diverse collection of single-celled and multicelled organisms that have a nucleus

• i.e. marine algae, protozoa

Linnaeus and Taxonomic Classification

in an effort to determine the relationships of all living things on earth, swedish botanist Carl von Linne - latinized his name to Carolus Linnaeus

• created a system in 1758 that is the basis of modern scientific system of classification used today

• developed a system similar to social hierarchy of his day w/ kingdoms, countries, provinces, parishes, villages

• organizational scheme can be visualized as a series of nested boxes

• systematic classification of organisms = taxonomy

taxonomy involves using physical characteristics and genetic information to recognize organism similarities and then group them into following increasingly specific categories:

• kingdom

  • phylum

    • class

      • order

        • family

          • genus

            • species

**most specific at top, least specific at bottom

• all organisms that cshare a common category have certain characteristics and evolution similarities

• in some cases, subdivisions used i.e. subphylum

• categories assigned to an individual species must be agreed upon by an international panel of experts

**for plants, term “division” is used instead of “phylum”

fundamental unit of taxonomic classification = species

• species consist of populations of genetically similar, interbreeding individuals that share a collection of inherited characteristics whose combination is unique

• sometimes species defined in otehr ways i.e. individuals coexisting in a population that are similar in appearance

**although species is a useful concept for classifying both present and past life, there’s still debate amongst biologists about what defines a species

linnaeus also invented binomial nomenclature - every living thing known by two latin names (previously organisms known by combo of as many as a dozen latin names) —> every type of organism has a unique two-word scientific name composed of its genus and species names, which is italicized w/ first letter of genus name capitalized i.e. Delphinus delphis

• Ex. Delphinus delphis = common dolphin

• Ex. Orcinus orca = killer whale / orca

Linnaean system = far from perfect but helpful to use

RECAP

Living organisms use energy, reproduce, adapt, and change through time. living things can be classified into one of 3 domains and 6 kingdoms, each of which is split into increasingly specific groupings of phylum, class, order, family, genus, and species.

CONCEPT CHECK 12.1

(1) what characteristics should be included in a good working definition of “life”?

• life can capture, store, and convert energy for use

• life can adapt

• life changes through time

• capable of reproduction

(2) list 3 major domains of life and 6 kingdoms of organisms. then describe fundamental criteria used in assigning organisms to these divisions.

3 domains = bacteria, archaea, eukarya

6 kingdoms = eubacteria, archaebacteria, animalia, plantae, protista, fungi

• eubacteria = single-celled organisms w/out discrete nucleus or other cell organelles, simplest organisms

• archaebacteria = single-celled, microscopic, bacteria-like creatures that live in areas w/ extreme temps and/or pressures, near sea-floor, most ancient life-forms

• animalia = multicelled animals

• plantae = multicelled plants, all photosynthesizers

• fungi = multicellular, eukaryotic heterotrophs

• protista = single-celled AND multicelled organisms WITH a nucleus i.e. protozoa and some marine algaes

(3) what are thought to be the ancestors of the three domains of life?

according to theory of evolution - all organisms on earth share a common genetic heritage, each being genealogical descendant of a single primitive species from distant past

• community of early primitive cells which acquired genetic material by engulfing microbial neighbors and stealing their genetic code

  • this happened over and over again until they became new organisms