Biologists use the following features of organisms to identify the major groupings of current classifications. (This book does not discuss animals and animal‐like protists beyond placing them in general perspective.) Presence or absence of a defined nucleus Unicellular or multicellular with specialized organelles Mode of nutrition Presence or absence of a cell wall Composition of the cell wall Motility Mode of reproduction Kind of life cycle Nucleus The most basic division of organisms separates the living world into two groups on the basis of those possessing and those lacking a defined nucleus (plural: nuclei). The nucleus is an organelle, which contains the major portion of the genetic material (DNA) of the cell and is surrounded by a nuclear membrane. The genetic material of Prokaryotes is not contained within a membrane‐bounded nucleus. Eukaryotes all have nuclei. Cellularity The form (morphology) of an organism can be unicellular (one‐celled) or multicellular (many‐celled). Some unicellular organisms form filaments (strings of cells), others form sheets of cells held together by pectins, and still others form colonies that give a superficial resemblance to multicellularity. Unicellular organisms do not form tissues (similar cells organized into a functional unit) nor organs (groups of tissues organized for a particular function). Some organisms alternate a unicellular stage with a multicellular stage in their life cycles. Eukaryotic organisms have organelles, membrane‐bounded structures within their cells specialized to perform certain functions. Nutrition All organisms need a source of energy to fuel their metabolism, the chemical processes that maintain life. Organisms obtain their nutrients for metabolism in one of two basic ways: 1.) Autotrophs are able to make the organic compounds they use for metabolism directly from inorganic materials; and 2.) Heterotrophs are unable to do this and obtain their nutrients from the organic materials manufactured by autotrophs. Some autotrophs are photoautotrophs. They use radiant energy from the sun in the process of photosynthesis to manufacture organic compounds. Chemoautotrophs use chemical energy in chemosynthesis, oxidizing inorganic compounds to manufacture organic nutrients. Chloroplasts are present in the photoautotrophs, absent in the chemoautotrophs. Animals are heterotrophs; they ingest (swallow) their food and then digest it internally. Fungi are heterotrophs, which release digestive enzymes into their surroundings and then absorb the nutrients into their cells. Many protists use phagotrophy, a type of nutrition in which single cells ingest food particles. Some fungi (and other organisms) are saprophages, heterotrophs that break down the organic materials of dead organisms. Cell wall Animals and the animal‐like protists have no cell walls, but most other organisms (with a few exceptions) have some kind of wall made from a variety of materials. Almost all of the prokaryote cells have walls, and a major distinction between the Bacteria and the Archaea is the presence of peptidoglycans (glycoprotein polymers) in the Bacteria and their absence in the Archaea cell walls. Fungi cell walls are made of chitin, the substance that makes the exoskeletons of lobsters, crabs, cockroaches, and other arthropods hard. The basic material of plant cells (and those of many algae) is cellulose. Lignin, suberin, waxes, and many other substances may be deposited additionally. Motility Plants in general and some animals don't move around; they are sessile (attached) to a substrate. But, many plant and sessile animal cells are motile, and they move using a variety of techniques. There are motile organisms in all of the kingdoms, so motility per se does not distinguish groups, but the kind and location of the devices employed for movement do determine groups. The organelle that propels most cells is the flagellum (plural: flagella) or, in the terminology of some biologists, the undulipodium (plural: undulipodia). A smaller, shorter flagellum is a cilium (plural: cilia). The flagella are long threads of protoplasm that extend outside of the cell and have the capability for limited movement. The prokaryotes have a single‐fiber flagellum that rotates; the flagella of eukaryotes are bundles that consist of nine pairs of microtubules wrapped around a central pair (a 9 + 2 configuration). A sliding action moves the microtubules. Type of reproduction Reproduction is the creation of new individuals from existing ones and can be either asexual—without special sex cells ( gametes)—or sexual, in which gametes fuse to produce new individuals. Gametes are usually haploid (with a single set of chromosomes) and their fusion ( fertilization) results in a diploid (with two sets of chromosomes) zygote (the cell formed by the fusion of two gametes). Variations of both sexual and asexual reproduction are legion throughout the living world. Asexual reproduction occurs in some members of all the kingdoms, whereas sexual reproduction is present in all but the Archaea. Many types of asexual reproduction exist. Fission, a splitting in two of the cell, is one type of asexual reproduction. In prokaryotes, division of the genetic material accompanies fission, whereas it does not accompany fission in the eukaryotes. Yeasts and some other organisms bud, simply by pushing out and breaking off pieces of the cell. Spore‐formation is a widespread method of asexual reproduction in which single‐celled spores, formed in specialized structures called sporangia, are produced in large numbers. They may undergo a resting stage first, or produce new individuals directly. Sexual spores are produced in some organisms. (See Figure .) Figure 1 Life cycle Three basic types of life cycles differentiate major groups of organisms (see Figure 2 ). All are variations on a general theme in which haploid cells alternate with diploid in the stages of the life cycle. Thus, meiotic (reduction) cell divisions alternate with fertilization (fusion of gametes). The three life Zygotic meiosis: The individual organisms are haploid, and only the zygote is diploid. The zygote produced by fertilization immediately undergoes meiosis, producing more haploid individuals. This life cycle appears in all fungi and some algae. Gametic meiosis: The mature, common individuals are diploid and produce haploid gametes that fuse. The zygote divides by ordinary mitosis, producing the adult diploid individuals. Animals, some brown and green algae, and many other organisms maintain this type of life cycle. Sporic meiosis: Also called alternation of generations because during the life cycle two kinds of individuals switch or alternate as the common individual, one diploid, one haploid. In plants the diploid individual, called the sporophyte, produces spore mother cells that divide by meiosis producing haploid spores. The spores germinate and produce haploid gametophytes. The latter then produce the haploid gametes, which fuse in fertilization, forming the diploid zygote that matures into the adult sporophyte. In addition to plants, this form of life cycle is present in many algae.