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.
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.
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.
(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.
The form (morphology) of an organism can be unicellular (one‐celled) or multicellular (man y‐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 ...
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.
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.)
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.
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 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.