Chapter XXVI

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Animal and plant cells are called “eukaryotic” because they have “ nuclei” (eu + kary + o + tic), that is, their nuclei and other organelles (“little organs”) are contained in an orderly way by membranes. (An alternative is the “” cell, whose nucleus and other organelles are not bounded by such a membrane.)

Animal cells are surrounded by a cell membrane, or membrane, which has selective permeability (from the Latin base meat-, “to go,” hence “able to admit or allow through”), a feature that allows them to absorb useful substances while repelling harmful ones. The membrane of some cells has cilia or a (“hairs” or a “little whip”) that allow for locomotion. The general term for the substance on the interior of the cell is “,” literally the “first creation” or “thing formed first.” This term encompasses two substances: the nucleoplasm is the substance that makes up most of the contents of the nucleus, and the cytoplasm is everything else in the cell, including its aqueous solution and any organelles. The cytoplasm is then further divided into the “” and “,” the latter referring to the cytoplasm adjacent to the cell membrane, the former to its interior portion. The cytoplasm contains protein-based fibers that make up the , which gives the cell structure while allowing for movement. The centrosomes aid in the maintenance of cell structure as the primary locus for the organization of microtubules, support structures that are part of the cytoskeleton.

Animals cells contain a variety of organelles that perform vital functions. Most important is the nucleus (“little nut”), which contains the cell’s genetic information. Inside the nucleus is the (“little little nut”), which transcribes RNA and helps to form ribosomes. “Ribosome,” derived from “ribonucleic (acid)” and the -some suffix, refers to a molecule that aids in the formation of polypeptides, chains of amino acids that are used in synthesis. The endoplasmic (“network”) is specialized for protein processing and lipid synthesis. The apparatus (discovered by the Italian anatomist of this name), is often compared to a post office, since it is responsible for packaging and transporting materials in the cell. (“dissolving bodies”) contain enzymes used in intracellular digestion. The small, rodlike mitochondria perform cellular respiration, that is, they convert biochemical energy (i.e., food) into adenosine triphosphate, a nucleotide that stores energy.

Plant cells have many of the same structures as animal cells, with some notable differences. For one, a rigid cell wall provides more structural stability than an animal cell has with only its flexible plasma membrane. The plant cell wall is not permeable, but small channels called “” allow for intercellular communication; the name is a combination of the Greek bases meaning "to form" and "bond, fetter.” Plant cells also contain very large , called by the Latin word meaning “small empty spaces,” even though these structures are actually used to hold materials. Although these can be found in animals cells, the ones in plant cell are much larger and have a structural function: as they store water, they create turgor pressure within the cell, adding durability. Finally, plant cells contain , a body that carries chlorophyll (“green leaf”), a substance that converts light energy into adenosine triphosphate in the process of , which also involves the conversion of carbon dioxide into oxygen.