Terminology

Antibodies - (Also called immunoglobulins) A set of related proteins that are produced by the immune system and bind with specificity to molecules called antigens. Antibodies provide the basis of immunity against disease (Figure 1.6)

Archaebacteria - A group of prokaryotes that are biochemically distinct from the true bacteria (Eubacteria) and that separated from them early in the history of life. Modern archaebacteria mostly live in extreme environments, such as acid hot springs.

Cell surface receptors - On the outer membrane of cells, there are special proteins that interact specifically with molecules. These proteins are called cell surface receptors. Cell surface receptors have many roles in communication, control, and identity for cells.

Chloroplast - The chloroplast is a specialized organelle in plant and algal cells that carries out photosynthesis (Figure 1.13).

Chromosome - Chromosomes are composed of DNA and proteins and carry the genetic information within the DNA.

DNA - DNA is an acronym for DeoxyriboNucleic Acid. It is a polymer of deoxyribonucleotides and carries the genetic information (Figure 1.7).
Endoplasmic Reticulum - A highly folded membranous compartment within the cytoplasm that is responsible for a great variety of cellular tasks, including the glycosylation and trafficking of proteins destined for secretion or for the cell membrane or some organelles. It also functions in lipid synthesis, and the enzymes of many pathways of intermediate metabolism are located on its surface (Figure 1.13).

Entropy - (Symbolized mathematically by 'S') A thermodynamic quantity that expresses the degree of disorder or randomness in a system. According to the second law of thermodynamics, the entropy of an open system tends to increase unless energy is expended to keep the system orderly.

Eukaryotes - Organisms whose cells are compartmentalized by internal cellular membranes to produce a nucleus and organelles.

Gene - The smallest unit of heredity. Genes are encoded by the DNA.

Golgi complex - A stack of flattened membranous vesicles in the cytoplasm of eukaryotic cells. It serves as a routing center for proteins destined for secretion or for Iysosomes or the cell membrane; it performs similar functions for membrane lipids, and it also modifies and finishes the oligosaccharide portions of glycoproteins (Figure 1.13).

Hormone - A substance that is synthesized and secreted by specialized cells and carried via the circulation to target cells, where it elicits specific changes in the metabolic behavior of the cell by interacting with a hormone-specific receptor.

Metabolism - Living cells contain thousands of chemical reactions occurring simultaneously. The term 'metabolism' is used to refer to this collection of reactions in its entirety.

Mitochondria - The organelles whose chief task it is to supply the cell with ATP via oxidative phosphorylation. They contain the enzymes for pyruvate oxidation, the citric acid cycle, the -oxidation of fatty acids, and oxidative phosphorylation, as well as the electron transport chain (Figure 1.13).

Nucleotide - A molecule that, upon complete hydrolysis, yields at least 1 mole per mole of a purine or pyrimidine base, a sugar, and inorganic phosphate.

Nucleus - The membrane-bound structure in a eukaryotic cell that contains the chromosomal genetic material and associated components. It is also the place where RNA molecules are processed and ribosomes are assembled (Figure 1.13).

Organelles - Membrane-bound compartments in the cytoplasm of eukaryotic cells. Each kind of organelle carries out a specific set of functions. Examples are mitochondria, chloroplasts, and nuclei (Figure 1.13).

Prokaryotes - Primitive single-celled organisms that are not compartmentalized by internal cellular membranes; the eubacteria and archaebacteria. Compare eukaryotes.

Ribosomes - Large protein-RNA complexes that are responsible for synthesizing polypeptides under the direction of mRNA templates (Figure 1.13).

Virus - Infectious entities that contain the nucleic acid to code for their own structure but that lack the enzymatic machinery of a cell; they replicate by invading a cell and using its machinery to express the viral genome. Most viruses consist of little but nucleic acid enclosed in a protein coat; some viruses also have an outer lipid-bilayer envelope.

X-ray Diffraction - A technique that is used to determine the three-dimensional structure of molecules, including macromolecules. A crystal or fiber of the substance is illuminated with a beam of x-rays, and the repeating elements of the structure scatter the x-rays to form a diffraction pattern that gives information on the molecule's structure.