Ectoderm: The outer covering of an embryo or adult. It usually consists of epithelial cells that serve a barrier function. It is one of the three major tissue layers of the embryo: ectoderm, mesoderm, and endoderm.

EGF (Epidermal Growth Factor): A hormone that stimulates a receptor, often causing cells to divide. The EGF receptor has a tyrosine kinase that can autophosphorylate tyrosine residues.

Electron Transport Chain: A linked group of intermediates or electron carriers that can sequentially pass electrons from higher- to lower-energy intermediates. In mitochondria and chloroplasts both a pH and transmembrane potential are created that can generate ATP through the ATP synthase.

Electronegative: Attracting electrons. An electronegative atom attracts electrons toward itself (or, similarly, is reluctant to give up electrons). Some elements are more electronegative than others.

Electronegativity: The arbitrary scale that measures the electronegative property of elements. In the scale Linus Pauling defined, fluorine has an electronegativity of 1, while all other elements have electronegativities less than 1.

ELISA (Enzyme-Linked Immunosorbent Assay): Assays that are often performed in 96-well plates using antibodies to detect the amount of antigen present in a solution. ELISAs are excellent quantitative assays for soluble proteins secreted or retained by cells.

Emulsify: Render hydrophobic substances soluble in an aqueous solution by formation of micelles through the mediation of amphipathic molecules like bile salts and detergents.

Enantiomers: Two nonsuperimposable structures that are mirror images of each other.

Endocytosis: The invagination of a membrane, ultimately forming a vesicle containing cargo molecules. Receptor-mediated endocytosis can import, for instance, iron and cholesterol.

Endonuclease: A family of enzymes that cleave DNA internally. Some, such as restriction enzymes, are found in bacteria and are very useful for molecular biology. Others that are endogenous to eukaryotic cells are calcium-activated and cleave DNA during programmed cell death, or apoptosis.

Endosomes: Closed vesicles that form subsequent to endocytosis. They deliver their cargo molecules to a particular destination such as a lysosome.

Endosymbiosis: The mutual beneficial relationship generated by the fusion of a protoprokaryote and a protoeukaryote that resulted in the formation of the current eukaryotic cell containing mitochondria and chloroplasts.

Endothelial Cell: A type of cell that surrounds blood vessels. These cells contain tight junctions between them limiting fluid movement across the endothelial layer; however, there is exchange of materials such as oxygen, glucose, and carbon dioxide across the layer. Endothelial cells are usually attached to basal laminas.

Enzymes: Biological molecules that act as catalysts by lowering the energy of activation of biological reactions. Most enzymes are proteins, and their specific shape is important to their activities.

Epidermal Growth Factor (EGF): A hormone that stimulates a receptor, often causing cells to divide. The EGF receptor has a tyrosine kinase that can autophosphorylate tyrosine residues.

Epithelial: Relating to epithelium. The epithelium is a continuous sheet of cells that covers the outside and inside surfaces of most animals.

Epithelium: A continuous sheet of cells that covers the outside and inside surfaces of most animals.

Erythrocyte (Red Blood Cell): A biconcave disk cell the size of a capillary. Mammalian erythrocytes are anucleate. Erythrocytes contain hemoglobin and are primarily responsible for carbon dioxide-oxygen exchange.

Erythrocyte Ghosts: Erythrocytes that have been emptied of their cytoplasmic contents. This occurs through the cytoplasmic lysis of the erythrocyte. Erythrocyte ghosts have been particularly useful in studying membrane phenomena owing to their simplicity and the fact that erythrocytes have few contaminating organelle membranes.

Ester: A molecule formed by the creation of a covalent bond between the carboxylic carbon of an acid and the oxygen of an alcohol.

Eukaryotic: Refering to eukaryotes. A eukaryote is a class of organisms that includes plants, animals, fungi, yeast, and others that are characterized by a membrane-bound nucleus and a variety of membrane-bound organelles.

Exon: Segment of a eukaryotic gene that when copied into an RNA transcript is spliced together with other exons to form mature mRNA.

Exonucleases: A family of DNA nucleases that cleave the ends of DNA.

Extracellular Matrix: A complex arrangement of many different types of molecules, such as collagen and laminin, that have been secreted by cells. The basal lamina is an example of an organized extracellular matrix. The extracellular matrix acts as a filter as well as a surface that encourages the differentiation of cells.