Cell Biology Terms Starting With E

Endocytosis is the process by which a cell internalizes extracellular material by deforming its plasma membrane inward to form a membrane-bound vesicle that pinches off into the cytoplasm.

Receptor-mediated endocytosis concentrates specific ligands at clathrin-coated pits and internalizes them at rates that allow a single mammalian cell to recycle its entire plasma membrane surface area roughly once every 30 minutes. Phagocytosis envelops particles larger than 0.5 micrometers, including bacteria and cellular debris, using actin-driven pseudopodia that surround and seal the target into a phagosome. Pinocytosis continuously samples bulk extracellular fluid in small vesicles without requiring specific receptors.

All forms of endocytosis require ATP hydrolysis and the coordinated activity of small GTPases such as Rac1 and Cdc42, which reorganize the actin cytoskeleton to drive membrane deformation.

Endoplasmic Reticulum is an interconnected network of membrane-enclosed cisternae and tubules extending from the nuclear envelope throughout the cytoplasm, where it synthesizes proteins and lipids, sequesters calcium, and carries out detoxification reactions.

Rough endoplasmic reticulum, studded with ribosomes on its cytoplasmic face, synthesizes approximately 30 percent of all cellular proteins, including secretory proteins and most integral membrane proteins, at rates of roughly 15 to 20 amino acids per second per ribosome. Smooth endoplasmic reticulum lacks ribosomes and instead handles phospholipid and steroid synthesis, calcium storage and release for muscle contraction, and cytochrome P450-mediated drug detoxification in liver cells. The ER lumen maintains a calcium concentration of 400 to 500 micromolar, roughly 5000 times higher than the resting cytoplasmic concentration of about 0.1 micromolar, a gradient that muscle cells exploit to trigger contraction.

Misfolded proteins accumulate in the ER lumen and trigger the unfolded protein response, a signaling cascade that either restores protein homeostasis or initiates apoptosis if damage is irreparable.

Endosome is a membrane-bound intracellular compartment that receives vesicles from the plasma membrane after endocytosis and sorts their cargo, directing some molecules back to the cell surface for reuse and others toward lysosomes for degradation.

Early endosomes receive incoming vesicles within minutes of endocytosis and maintain a mildly acidic lumen near pH 6.0 to 6.5, which weakens ligand-receptor bonds and begins cargo sorting. Recycling endosomes return membrane receptors such as transferrin receptors and LDL receptors to the plasma membrane, completing a round trip in roughly 10 to 15 minutes and allowing each receptor to be reused many times. Late endosomes mature over 15 to 30 minutes, accumulating intraluminal vesicles and acidifying further to around pH 5.5 before fusing with lysosomes that expose cargo to hydrolases active below pH 5.0.

The Rab GTPase family, particularly Rab5 on early endosomes and Rab7 on late endosomes, coordinates membrane identity and fusion specificity at each stage.

Epithelial Cell is a cell that forms continuous sheets covering body surfaces and lining internal cavities, characterized by tight intercellular junctions, structural polarity between apical and basolateral surfaces, and attachment to an underlying basement membrane.

Epithelial cells seal their lateral borders with tight junctions that restrict paracellular movement of ions and molecules, desmosomes that resist mechanical stress, and adherens junctions that coordinate sheet integrity. Their apical surface faces the lumen or external environment and often carries specialized structures such as microvilli, which increase absorptive area roughly 20-fold in intestinal cells, or cilia that move overlying fluid. The basolateral surface anchors to the basement membrane through integrins and communicates with underlying connective tissue and blood vessels.

Intestinal epithelial cells renew from stem cells in the crypts of Lieberkühn and migrate to the villus tip within 3 to 5 days, making them among the fastest-cycling somatic cells in the human body.

Epithelium is a tissue composed of one or more layers of tightly packed cells that covers external body surfaces, lines internal cavities and organs, and forms glands, with its cells connected by specialized junctions and anchored to a basement membrane.

Epithelial tissues are classified by cell shape and layer number: squamous cells form thin sheets that line blood vessels and alveoli for rapid diffusion; cuboidal cells line kidney tubules and small glands; columnar cells line the intestine and stomach for secretion and absorption; and pseudostratified columnar epithelium lines much of the respiratory tract, where ciliated cells beat at 10 to 20 cycles per second to clear inhaled particles. Simple epithelia consist of a single cell layer, while stratified epithelia stack multiple layers to resist abrasion, as in the 15 to 20 cell layers of the oral mucosa. Epithelial cells in the small intestine renew completely every 3 to 5 days, driven by stem cells in the crypts of Lieberkühn.

Tight junctions between cells restrict paracellular movement of ions and pathogens, giving epithelia their barrier function.

Eukaryote is an organism whose cells contain a membrane-bound nucleus housing the genetic material, along with other specialized membrane-enclosed compartments; animals, plants, fungi, and protists all belong to this group.

Eukaryotic cells compartmentalize DNA within a nucleus surrounded by a double nuclear envelope pierced by nuclear pore complexes, a feature absent in prokaryotes. Mitochondria generate ATP through oxidative phosphorylation, the endoplasmic reticulum synthesizes and processes proteins and lipids, and lysosomes digest cellular waste using hydrolytic enzymes. These organelles dramatically increase the functional complexity a single cell can achieve, and eukaryotic cells typically range from 10 to 100 micrometers in diameter, far larger than most bacteria.

Single-celled eukaryotes such as Paramecium and Amoeba proteus demonstrate that eukaryotic organization exists independently of multicellularity.

Exocytosis is the cellular process by which membrane-bound vesicles fuse with the plasma membrane and release their contents into the extracellular space.

Exocytosis moves cargo-containing vesicles to the plasma membrane, where SNARE proteins mediate bilayer fusion and release contents into the extracellular space. Constitutive exocytosis continuously delivers membrane lipids and proteins to the cell surface, replacing roughly 0.1 percent of total membrane per minute. Regulated exocytosis concentrates neurotransmitters or hormones in secretory vesicles that fuse only upon receiving activation signals such as a rise in cytosolic calcium.

Beyond secretion, both pathways recycle vesicle membrane components back into the cell after fusion, maintaining plasma membrane composition.

Extracellular matrix is the organized network of proteins and polysaccharides secreted by cells into the space surrounding them, providing structural support and transmitting biochemical and mechanical signals that regulate cell behavior.

Collagen, the most abundant matrix protein in animals, accounts for roughly 70 percent of skin dry weight and assembles into fibrils 50 to 500 nanometers in diameter that resist tensile forces exceeding 10 megapascals. Proteoglycans carry long glycosaminoglycan chains that bind water and give tissues the ability to resist compression, while fibronectin and laminin anchor cells to the matrix through integrin receptors. Matrix metalloproteinases remodel the matrix during wound healing and tissue development at controlled rates that regulate cell migration and signaling.

Cell surface integrins bind matrix components and transmit mechanical and chemical signals that influence cell shape, gene expression, survival, and differentiation.