Glossary: The Cellular Level of Organization

active transport: form of transport across the cell membrane that requires input of cellular energy

amphipathic: describes a molecule that exhibits a difference in polarity between its two ends, resulting in a difference in water solubility

anaphase: third stage of mitosis (and meiosis), during which sister chromatids separate into two new nuclear regions of a dividing cell

anticodon: consecutive sequence of three nucleotides on a tRNA molecule that is complementary to a specific codon on an mRNA molecule

autolysis: breakdown of cells by their own enzymatic action

autophagy: lysosomal breakdown of a cell’s own components

cell cycle: life cycle of a single cell, from its birth until its division into two new daughter cells

cell membrane: membrane surrounding all animal cells, composed of a lipid bilayer interspersed with various molecules; also known as plasma membrane

centriole: small, self-replicating organelle that provides the origin for microtubule growth and moves DNA during cell division

centromere: region of attachment for two sister chromatids

centrosome: cellular structure that organizes microtubules during cell division

channel protein: membrane-spanning protein that has an inner pore which allows the passage of one or more substances

checkpoint: progress point in the cell cycle during which certain conditions must be met in order for the cell to proceed to a subsequence phase

chromatin: substance consisting of DNA and associated proteins

chromosome: condensed version of chromatin

cilia: small appendage on certain cells formed by microtubules and modified for movement of materials across the cellular surface

cleavage furrow: contractile ring that forms around a cell during cytokinesis that pinches the cell into two halves

codon: consecutive sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid

concentration gradient: difference in the concentration of a substance between two regions

cyclin-dependent kinase (CDK): one of a group of enzymes associated with cyclins that help them perform their functions

cyclin: one of a group of proteins that function in the progression of the cell cycle

cytokinesis: final stage in cell division, where the cytoplasm divides to form two separate daughter cells

cytoplasm: internal material between the cell membrane and nucleus of a cell, mainly consisting of a water-based fluid called cytosol, within which are all the other organelles and cellular solute and suspended materials

cytoskeleton: “skeleton” of a cell; formed by rod-like proteins that support the cell’s shape and provide, among other functions, locomotive abilities

cytosol: clear, semi-fluid medium of the cytoplasm, made up mostly of water

DNA polymerase: enzyme that functions in adding new nucleotides to a growing strand of DNA during DNA replication

DNA replication: process of duplicating a molecule of DNA

diffusion: movement of a substance from an area of higher concentration to one of lower concentration

diploid: condition marked by the presence of a double complement of genetic material (two sets of chromosomes, one set inherited from each of two parents)

electrical gradient: difference in the electrical charge (potential) between two regions

endocytosis: import of material into the cell by formation of a membrane-bound vesicle

endoplasmic reticulum (ER): cellular organelle that consists of interconnected membrane-bound tubules, which may or may not be associated with ribosomes (rough type or smooth type, respectively)

exocytosis: export of a substance out of a cell by formation of a membrane-bound vesicle

exon: one of the coding regions of an mRNA molecule that remain after splicing

extracellular fluid (ECF): fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluid found in other reservoirs in the body

facilitated diffusion: diffusion of a substance with the aid of a membrane protein

flagellum: appendage on certain cells formed by microtubules and modified for movement

G₀ phase: phase of the cell cycle, usually entered from the G₁ phase; characterized by long or permanent periods where the cell does not move forward into the DNA synthesis phase

G₁ phase: first phase of the cell cycle, after a new cell is born

G₂ phase: third phase of the cell cycle, after the DNA synthesis phase

Golgi apparatus: cellular organelle formed by a series of flattened, membrane-bound sacs that functions in protein modification, tagging, packaging, and transport

gene expression: active interpretation of the information coded in a gene to produce a functional gene product

gene: functional length of DNA that provides the genetic information necessary to build a protein

genome: entire complement of an organism’s DNA; found within virtually every cell

glycocalyx: coating of sugar molecules that surrounds the cell membrane

glycoprotein: protein that has one or more carbohydrates attached

helicase: enzyme that functions to separate the two DNA strands of a double helix during DNA replication

histone: family of proteins that associate with DNA in the nucleus to form chromatin

homologous: describes two copies of the same chromosome (not identical), one inherited from each parent

hydrophilic: describes a substance or structure attracted to water

hydrophobic: describes a substance or structure repelled by water

hypertonic: describes a solution concentration that is higher than a reference concentration

hypotonic: describes a solution concentration that is lower than a reference concentration

integral protein: membrane-associated protein that spans the entire width of the lipid bilayer

intermediate filament: type of cytoskeletal filament made of keratin, characterized by an intermediate thickness, and playing a role in resisting cellular tension

interphase: entire life cycle of a cell, excluding mitosis

interstitial fluid (IF): fluid in the small spaces between cells not contained within blood vessels

intracellular fluid (ICF): fluid in the cytosol of cells

intron: non-coding regions of a pre-mRNA transcript that may be removed during splicing

isotonic: describes a solution concentration that is the same as a reference concentration

kinetochore: region of a centromere where microtubules attach to a pair of sister chromatids

ligand: molecule that binds with specificity to a specific receptor molecule

lysosome: membrane-bound cellular organelle originating from the Golgi apparatus and containing digestive enzymes

messenger RNA (mRNA): nucleotide molecule that serves as an intermediate in the genetic code between DNA and protein

metaphase plate: linear alignment of sister chromatids in the center of the cell, which takes place during metaphase

metaphase: second stage of mitosis (and meiosis), characterized by the linear alignment of sister chromatids in the center of the cell

microfilament: the thinnest of the cytoskeletal filaments; composed of actin subunits that function in muscle contraction and cellular structural support

microtubule: the thickest of the cytoskeletal filaments, composed of tubulin subunits that function in cellular movement and structural support

mitochondrion: one of the cellular organelles bound by a double lipid bilayer that function primarily in the production of cellular energy (ATP)

mitosis: division of genetic material, during which the cell nucleus breaks down and two new, fully functional, nuclei are formed

mitotic phase: phase of the cell cycle in which a cell undergoes mitosis

mitotic spindle: network of microtubules, originating from centrioles, that arranges and pulls apart chromosomes during mitosis

multipotent: describes the condition of being able to differentiate into different types of cells within a given cell lineage or small number of lineages, such as a red blood cell or white blood cell

mutation: change in the nucleotide sequence in a gene within a cell’s DNA

nuclear envelope: membrane that surrounds the nucleus; consisting of a double lipid-bilayer

nuclear pore: one of the small, protein-lined openings found scattered throughout the nuclear envelope

nucleolus: small region of the nucleus that functions in ribosome synthesis

nucleosome: unit of chromatin consisting of a DNA strand wrapped around histone proteins

nucleus: cell’s central organelle; contains the cell’s DNA

oligopotent: describes the condition of being more specialized than multipotency; the condition of being able to differentiate into one of a few possible cell types

organelle: any of several different types of membrane-enclosed specialized structures in the cell that perform specific functions for the cell

osmosis: diffusion of molecules down their concentration across a selectively permeable membrane

passive transport: form of transport across the cell membrane that does not require input of cellular energy

peripheral protein: membrane-associated protein that does not span the width of the lipid bilayer, but is attached peripherally to integral proteins, membrane lipids, or other components of the membrane

peroxisome: membrane-bound organelle that contains enzymes primarily responsible for detoxifying harmful substances

phagocytosis: endocytosis of large particles

pinocytosis: endocytosis of fluid

pluripotent: describes the condition of being able to differentiate into a large variety of cell types

polypeptide: chain of amino acids linked by peptide bonds

polyribosome: simultaneous translation of a single mRNA transcript by multiple ribosomes

promoter: region of DNA that signals transcription to begin at that site within the gene

prophase: first stage of mitosis (and meiosis), characterized by breakdown of the nuclear envelope and condensing of the chromatin to form chromosomes

proteome: full complement of proteins produced by a cell (determined by the cell’s specific gene expression)

RNA polymerase: enzyme that unwinds DNA and then adds new nucleotides to a growing strand of RNA for the transcription phase of protein synthesis

reactive oxygen species (ROS): a group of extremely reactive peroxides and oxygen-containing radicals that may contribute to cellular damage

receptor-mediated endocytosis: endocytosis of ligands attached to membrane-bound receptors

receptor: protein molecule that contains a binding site for another specific molecule (called a ligand)

ribosomal RNA (rRNA): RNA that makes up the subunits of a ribosome

ribosome: cellular organelle that functions in protein synthesis

S phase: stage of the cell cycle during which DNA replication occurs

selective permeability: feature of any barrier that allows certain substances to cross but excludes others

sister chromatid: one of a pair of identical chromosomes, formed during DNA replication

sodium-potassium pump: (also, Na⁺/K⁺ ATP-ase) membrane-embedded protein pump that uses ATP to move Na⁺ out of a cell and K⁺ into the cell

somatic cell: all cells of the body excluding gamete cells

spliceosome: complex of enzymes that serves to splice out the introns of a pre-mRNA transcript

splicing: the process of modifying a pre-mRNA transcript by removing certain, typically non-coding, regions

stem cell: cell that is oligo-, multi-, or pleuripotent that has the ability to produce additional stem cells rather than becoming further specialized

telophase: final stage of mitosis (and meiosis), preceding cytokinesis, characterized by the formation of two new daughter nuclei

totipotent: embryonic cells that have the ability to differentiate into any type of cell and organ in the body

transcription factor: one of the proteins that regulate the transcription of genes

transcription: process of producing an mRNA molecule that is complementary to a particular gene of DNA

transfer RNA (tRNA): molecules of RNA that serve to bring amino acids to a growing polypeptide strand and properly place them into the sequence

translation: process of producing a protein from the nucleotide sequence code of an mRNA transcript

triplet: consecutive sequence of three nucleotides on a DNA molecule that, when transcribed into an mRNA codon, corresponds to a particular amino acid

unipotent: describes the condition of being committed to a single specialized cell type

vesicle: membrane-bound structure that contains materials within or outside of the cell