Cell Structures: in plants and animals, each cell has an internal order, and the cells within the body have specific arrangements and functions. (Brooker, 3) Cell structure relies on four critical phenomena: matter, energy, organization, and information. (Brooker, 64)

The "cytoplasm" contains a "cytoskeleton" and specialized membrane-bounded "organelles." (Lawrence) Each cell contains a unique mix of "proteins" that enables it to perform its specific biological functions. (Kandel, 257) By studying mammalian cells in the laboratory, geneticists have determined that the profound "morphological" differences between...  "differentiated" cells, such as "muscle" and "nerve" cells, arise through “gene regulation.” Though nerve and muscle cells contain the same set of genes, they regulate the expression of their genes in very different ways.  (Brooker, 403)


Cell Membranes: any thin, pliable, usually fibrous sheet or layer of tissue, frequently serving to separate other structures or to line a part or organ.(Oxford) The basic framework of a plasma membrane is a “phospholipid bilayer.” The two primary components of the cell membrane are "lipids," which form the basic matrix of a membrane, and proteins, which are embedded in the membrane or loosely attached to its surface. A third component is “carbohydrate,” which may be attached to membrane lipids and proteins. (Brooker, 85-86) Every cell in a multicellular organism is surrounded by (this) oily membrane that separates it from other cells and from the “extracellular fluid” that bathes all cells. (Kandel, 58) The structure of cellular membranes enables them to compartmentalize the cell while selectively importing and exporting vital substances. (Brooker, 85) Defines the boundary of an individual cell. (LeDoux, 54) Cells adapt to changes in temperature by altering the lipid composition of their membranes. Allows the cell to maintain a favorable internal environment. (Brooker, 89-92) Also referred to as 'membrane,' 'biomembrane,' 'biological membrane,' 'cellular membrane,' and 'plasma membrane.’

Cell Surface Membrane: the cell membrane found on the outside of the cell. (Indge, 210) The cell surface membrane is permeable to certain substances, thereby allowing an exchange of nutrients to take place between the interior of the cell and the fluid surrounding it. (Kandel, 58)

Endomembrane System: network of membranes. These include not only the “nuclear envelope,” which encloses the “nucleus,” but also the “endoplasmic reticulum,” the “Golgi apparatus,” the “lysosomes,” and the “vacuoles.” The nuclear envelope is considered part of the endomembrane system, but the interior of the nucleus is not. The (cell membrane) is also part of this integrated membrane system. (Brooker, 75) 

Fatty Acyl Tails: the “hydrophobic” tail region (of the phospholipid bilayer). (Brooker, 86)

Fluidity: a quality of the (membrane) that means that individual molecules remain in close association yet have the ability to move laterally or rotationally within the plane of the membrane. Membranes are semi-fluid. (Brooker, G-14) Factors affecting fluidity include the length of fatty acyl tails (shorter - more fluidity), the presence of “double bonds” (more - more fluidity), and the presence of “cholesterol” (more means less fluidity). (Norton Lectures, 6/3/09)

Fluid Mosaic Model: the way we describe the membrane and know what is in it. (Norton Lectures, 6/3/09) Membranes exhibit properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane. (Brooker, 86) Editor's note - originally proposed by Jonathan Singer and Garth Nicolson in 1972.

Glycocalyx: a term used to describe the carbohydrate-rich zone on the cell surface that shields the cell from mechanical and physical damage. (Brooker, 91) Meaning ‘sugar-coat.’ It is ‘sticky’ so that substances trying to get into the cell, can get stuck to it. (Norton Lectures, 6/3/09) Outer viscous covering surrounding many bacteria. The glycocalyx traps water and helps protect bacteria from drying out. (Brooker, 64)

Glycosylation: the process of “covalently” attaching a carbohydrate to a protein or lipid in the phospholipid bilayer of a cell membrane. (Brooker, 90) Additions of side chains to proteins to form "glycoproteins." Takes place in the endoplasmic reticulum and the golgi apparatus. (Lawrence) Also referred to as 'tagging.'

Permeability: permitting the passage or diffusion of something; penetrable. (Oxford) The plasma membrane is a selectively permeable barrier between the cell and its external environment. The relative permeability of an phospholipid bilayer to a variety of “solutes” (can be characterized as) ‘high’ (gasses, ethanol,) ‘moderate’ (water,) ‘low’ (“glucose,”) and ‘very low’ (“ions” and charged “polar molecules” and “macromolecules.”) (Brooker, 92)

Permeable: allowing a given substance to diffuse freely across, for example, a membrane. (Lawrence)

Semipermeable: partially permeable, allowing some substances through but not others. A membrane permeable to a "solvent," especially water, but not to "solutes." (Lawrence)

P-Face: the interior or plasma face of a cell membrane. (Norton Lectures, 6/9/09)

Phospholipid Bilayer: framework of the cellular membrane which consists of two layers of lipids called “leaflets.” Acts as a barrier to “hydrophilic” substances. (Brooker, 85)

Polar Heads: the hydrophilic head group on the surface (of the phospholipid bilayer). (Brooker, 86)

Receptor(s): a molecule inside or on the surface of a cell that binds to a specific substance and causes a specific physiologic effect in the cell. (GHR) A protein located on the cell surface, or in the cytoplasm, that binds to a specific signaling factor, such as a “hormone,” “antigen,” or “neurotransmitter,” causing a “conformational” and functional change in the receptor molecule. The ligand-bound receptor then alters its interaction with target molecules, which leads to changes in cellular physiology through modification of the activity. (NCIt) Editor’s note - for receptor’s binding neurotransmitters, see “synaptic receptors.”

Villi: any of numerous slender hairlike processes or minute projections closely set on a surface, especially in the “mucous membrane” of the intestines. (Oxford) Adjective - ‘villous.’

Cytoplasm: the region of the cell that is contained within the plasma membrane. (Brooker, 64) The liquid part of the cell. Within the cytoplasm are found all the “organelles” of the cell except the nucleus, which is separated from the cytoplasm by its own membrane. (Bynum, 292) The actual machinery for making proteins is located in the cytoplasm. (Kandel, 58) From ‘cyto’ meaning cell and ‘plasm’ meaning fluid. Mostly water. Contains high levels of "potassium,"  "amino acids," and lipids. Contains low levels of "sodium," and "carbohydrates." Different concentrations of "ionic" "solutes" give the interior of the cell a negative charge compared to extracellular fluid. (Norman, 6/2/09)

Cytoplasmic Streaming: phenomenon in which the cytoplasm circulates throughout the cell to distribute resources efficient in large cells. (Brooker, 74)

Cytoskeleton: a network of three different types of protein 'filaments.' Provides cell organization, shape, and movement. (Brooker, 70) Gives shape to a cell and provides support for cell extensions such as "villi" and "axons" of nerve cells. Elements of the cytoskeleton form the "mitotic" and "meiotic" "spindles" and are involved in "cellular transport." (Lawrence)  Includes “actin filaments,” “intermediate filaments,” and “microtubules.” (Norman, 6/2/09) 

Actin Filaments: protein filaments. Long thin fibers approximately seven “nanometers” in diameter. Each fiber is composed of two strands of actin subunits that spiral around each other. Although dispersed throughout the cytosol, they tend to be highly concentrated near the plasma membrane. Support the plasma membrane and provide strength and shape to the cell. Involved in cell "division" and intracellular movement of cargo. Have plus and minus ends. Very dynamic structures that grow at the plus end. The minus ends are usually anchored at the plasma membrane. (Brooker, G71) Anchor membranes to (the) cytoplasm. Aid in muscle contraction via "motor" proteins. Provide localized cell ‘cinch’ or cutting. Involved in cytoplasmic streaming. (Norton Lectures, 6/2/09) Also referred to as 'microfilaments.'

Dynamic Instability: the property displayed by "actin filaments" and "microtubules" that are in a constant state of assembly and disassembly. (Lawrence) Process by which a filament can oscillate between growing and shortening. (Norman, 6/2/09)

Intermediate Filaments: protein filaments that influence shape of the cell. (Norman, 5/26/09) Bind to each other in a staggered array to form a twisted, rope-like structure  with a diameter of approximately 10 nanometers. Function as tension-bearing fibers that help maintain cell shape and rigidity. Involved in muscle cells, skin, intestinal, and kidney cells, hair and nails. In addition, line the inner nuclear membrane and provide anchorage points for the nuclear pores. (Brooker, 71) Provide cells with mechanical strength. Fix certain organelles in place. (Norman, 6/2/09)

Microtubules: protein filaments. Long, hollow, cylindrical structures about 25 nanometers in diameter composed of the protein ‘tubulin.’  “Polar” structure with a plus end and a minus end. A single microtubule can "oscillate" between growing and shortening. Growth occurs at the plus end while shortening can occur at either the plus end or the minus end. Important for cell shape and organization. Involved in the organization of chromosomes during “mitosis” and in the orientation of cells during cell division. Organelles such as the “golgi apparatus” often are attached to microtubules. (Brooker, 70) Involved in the intracellular movement of cargo, providing ‘tracks’ for motor proteins. Involved in cell “motility” in “flagella” or “cilia.” (Norman, 6/2/09)

Cytosol: the fluid part of the cytoplasm outside the membrane-bounded organelles. (Lawrence) The region of the cell that is outside the cell organelles but inside the plasma membrane. The cytosol is a central coordinating region for many “metabolic” activities of cells. (Brooker, 68-69)

Nucleus: an organelle that contains most of the cell’s "genetic material." The primary function of the nucleus involves the protection, organization, and expression of the genetic material. (Brooker, 75)  Has a membrane of its own and is surrounded by intracellular fluid. Contains the “chromosomes" that carry genes like beads on a string. (Kandel, 58) Along with axons and “dendrites,” one of the three main parts of the “neuron.” Sustains the life of the cell and contains its “DNA.” (Doidge, 53) Plural - ‘nuclei.’ Adjective - ‘nuclear.’ Also referred to as 'cell nucleus,'  'cell body,' and 'neuronal body.'

Nuclear Envelope: the membrane that encloses the nucleus. A double-membrane structure. (Brooker, 75) Also referred to as the ‘nuclear membrane.'

Nuclear Matrix: a network of proteins. Serves to organize the chromosomes within the nucleus. (Brooker, 76) The "insoluble" ... material left in the nucleus after certain biochemical extraction procedures. It has been proposed to form a framework on which the chromosomes are organized. (Lawrence)

Nuclear Pores: passageways for molecules into and out of the nucleus. (Brooker, 66) Formed where the inner and outer nuclear membranes make contact with each other. (Brooker, 75) They are present in large numbers in the nuclear envelope and form channels through which "macromolecules" pass from (the) nucleus to (the) cytoplasm of vice versa. (Lawrence)

Nucleoid: the region of a cell where DNA is located. (Brooker, 64)

Nucleolus: a prominent region in the nucleus of non-dividing cells. "Ribosome" assembly occurs in the nucleolus. (Brooker, 76) Region of the nucleus where "ribosomal RNA" is synthesized. (Lawrence)

Organelles: sub-cellular structures, or membrane-bounded compartments with (their) own unique structure and function. (Brooker, 64) Non-membranous organelles have direct contact with the "cytoplasm." They include the "cytoskeleton" and "ribosomes." (Norman, 6/2/09) Membrane-bound organelles have a single or double "phospholipid bilayer" surrounding them. They include "mitochondria," the smooth and rough “endoplasmic reticulum,” the "golgi apparatus," "vacuoles," "lysosomes," "peroxisomes," and the "nucleus." (Norman, 6/2/09)

Centrioles: a pair of structures within the "centrosome" of animal cells. Most plant cells lack centrioles. (Oxford) Formed of short microtubules and found just outside the nuclear envelope. (Lawrence)

Centrosome: site where "microtubules" grow and centrioles are formed. (Booker, 66) Organelle situated near the nucleus , and which organizes the microtubule cytoskeleton. (Lawrence)

Endoplasmic Reticulum (ER): cellular structure involved in "detoxification," lipid synthesis, protein synthesis, sorting, and "secretion." (Brooker, 67) A convoluted network of membranes that form flattened, fluid-filled "tubules." The terms endoplasmic (in the cytoplasm) and reticulum (little net), refer to the location and shape of this organelle when viewed under a microscope. (Brooker, 76) Network of tubules and sacs characterized by a single membrane. Consists of alternating rough and smooth forms. (Norman, 6/2/09) Acts as a quality control center for the cell. Its chemical environment enables the forming protein to start folding into the three-dimensional shape necessary for its specific function. Misfolded proteins (which can cause disease) are pulled out of the ER and degraded. (Lewis, 23) 

Endoplasmic Reticulum Lumen: a single compartment enclosed by the endoplasmic reticulum membrane. (Brooker, 77)

Endoplasmic Reticulum Signal Sequence: a sorting signal which is usually located near the 'amino terminus.' (Brooker, 117)

Rough Endoplasmic Reticulum: section of ER nearest the nucleus, which is flattened and studded with ribosomes. "Messenger RNA" attaches to the ribosomes. "Amino acids" from the cytoplasm are then linked, following the instructions in the messenger RNA's sequence, to form particular proteins. (Lewis, 20) Plays a key role in the initial synthesis and sorting of proteins that are destined for the golgi apparatus, lysosomes, vacuoles, plasma membrane, or outside the cell. To reach any of these locations, a protein must first be directed to the endoplasmic reticulum membrane. A second function is the insertion of certain newly made proteins into the membrane. A third important function is the attachment of carbohydrate to proteins and lipids. (Brooker, 77) Manufactures proteins typically for excretion. Also manufactures membranes. Characterized by the presence of ribosomes on its membrane which look like bumps. After the ribosome does its work, it goes away, so photos of the organelle taken at different times will appear different. It's membrane is continuous with the “nuclear envelope.” (Norton Lectures, 6/2/09)

Smooth Endoplasmic Reticulum: endoplasmic reticulum with no interactions with ribosomes. Synthesizes lipids, phospholipids, “steroids," and “hormones.” Detoxifies some drugs and poisons by converting “hydrophobic” molecules into “hydrophilic” molecules. Stores "calcium" needed for muscle contraction. “Metabolizes” carbohydrate. (Norman, 6/2/09)

Golgi Apparatus: cellular structure involved in modification, sorting, and secretion of lipids and proteins. (Brooker, 67) Composed of a stack of flattened membranes. Each flattened membrane encloses a single compartment or "sac." The stacks are named according to their orientation in the cell. (Brooker, 78) Modifies, stores, ‘tags,’ and routes products made in the endoplasmic reticulum. (Norman, 6/2/09) Mostly concerned with the packaging and processing of molecules produced by the cell. (Indge, 192)

Lumen: the internal space of an organelle. (Brooker, 77) The space within a tubular or sac-like part, such as a blood vessel, the intestine, or the stomach. (OxfordMed)

Lysosome: a small sac surrounded by a membrane and containing "digestive" enzymes. (Indge, 192) Cellular structure involved in the "degradation" of macromolecules. (Brooker, 66) Lysosomes are 'trash centers.' Fuse with vesicles or damaged organelles, activating the enzymes within to recycle the molecules. (They) dismantle bacterial remnants, worn-out organelles, and other material such as excess cholesterol. The enzymes also break down some digested nutrients into forms the cell can use. (Lewis, 24)  From 'lyso' meaning 'to break apart' and 'some' meaning ‘body.’ Involved in "hydrolysis" of macromolecules and the recycling of cell components. (Norman, 6/2/09)

Ribosome: large molecule composed of "RNA" and protein. The major cellular component required for protein synthesis. (Micklos, 44) Structure found in the cell’s cytoplasm that translates messenger RNA into protein. (Kandel, 258) Site of protein synthesis. (Indge, 193) The role of the ribosome is to facilitate the binding between messenger RNA and "transfer RNA" molecules, and to catalyze the formation of "covalent bonds" between adjacent amino acids. (Brooker, 69) Functions as a machine to attach amino acids to proteins. (Lewis, 182) Composed of two subunits, one small and one large; each subunit contains one or more RNA molecules and several types of proteins. The ribosomal proteins are produced in the cytosol, imported into the nucleus through the nuclear pores, and assembled with the RNA molecules to form the ribosomal subunits. The subunits then exit through the nuclear pores into the cytosol, where they are needed for protein synthesis. (Brooker, 76) Reads the messenger RNA sequence, one "codon" at a time, and matches it to the "anticodon" on each transfer RNA, lining up their cargo of amino acids in the proper order. (Venter, 37)

Semiautonomous Organelles: those organelles that can grow and divide to reproduce themselves, but they are not completely autonomous because they depend on other parts of the cell for their internal components. They include mitochondria and peroxisomes. (Brooker, 80)

Mitochondria: provide energy by breaking the chemical bonds that hold together the nutrient molecules in food. Has an outer membrane similar to those in the ER and Golgi apparatus and an inner membrane that forms folds. These folds hold enzymes that catalyze the biochemical reactions that release energy from nutrient molecules. The freed energy is captured and stored in the bonds that hold together a molecule called "ATP." (Lewis, 25) Tiny structures within every cell nucleus that act as the cell’s furnace, using oxygen to convert “glucose” into a usable fuel during “aerobic" metabolism. When oxygen levels are insufficient, fuel conversion is shifted outside the mitochondria for “anaerobic metabolism,” a far less efficient process than aerobic metabolism. (Ratey, 280) Site of “cellular respiration.” Characterized by a double membrane. Contains its own ribosomes, so it can make its own DNA. Dynamic organelle - moves, divides, and changes shape. Transforms energy acquired from surroundings into forms useful for cellular work. Involved in the synthesis, modification and breakdown of certain molecules. (Norman, 6/2/09)

Peroxisome: sacs with outer membranes that are studded with several types of enzymes. These enzymes (are involved in) breaking down certain lipids and rare biochemicals, synthesizing "bile" acids used in fat digestion, and detoxifying compounds that result from exposure to oxygen "free radicals." (Lewis, 24) Relatively small organelles. Consist of a single membrane that encloses a fluid-filled lumen. The general function of peroxisomes is to catalyze certain chemical reactions, typically those that break down molecules by removing "hydrogen" or adding oxygen. (Brooker, 82) Converts toxins to ‘hydrogen peroxide’ then to water. Involved in the hydrolysis of macromolecules. (Norman, 6/2/09) Large and abundant in "liver" and "kidney cells." (Lewis, 25)

Vacuoles: cellular structures involved in storage and regulation of cell volume. (Broker, 67)

Vesicle(s): a small sac in the cytoplasm of a cell, surrounded by a membrane. The golgi apparatus is responsible for the processing and packaging of substances produced by a cell. These substances are enclosed in small vesicles which are continually being formed by being pinched off from the flattened sacs which make up the golgi apparatus. A vesicle which is pinched off, moves into the cytoplasm. (Indge, 284) The movement of vesicles occurs in both directions (out of the cell, and into the cell.) (Brooker, 75)

Secretory Vesicles: vesicles derived from the golgi apparatus containing material to be released at the cell surface. (MeSH) Also referred to as ‘secretory granules.’