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| 2380437986 | plasma membrane | The membrane at the boundary of every cell that acts as a selective barrier, regulating the cell's chemical composition. | 0 | |
| 2380437987 | organelles | Any of several membrane-enclosed structures with specialized functions, suspended in the cytosol of eukaryotic cells. | 1 | |
| 2380438223 | nucleus |  | 2 | |
| 2380438224 | ribosome |  | 3 | |
| 2380439168 | endoplasmic reticulum |  | 4 | |
| 2380445579 | smooth ER | synthesis of lipids, detoxification of drugs and poisons | 5 | |
| 2380439170 | rough ER | synthesis of secretory and other proteins from bound ribosomes | 6 | |
| 2380440678 | Golgi apparatus |  | 7 | |
| 2380440679 | lysosome |  | 8 | |
| 2380440680 | vacuole |  | 9 | |
| 2380440681 | mitochondrion |  | 10 | |
| 2380440928 | chloroplast |  | 11 | |
| 2380440929 | peroxisome |  | 12 | |
| 2380462222 | endosymbiotic theory |  | 13 | |
| 2404734576 | fluid mosaic model | The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids. | 14 | |
| 2404735069 | amphipathic | Having both a hydrophilic region and a hydrophobic region. | 15 | |
| 2404743966 | diffusion | The spontaneous movement of a substance down its concentration or electrochemical gradient, from a region where it is more concentrated to a region where it is less concentrated. | 16 | |
| 2404746743 | osmosis | The diffusion of free water across a selectively permeable membrane. | 17 | |
| 2404747392 | hypertonic | Referring to a solution that, when surrounding a cell, will cause the cell to lose water. | 18 | |
| 2404748065 | hypotonic | Referring to a solution that, when surrounding a cell, will cause the cell to take up water. | 19 | |
| 2404748867 | isotonic | Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell. | 20 | |
| 2404751682 | facilitated diffusion | The passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure. | 21 | |
| 2404756397 | active transport | The movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy. | 22 | |
| 2404757889 | sodium-potassium pump | A transport protein in the plasma membrane of animal cells that actively transports sodium out of the cell and potassium into the cell. | 23 | |
| 2404759489 | proton gradient | The concentration difference of H+ ions, or protons, across a membrane, producing a force | 24 | |
| 2404774073 | phagocytosis | A type of endocytosis in which large particulate substances or small organisms are taken up by a cell. It is carried out by some protists and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells). | 25 | |
| 2404777015 | metabolism | The totality of an organism's chemical reactions, consisting of catabolic and anabolic pathways, which manage the material and energy resources of the organism. | 26 | |
| 2404777016 | enzyme | A macromolecule serving as a catalyst, a chemical agent that increases the rate of a reaction without being consumed by the reaction. Most enzymes are proteins. | 27 | |
| 2404778462 | catabolic pathway | A metabolic pathway that releases energy by breaking down complex molecules to simpler molecules. | 28 | |
| 2404778463 | anabolic pathway | A metabolic pathway that consumes energy to synthesize a complex molecule from simpler molecules. | 29 | |
| 2404781192 | first law of thermodynamics | The principle of conservation of energy: Energy can be transferred and transformed, but it cannot be created or destroyed. | 30 | |
| 2404781193 | second law of thermodynamics | The principle stating that every energy transfer or transformation increases the entropy of the universe. Usable forms of energy are at least partly converted to heat. | 31 | |
| 2404784278 | exergonic reaction | A spontaneous chemical reaction, in which there is a net release of free energy. | 32 | |
| 2404785367 | endergonic reaction | A nonspontaneous chemical reaction, in which free energy is absorbed from the surroundings. | 33 | |
| 2404785950 | ATP (adenosine triphosphate) | An adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed. This energy is used to drive endergonic reactions in cells. | 34 | |
| 2404789862 | activation energy | The amount of energy that reactants must absorb before a chemical reaction will start; also called free energy of activation. | 35 | |
| 2404794220 | allosteric regulation | The binding of a regulatory molecule to a protein at one site that affects the function of the protein at a different site. | 36 | |
| 2404795516 | feedback inhibition | A method of metabolic control in which the end product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway. | 37 | |
| 2404796600 | fermentation | A catabolic process that makes a limited amount of ATP from glucose (or other organic molecules) without an electron transport chain and that produces a characteristic end product, such as ethyl alcohol or lactic acid. | 38 | |
| 2404797778 | cellular respiration | The catabolic pathways of aerobic and anaerobic respiration, which break down organic molecules and use an electron transport chain for the production of ATP. | 39 | |
| 2404801645 | oxidation | The complete or partial loss of electrons from a substance involved in a redox reaction. | 40 | |
| 2404801646 | reduction | The complete or partial addition of electrons to a substance involved in a redox reaction. | 41 | |
| 2404802629 | electron transport chain | A sequence of electron carrier molecules (membrane proteins) that shuttle electrons down a series of redox reactions that release energy used to make ATP. | 42 | |
| 2404803799 | NAD+ | Nicotinamide adenine dinucleotide, a coenzyme that cycles easily between oxidized (NAD+) and reduced (NADH) states, thus acting as an electron carrier. | 43 | |
| 2404804727 | glycolysis | A series of reactions that ultimately splits glucose into pyruvate. Glycolysis occurs in almost all living cells, serving as the starting point for fermentation or cellular respiration. | 44 | |
| 2404805694 | citric acid cycle | A chemical cycle involving eight steps that completes the metabolic breakdown of glucose molecules begun in glycolysis by oxidizing acetyl CoA (derived from pyruvate) to carbon dioxide; occurs within the mitochondrion in eukaryotic cells and in the cytosol of prokaryotes; together with pyruvate oxidation, the second major stage in cellular respiration. | 45 | |
| 2404806266 | oxidative phosphorylation | The production of ATP using energy derived from the redox reactions of an electron transport chain; the third major stage of cellular respiration. | 46 | |
| 2404807575 | ATP synthase | A complex of several membrane proteins that functions in chemiosmosis with adjacent electron transport chains, using the energy of a hydrogen ion (proton) concentration gradient to make ATP. ATP synthases are found in the inner mitochondrial membranes of eukaryotic cells and in the plasma membranes of prokaryotes. | 47 | |
| 2404808265 | chemiosmosis | An energy-coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP. Under aerobic conditions, most ATP synthesis in cells occurs by chemiosmosis. | 48 | |
| 2404812284 | photosynthesis | The conversion of light energy to chemical energy that is stored in sugars or other organic compounds; occurs in plants, algae, and certain prokaryotes. | 49 | |
| 2404814298 | light reactions | The first of two major stages in photosynthesis (preceding the Calvin cycle). These reactions, which occur on the thylakoid membranes of the chloroplast or on membranes of certain prokaryotes, convert solar energy to the chemical energy of ATP and NADPH, releasing oxygen in the process. | 50 | |
| 2404814299 | Calvin cycle | The second of two major stages in photosynthesis (following the light reactions), involving fixation of atmospheric CO2 and reduction of the fixed carbon into carbohydrate. | 51 | |
| 2404816403 | NADP+ | Nicotinamide adenine dinucleotide phosphate, an electron acceptor that, as NADPH, temporarily stores energized electrons produced during the light reactions. | 52 | |
| 2404817219 | photosystem | A light-capturing unit located in the thylakoid membrane of the chloroplast or in the membrane of some prokaryotes, consisting of a reaction-center complex surrounded by numerous light-harvesting complexes. There are two types of photosystems, I and II; they absorb light best at different wavelengths. | 53 | |
| 2404818192 | glyceraldehyde 3-phosphate (G3P) | A three-carbon carbohydrate that is the direct product of the Calvin cycle; it is also an intermediate in glycolysis. | 54 | |
| 2404820271 | signal transduction pathway | A series of steps linking a mechanical, chemical, or electrical stimulus to a specific cellular response. | 55 | |
| 2404820930 | hormone | In multicellular organisms, one of many types of secreted chemicals that are formed in specialized cells, travel in body fluids, and act on specific target cells in other parts of the body, changing the target cells' functioning. Hormones are thus important in long-distance signaling. | 56 | |
| 2404822383 | G protein-coupled receptor (GPCR) | A signal receptor protein in the plasma membrane that responds to the binding of a signaling molecule by activating a G protein. Also called a G protein-linked receptor. | 57 | |
| 2404823027 | receptor tyrosine kinase (RTK) | A receptor protein spanning the plasma membrane, the cytoplasmic (intracellular) part of which can catalyze the transfer of a phosphate group from ATP to a tyrosine on another protein. Receptor tyrosine kinases often respond to the binding of a signaling molecule by dimerizing and then phosphorylating a tyrosine on the cytoplasmic portion of the other receptor in the dimer. The phosphorylated tyrosines on the receptors then activate other signal transduction proteins within the cell. | 58 | |
| 2404823605 | ligand-gated ion channel | A transmembrane protein containing a pore that opens or closes as it changes shape in response to a signaling molecule (ligand), allowing or blocking the flow of specific ions; also called an ionotropic receptor. | 59 | |
| 2404824417 | protein kinase | An enzyme that transfers phosphate groups from ATP to a protein, thus phosphorylating the protein. | 60 | |
| 2404824418 | protein phosphatase | An enzyme that removes phosphate groups from (dephosphorylates) proteins, often functioning to reverse the effect of a protein kinase. | 61 | |
| 2404825575 | secondary messengers | A small, nonprotein, water-soluble molecule or ion, such as a calcium ion (Ca2+) or cyclic AMP, that relays a signal to a cell's interior in response to a signaling molecule bound by a signal receptor protein. | 62 | |
| 2404826394 | cyclic AMP (cAMP) | Cyclic adenosine monophosphate, a ring-shaped molecule made from ATP that is a common intracellular signaling molecule (second messenger) in eukaryotic cells. It is also a regulator of some bacterial operons. | 63 | |
| 2404827767 | adenylyl cyclase | An enzyme that converts ATP to cyclic AMP in response to an extracellular signal. | 64 | |
| 2404832538 | mitosis | A process of nuclear division in eukaryotic cells conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis conserves chromosome number by allocating replicated chromosomes equally to each of the daughter nuclei. | 65 | |
| 2404833910 | cell cycle | An ordered sequence of events in the life of a cell, from its origin in the division of a parent cell until its own division into two. The eukaryotic cell cycle is composed of interphase (including GG1, S, and GG2 subphases) and M phase (including mitosis and cytokinesis). | 66 | |
| 2404834712 | chromosome | A cellular structure carrying genetic material, found in the nucleus of eukaryotic cells. Each chromosome consists of one very long DNA molecule and associated proteins. (A bacterial chromosome usually consists of a single circular DNA molecule and associated proteins. It is found in the nucleoid region, which is not membrane bounded.) See also chromatin. | 67 | |
| 2404834713 | chromatin | The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope. | 68 | |
| 2404835941 | somatic cell | Any cell in a multicellular organism except a sperm or egg or their precursors. | 69 | |
| 2404836503 | sister chromatids | Two copies of a duplicated chromosome attached to each other by proteins at the centromere and, sometimes, along the arms. While joined, two sister chromatids make up one chromosome. Chromatids are eventually separated during mitosis or meiosis II. | 70 | |
| 2404837111 | centromere | In a duplicated chromosome, the region on each sister chromatid where they are most closely attached to each other by proteins that bind to specific DNA sequences; this close attachment causes a constriction in the condensed chromosome. (An uncondensed, unduplicated chromosome has a single centromere, identified by its DNA sequence.) | 71 | |
| 2404837626 | cytokinesis | The division of the cytoplasm to form two separate daughter cells immediately after mitosis, meiosis I, or meiosis II. | 72 | |
| 2404838232 | interphase | The period in the cell cycle when the cell is not dividing. During interphase, cellular metabolic activity is high, chromosomes and organelles are duplicated, and cell size may increase. Interphase often accounts for about 90% of the cell cycle. | 73 | |
| 2404838875 | mitotic spindle | An assemblage of microtubules and associated proteins that is involved in the movement of chromosomes during mitosis. | 74 | |
| 2404839656 | cell plate | A membrane-bounded, flattened sac located at the midline of a dividing plant cell, inside which the new cell wall forms during cytokinesis. | 75 | |
| 2404841149 | cleavage furrow | The pinching of the plasma membrane, during the the process of cytokinesis in animal cells. | 76 | |
| 2404841826 | cyclin | A cellular protein that occurs in a cyclically fluctuating concentration and that plays an important role in regulating the cell cycle. | 77 | |
| 2404841827 | cyclin-dependent kinases (Cdk) | A protein kinase that is active only when attached to a particular cyclin. | 78 | |
| 2404842774 | checkpoint | A control point in the cell cycle where stop and go-ahead signals can regulate the cycle. | 79 |
