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Endosperm : The tissue produced inside the seeds of most of the flowering plants following fertilization.

Endoplasmic reticulum : A type of organelle found in eukaryotic cells that forms an interconnected network of flattened, membrane-enclosed sacs or tube-like structures known as cisternae.

Endogenous : (of a substance or process) Originating from within a system (such as an organism, tissue, or cell), as with endogenous cannabinoids and circadian rhythms. Contrast exogenous.

Endoderm : One of the three primary germ layers in the very early human embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer), with the endoderm being the innermost layer.

Endocytosis : A form of active transport in which a cell transports molecules such as proteins into the cell’s interior by engulfing them in an energy-consuming process.

Endocrine system : The collection of glands that produce hormones which regulate metabolism, growth and development, tissue function, and a wide variety of other biological processes.

Endocrine gland :A gland of the animalian endocrine system that secretes hormones directly into the blood rather than through a duct. In humans, the major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, and adrenal glands.

Endergonic reaction :Also called a nonspontaneous reaction or unfavorable reaction.

A type of chemical reaction in which the standard change in free energy is positive, and energy is absorbed.

Endemism : The ecological state of an organism or species being unique to a defined geographic location, such as an island, nation, country, habitat type, or other defined zone. Organisms are said to be endemic to a place if they are indigenous to it and found nowhere else.

Endangered species : Any species which is very likely to become extinct in the near future, either worldwide or in a particular area. Such species may be threatened by factors such as habitat loss, hunting, disease, and climate change, and most have a declining population or a very limited range.

Embryology : The branch of biology that studies the development of gametes (sex cells), fertilization, and development of embryos and fetuses. Additionally, embryology involves the study of congenital disorders that occur before birth.

Embryo : A developing stage of a multicellular organism.

Electron transport chain : The process of oxidative phosphorylation, by which the NADH and succinate generated by the citric acid cycle are oxidized and electrons are transferred sequentially down a long series of proteins, ultimately to the enzyme ATP synthase, which uses the electrical energy to catalyze the synthesis of ATP by the addition of a phosphate group to ADP. The process takes place in the cell’s mitochondria and is the primary means of energy generation in most eukaryotic organisms.

Electron microscope : A type of microscope that uses a beam of electrons to create an image of a sample or specimen. Electron microscopes are capable of much higher magnifications and have greater resolving power than conventional light microscopes, allowing them to see much smaller objects in finer detail.

Electron donor : A chemical entity that donates electrons to another chemical entity. It is a reducing agent that, by virtue of its giving up its electrons, is itself oxidized in the process. Contrast electron acceptor.

Electron carrier : Any of various molecules that are capable of accepting one or two electrons from one molecule and donating them to another in the process of electron transport. As the electrons are transferred from one electron carrier to another, their energy level decreases, and energy is released.

Electrochemical gradient : A gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts: the electrical potential and the difference in chemical concentration across the membrane.

Egg : The organic vessel containing the zygote in which an animal embryo develops until it can survive on its own, at which point the developing organism emerges from the egg in a process known as hatching.

Effector :A small molecule that selectively binds to a protein and regulates its biological activity. In this manner, effector molecules act as ligands that can increase or decrease enzyme activity, gene expression, or cell signaling.

Ectotherm : An organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature compared to ambient sources of heat. Ectotherms generally experience changes in body temperature that closely match changes in the temperature of their environment; colloquially, these organisms are often referred to as “cold-blooded”. Contrast endotherm.

Ectoderm : The outermost layer of cells or tissue of an embryo in early development, or the parts derived from this, which include the epidermis, nerve tissue, and nephridia.

Ecosystem : A community of living organisms in conjunction with the non-living components of their physical environment, interacting as a system.

Ecophysiology : A biological discipline that studies the adaptation of an organism’s physiology to environmental conditions.

Ecology: The scientific analysis and study of interactions between organisms and their environment. It is an interdisciplinary field that combines concepts from biology, geography, and Earth science.

Ecological succession : The more or less predictable and orderly set of changes that occurs in the composition or structure of an ecological community over time.

Ecological pyramid: Also called a trophic pyramid, eltonian pyramid, energy pyramid, or sometimes food pyramid.

A graphical representation of the biomass or bio-productivity generated at each trophic level in a given ecosystem.

Ecological efficiency : The efficiency with which energy is transferred from one trophic level to the next. It is determined by a combination of efficiencies relating to organismic resource acquisition and assimilation in an ecosystem.

Dynein : A motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement.

Dimorphism : The existence of a morphological distinction between organisms of the same species, such that individuals of that species occur in one of two distinct forms which differ in one or more characteristics, such as colour, size, shape, or any other phenotypic trait. Dimorphism based on sex – e.g. male vs. female – is common in sexually reproducing organisms such as plants and animals.

Drug : Any substance that causes a change in an organism’s physiology or psychology when consumed. Drugs may be naturally occurring or artificially produced, and consumption may occur in a number of different ways. Drugs are typically distinguished from substances that provide nutritional support such as food.

DNA sequencing : The process of determining the precise order of nucleotides within a DNA molecule.

DNA replication : The chemical duplication or copying of a DNA molecule; the process of producing two identical copies from one original DNA molecule, in which the double helix is unwound and each strand acts as a template for the next strand. Complementary nucleotide bases are matched to synthesize the new partner strands.

DNA : See deoxyribonucleic acid.

Disease : Any particular abnormal condition that negatively affects the structure or function of all or part of a living organism and that is not the result of any immediate external injury. Diseases are medical conditions that are often identifiable by specific signs and symptoms. They may be caused by external factors such as infectious pathogens or by internal dysfunctions such as immune deficiency or senescence.

Developmental biology : The branch of biology that studies the processes by which living organisms grow and develop over time. The field may also encompass the study of reproduction, regeneration, metamorphosis, and the growth and differentiation of stem cells in mature tissues.

Desmosome : Also called the macula adhaerens.

A cell structure specialized for cell-to-cell adhesion.

Depolarization : The process of reversing the charge across a cell membrane (such as that of a neuron), thereby causing an action potential. In depolarization, the inside of the membrane, which is normally negatively charged, becomes positive and the outside becomes negative. This is brought about by positively charged sodium ions rapidly passing into the axon.

Deoxyribonucleic acid (DNA) : A nucleic acid polymer that serves as the fundamental hereditary material in all living organisms. Each DNA molecule is composed of long sequences of nucleotides, each of which includes one of four nitrogenous bases – adenine (abbreviated A), cytosine (C), guanine (G), and thymine (T) – attached to a sugar-phosphate complex which acts as a “backbone” for the long-chain polymer. DNA most commonly occurs in “double-stranded” form, i.e. as a pair of nucleotide polymers bound together by complementary base pairing.

Denitrification : The microbially facilitated process of nitrate reduction that ultimately produces molecular nitrogen (N₂) through a series of intermediate gaseous nitrogen oxide products. It is performed by a large group of heterotrophic facultative anaerobic bacteria and is a fundamental component of the nitrogen cycle.

Dendrite : A short branched extension of a nerve cell, along which impulses received from other cells at synapses are transmitted to the cell body.

Denaturation : A process in which proteins or nucleic acids lose the quaternary, tertiary, and secondary structure which is present in their native state, when exposed to some external stress or chemical compound such as a strong acid or base, a concentrated inorganic salt, or an organic solvent.

Dehydration reaction : A chemical reaction that involves the loss of a water molecule from the reacting molecule.

Deciduous : Deciduous means “falling off at maturity” or “tending to fall off”, and it is typically used in botany in order to refer to trees or shrubs that lose their leaves seasonally (most commonly during autumn) and to the shedding of other plant structures such as petals after flowering or fruits when ripe.

Darwinian fitness : The genetic contribution of an individual to the next generation‘s gene pool relative to the average for the population, usually measured by the number of offspring or close kin that survive to reproductive age.

Cytoskeleton : A complex, dynamic network of interlinking protein filaments that extends from the cell nucleus to the cell membrane and which is present in the cytoplasm of all cells, including bacteria and archaea.^[3] The cytoskeletal systems of different organisms are composed of similar proteins. In eukaryotes, the cytoskeletal matrix is a dynamic structure composed of three main proteins, which are capable of rapid growth or disassembly dependent on the cell’s requirements.^[4]

Cytosine : One of the four main nitrogenous bases found in both DNA and RNA, along with adenine, guanine, thymine, and uracil (in RNA); it is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amine group at position 4 and a keto group at position 2).

Cytoplasm : All of the material within a cell and enclosed by the cell membrane, except for the nucleus. The cytoplasm consists mainly of water, the gel-like cytosol, various organelles, and free-floating granules of nutrients and other biomolecules.

Cytology : See cell biology.

Crista : A fold in the inner membrane of a mitochondrion.

Convergent evolution : An evolutionary process by which species of different lineages independently develop similar characteristics, often to the point that the species appear to be more closely related than they actually are.

Conservation biology : The scientific study of nature and of Earth’s biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions.

Comparative biology : The use of comparative methods to study the similarities and differences between two or more biological organisms (e.g. two organisms from the same time period but different taxa, or two organisms from the same taxon but different times in evolutionary history). The side-by-side comparison of morphological or molecular characteristics of different organisms is the basis from which biologists infer the organisms’ genetic relatedness and their natural histories. It is a fundamental tool in many biological disciplines, including anatomy, physiology, paleontology, and phylogenetics.

Coat, coating : In the context of virus capsid, may refer colloquially to the defined geometric structure of a capsid, or the membrane of an endosome containing an intact virion. The coat of a virus is used in descriptions for the general public. Related slang: uncoating.

Cloning : The process of producing individual organisms or molecules with identical or virtually identical DNA, either naturally or artificially. Many organisms, such as bacteria, insects, and plants, are capable of naturally producing clones through asexual reproduction. In biotechnology, cloning refers to the artificial creation of copies of cells, DNA fragments, or other biomolecules by various laboratory techniques.

Clonal selection : A scientific theory in immunology that explains the functions of cells (lymphocytes) of the immune system in response to specific antigens invading the body. The theory has become the widely accepted model for how the immune system responds to infection and how certain types of B and T lymphocytes are selected for destruction of specific antigens.^[2]

Citric acid cycle : Also called the Krebs cycle and tricarboxylic acid cycle (TCA).

A series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide and chemical energy in the form of guanosine triphosphate (GTP). In addition, the cycle provides the chemical precursors for certain amino acids as well as the reducing agent NADH that is used in numerous other biochemical reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest established components of cellular metabolism and may have originated abiogenically.

Chromosome : A threadlike strand of DNA in the cell nucleus that carries the genes in a linear order.

Cholesterol : A type of lipid molecule that is biosynthesized by all animal cells because it is an essential structural component of animal cell membranes, essential for maintaining both membrane structural integrity and fluidity.

Chloroplast : A type of highly specialized organelle in the cells of plants and algae, the main role of which is to conduct photosynthesis, by which the photosynthetic pigment chlorophyll captures the energy from sunlight and converts and stores it in the molecules ATP and NADPH while freeing oxygen from water.

Chlorophyll : Any of several photosynthetic pigments found in cyanobacteria, algae, or plants.

Chemistry : A branch of the physical sciences that studies the composition, structure, properties, and change of matter. Chemical interactions underlie all biological processes.

Chemical reaction : A process that leads to the transformation of one set of chemical substances to another.

Chemical equilibrium : The state in which both reactants and products are present in concentrations which have no further tendency to change with time in a chemical reaction.

Chemical compound : A chemical substance consisting of two or more different chemically bonded elements, with a fixed ratio determining the composition. The ratio of each element is usually expressed by a chemical formula.

Centrosome : An organelle that is the primary site at which microtubules are organized. They occur only in plant and animal cells and help to regulate cell division.

Centriole : A cylindrical cell structure found in most eukaryotic cells, composed mainly of a protein called tubulin.

Cellular : Of or relating to a cell.

Cell wall : A tough, often rigid structural barrier surrounding certain types of cells (such as in fungi, plants, and most prokaryotes) that is immediately external to the cell membrane.

Cell plate : Grown in the cell’s center, it fuses with the parental plasma membrane, creating a new cell wall that enables cell division.

Cell nucleus : The “control room” for the cell. The nucleus gives out all the orders.