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ionic bond : chemical bond that forms between ions with opposite charges (cations and anions)
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irreversible chemical reaction : chemical reaction where reactants proceed uni-directionally to form products
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isomers : molecules that differ from one another even though they share the same chemical formula
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isotope : one or more forms of an element that have different numbers of neutrons
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law of mass action : chemical law stating that the rate of a reaction is proportional to the concentration of the reacting substances
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litmus paper : (also, pH paper) filter paper that has been treated with a natural water-soluble dye that changes its color as the pH of the environment changes so it can be used as a pH indicator
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mass number : total number of protons and neutrons in an atom
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matter : anything that has mass and occupies space
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molecule : two or more atoms chemically bonded together
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neutron : uncharged particle that resides in the nucleus of an atom; has a mass of one amu
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noble gas : see inert gas
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nonpolar covalent bond : type of covalent bond that forms between atoms when electrons are shared equally between them
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nucleus : core of an atom; contains protons and neutrons
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octet rule : rule that atoms are most stable when they hold eight electrons in their outermost shells
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orbital : region surrounding the nucleus; contains electrons
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organic molecule : any molecule containing carbon (except carbon dioxide)
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periodic table : organizational chart of elements indicating the atomic number and atomic mass of each element; provides key information about the properties of the elements
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pH paper : see litmus paper
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pH scale : scale ranging from zero to 14 that is inversely proportional to the concentration of hydrogen ions in a solution
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polar covalent bond : type of covalent bond that forms as a result of unequal sharing of electrons, resulting in the creation of slightly positive and slightly negative charged regions of the molecule
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product : molecule found on the right side of a chemical equation
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proton : positively charged particle that resides in the nucleus of an atom; has a mass of one amu and a charge of +1
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radioisotope : isotope that emits radiation composed of subatomic particles to form more stable elements
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reactant : molecule found on the left side of a chemical equation
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reversible chemical reaction : chemical reaction that functions bi-directionally, where products may turn into reactants if their concentration is great enough
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solvent : substance capable of dissolving another substance
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specific heat capacity : the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius
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sphere of hydration : when a polar water molecule surrounds charged or polar molecules thus keeping them dissolved and in solution
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structural isomers : molecules that share a chemical formula but differ in the placement of their chemical bonds
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substituted hydrocarbon : hydrocarbon chain or ring containing an atom of another element in place of one of the backbone carbons
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surface tension : tension at the surface of a body of liquid that prevents the molecules from separating; created by the attractive cohesive forces between the molecules of the liquid
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valence shell : outermost shell of an atom
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van der Waals interaction : very weak interaction between molecules due to temporary charges attracting atoms that are very close together
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Proteins, carbohydrates, nucleic acids, and lipids are the four major classes of biological macromoleculesâlarge molecules necessary for life that are built from smaller organic molecules. Macromolecules are made up of single units known as monomers that are joined by covalent bonds to form larger polymers. The polymer is more than the sum of its parts: it acquires new characteristics, and leads to an osmotic pressure that is much lower than that formed by its ingredients; this is an important advantage in the maintenance of cellular osmotic conditions. A monomer joins with another monomer with the release of a water molecule, leading to the formation of a covalent bond. These types of reactions are known as dehydration or condensation reactions. When polymers are broken down into smaller units (monomers), a molecule of water is used for each bond broken by these reactions; such reactions are known as hydrolysis reactions. Dehydration and hydrolysis reactions are similar for all macromolecules, but each monomer and polymer reaction is specific to its class. Dehydration reactions typically require an investment of energy for new bond formation, while hydrolysis reactions typically release energy by breaking bonds.
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Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that are formed as a result of dehydration reactions, forming disaccharides and polysaccharides with the elimination of a water molecule for each bond formed. Glucose, galactose, and fructose are common monosaccharides, whereas common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be branched or unbranched. Cellulose is an example of an unbranched polysaccharide, whereas amylopectin, a constituent of starch, is a highly branched molecule. Storage of glucose, in the form of polymers like starch of glycogen, makes it slightly less accessible for metabolism; however, this prevents it from leaking out of the cell or creating a high osmotic pressure that could cause excessive water uptake by the cell.
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Lipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and oils, waxes, phospholipids, and steroids. Fats are a stored form of energy and are also known as triacylglycerols or triglycerides. Fats are made up of fatty acids and either glycerol or sphingosine. Fatty acids may be unsaturated or saturated, depending on the presence or absence of double bonds in the hydrocarbon chain. If only single bonds are present, they are known as saturated fatty acids. Unsaturated fatty acids may have one or more double bonds in the hydrocarbon chain. Phospholipids make up the matrix of membranes. They have a glycerol or sphingosine backbone to which two fatty acid chains and a phosphate-containing group are attached. Steroids are another class of lipids. Their basic structure has four fused carbon rings. Cholesterol is a type of steroid and is an important constituent of the plasma membrane, where it helps to maintain the fluid nature of the membrane. It is also the precursor of steroid hormones such as testosterone.
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Proteins are a class of macromolecules that perform a diverse range of functions for the cell. They help in metabolism by providing structural support and by acting as enzymes, carriers, or hormones. The building blocks of proteins (monomers) are amino acids. Each amino acid has a central carbon that is linked to an amino group, a carboxyl group, a hydrogen atom, and an R group or side chain. There are 20 commonly occurring amino acids, each of which differs in the R group. Each amino acid is linked to its neighbors by a peptide bond. A long chain of amino acids is known as a polypeptide.
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Proteins are organized at four levels: primary, secondary, tertiary, and (optional) quaternary. The primary structure is the unique sequence of amino acids. The local folding of the polypeptide to form structures such as theαhelix andβ-pleated sheet constitutes the secondary structure. The overall three-dimensional structure is the tertiary structure. When two or more polypeptides combine to form the complete protein structure, the configuration is known as the quaternary structure of a protein. Protein shape and function are intricately linked; any change in shape caused by changes in temperature or pH may lead to protein denaturation and a loss in function.
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Nucleic acids are molecules made up of nucleotides that direct cellular activities such as cell division and protein synthesis. Each nucleotide is made up of a pentose sugar, a nitrogenous base, and a phosphate group. There are two types of nucleic acids: DNA and RNA. DNA carries the genetic blueprint of the cell and is passed on from parents to offspring (in the form of chromosomes). It has a double-helical structure with the two strands running in opposite directions, connected by hydrogen bonds, and complementary to each other. RNA is single-stranded and is made of a pentose sugar (ribose), a nitrogenous base, and a phosphate group. RNA is involved in protein synthesis and its regulation. Messenger RNA (mRNA) is copied from the DNA, is exported from the nucleus to the cytoplasm, and contains information for the construction of proteins. Ribosomal RNA (rRNA) is a part of the ribosomes at the site of protein synthesis, whereas transfer RNA (tRNA) carries the amino acid to the site of protein synthesis. microRNA regulates the use of mRNA for protein synthesis.
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alpha-helix structure (α-helix) : type of secondary structure of proteins formed by folding of the polypeptide into a helix shape with hydrogen bonds stabilizing the structure
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amino acid : monomer of a protein; has a central carbon or alpha carbon to which an amino group, a carboxyl group, a hydrogen, and an R group or side chain is attached; the R group is different for all 20 amino acids
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beta-pleated sheet (β-pleated) : secondary structure found in proteins in which âpleatsâ are formed by hydrogen bonding between atoms on the backbone of the polypeptide chain
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biological macromolecule : large molecule necessary for life that is built from smaller organic molecules
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carbohydrate : biological macromolecule in which the ratio of carbon to hydrogen and to oxygen is 1:2:1; carbohydrates serve as energy sources and structural support in cells and form the a cellular exoskeleton of arthropods
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cellulose : polysaccharide that makes up the cell wall of plants; provides structural support to the cell
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chaperone : (also, chaperonin) protein that helps nascent protein in the folding process
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chitin : type of carbohydrate that forms the outer skeleton of all arthropods that include crustaceans and insects; it also forms the cell walls of fungi
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dehydration synthesis : (also, condensation) reaction that links monomer molecules together, releasing a molecule of water for each bond formed
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denaturation : loss of shape in a protein as a result of changes in temperature, pH, or exposure to chemicals
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deoxyribonucleic acid (DNA) : double-helical molecule that carries the hereditary information of the cell
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disaccharide : two sugar monomers that are linked together by a glycosidic bond
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enzyme : catalyst in a biochemical reaction that is usually a complex or conjugated protein
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glycogen : storage carbohydrate in animals
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glycosidic bond : bond formed by a dehydration reaction between two monosaccharides with the elimination of a water molecule
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hormone : chemical signaling molecule, usually protein or steroid, secreted by endocrine cells that act to control or regulate specific physiological processes
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hydrolysis : reaction causes breakdown of larger molecules into smaller molecules with the utilization of water
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lipid : macromolecule that is nonpolar and insoluble in water
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messenger RNA (mRNA) : RNA that carries information from DNA to ribosomes during protein synthesis
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monomer : smallest unit of larger molecules called polymers
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monosaccharide : single unit or monomer of carbohydrates
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nucleic acid : biological macromolecule that carries the genetic blueprint of a cell and carries instructions for the functioning of the cell
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nucleotide : monomer of nucleic acids; contains a pentose sugar, one or more phosphate groups, and a nitrogenous base
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omega fat : type of polyunsaturated fat that is required by the body; the numbering of the carbon omega starts from the methyl end or the end that is farthest from the carboxylic end
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peptide bond : bond formed between two amino acids by a dehydration reaction
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phosphodiester : linkage covalent chemical bond that holds together the polynucleotide chains with a phosphate group linking two pentose sugars of neighboring nucleotides
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phospholipid : major constituent of the membranes; composed of two fatty acids and a phosphate-containing group attached to a glycerol backbone
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polymer : chain of monomer residues that is linked by covalent bonds; polymerization is the process of polymer formation from monomers by condensation
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polynucleotide : long chain of nucleotides
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polypeptide : long chain of amino acids linked by peptide bonds
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polysaccharide : long chain of monosaccharides; may be branched or unbranched
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primary structure : linear sequence of amino acids in a protein
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protein : biological macromolecule composed of one or more chains of amino acids
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purine : type of nitrogenous base in DNA and RNA; adenine and guanine are purines
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pyrimidine : type of nitrogenous base in DNA and RNA; cytosine, thymine, and uracil are pyrimidines
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quaternary structure : association of discrete polypeptide subunits in a protein
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ribonucleic acid (RNA) : single-stranded, often internally base paired, molecule that is involved in protein synthesis
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ribosomal RNA (rRNA) : RNA that ensures the proper alignment of the mRNA and the ribosomes during protein synthesis and catalyzes the formation of the peptide linkage
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saturated fatty acid : long-chain of hydrocarbon with single covalent bonds in the carbon chain; the number of hydrogen atoms attached to the carbon skeleton is maximized
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secondary structure : regular structure formed by proteins by intramolecular hydrogen bonding between the oxygen atom of one amino acid residue and the hydrogen attached to the nitrogen atom of another amino acid residue
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starch : storage carbohydrate in plants
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steroid : type of lipid composed of four fused hydrocarbon rings forming a planar structure
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tertiary structure : three-dimensional conformation of a protein, including interactions between secondary structural elements; formed from interactions between amino acid side chains
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trans fat : fat formed artificially by hydrogenating oils, leading to a different arrangement of double bond(s) than those found in naturally occurring lipids
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transcription : process through which messenger RNA forms on a template of DNA
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transfer RNA (tRNA) : RNA that carries activated amino acids to the site of protein synthesis on the ribosome
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translation : process through which RNA directs the formation of protein
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triacylglycerol (also, triglyceride) : fat molecule; consists of three fatty acids linked to a glycerol molecule
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unsaturated fatty acid : long-chain hydrocarbon that has one or more double bonds in the hydrocarbon chain
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wax : lipid made of a long-chain fatty acid that is esterified to a long-chain alcohol; serves as a protective coating on some feathers, aquatic mammal fur, and leaves
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A cell is the smallest unit of life. Most cells are so tiny that they cannot be seen with the naked eye. Therefore, scientists use microscopes to study cells. Electron microscopes provide higher magnification, higher resolution, and more detail than light microscopes. The unified cell theory states that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells.
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Prokaryotes are predominantly single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, and DNA that is not membrane-bound. Most have peptidoglycan cell walls and many have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1 to 5.0 μm.
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As a cell increases in size, its surface area-to-volume ratio decreases. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume.
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Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleusâs nucleolus is the site of ribosome assembly. Ribosomes are either found in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration; they are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.
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Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of animal cells.
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Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more cytoplasm.
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The endomembrane system includes the nuclear envelope, lysosomes, vesicles, the ER, and Golgi apparatus, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport proteins and lipids that form the membranes.
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The RER modifies proteins and synthesizes phospholipids used in cell membranes. The SER synthesizes carbohydrates, lipids, and steroid hormones; engages in the detoxification of medications and poisons; and stores calcium ions. Sorting, tagging, packaging, and distribution of lipids and proteins take place in the Golgi apparatus. Lysosomes are created by the budding of the membranes of the RER and Golgi. Lysosomes digest macromolecules, recycle worn-out organelles, and destroy pathogens.
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The cytoskeleton has three different types of protein elements. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Microfilaments are often associated with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia.
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Animal cells communicate via their extracellular matrices and are connected to each other via tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other via plasmodesmata.
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When protein receptors on the surface of the plasma membrane of an animal cell bind to a substance in the extracellular matrix, a chain of reactions begins that changes activities taking place within the cell. Plasmodesmata are channels between adjacent plant cells, while gap junctions are channels between adjacent animal cells. However, their structures are quite different. A tight junction is a watertight seal between two adjacent cells, while a desmosome acts like a spot weld.
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