Definitions for Chapter 3—The Molecules of Life
Adenine—One of the nucleotides that make up DNA and RNA.
Amino acid—A small organic molecule which has a central carbon atom bonded to a carboxyl group, an amino group, a variable side group (or R group), and a single hydrogen atom. Amino acids are the monomers which make up proteins. There are about 20 amino acids in most living organisms.
Amino group—A functional group consisting of two hydrogen atoms and a nitrogen atom. Amino groups are one of the five components of an amino acid.
ATP—Adenosine triphosphate, a nucleotide which is the most important short-term energy storage compound in cells. It is the “energy currency” of the cell, necessary for practically all metabolic activities.
Carbohydrate—A type of organic molecule made of carbon, hydrogen, and oxygen. Carbohydrates are polymers made of monomers called sugars. They are important energy storage molecules and structural molecules. They may be classified as monosaccharides, oligosaccharides, or polysaccharides.
Carboxyl—A functional group often found on the end of a fatty acid which connects to a glycerol group in fats. Carboxyls are also one of the five components of an amino acid.
Cellulose—A polysaccharide composed of glucose units linked together. Cellulose is the primary component of the cell wall of plants. It is difficult for animals to digest without the help of bacteria which can make the enzyme cellulase.
Chitin—A polysaccharide composed of glucose modified with nitrogen. It is flexible and strong. It makes up the cell walls of fungi, and the exoskeleton of arthropods (insects, spiders, crustaceans, and their relatives).
Cholesterol—A type of steroid molecule which is an important component of cell membranes in animals, and which can be modified into sex hormones like testosterone and estrogen.
Cleavage—One of the mechanisms of enzyme function; it refers to splitting a molecule into smaller molecules.
Collagen—A protein molecule found in skin and other tissues which provides flexibility.
Condensation—One of the mechanisms of enzyme function; it refers to the joining of two molecules into a larger molecule.
Cytosine—One of the nucleotides that make up DNA and RNA.
Denaturation—Destruction of the shape of a protein molecule due to excessive heat, cold, salinity, or acidity. Once a protein becomes denatured, it is useless and cannot be repaired.
Deoxyribose—A five-carbon sugar which is an important component of DNA (deoxyribonucleic acid).
Disaccharide—A carbohydrate made of two sugar subunits. Common disaccharides include lactose, sucrose, and maltose.
Electron transfer—One of the mechanisms of enzyme function; it refers to movement of electrons from one molecule to another, often to release energy associated with the electron.
Enzyme—A protein molecule which acts as a catalyst; in other words, it speeds up metabolic reactions.
Fat—A type of lipid made up of a glycerol group connected to one, two, or three fatty acids. Fats are important energy storage molecules. They may be saturated or unsaturated.
Fatty acid—An organic molecule with a long hydrocarbon chain attached to a carboxyl head. Fatty acids are monomers which are part of the make-up of fats. They contain a great deal of chemical energy. Some fatty acids have a kink in their shape due to double bonding of carbon atoms.
Functional group—Groups of atoms that may be added to molecules to change their chemical properties. Examples include carboxyl groups, amino groups, and hydroxyl groups.
Functional group transfer—One of the mechanisms of enzyme function; it refers to the transfer of a functional group from one molecule to another.
Glucose—A monosaccharide which is the primary medium-term energy source for metabolism. Glucose is a product of photosynthesis, and a starting point for many types of cellular respiration.
Glycerol—A sugar alcohol which is part of many lipid molecules.
Glycogen—A branched polysaccharide which is an important long-term energy storage compound in animals.
Glycoprotein—The combination of a carbohydrate and a protein which allows for transport of the carbohydrate.
Guanine—One of the nucleotides that make up DNA and RNA.
Hemoglobin—A protein molecule which carries oxygen in the blood.
Hydrocarbon—Molecules made only of carbon and hydrocarbon. They are stable and contain large amounts of chemical energy in their chemical bonds. Other types of organic chemicals may be thought of as modified hydrocarbons.
Hydrolysis—A type of cleavage reaction in which a water molecule is split at the same time as the larger molecule is split.
Keratin—A protein molecule which makes up hair, claws, and fingernails, and which is found in skin cells. It makes the cells flexible, waterproof, and resistant to abrasion.
Lactose—A disaccharide which consists of a combination of glucose and galactose. It is found in milk, and commonly called “milk sugar.”
Lipid—A type of organic molecule which is non-polar. Lipids are polymers; the monomers which make them up are usually fatty acids. They are important energy storage molecules, structural molecules, and hormones.
Lipoprotein—The combination of a lipid molecule with a protein which allows the lipid to be easily transported through the bloodstream.
Maltose—A disaccharide composed of two molecules of glucose. Maltose is sometimes called “malt sugar,” and is found in grains, germinating seeds, and in many beer-making processes.
Monomer—Small organic molecules which can act as subunits which bind together to form larger molecules called polymers.
Monosaccharide—A carbohydrate that consists of a single sugar subunit. Examples of monosaccharides include ribose, deoxyribose, glucose, fructose, galactose, and many others.
Nucleic acid—A type of organic molecule which is used to transmit information in the cell. Nucleic acids are polymers; the monomers which make them up are nucleotides. The two most important nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Nucleotide—A small organic molecule consisting of a five-carbon sugar bonded to a nitrogenous base and a phosphate group. Nucleotides (adenine, guanine, cytosine, thymine, and uracil) are the monomers which make up DNA and RNA. Other nucleotides have uses unrelated to nucleic acids (for example, ATP).
Oligosaccharide—A carbohydrate that consists of two or three sugar subunits. Most oligosaccharides are disaccharides.
Organic chemistry—The chemistry of molecules that contain carbon.
Peptide bond—A covalent bond between the carboxyl group of one amino acid and the amino group of another. Such a bond results in a dipeptide; a series of such bonds results in a polypeptide.
Phospholipid—A type of lipid consisting of a hydrophilic head (consisting of a phosphate group and a glycerol) and a hydrophobic tail (consisting of two fatty acid chains). When mixed with water, phospholipids naturally form a bilayer which is the structural basis of most cellular membranes.
Polymer—Large organic molecules made of subunits called monomers.
Polypeptide—A long chain of amino acids joined together by peptide bonds; a polypeptide makes up the primary structure of a protein.
Polysaccharide—A carbohydrate made of many sugar subunits. Important polysaccharides include cellulose, chitin, glycogen, and starch.
Primary structure—The specific chain of amino acids, in the proper order, which makes up the polypeptide backbone of a protein.
Protein—A type of organic molecule which is large and has a complex structure. Proteins are polymers; the monomers which make them up are amino acids. Proteins may be used as enzymes, structural molecules, energy storage molecules, transport molecules, hormones, and many other metabolic uses. They may have up to four levels of structure.
Quarternary structure—Higher level structure of protein molecules caused by bonding together of separate polypeptide chains into a larger molecule.
Rearrangement—One of the mechanisms of enzyme function; rearrangement refers to the juggling of internal bonds of a molecule to change its structure or shape.
R-group—Another name for the side group of an amino acid.
Ribose—A five-carbon monosaccharide which is an important component of RNA (ribonucleic acid).
Saturated fat—A fat molecule in which all of the carbon-carbon bonds in the fatty acids are single bonds. This means that the fatty acids hold all of the hydrogen they possibly can (they are “saturated” with hydrogen). Saturated fats are loaded with chemical energy which is easily released. Saturated fats, like butter and lard, are often solid at room temperature.
Secondary structure—Higher-level structure of a protein molecule caused by hydrogen bonds along the backbone of the molecule. Secondary structure can give parts of a protein molecule shape such as helical portions and portions that look like pleated sheets.
Side-group—One of the five components of an amino acid. The side group is the only part of the 20 amino acids which varies; interactions between side groups help give protein molecules their higher-level structures.
Starch—A polysaccharide made of large numbers of glucose molecules joined together. Starch is the long-term energy storage compound in plants.
Steroid—A type of lipid which consists of a series of carbon rings. Sterols are important components of cellular membranes, and can be used as hormones. Important examples include cholesterol, testosterone, and estrogen.
Sucrose—A disaccharide which consists of a combination of glucose and fructose. Sucrose is commonly called “table sugar.”
Sugar—A small molecule made of carbon, oxygen, and hydrogen which acts as a monomer; sugars are the subunits that make up larger carbohydrates.
Tertiary structure—Higher-level structure of a protein molecule caused by bonding between side groups.
Thymine—A nucleotide found in DNA but not RNA. In RNA, thymine is replaced by uracil.
Triglyceride—A fat molecule with three fatty acid chains. Triglycerides have large amounts of available chemical energy.
Unsaturated fat—A fat molecule in which one or more of the carbon-carbon bonds in the fatty acids are double bonds. This means they hold less hydrogen then possible (thus, are “unsaturated” with hydrogen), and have less obtainable chemical energy than saturated fats. Unsaturated fats are usually liquid at room temperature.
Uracil—A nucleotide found in RNA, but not DNA. DNA has thymine instead of uracil.
Wax—A type of lipid which has important waterproofing qualities in animals and plants.