Disaccharides consist of two monosaccharide monomers covalently linked by a glycosidic bond. They function in sugar transport.

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1 1. The fundamental life processes of plants and animals depend on a variety of chemical reactions that occur in specialized areas of the organism s cells. As a basis for understanding this concept: 1. h. Students know most macromolecules (polysaccharides, nucleic acids, proteins, lipids) in cells and organisms are synthesized from a small collection of simple precursors. Many of the large carbon compound molecules necessary for life (e.g., polysaccharides, nucleic acids, proteins, and lipids) are polymers of smaller monomers. Polysaccharides are composed of monosaccharides; proteins are composed of amino acids; lipids are composed of fatty acids, glycerol, and other components; and nucleic acids are composed of nucleotides. Notes: Four main types of organic molecules predominate in living organisms: carbohydrates (polymers of simple sugars) lipids (fatty acids linked by glycerol) polypeptides (made of amino acids) nucleic acids (DNA or RNA - polymers of nucleotides) Carbohydrates Most complex carbohydrates are made of repeating units called sugars. Most simple sugars (monosaccharides) conform to the basic formula (CH 2 O)n and possess an aldehyde or ketone functional group. Monosaccharides (the simplest carbohydrates) can: provide ready energy, be converted to other types of organic molecules, be used as monomers for polymers (macromolecules). Examples: glucose, fructose, galactose Disaccharides consist of two monosaccharide monomers covalently linked by a glycosidic bond. They function in sugar transport. Examples: maltose (2 glucoses), lactose (glucose+galactose), sucrose (glucose+fructose) Polysaccharides serve as storage or structural molecules. Many common polysaccharides are made of the monosaccharide glucose. Examples:

2 structural polysaccharides: cellulose, chitin storage polysaccharides: glycogen (animal starch); amylose (plant starch) Glucose Glucose like other sugars can exist in a straight chain form or it can loop back on itself and become a ring. If the hydroxyl group on the first carbon atom points down when the ring is formed alpha glucose results and if the hydroxyl group points up the result is beta glucose. alpha glucose When glucose molecules are enzymatically joined to form a disaccharide a water molecule is removed. This process is called dehydration synthesis. The covalent bond that holds sugar molecules together is called a glycosidic linkage. When glucose molecules are combined in a straight chain by alpha 1-4 linkages the resulting polysaccharide is the edible, somewhat soluble starch called amylose. If the beta form of glucose is used instead, the resulting polysaccharide is an insoluble, indigestible, and tough fiber called cellulose. The polymer strands of cellulose can hydrogen bonds in groups of 60 or 70 to produce microfibrils, which in turn can join with other microfibrils to make strong cord-like fibers. Plant cell walls are made of crisscross layers of these fibers. Lipids Lipids are hydrocarbons insoluble in polar solvents. They constitute a heterogeneous group of hydrophobic molecules that include the neutral fats or triglycerides, the steroids, and the phospholipids. Lipids serve as energy-storage molecules, as major components of cell membranes, and as hormones. Fats or triglycerides are formed by three fatty acids each bonded by an ester linkage to glycerol. Fats and oils contain a higher proportion of energy-rich carbon-hydrogen bonds than carbohydrates or proteins. Many seeds are rich in oils.

3 Saturated fatty acids have the maximum number of hydrogen atoms because of single bonding between all the carbons. Unsaturated and polyunsaturated fatty acids (present in oils) have one or more double bonds between the carbons. Fats are ideal for energy storage requiring only half the mass of glycogen. Fats are also important in cushioning delicate organs like kidneys against shock and for insulation. Waxes are a form of structural lipid. They form protective coatings on skin, fur, feathers, on the leaves of land plants, and on the exoskeletons of many insects. Phospholipids substitute the third fatty acid of a triglyceride with a negatively charged phosphate group, which may be joined to another small molecule. Phospholipids may have a hydrophilic and a hydrophobic end making them ideally suited for construction of cell membranes. Steroids, such as cholesterol and the sex hormones, are classified as lipids. These lipids are characterized by a carbon skeleton consisting of four interconnected rings. Steroids often have a hydroxyl functional group. Polypeptides and Proteins Proteins consist of one or more chains of amino acids linked by peptide bonds. These chains are known as polypeptides. Proteins are the most complex and versatile macromolecules. Each amino acid contains a central carbon singly bonded to four different groups: a hydrogen atom, an amino group, a carboxyl group, and some other chemical group which confers on it unique properties. Proteins exhibit three or four levels of structural organization. Primary structure is the first level and is determined by a unique linear sequence of amino acids.

4 Secondary structure of proteins describes how the primary structure is folded into particular, localized configurations, the alpha helix and the beta pleated sheet, which result from hydrogen bonding. Tertiary structure describes the additional, less regular contortions of the molecule caused by the side groups in hydrophobic interactions, hydrogen bonds, and disulfide linkages. In many proteins, the tertiary structure produces an intricately folded, globular shape. Quaternary structure describes how two or more polypeptide chains interact to form a functional structure. Proteins are generally classified as either fibrous or globular. Proteins may also be characterized by their function. Examples of types of proteins include: Structural proteins - collagen, silk, microtubules Regulatory proteins (hormones) - insulin, growth hormones Contractile proteins - Actin, myosin, dynein Transport proteins - hemoglobin, myoglobin Storage proteins - egg white, seed protein Protective proteins - antibodies Membrane proteins - membrane-transport, channels Enzymes - most proteins ending in -ase

5 The function of a protein is an emergent property of its conformation, which is sensitive to conditions such as ph, salt concentration, and temperature. If these conditions exceed certain limits the protein's shape may be altered or denatured rendering it biologically inactive. Nucleic Acids Nucleic acids are polymers of nucleotides, complex monomers consisting of a pentose (five carbon sugar) covalently bonded to a phosphate group and to one of five different kinds of nitrogenous bases. DNA and RNA are the only two nucleic acids found in living matter. These large polymers are formed when the pentose of one nucleotide joins to the phosphate of another forming a sugar-phosphate backbone from which the nitrogenous bases project. The five nitrogenous bases are members of two families, the purines (A and G) and the pyrimidines (C, T, and U). Four nucleotides (A,T,C,G) are chemically joined through sugar and phosphate molecules in the backbone of DNA. Base pairs across the double helix are joined by complementary base-pairing: A base pairs with T, C base pairs with G. The complementary base pairs direct the addition of nucleotides during synthesis of new DNA strands or synthesis of mrna (where U is used instead of T) or hybridization of two different molecules. The overall directionality (seen best by looking at the sugar molecules) is antiparallel in the two strands. One strand has a 5'-3' direction; the other a 3'-5' direction. This has consequences for enzymes that work on the DNA (e.g. DNA polymerase, restriction enzymes). DNA Structure DNA contains the genetic information that codes for the RNA and proteins necessary for cell function. All DNA in the chromosomes has to be copied (replicated) and transmitted to daughter cells via mitosis. Non-perfect replication or inability to correct errors and damage to DNA results in mutations.