Biological Molecules

Chapter 3 Biological Molecules

Why Is Carbon So Important in Biological Molecules? • Organic/inorganic molecules and functional groups • Organic refers to molecules containing a carbon skeleton bonded to hydrogen atoms • Inorganic refers to carbon dioxide and all molecules without carbon

Why Is Carbon So Important in Biological Molecules? • The carbon atom is key • Is versatile because it has four electrons in the outer shell • Is stable by forming up to four bonds • single, double, or triple covalent • Result organic molecules can assume complex shapes: • branched chains, rings, sheets, and helices • Functional groups in organic molecules • Are less stable than the carbon backbone and are more likely to participate in chemical reactions • Determine the characteristics and chemical reactivity of organic molecules

Table 3-1

Dehydration Synthesis • Small organic molecules (called monomers) are joined to form longer molecules (called polymers) • Monomers are joined together through dehydration synthesis, at the site where an H+and an OH-are removed, resulting in the loss of a water molecule (H2O) • The openings in the outer electron shells of the two subunits are filled when the two subunits share electrons, creating a covalent bond dehydration synthesis  

Hydrolysis • Polymers are broken apart through hydrolysis (“water cutting”) • Water is broken into H+and OH-and is used to break the bond between monomers • Digestive enzymes use to break down food hydrolysis 

Biological Molecules The important organic molecules found in living things: Carbohydrates Lipids Proteins Nucleic Acids Sugar Fat Enzymes Deoxyribonucleic Starch Structural Proteins Ribonucleic Acid Cellulose All contain Carbon, Hydrogen, and generally Oxygen.

Carbohydrates • Carbohydrate molecules are composed of C, H, and O in the ratio of 1:2:1 • Monosaccharide - consists of just one sugar molecule • Disaccharide - Two linked monosaccharides • Polysaccharide - polymer of many monosaccharides

Carbohydrates • Hydrophilic due to the polar OH- functional group • Only mono- and disaccharides • Functions: • Energy source • Combine with other molecules through dehydration synthesis (plasma membrane, cell wall, exoskeletons)

Carbohydrates • There are several monosaccharides with slightly different structures • The basic monosaccharide structure is: • A backbone of 3–7 carbon atoms • Most of the carbon atoms have both a hydrogen (-H) and an hydroxyl group (-OH) attached to them • Chemical formula (CH2O)n water hydrogen bond hydroxyl group

Monosaccharides • When dissolved in the cytoplasmic fluid of a cell, the carbon backbone usually forms a ring • Glucose (C6H12O6) is the most common monosaccharide in living organisms • Fructose (“fruit sugar” found in fruits, corn syrup, and honey) • Galactose (“milk sugar” found in lactose) • Ribose and deoxyribose (found in RNA and DNA) Note “missing” oxygen atom fructose galactose ribose deoxyribose

Disaccharides • Functions: • They are used for short-term energy storage • When energy is required, they are broken apart by hydrolysis • Examples: • Sucrose (table sugar) = glucose + fructose • Lactose (milk sugar) = glucose + galactose • Maltose (malt sugar) = glucose + glucose sucrose glucose fructose dehydration synthesis

Polysaccharides • Storage polysaccharides include: • Starch, an energy-storage molecule in plants, formed in roots and seeds • Glycogen,an energy-storage molecule in animals, found in the liver and muscles starch grains (a) Potato cells (b) A starch molecule (c) Detail of a starch molecule

Polysaccharides • Polysaccharides as a structural material • Cellulose (a polymer of glucose) • It is found in the cell walls of plants • Most abundant organic molecule on Earth • It is indigestible for most animals due to the orientation of the bonds between glucose molecules

Polysaccharides • Chitin(a polymer of modified glucose units) • Nitrogen-containing functional group • The outer coverings (exoskeletons) of insects, crabs, and spiders • The cell walls of many fungi

Review What are carbohydrates used for? What are the major classes of carbohydrates? What are the types and functions of polysaccharides?

Lipids • Lipids are a diverse group of molecules that contain regions composed almost entirely of hydrogen and carbon • All lipids contain large chains of non-polar hydrocarbons • hydrophobic

Lipids • Lipids are diverse in structure and serve a variety of functions: • energy storage • waterproof coverings on plant and animal bodies • primary component of cellular membranes • hormones • Lipids are classified into three major groups • Oils, fats, and waxes • Phospholipids • Steroids

Lipids • Oils, fats, and waxes • Carbon, hydrogen, oxygen • Made of one or more fatty acid subunits • Fats and oils • Are used primarily as energy-storage molecules, containing twice as many calories per gram as carbyhydrates and proteins • Are formed by dehydration synthesis • Three fatty acids + glycerol triglyceride

Synthesis of a Triglyceride  glycerol fatty acids Dehydration synthesis  triglyceride Fig. 3-12

Lipids • Oils, fats, and waxes • Fatsare solid at room temperature are saturated (the carbon chain has as many hydrogen atoms as possible, and mostly or all C-C bonds); for example, beef fat • Produced in animals

Lipids • Oils, fats, and waxes • Oilsare liquid at room temperature are unsaturated (with fewer hydrogen atoms, and many C=C bonds); for example, corn oil • Produced by plants • Unsaturated trans fats have been linked to heart disease

Lipids • Oils, fats, and waxes • Waxes are similar to fats • Most animals don’t have the enzymes to break them down. • Functions: • form waterproof coatings such as on: • Leaves and stems of landplants • Fur in mammals • Insect exoskeletons • used to build honeycomb structures

Lipids • Phospholipids • These form plasma membranes around all cells • Phospholipids consist of two fatty acids + glycerol + a short polar functional group containing nitrogen • Hydrophilicpolar functional groups form the “head” • Hydrophobic non-polar fatty acids form the “tails” variable functional group phosphate group fatty acid tails polar head glycerol backbone (hydrophobic) (hydrophilic)

Lipids • Steroids • Composed of four carbon rings fused together with various functional groups protruding from them • Examples: • Cholesterol (animal cell membranes) • Male and female sex hormones

Proteins • Functions • Enzymes are proteins that promote chemical reactions • Structural proteins (e.g., elastin, keratin) provide support • Hormones • Antibodies • Toxins

Proteins • Polymers of amino acids joined by peptide bonds • All amino acids have a similar structure • All contain amino and carboxyl groups • All have a variable “R” group • Some R groups are hydrophobic • Some are hydrophilic • Cysteine R groups can form disulfide bridges variable group amino group carboxylic acid group hydrogen

Proteins • The sequence of amino acids in a protein dictates its function • Amino acids are joined to form chains by dehydration synthesis • An amino group reacts with a carboxyl group, and water is lost dehydration synthesis water amino acid amino acid peptide  amino group amino group carboxylic acid group peptide bond

Proteins • Amino acids are joined to form chains by dehydration synthesis • The covalent bond resulting after the water is lost is a peptide bond, and the resulting chain of two amino acids is called a peptide • Long chains of amino acids are known as polypeptides

Proteins • Proteins exhibit up to four levels of structure • Primary structure is the sequence of amino acids • Secondary structure is a helix, or a pleated sheet • Repeating structure with hydrogen bonds • Tertiary structure refers to complex foldings of the protein chain held together by disulfide bridges, hydrophobic/hydrophilic interactions, and other bonds • Quaternary structure occurs where multiplepolypeptides are linked together

The Four Levels of Protein Structure (b) Secondary structure: Usually maintained by hydrogen bonds, which shape this helix (a) Primary structure: The sequence of amino acids linked by peptide bonds leu val heme group lys lys gly his hydrogen bond ala lys val (d) Quaternary structure: Individual polypeptides are linked to one another by hydrogen bonds or disulfide bridges (c) Tertiary structure: Folding of the helix results from hydrogen bonds with surrounding water molecules and disulfide bridges between cysteine amino acids lys helix pro Fig. 3-21

Proteins • The functions of proteins are linked to their three-dimensional structures • Precise positioning of amino acid R groups leads to bonds that determine secondary and tertiary structure • Disruption of secondary and tertiary bonds leads to denatured proteins and loss of function • Extreme heat • Extreme changes in pH • UV radiation

Nucleotides and Nucleic Acids • Nucleotides act as energy carriers and intracellular messengers • Nucleotides are the monomers of nucleic acid chains • Three parts: • Phosphate group • Five-carbon sugar • Nitrogen-containing base phosphate Deoxyribose Nucleotide base sugar

Nucleotides and Nucleic Acids • Nucleotides act as energy carriers • Adenosine triphosphate (ATP) is a ribose nucleotide with three phosphate functional groups • ADP and cAMP • NAD and FAD: electron carriers

Nucleic Acids • DNA and RNA, the molecules of heredity • Polymers of nucleotides • DNA(deoxyribonucleic acid) is found in chromosomes and carries genetic information needed for protein construction • RNA (ribonucleic acid) makes copies of DNA and is used directly in the synthesis of proteins

Nucleic Acids • Each DNA molecule consists of two chains of millions of nucleotides that form a double helix linked by hydrogen bonds hydrogen bond

Biological Molecules