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Chapter 3 The Molecules of Life Biological molecules Carbon is a crucial element in biological molecules Carbon can share electrons in as many as four covalent bonds Since two or more carbons can bond together, it is possible to construct an endless variety of carbon skeletons Functional Groups Some combinations of atoms act together as a functional group Often, the functional group imparts an electrical charge or polarity onto molecules affecting their chemical properties Chemical reactions So chemical reactions are all about breaking existing covalent bonds and forming new ones It’s about rearranging atoms in molecules Many of our key biological molecules are large molecules (macromolecules) composed of multiple smaller subunits Four categories of macromolecules are carbohydrates proteins nucleic acids lipids Macromolecules are polymers, made by connecting many smaller monomers A dehydration reaction links two monomers together removes a molecule of water Organisms also have to break down macromolecules During digestion, macromolecules are broken down to monomers via chemical reactions to make the monomers available to our cells Hydrolysis breaks the bonds between monomers by adding a molecule of water Carbohydrates Carbohydrates are our primary energy source for cellular functions Carbs range from simple (monosaccharides) through disaccharides to complex carbs Glucose and fructose are isomers of each other - sharing the same chemical formula but having different structures Monosaccharides, particularly glucose, are the main fuels for cellular work In water, many monosaccharides form rings Disaccharides form when two monosaccharides are bonded together via a dehydration reaction Polysaccharides are complex carbohydrates are made of long chains of sugars - polymers of monosaccharides Starch is a familiar example of a polysaccharide It is used by plant cells to store energy Potatoes and grains Glycogen is used by animal cells to store energy It is hydrolyzed to release glucose when we need energy Cellulose is the most abundant organic compound on Earth It makes up the walls of plant cells It cannot be chemically broken by any enzyme produced by animals Lipids Lipids have several functions, including energy storage, components of biological membranes ... and insulation All lipids are hydrophobic Triglycerides contain three fatty acid tails Fatty acid chains, and therefore the fat molecules, can be either saturated (C’s bonded to the maximum number of H’s) or unsaturated (some C’s sharing double bonds) Most animal fats have a high proportion of saturated fatty acids can easily stack making them solid at room temperature contribute to atherosclerosis, in which lipid-containing plaques accumulate along the inside walls of blood vessels, decreasing blood flow Most plant and fish fats have a high proportion of unsaturated fatty acids are usually liquid at room temperature Good fat ... bad fat? Hydrogenation adds hydrogen atoms converts unsaturated fats to saturated fats produces trans fats, a type of unsaturated fat that is particularly bad for cardiovascular health Steroids are also hydrophobic Proteins Proteins are polymers of amino acids account for >50% of the dry weight of most cells are extremely versatile molecules All proteins are composed of the same 20 amino acids All amino acids share a common basic chemical structure The 20 amino acids differ in their side chain or R group So it’s the properties of the R group that determine the properties of the amino acid Some amino acids are hydrophobic; some are hydrophilic Others have unique properties Cells join amino acids together using dehydration reactions The linkage between two amino acids is a peptide bond, eventually forming long chains of amino acids called polypeptides A functional protein consists of one or more polypeptide chain(s) precisely twisted, folded, and coiled into a particular 3-dimensional shape The 3-D shape of proteins is crucial to their function Changes to the 3-D shape can drastically affect the functionality of the protein The 3-D shape is dependent upon the order of the amino acids in the polypeptide chain Levels of protein 3-D structure Primary is based on the sequence of amino acids in the chain Secondary is based on H-bonding between amino acids in the chain Tertiary is based on other interactions (hydrophobic and hydrophilic interactions, covalent, and non-covalent bonds) Quaternary is based on the various interactions between two or more polypeptides to give a functional protein Because the 3-D shape of a protein is crucial to its function, even slight changes in the sequence can drastically affect protein function Single amino acid changes can lead to changes in protein structure that can cause disease Misfolded proteins are associated with many diseases, including some severe nervous system disorders Some treatments can cause proteins to lose their 3-D shape The change can be reversible or irreversible Nucleic Acids Nucleic acids are genetic molecules They store and transfer genetic information The monomers (nucleotides) can also be important energy carriers A gene is a unit of inheritance encoded in a specific stretch of DNA that dictates the amino acid sequence of a polypeptide Each nucleotide consists of three parts a 5-C sugar (ribose or deoxyribose) a phosphate functional group a nitrogen-containing base There are 4 nitrogen-containing bases in DNA A molecule of DNA is double-stranded, consisting of two polynucleotide strands coiled around each other to form a double helix In contrast to DNA, RNA incorporates the sugar ribose rather than deoxyribose has the similar base uracil (U) rather than thymine is usually present in a single-stranded form