Biological Macromolecules: Polymers and Carbohydrates

• Living things are composed of 4 classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids.
• Macromolecules are large and complex molecules with properties arising from their atomic arrangement.
• A polymer is a long molecule of similar building blocks; a monomer is a repeating unit serving as a building block.
• Three-quarters of life's organic molecules are polymers, namely carbohydrates, proteins, and nucleic acids.

Synthesis & Breakdown of Polymers

• Enzymes are specialized macromolecules that speed up chemical reactions by making or breaking down polymers.
• A dehydration reaction occurs when two monomers bond together through water loss (Make Polymer > Dehydration (remove H2O)).
• Hydrolysis is when polymers are disassembled into monomers and it reserves the dehydration reaction.

Carbohydrates

• Fuel and building material which includes sugars and sugar polymers.
• Monosaccharides are simple, single sugars, such as glucose (C6H12O6), with molecular formulas which are multiples of CH2O and are classified by carbonyl groups and carbon skeleton number.
• They form rings in aqueous solutions and are a major fuel for cells as well as raw materials for building molecules.
• Disaccharides are double sugars which form when dehydration joins two monosaccharides.
• A glycosidic linkage is a covalent bond.
• Polysaccharides are polymers of many sugar building blocks that have storage and structural roles determined by sugar monomers and glycosidic linkages.
• Starch (polymer) is a storage polysaccharide in plants entirely of glucose monomers which includes amylose (unbranched) and amylopectin (branched).
• Plastids include Chloroplasts, Chromoplasts, and Leuceplasts.
  • Chloroplasts contain Chlorophyll (green).
  • Chromoplasts appear as fruits and flowers.
  • Leuceplasts contain starch.
• Glycogen is a storage polysaccharide in animals, mainly stored in the liver and muscle cells where hydrolysis releases glucose as an energy source.
• Polysaccharide cellulose protects the plant cell wall from breaking and is a major component of the cell wall and is a polymer of glucose; glycosidic linkages differ.
• Cellulose molecules form microfibrils and cellulose can form hydrogen bonds with hydroxyl of parallel cellulose molecules.
• Starch-digesting enzymes cannot digest cellulose because of different glycosidic linkages.
• Cellulose passes through the human digestive tract, working as an insoluble fiber, while some microbes use enzymes to digest cellulose.
• Many herbivores have symbiotic relationships with digesting microbes.
• Chitin is found in exoskeletons of arthropods providing structural support for fungi cell walls.

Lipids

• A class of large biological molecules that does not include true polymers and mixes poorly with water, making it hydrophobic due to hydrocarbon composition.
• Important lipids include fats, phospholipids, and steroids.
• Fats are constructed from glycerol and fatty acids.
  • Glycerol is a 3-carbon alcohol with a hydroxyl group attached to each carbon.
  • Fatty acids consist of a carboxyl group attached to a long carbon skeleton.
  • Three fatty acids join glycerol via an ester linkage, forming triacylglycerol or triglyceride.
• Fats separate from water because the water molecules hydrogen bond to each other and exclude fats.
• Fatty acids in fats can be all the same, or made of two or three different kinds.
• Fatty acids vary in length, by number of carbons, and by the number and location of the double bonds.
• Saturated fatty acids have the maximum number of hydrogen atoms and no double bonds with most animals being saturated.
• Unsaturated fatty acids have one or more double bonds, commonly found in plant fats and fish fats.
• Hydrogenation converts unsaturated fats to saturated fats by adding hydrogen and hydrogenating vegetable oils creating unsaturated fats with trans double bonds.
• Trans fats may contribute more to cardiovascular disease than saturated fats.
• Major functions of fats are energy storage, insulation, and cushioning of vital organs. Humans and other mammals store long-term food reserves in adipose cells.
• Phospholipids consist of 2 hydrophobic fatty acids, a hydrophilic phosphate group and are attached to glycerol, self- assembling into lipid bilayers when added to water.
• Steroids are lipids with a carbon skeleton of four fused rings.
  • Cholesterol is a component in animal cell membranes, where it is a precursor to other steroids, and high levels cause cardiovascular disease.

Proteins

• Biologically functional molecules consisting of one or more polypeptides.
• Proteins have diverse structures, resulting in various functions, making up 50% of the dry mass of most cells.
  • Enzymatic proteins accelerate chemical reactions selectively.
  • Defensive proteins protect against diseases.
  • Storage proteins store amino acids.
  • Transport proteins transport substances.
  • Hormonal proteins coordinate organism activities.
  • Receptor proteins respond to cellular chemical stimuli.
  • Contractile and motor proteins facilitate movement.
  • Structural proteins provide support.
• Proteins are polymers constructed from the same set of 20 amino acids.
• Polypeptides are unbranched polymers built from amino acids where amino acids are linked by covalent bonds called peptide bonds.
• Polypeptides are polymers of amino acids that range in length from a few to over a thousand monomers.
• Amino acids are organic molecules with amino and carboxyl groups differing by their side chains (R groups), which are nonpolar (hydrophobic) or polar (hydrophilic).
• Electrically charged side chains are hydrophilic.
• The protein structure and function determine how it works where amino acid sequence determines the protein's 3D structure.
• A protein's function usually relies on its ability to recognize and bind to other molecules.

Four Protein Structure Levels

• Primary structure is a unique sequence of amino acids determined by inherited genetic information with the order of letters in a long word.
• Secondary structure is found in most proteins consisting of coils and folds in the polypeptide chain; caused results from hydrogen bonds between repeating constituents of the polypeptide backbone.
  • A coil is an “a” helix.
  • A fold is a “B” pleated sheet.
• Tertiary structure is determined by interactions among variable side chains (R groups)
  • The overall shape of polypeptide results from interactions between amino acids R groups and/or interactions between backbone parts.
  • This includes covalent bonds (disulfide bridges), hydrophobic interactions, Van der Waals interactions, Hydrogen Bonds & ionic Bonds.
• Quaternary structure results when a protein consists of multiple polypeptide chains.
• Collagen is a fibrous protein of three polypeptides coiled like a rope.
• Hemoglobin is a globular protein of four polypeptides: two alpha and two beta chains.
• Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin.
• Protein structure is affected by physical and chemical conditions, such as alterations in pH, salt concentration, temperature, or other environmental factors, causing it to unravel; termed as denaturation
• Denaturation is the loss of a protein's native structure, making it biologically inactive.

Nucleic Acids

• Stores, transmits, and helps express hereditary information.
• A gene is the amino acid sequence of a polypeptide, programmed by a unit of inheritance.
• A gene consists of DNA, a nucleic acid which itself is made of monomers called nucleotides.
• Two types of nucleic acids: deoxyribonucleic acid [DNA] and ribonucleic acid [RNA].
• Both participate in gene expression where DNA > RNA> Protein.
• Polynucleotides: nucleic acids are polymers.
• Each polynucleotide consists of monomers termed nucleotides composed of:
  • A nitrogenous base.
  • A pentose sugar.
  • One or more phosphate groups.
• Nitrogenous base plus sugar equals nucleoside.
• Nucleoside plus phosphate group equals nucleotide.
• There are two families of nitrogenous bases: pyrimidines and purines.
  • Pyrimidines (cytosine, thymine, and uracil) have a single 6-membered ring.
  • Purines (adenine & guanine) have a 6-membered ring fused to a 5-membered ring.
• Nucleotides are linked together to build polynucleotides with adjacent nucleotides joined by:
  • A phosphodiester linkage consisting of a phosphate group links which links the sugar of two nucleotides.
  • A backbone creates sugar-phosphate units with nitrogenous bases as appendages.
• The sequence of bases along a DNA or mRNA polymer is unique for each gene.

DNA Structures

• DNA molecules have two polynucleotides spiraling around an imaginary axis, forming a double helix.
• Backbones run in opposite 5'>3' directions, an arrangement known as antiparallel.
• One DNA molecule has many genes.
• DNA has complementary base pairing, where Adenine (A) always pairs with Thymine (T) and Guanine (G) always pairs with Cytosine (C).

RNA Structures

• RNA is single-stranded.
• Complementary pairing can occur between two RNA molecules or between parts of the same molecule.
• Thymine (T) is replaced by Uracil (U), so A & U pair.
• DNA always exists as a double helix while RNA molecules are more variable in form.