Biological Molecules

• Covalent bonds occur when non-metal atoms share electrons in their outer shells, forming a stable compound.
• Ionic bonds occur due to the electrostatic attraction shared by atoms with opposite charges (cations and anions) e.g. sodium and chlorine bond to form sodium chloride. These bonds are weaker than covalent bonds.
• Hydrogen bonds occur when electrons in a molecule are unevenly distributed, causing negative and positive polar regions, rendering the molecule polar. The regions share a weak electrostatic attraction. These bonds are very weak (1/10th the strength of covalent bonds) but a collection of H-bonds can alter the properties of molecules and form very important forces.
• Monomers are smaller, carbon-based molecules or atoms that bond together to form subunits in long chains (polymers) in a process called polymerisation.
• Polyesters and polythene are examples of industrially produced polymers. Polypeptides (peptides), polysaccharides (monosaccharides), polynucleotides (nucleotides) are examples of organically produced polymers (i.e. carbon compounds).
• Macromolecules have 1000+ atoms and therein a very high molecular mass. Not all macromolecules are polymers as the subunits of polymers are always the same type of repeating units (e.g. mononucleotides).
• An atom is the smallest unit of a chemical element that can exist independently. It contains an atomic nucleus with protons and neutrons. Electrons orbit the nucleus in 3D shells and have a negligible mass. The number of electrons in an atom determines its properties. The atomic number is equal to the number of protons. The mass number is equal to the combined number of protons and neutrons. Isotopes are different variations of chemical elements with different numbers of neutrons. Some isotopes are radioactive and this property can be very useful scientifically.
• Ion formation occurs through the loss or receipt of electrons. An anion is negative, having received an electron (Cl-). A cation is positive, having lost an electron (H+). More than one electron can be gained or lost (e.g. in the case of Ca2+). Ions may be molecules - e.g. the sulfate ion is formed of one sulfur atom, four oxygen atoms and 2 electrons.
• All living organisms share the same base biochemical composition (biomolecules) because they are descended from the same common ancestor 3.4 billion years ago (LUCA).
• A nucleotide comprises a phosphate group, a pentose sugar (deoxyribose or ribose) and a nitrogenous base.

  A) Phosphate Group

  B) Pentose Sugar

  C) Nitrogenous Base

  D) Nucleotide

  
• For each subunit attached during polymerisation, a molecule of water is formed. Polymers are broken down into their constituent parts via the addition of water - via hydrolysis. Hydrolysis allows the large biological molecules we ingest to be broken down into smaller components which can diffuse into cells through protein channels.
• A mole (mol) is an SI unit measuring the amount of a substance, expressing molecular mass in grams. 1 mol is equal to the total particles in 12g of carbon-12 atoms. A molar solution has 1 mol of solute/litre solution. 6.022 x 10²³ is Avogadro's number/constant and reflects the number of atoms in a mol of any substance.
• Carbon readily bonds to other carbon, allowing for the creation of a long 'backbone', to which a limited number of other molecules can readily attach, which forms the basis of all organic molecules. This is the reason for the limited biochemical composition of living organisms.
• Most polymers contain just 4 elements - carbon, hydrogen, oxygen and nitrogen.
• Monosaccharides are sweet tasting and soluble reducing sugars, meaning that they can donate electrons to reduce another chemical (e.g. Benedict's reagent). Glucose is a hexose sugar (meaning that it has 6 carbons), although the arrangement of atoms varies, leading to the formation of 2 isomers - alpha and beta glucose.
• Maltose comprises 2 glucose molecules. Sucrose comprises glucose and fructose. Lactose comprises glucose and galactose. Condensation reactions between monosaccharides form a glycosidic bond, producing a water molecule. The addition of water to a disaccharide under suitable conditions breaks this bond, releasing the constituent monosaccharides.
• Polysaccharides comprise long chains of glycosidic bonds, so large that they are insoluble and thus suitable for storage e.g. starch is found in plant structures in granular form, formed of 20 to 10000 alpha glucose molecules. Others, such as cellulose, can provide structural support.
• Isomers have the same molecular formula but different arrangements of atoms. The hydroxyl (OH) groups in alpha and beta glucose are in different orientations around the alpha carbon.
• Benedict's Test provides a semi-quantitative estimate of the amount of reducing sugar in a substance. When a liquid food sample containing reducing sugar is heated with an equal volume of Benedict's solution in a gently boiling water bath for 5 minutes, an insoluble precipitate of copper oxide is formed, turning the solution brick red. If no reducing sugar is present, the sample remains clear. If the level is very low, the sample appears green.
• The iodine test indicates the presence of starch in a sample. The addition of an aqueous potassium iodide solution to a sample containing starch changes the colour of the solution from yellow to blue-black at room temperature.
• Carbohydrates contain hydrogen and oxygen atoms in the ratio 2:1. Disaccharides are the sugars found in germinating seeds (maltose), mammal milk (lactose) and sugarcane (sucrose), whilst polysaccharides primarily serve as energy storage molecules.
• Lipid molecules are made of three fatty acids joined to a glycerol molecule by ester bonds. They are therefore not polymers as they do not comprise the same monomers and are not joined by covalent bonds.
• Non-reducing sugars cannot donate electrons and therein cannot become oxidised. Reducing sugars can donate electrons and therein oxidise their carboxyl groups.
• When in aqueous solution, glucose forms a ring structure. In alpha glucose, the carboxyl group is below the ring - whereas in beta glucose, it is above the ring.
• Some disaccharides are non-reducing sugars, such as sucrose. If Benedict's test is performed and the sample solution remains clear, there is no reducing sugar present. The food sample is then heated in a water bath for 5 minutes with an equal volume of dilute hydrochloric acid to hydrolyse glycosidic bonds. Sodium hydrogencarbonate solution is then added to neutralise the acid (the alkalinity of the solution tested with pH paper). The solution is then re-tested with Benedict's and if the non-reducing sugar was initially present, the sample will now appear brick red.
• Water has no overall charge as oxygen has a slight negative charge and hydrogen has a slight positive charge but its molecule is dipolar, forming a singular hydrogen bond. The collective strength of multiple hydrogen bonds can cause water molecules to readily stick together.
• The collective strength of hydrogen bonds shared between water molecules means that it takes considerable energy to heat a given mass of water - i.e. a high specific heat capacity. Without these bonds, life as we know it could not exist on Earth, as water would exist only in gaseous form. This means that water can act as a buffer to sudden temperature variations - internally in organisms' bodies or externally, e.g. maintaining the stability of aquatic environments.
• Water has a high latent heat of vaporisation due to the collective strength of hydrogen bonds, meaning that a considerable amount of energy is required to convert a gram of water to a vapour. This means that sweating is an effective cooling mechanism.
• The intermolecular forces shared between water molecules is referred to as cohesion. Hydrogen bonds thus have large cohesive forces, allowing water to be pulled through xylem vessels.