Biological molecules (1.2, 1.12, 1.13, 1.14)
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Water, , is a simple covalent molecule. The central oxygen shares a single covalent bond with each hydrogen.
Water is polar because the oxygen atom attracts the shared electrons more strongly, meaning that it has a partial negative charge, while the hydrogen atoms are partially positive. This polarity results in water molecules interacting with each other through hydrogen bonds.
Hydrogen bonds are strong electrostatic intermolecular interactions. They form between water molecules as the partially positively charged hydrogen atom is attracted to the lone electron pair of the electronegative oxygen atom.
Hydrogen bonds are responsible for some unique properties of water, such as cohesion, adhesion, heat capacity, and surface tension.
Water is a polar molecule, so it interacts with charged species, behaving as a solvent. This property facilitates numerous metabolic reactions that occur in aqueous environments.
In ionic compounds such as sodium chloride, the positive ions are attracted to the partially negative oxygen of the water molecules, while ions are attracted to the partially positive hydrogens.
As water freezes, it becomes less dense as hydrogen bonds lock the molecules into a fixed crystalline structure, where there is increased spacing between molecules. This reduced density causes ice to float.
A lot of energy is needed to break the hydrogen bonds between water molecules, which means that the structure of water also results in a high boiling point.
Carbohydrates are biological molecules made from sugars.
Monosaccharides are the simplest form of sugar, consisting of a single monomer. Examples include glucose, fructose, and galactose.
Two monosaccharides join via a condensation reaction to form a disaccharide. Many monosaccharides join via condensation reactions to form a polysaccharide.
Monosaccharides are linked by glycosidic bonds, which form in a condensation reaction between hydroxyl () groups on adjacent monosaccharides, releasing a molecule of water.
The name of the glycosidic bond indicates the carbon atoms involved in the linkage. For example, an 1,4 glycosidic bond forms between carbon 1 of one -glucose molecule and carbon 4 of another.
Two -glucose molecules combine to form the disaccharide maltose, joined by an 1,4 glycosidic bond.
Two monosaccharides bonded together via a glycosidic bond form disaccharides:
- Maltose = -glucose + -glucose
- Sucrose = -glucose + -fructose
- Lactose = -galactose + glucose.
The glycosidic bonds between monosaccharides can be broken down through hydrolysis reactions, where one molecule of water is added to break the bond.
Hydrolysis is crucial in biological processes, such as digestion and the breakdown of stored carbohydrates for cellular respiration.
Starch is the insoluble, primary storage polysaccharide in plants and is formed from a few hundred to a few thousand -glucose monosaccharides bonded together. It is made from a mixture of two polysaccharides.
The two polysaccharides which form starch are:
- Amylose constitutes 10–30 of starch and has an unbranched, helix-shaped chain of -glucose molecules linked by 1,4 glycosidic bonds.
- Amylopectin constitutes 70–90 of starch and has both 1,4 glycosidic bonds and 1,6 glycosidic bonds, creating a branched molecule. Enzymes involved in breaking down these molecules can simultaneously work at multiple branch points, leading to rapid release of glucose for energy. More accessible hydroxyl groups make amylopectin more soluble in water.
Glycogen is the primary storage polysaccharide in animals and fungi, characterised by a highly branched and uncoiled structure.
The highly branched structure of glycogen means there are numerous terminal glucose units leading to rapid release of glucose for energy. Branched polysaccharides with more accessible hydroxyl groups are also more soluble in water.
Glycogen’s structure is similar to amylopectin, but its branching is more extensive. This makes it more water-soluble and its terminal glucose molecules more accessible to enzymes.
Amylose is coiled, making it insoluble and compact which is ideal for storage.
Amylopectin and glycogen are both highly branched molecules, making them compact and allowing more energy to be stored. Their branches provide multiple free ends where glucose molecules can be rapidly added in condensation reactions or removed in hydrolysis reactions, enabling quick adaptation to cellular energy demands.
The bonding and structure of the key polysaccharides in plants and animals are summarised below.
Triglycerides are non-polar, hydrophobic molecules consisting of one glycerol molecule (an alcohol) and three fatty acid chains. They are the main components of fats and oils and are insoluble in water, but soluble in organic solvents, e.g., ethanol.
Fatty acids have a methyl group () at one end of a hydrocarbon chain (R group) and a carboxyl group () at the other end.
Triglycerides contain a high ratio of hydrogen:oxygen atoms, which means that they release water when oxidised. This is called metabolic water and can be a source of hydration, especially for animals living in arid environments.
Triglycerides are a source of energy owing to the high number of bonds which store energy.
Triglycerides are formed through a condensation reaction where each hydroxyl group () of glycerol reacts with a carboxyl group () of a fatty acid to form an ester bond.
Three fatty acids are attached to one glycerol molecule, three water molecules are released per triglyceride formed.
Ester bonds are broken during the digestion of the triglyceride molecule through hydrolysis reactions consuming three water molecules per triglyceride hydrolysed.
Fatty acids can differ in hydrocarbon chain length (R group) and in their saturation.
Saturated fatty acids have no double bonds. They form a straighter structure and pack tightly together, resulting in higher melting points and being in a solid state at room temperature. They are primarily found in animal fats.
Unsaturated fatty acids contain one (monounsaturated) or more (polyunsaturated) double bonds, leading to ‘kinks’ in the hydrocarbon chains that prevent tight packing. This lowers the melting point, so they are usually liquids at room temperature. They are typically found in vegetable oils.
