Unit 11: Introduction to Biochemistry
Exploring the fundamental organic molecules of life: lipids, carbohydrates, and proteins.
11.22 Fats, Oils, Fatty Acids & Glycerol
Fats and oils are natural esters known as triglycerides. They are formed from a condensation reaction between one molecule of glycerol (propan-1,2,3-triol) and three molecules of fatty acids (long-chain carboxylic acids).
The difference between a fat and an oil lies in the nature of the fatty acid chains:
- Fats (solid at room temperature) are typically derived from saturated fatty acids. The straight, saturated chains allow the molecules to pack closely together, leading to stronger intermolecular forces and a higher melting point.
- Oils (liquid at room temperature) are typically derived from unsaturated fatty acids (containing C=C double bonds). The 'kinks' in the unsaturated chains prevent close packing, leading to weaker intermolecular forces and a lower melting point.
Oils can be converted into fats by hydrogenation (an addition reaction with hydrogen and a nickel catalyst), which saturates the C=C bonds. This is how margarine is made from vegetable oils. In the body, fats are broken down (hydrolysed) back into glycerol and fatty acids for energy and to build cell membranes.
Solved Examples:
- What is the chemical name for a fat or
oil?
Solution: A triglyceride. - What two types of molecules combine to form a
triglyceride?
Solution: One glycerol molecule and three fatty acid molecules. - What is the functional group that links the components of a
triglyceride?
Solution: An ester link. - What is the key structural difference between a saturated and an unsaturated
fatty acid?
Solution: An unsaturated fatty acid contains at least one C=C double bond in its hydrocarbon chain, while a saturated fatty acid does not. - Why are oils liquid at room
temperature?
Solution: Their unsaturated fatty acid chains have kinks, which prevent close packing and result in weaker intermolecular forces. - What is the process of converting an oil into a solid fat
called?
Solution: Hydrogenation. - What happens to fats during
digestion?
Solution: They are hydrolysed back into glycerol and fatty acids. - What is the alcohol component of all
triglycerides?
Solution: Glycerol (propan-1,2,3-triol). - What is a fatty acid?
Solution: A long-chain carboxylic acid. - What type of reaction is the formation of a
triglyceride?
Solution: A condensation (or esterification) reaction.
11.23 Carbohydrates
Carbohydrates are a major class of biological molecules with the general formula $C_x(H_2O)_y$. They are the primary source of energy for most organisms.
Classification:
- Monosaccharides: The simplest carbohydrates, also known as simple sugars. They are the monomers of larger carbohydrates. Examples include glucose and fructose (both $C_6H_{12}O_6$).
- Disaccharides: Formed when two monosaccharides join via a condensation reaction. Examples include sucrose (table sugar) and maltose.
- Polysaccharides: Large polymers made of many monosaccharide units joined together. They are used for energy storage and structural support. Examples include starch (plant energy store), glycogen (animal energy store), and cellulose (plant structural component).
Tests for Carbohydrates:
- Test for Reducing Sugars: All monosaccharides and some disaccharides (like maltose) are reducing sugars. When heated with Benedict's solution (a blue solution containing $Cu^{2+}$ ions), they reduce the copper(II) ions to copper(I) oxide, forming a precipitate that changes colour from blue to green to yellow to brick-red.
- Test for Starch: Starch gives a characteristic blue-black colour when a few drops of iodine solution are added.
Solved Examples:
- What are the monomers of
polysaccharides?
Solution: Monosaccharides. - Name a common monosaccharide.
Solution: Glucose or fructose. - What is the function of starch in
plants?
Solution: It is a long-term store of energy. - What is the positive test result for a reducing sugar with Benedict's
solution?
Solution: The blue solution forms a brick-red precipitate upon heating. - What is the test for starch?
Solution: Add iodine solution; a positive result is a blue-black colour. - What two monosaccharides make up
sucrose?
Solution: Glucose and fructose. - What is the animal equivalent of
starch?
Solution: Glycogen. - Is sucrose a reducing sugar?
Solution: No, it is a non-reducing sugar and will not give a positive Benedict's test. - What is the structural polysaccharide found in plant cell
walls?
Solution: Cellulose. - What type of reaction breaks down a polysaccharide into
monosaccharides?
Solution: Hydrolysis.
11.24 Proteins
Proteins are extremely complex and versatile natural polymers that are essential for virtually all life processes. They have a wide range of functions, including acting as structural components (e.g., muscle, hair), enzymes (biological catalysts), and antibodies.
Proteins are polymers of amino acid monomers. There are 20 common amino acids that are linked together in long chains called polypeptides via a condensation reaction. The bond that links the amino acids is an amide link, known as a peptide bond.
A protein consists of one or more polypeptide chains folded into a specific and complex three-dimensional shape, which is crucial for its biological function.
Test for Proteins:
The Biuret test is used to detect the presence of proteins.
Procedure: Add a few drops of Biuret solution (an alkaline copper(II)
sulphate solution) to the sample.
Positive Result: The solution turns from blue to
purple/lilac. The test works by detecting the presence of peptide bonds.
Solved Examples:
- What are the monomers of proteins?
Solution: Amino acids. - What is the name of the bond that links amino acids together in a
protein?
Solution: A peptide bond. - What is a long chain of amino acids
called?
Solution: A polypeptide. - Name two functions of proteins in the
body.
Solution: Acting as enzymes (catalysts) and providing structural support (e.g., in muscle). - What is the name of the laboratory test for
proteins?
Solution: The Biuret test. - What is the positive result for the Biuret
test?
Solution: The solution turns from blue to purple or lilac. - What specific feature of a protein does the Biuret test
detect?
Solution: The peptide bonds. - What type of polymerisation forms
proteins?
Solution: Condensation polymerisation. - Would a single amino acid give a positive Biuret
test?
Solution: No, because it does not contain any peptide bonds. - What happens to proteins during
digestion?
Solution: They are hydrolysed back into their constituent amino acids.