Biology · Biological molecules
This chapter explores the essential biological molecules: carbohydrates, lipids, proteins, and water. It details their structures, functions, and how large macromolecules are built from smaller monomers via condensation reactions and broken down by hydrolysis. Key properties of water and biochemical identification tests are also covered.
macromolecule — A large molecule such as a polysaccharide, protein or nucleic acid.
Macromolecules are giant molecules formed from smaller repeating subunits. They are essential for life, performing diverse functions from energy storage to structural support and genetic information. Think of a macromolecule like a long train, where each carriage is a smaller subunit (monomer) linked together to form a much larger structure.
Students often think all large molecules are macromolecules, but actually the term specifically refers to polymers like polysaccharides, proteins, and nucleic acids, which are built from repeating monomer units.
polymer — A giant molecule made from many similar repeating subunits joined together in a chain; the subunits are much smaller and simpler molecules known as monomers; examples of biological polymers are polysaccharides, proteins and nucleic acids.
Polymers are formed through condensation reactions where monomers are linked by covalent bonds, with the removal of water. This repeating process allows for the creation of complex structures from simple building blocks. A polymer is like a pearl necklace, where each pearl is a monomer, and the entire necklace is the polymer.
When identifying polymers, remember to state that they are made of 'repeating subunits' (monomers) and give specific biological examples like starch or protein, not just 'large molecules'.
monomer — A relatively simple molecule which is used as a basic building block for the synthesis of a polymer; many monomers are joined together by covalent bonds to make the polymer, usually by condensation reactions; common examples of monomers are monosaccharides, amino acids and nucleotides.
Monomers are the fundamental units that link together to form polymers. The specific type of monomer determines the properties and function of the resulting polymer. A monomer is like a single LEGO brick; many identical or similar bricks can be joined together to build a larger, more complex LEGO model (the polymer).
When asked to name monomers, be precise: 'monosaccharides' for carbohydrates, 'amino acids' for proteins, and 'nucleotides' for nucleic acids. Avoid generic terms.
condensation reaction — A chemical reaction involving the joining together of two molecules by removal of a water molecule.
This reaction is crucial for building larger biological molecules (polymers) from smaller ones (monomers). The formation of glycosidic, ester, and peptide bonds are all examples of condensation reactions. Imagine two people holding hands; a condensation reaction is like them joining hands and a drop of water falling away as they connect.
Always mention the 'removal of a water molecule' when describing condensation reactions, as this is a key part of the definition and mechanism.
hydrolysis — A chemical reaction in which a chemical bond is broken by the addition of a water molecule; commonly used to break down complex molecules into simpler molecules.
Hydrolysis is the reverse of a condensation reaction, essential for digestion and breaking down polymers into their constituent monomers, making nutrients available for absorption or recycling. If condensation is like two people holding hands and a drop of water falling away, hydrolysis is like adding a drop of water to their hands, causing them to let go.
Large biological molecules, or macromolecules, are fundamental to life. These include carbohydrates, lipids, and proteins. They are typically polymers, meaning they are constructed from smaller, repeating monomer units. The synthesis of these polymers from monomers occurs through condensation reactions, where a water molecule is removed. Conversely, these polymers can be broken down into their constituent monomers via hydrolysis, a reaction that involves the addition of a water molecule.
monosaccharide — A molecule consisting of a single sugar unit and with the general formula (CH2O)n.
Monosaccharides are the simplest carbohydrates, serving as primary energy sources (e.g., glucose) and building blocks for disaccharides and polysaccharides. They are typically sweet-tasting and water-soluble. A monosaccharide is like a single sugar cube; it's the smallest, most basic unit of sugar.
When asked for examples of monosaccharides, name specific ones like glucose, fructose, galactose (hexoses) or ribose, deoxyribose (pentoses), and remember their general formula (CH2O)n.
disaccharide — A sugar molecule consisting of two monosaccharides joined together by a glycosidic bond.
Disaccharides are formed by a condensation reaction between two monosaccharides, such as maltose (glucose + glucose) or sucrose (glucose + fructose). They are soluble sugars with roles in energy transport and diet. A disaccharide is like two sugar cubes stuck together; it's still sweet and soluble, but a bit larger than a single cube.
Students often think all disaccharides are reducing sugars, but actually sucrose is a common non-reducing sugar.
glycosidic bond — A C–O–C link between two sugar molecules, formed by a condensation reaction; it is a covalent bond.
This covalent bond is formed when two hydroxyl groups from monosaccharides react, releasing a water molecule. It is the fundamental linkage in disaccharides and polysaccharides. It's like a small oxygen bridge connecting two sugar molecules, holding them firmly together.
Benedict’s test — A test for the presence of reducing sugars; the unknown substance is heated with Benedict’s reagent, and a change from a clear blue solution to the production of a yellow, red or brown precipitate indicates the presence of reducing sugars such as glucose.
Reducing sugars donate electrons to reduce the blue copper(II) ions in Benedict's reagent to brick-red copper(I) oxide precipitate. The intensity of the colour change can be used for semi-quantitative estimation. It's like a chemical 'traffic light' for sugar: blue means no sugar, and green, yellow, orange, or red means increasing amounts of reducing sugar are present.
For Benedict's test, remember to state 'heat in a water bath' and describe the colour change from 'blue to green/yellow/orange/brick-red precipitate' for a positive result. Mentioning 'excess Benedict's reagent' is key for semi-quantitative use.
polysaccharide — A polymer whose subunits are monosaccharides joined together by glycosidic bonds.
Polysaccharides are large, complex carbohydrates formed from many monosaccharide units. They serve as energy stores (starch, glycogen) or structural components (cellulose) and are generally insoluble and unreactive. A polysaccharide is like a long, complex chain made of many identical or similar beads (monosaccharides).
Students often think polysaccharides are sweet like sugars, but actually they are not sugars and do not taste sweet.
glycogen — A polysaccharide made of many glucose molecules linked together, that acts as a glucose store in liver and muscle cells.
Glycogen is the primary energy storage carbohydrate in animals, highly branched like amylopectin but more so, allowing for rapid glucose release when needed. It forms granules in liver and muscle cells. Glycogen is like a highly branched tree, where each leaf is a glucose molecule, allowing for quick access to many leaves (glucose) from many points.
Highlight that glycogen is the 'animal storage carbohydrate' and mention its 'highly branched structure' (1,4 and 1,6 linkages of α-glucose) for efficient glucose release.
cellulose — A polysaccharide made from beta-glucose subunits; used as a strengthening material in plant cell walls.
Cellulose is a structural polysaccharide in plants, formed from β-glucose monomers linked by 1,4 glycosidic bonds, with alternate glucose units inverted. This arrangement allows extensive hydrogen bonding, forming strong microfibrils and fibres. Cellulose is like a strong, interwoven fabric made of many long, parallel threads (cellulose molecules), giving plant cell walls immense strength.
Emphasise that cellulose is a polymer of 'β-glucose' and that the 'alternating 180° rotation' of glucose units enables extensive 'hydrogen bonding' between parallel chains, leading to high 'tensile strength' in plant cell walls.
Carbohydrates are essential biological molecules, ranging from simple monosaccharides like glucose, which serve as immediate energy sources, to complex polysaccharides. Monosaccharides can join together via condensation reactions to form disaccharides, such as maltose or sucrose, linked by glycosidic bonds. Polysaccharides, like starch and glycogen, are large polymers of glucose used for energy storage in plants and animals respectively. Cellulose, another polysaccharide, provides structural support in plant cell walls due to its unique β-glucose linkages and extensive hydrogen bonding.
hydrogen bond — A relatively weak bond formed by the attraction between a group with a small positive charge on a hydrogen atom (Hδ+) and another group carrying a small negative charge (δ−), e.g. between two –Oδ– Hδ+ groups.
Hydrogen bonds are crucial intermolecular forces, weaker than covalent bonds but collectively strong. They are responsible for many unique properties of water and play vital roles in maintaining the structure of large biological molecules like proteins and nucleic acids. Think of hydrogen bonds as weak magnetic attractions between molecules; individually they are easily broken, but many together can create a strong overall force.
Students often think hydrogen bonds are covalent bonds, but actually they are intermolecular forces of attraction, much weaker than the intramolecular covalent bonds that hold atoms together within a molecule.
ester bond / ester linkage — A chemical bond, represented as –COO– , formed when an acid reacts with an alcohol.
Ester bonds are formed via condensation reactions, typically between a carboxyl group of a fatty acid and a hydroxyl group of an alcohol (like glycerol). They are characteristic linkages in lipids, particularly triglycerides. An ester bond is like a chemical 'clasp' that joins a fatty acid 'chain' to a glycerol 'backbone', forming a lipid molecule.
triglyceride — A type of lipid formed when three fatty acid molecules combine with glycerol, an alcohol with three hydroxyl (−OH) groups.
Triglycerides are the most common lipids, serving as efficient energy stores due to their high number of C-H bonds. They are hydrophobic and insoluble in water, also providing insulation and buoyancy. A triglyceride is like a three-pronged fork (glycerol) with three long spaghetti strands (fatty acids) attached to it.
For triglycerides, remember 'three fatty acids' and 'one glycerol' joined by 'ester bonds' via 'condensation reactions'. Emphasize their 'hydrophobic' nature and role as 'energy stores' and 'insulators'.
Lipids are a diverse group of biological molecules, characterized by their insolubility in water. Triglycerides, formed from one glycerol molecule and three fatty acids linked by ester bonds through condensation reactions, are primary energy storage molecules. Their hydrophobic nature also makes them excellent insulators. Phospholipids, with their hydrophilic head and hydrophobic tails, are crucial components of cell membranes, forming a bilayer that regulates substance passage.
peptide bond — The covalent bond joining neighbouring amino acids together in proteins; it is a C–N link between two amino acid molecules, formed by a condensation reaction.
Peptide bonds are the fundamental linkages in proteins, formed between the carboxyl group of one amino acid and the amino group of another, with the release of water. A chain of amino acids linked by peptide bonds is a polypeptide. A peptide bond is like a strong clasp connecting two beads (amino acids) on a necklace (polypeptide chain).
When describing peptide bond formation, specify the reaction between the 'carboxyl group' of one amino acid and the 'amino group' of another, and mention it's a 'condensation reaction' forming a 'C–N link'.
polypeptide — A long chain of amino acids formed by condensation reactions between the individual amino acids; proteins are made of one or more polypeptide chains; see peptide bond.
Polypeptides are linear sequences of amino acids linked by peptide bonds. They are the primary structural component of proteins, which may consist of one or more polypeptide chains folded into a specific 3D shape. A polypeptide is like a string of different coloured beads, where each bead is an amino acid, and the string itself is the chain.
primary structure — The sequence of amino acids in a polypeptide or protein.
The primary structure is the most fundamental level of protein organization, determined by the genetic code. It dictates all subsequent levels of folding and ultimately the protein's final 3D shape and function. The primary structure is like the specific order of letters in a word; changing even one letter can completely change the meaning (function) of the word.
secondary structure — The structure of a protein molecule resulting from the regular coiling or folding of the chain of amino acids (an α-helix or β-pleated sheet).
Secondary structures are local, repeating patterns formed by hydrogen bonding between the backbone atoms (C=O and N-H groups) of amino acids. The two main types are the α-helix and β-pleated sheet. Secondary structure is like the basic patterns you can make with a flexible wire, such as coiling it into a spring (α-helix) or folding it into zig-zags (β-pleated sheet).
Students often think R groups are involved in secondary structure, but actually secondary structure is formed by hydrogen bonds between the 'backbone' atoms (C=O and N-H) of the polypeptide chain, not the R groups.
α-helix — A helical structure formed by a polypeptide chain, held in place by hydrogen bonds; an α-helix is an example of secondary structure in a protein.
In an α-helix, the polypeptide chain coils into a corkscrew shape, stabilized by hydrogen bonds between the oxygen of a C=O group and the hydrogen of an N-H group four amino acids ahead in the chain. All backbone C=O and N-H groups are involved. An α-helix is like a coiled telephone cord, where the coils are held in shape by invisible 'sticky spots' (hydrogen bonds) along the cord.
β-pleated sheet — A loose, sheet-like structure formed by hydrogen bonding between parallel polypeptide chains; a β-pleated sheet is an example of secondary structure in a protein.
In a β-pleated sheet, polypeptide chains lie side-by-side in a zig-zag pattern, forming a sheet-like structure. This is stabilized by hydrogen bonds between C=O and N-H groups of adjacent parallel segments of the polypeptide. A β-pleated sheet is like a folded fan or a corrugated cardboard sheet, where the folds are held in place by hydrogen bonds.
tertiary structure — The compact structure of a protein molecule resulting from the three-dimensional coiling of the chain of amino acids.
Tertiary structure is the overall 3D shape of a single polypeptide chain, formed by interactions between R groups of amino acids. These interactions include hydrogen bonds, disulfide bonds, ionic bonds, and hydrophobic interactions, which stabilize the precise shape. Tertiary structure is like taking a coiled spring (secondary structure) and then bending, twisting, and knotting it into a specific, complex sculpture.
When explaining tertiary structure, list the four types of bonds/interactions (hydrogen, disulfide, ionic, hydrophobic) and state that they occur between 'R groups' to maintain the 'precise 3D shape'.
quaternary structure — The three-dimensional arrangement of two or more polypeptides, or of a polypeptide and a non-protein component such as haem, in a protein molecule.
Quaternary structure applies to proteins composed of multiple polypeptide chains (subunits) or those with non-protein prosthetic groups. It describes how these subunits are arranged and interact to form the complete functional protein. Quaternary structure is like several individual sculptures (tertiary structures of polypeptide chains) coming together and arranging themselves into a larger, functional art installation.
haemoglobin — The red pigment found in red blood cells, whose molecules contain four iron atoms within a globular protein made up of four polypeptides; it combines reversibly with oxygen.
Haemoglobin is a globular protein with a quaternary structure, consisting of two α-globin and two β-globin polypeptide chains, each associated with a haem prosthetic group containing an iron atom. It is responsible for oxygen transport in the blood. Haemoglobin is like a tiny oxygen taxi, with four seats (haem groups) that can each pick up and drop off one oxygen molecule.
When describing haemoglobin, mention its 'globular' nature, 'quaternary structure' (four polypeptide chains), and the presence of 'four haem groups' each with an 'iron atom' for 'reversible oxygen binding'.
globular protein — A protein whose molecules are folded into a relatively spherical shape, often has physiological roles and is often water-soluble and metabolically active, e.g. insulin, haemoglobin and enzymes.
Globular proteins typically have hydrophobic R groups folded into the interior and hydrophilic R groups on the exterior, making them soluble in water. Their precise 3D shape is critical for their diverse physiological functions, such as catalysis (enzymes) or transport (haemoglobin). A globular protein is like a tightly wound ball of yarn, with the 'water-hating' parts tucked inside and the 'water-loving' parts on the surface, allowing it to dissolve in water.
sickle cell anaemia — A genetic disease caused by a faulty gene coding for haemoglobin, in which haemoglobin tends to precipitate when oxygen concentrations are low.
This condition results from a single amino acid substitution (valine for glutamic acid) on the surface of the β-chain of haemoglobin. This makes haemoglobin less soluble, causing red blood cells to become sickle-shaped, leading to blockages and reduced oxygen transport. It's like a tiny, crucial bolt on a machine being replaced with a slightly wrong one, causing the whole machine (red blood cell) to malfunction and change shape under stress.
collagen — The main structural protein of animals; known as ‘white fibres’, the fundamental unit of the fibre consists of three helical polypeptide chains wound around each other, forming a ‘triple helix’ with high tensile strength.
Collagen is an insoluble fibrous protein, highly abundant in animals, providing strength and flexibility to tissues like skin, tendons, and bones. Its triple helix structure, stabilized by hydrogen and covalent bonds, and staggered arrangement of fibrils contribute to its immense tensile strength. Collagen is like a super-strong, braided rope made of three smaller ropes (polypeptide helices), which are then woven together with other ropes to form an even stronger cable.
fibrous protein — A protein whose molecules have a relatively long, thin structure that is generally insoluble and metabolically inactive, and whose function is usually structural, e.g. keratin and collagen.
Fibrous proteins are elongated and often form strong, insoluble fibres, making them ideal for structural roles in organisms. Their insolubility arises from having many hydrophobic R groups exposed on their surface. A fibrous protein is like a long, strong piece of string or a hair strand, designed for strength and support rather than for dissolving or reacting quickly.
biuret test — A test for the presence of amine groups and thus for the presence of protein; biuret reagent is added to the unknown substance, and a change from pale blue to purple indicates the presence of protein.
The biuret test detects peptide bonds (which contain nitrogen atoms in amine groups). Copper(II) ions in the alkaline biuret reagent form a purple complex with these bonds. No heating is required. It's like a chemical 'purple alert' for protein; if the solution turns purple, protein is present.
Students often think the biuret test detects amino acids, but actually it specifically detects the 'peptide bonds' between amino acids, meaning it tests for polypeptides or proteins.
Proteins are polymers of amino acids linked by peptide bonds, formed through condensation reactions. Their function is intimately tied to their complex three-dimensional structure, which is described at four levels. The primary structure is the unique sequence of amino acids. This sequence dictates the secondary structure, which involves regular coiling (α-helix) or folding (β-pleated sheet) stabilized by hydrogen bonds in the polypeptide backbone. The tertiary structure is the overall 3D shape of a single polypeptide, maintained by interactions between R groups, including hydrogen, ionic, disulfide bonds, and hydrophobic interactions. Some proteins, like haemoglobin, exhibit quaternary structure, involving the arrangement of multiple polypeptide chains or non-protein components.
Water is an indispensable biological molecule, crucial for life due to its unique properties. Its polarity, arising from hydrogen bonding, makes it an excellent solvent, allowing many substances to dissolve and be transported within organisms. Water also possesses a high specific heat capacity, meaning it can absorb or release large amounts of heat with only a small change in its own temperature, helping to stabilize internal body temperatures. Furthermore, its high latent heat of vaporisation allows organisms to cool effectively through evaporation.
When comparing molecules (e.g., starch vs. cellulose), always link their structural differences (e.g., α/β-glucose, branching) directly to their functional differences (e.g., energy storage vs. structural support).
Practice drawing and labelling: α-glucose, a triglyceride (showing ester bonds), and the general structure of an amino acid. Be able to show how a peptide or glycosidic bond is formed.
For proteins like haemoglobin and collagen, explicitly relate their structure (globular vs. fibrous, quaternary structure, prosthetic groups) to their specific biological role.
macromolecule
A large molecule such as a polysaccharide, protein or nucleic acid.
polymer
A giant molecule made from many similar repeating subunits joined together in a chain; the subunits are much smaller and simpler molecules known as monomers; examples of biological polymers are polysaccharides, proteins and nucleic acids.
monomer
A relatively simple molecule which is used as a basic building block for the synthesis of a polymer; many monomers are joined together by covalent bonds to make the polymer, usually by condensation reactions; common examples of monomers are monosaccharides, amino acids and nucleotides.
condensation reaction
A chemical reaction involving the joining together of two molecules by removal of a water molecule.
hydrolysis
A chemical reaction in which a chemical bond is broken by the addition of a water molecule; commonly used to break down complex molecules into simpler molecules.
monosaccharide
A molecule consisting of a single sugar unit and with the general formula (CH2O)n.
disaccharide
A sugar molecule consisting of two monosaccharides joined together by a glycosidic bond.
glycosidic bond
A C–O–C link between two sugar molecules, formed by a condensation reaction; it is a covalent bond.
Benedict’s test
A test for the presence of reducing sugars; the unknown substance is heated with Benedict’s reagent, and a change from a clear blue solution to the production of a yellow, red or brown precipitate indicates the presence of reducing sugars such as glucose.
polysaccharide
A polymer whose subunits are monosaccharides joined together by glycosidic bonds.
glycogen
A polysaccharide made of many glucose molecules linked together, that acts as a glucose store in liver and muscle cells.
cellulose
A polysaccharide made from beta-glucose subunits; used as a strengthening material in plant cell walls.
hydrogen bond
A relatively weak bond formed by the attraction between a group with a small positive charge on a hydrogen atom (Hδ+) and another group carrying a small negative charge (δ−), e.g. between two –Oδ– Hδ+ groups.
ester bond / ester linkage
A chemical bond, represented as –COO– , formed when an acid reacts with an alcohol.
triglyceride
A type of lipid formed when three fatty acid molecules combine with glycerol, an alcohol with three hydroxyl (−OH) groups.
peptide bond
The covalent bond joining neighbouring amino acids together in proteins; it is a C–N link between two amino acid molecules, formed by a condensation reaction.
polypeptide
A long chain of amino acids formed by condensation reactions between the individual amino acids; proteins are made of one or more polypeptide chains; see peptide bond.
primary structure
The sequence of amino acids in a polypeptide or protein.
secondary structure
The structure of a protein molecule resulting from the regular coiling or folding of the chain of amino acids (an α-helix or β-pleated sheet).
α-helix
A helical structure formed by a polypeptide chain, held in place by hydrogen bonds; an α-helix is an example of secondary structure in a protein.
β-pleated sheet
A loose, sheet-like structure formed by hydrogen bonding between parallel polypeptide chains; a β-pleated sheet is an example of secondary structure in a protein.
tertiary structure
The compact structure of a protein molecule resulting from the three-dimensional coiling of the chain of amino acids.
quaternary structure
The three-dimensional arrangement of two or more polypeptides, or of a polypeptide and a non-protein component such as haem, in a protein molecule.
haemoglobin
The red pigment found in red blood cells, whose molecules contain four iron atoms within a globular protein made up of four polypeptides; it combines reversibly with oxygen.
globular protein
A protein whose molecules are folded into a relatively spherical shape, often has physiological roles and is often water-soluble and metabolically active, e.g. insulin, haemoglobin and enzymes.
sickle cell anaemia
A genetic disease caused by a faulty gene coding for haemoglobin, in which haemoglobin tends to precipitate when oxygen concentrations are low.
collagen
The main structural protein of animals; known as ‘white fibres’, the fundamental unit of the fibre consists of three helical polypeptide chains wound around each other, forming a ‘triple helix’ with high tensile strength.
fibrous protein
A protein whose molecules have a relatively long, thin structure that is generally insoluble and metabolically inactive, and whose function is usually structural, e.g. keratin and collagen.
biuret test
A test for the presence of amine groups and thus for the presence of protein; biuret reagent is added to the unknown substance, and a change from pale blue to purple indicates the presence of protein.
| Command word | What examiners expect |
|---|---|
| Describe | For structures, describe specific features (e.g., 'alpha-glucose with hydroxyl group on carbon 1 below the ring'). For tests, describe the reagent, conditions, and exact colour changes (e.g., 'Benedict's reagent, heat in water bath, blue to brick-red precipitate'). |
| Explain | Provide reasons and mechanisms. For example, 'Explain how cellulose provides strength' requires linking β-glucose, 180° rotation, hydrogen bonding, microfibrils, and tensile strength. For water properties, link hydrogen bonding to specific heat capacity or latent heat of vaporisation. |
| Compare | State both similarities and differences, often linking structure to function. For example, comparing starch and cellulose requires mentioning α vs β glucose, branching vs linear, and energy storage vs structural roles. |
| Suggest | Apply knowledge to a novel situation. For example, 'Suggest why a particular molecule is soluble' would require referring to polar groups and hydrogen bonding with water. |
Mistake
Thinking all large molecules are macromolecules.
Correction
Macromolecules specifically refer to polymers like polysaccharides, proteins, and nucleic acids, which are built from repeating monomer units.
Mistake
Believing all disaccharides are reducing sugars.
Correction
Sucrose is a common non-reducing sugar and requires prior hydrolysis to be detected by Benedict's test.
Mistake
Thinking polysaccharides are sweet like sugars.
Correction
Polysaccharides are not sugars and do not taste sweet; they are large, complex carbohydrates.
Mistake
Confusing hydrogen bonds with covalent bonds.
Correction
Hydrogen bonds are weak intermolecular forces of attraction, much weaker than the intramolecular covalent bonds that hold atoms together within a molecule.
Mistake
Believing R groups are involved in secondary protein structure.
Correction
Secondary structure (α-helices/β-sheets) is formed by hydrogen bonds between the C=O and N-H groups of the polypeptide backbone, not the R groups.
Mistake
Thinking the biuret test detects amino acids.
Correction
The biuret test specifically detects the 'peptide bonds' between amino acids, meaning it tests for polypeptides or proteins, not individual amino acids.