Welcome to AS Biology: Molecules
Welcome to the building blocks of life! In this chapter of AS 1: Molecules and Cells, we will explore the essential chemical components that make up every living organism. Don't worry if biochemistry feels a bit daunting at first—we will break down every molecule step-by-step, from simple inorganic ions to complex genetic material. By understanding how these molecules are shaped and how they interact, you will unlock the core foundations of biology!
---1. Inorganic Ions and Water
Inorganic Ions
Inorganic ions are charged chemical elements that play vital biological roles in both plants and animals. You need to know these four key ions for CCEA AS 1:
• Magnesium (\(\text{Mg}^{2+}\)): A crucial central component of the chlorophyll molecule. Without magnesium, plants cannot synthesise chlorophyll and cannot perform photosynthesis efficiently.
• Iron(II) (\(\text{Fe}^{2+}\)): Located at the centre of the haem group in haemoglobin. It binds reversibly to oxygen, enabling red blood cells to transport oxygen throughout the body.
• Calcium (\(\text{Ca}^{2+}\)): Provides structural strength in bones and teeth in animals. In plants, it forms calcium pectate in the middle lamella to cement adjacent cell walls together. It also plays an essential role in muscle contraction and blood clotting mechanisms.
• Phosphate (\(\text{PO}_4^{3-}\)): An essential structural component of nucleotides (found in \(\text{DNA}\), \(\text{RNA}\), and \(\text{ATP}\)) and forms the hydrophilic heads of phospholipids in biological membranes.
Water: The Medium of Life
Water (\(\text{H}_2\text{O}\)) covers most of our planet and makes up a huge portion of living cells. Its remarkable properties arise from its structure:
• Dipolar Nature and Hydrogen Bonding: Oxygen atoms pull electrons more strongly than hydrogen atoms, giving the oxygen atom a partial negative charge (\(\delta^-\)) and the hydrogen atoms a partial positive charge (\(\delta^+\)). Because water is a dipole, weak electrostatic attractions called hydrogen bonds form between the \(\delta^-\) oxygen of one water molecule and the \(\delta^+\) hydrogen of a neighbouring molecule.
• Universal Aqueous Solvent: Because water is polar, it readily surrounds and dissolves ionic compounds and polar (hydrophilic) molecules. This allows water to act as an aqueous transport medium (e.g., in blood plasma and plant xylem/phloem) and a medium for metabolic reactions.
• High Specific Heat Capacity: A large amount of thermal energy is required to raise the temperature of water because many hydrogen bonds must be broken. Biological significance: Water acts as a thermal buffer, preventing rapid temperature swings in aquatic habitats and within cellular cytoplasm.
• High Latent Heat of Vaporisation: It takes significant heat energy to vaporise liquid water into steam. Biological significance: Organisms can cool down effectively via evaporation (e.g., sweating in humans, transpiration in plants) with minimal loss of body water.
• Density and Freezing Behaviour: Water reaches its maximum density at \(4\,^\circ\text{C}\). As water freezes, hydrogen bonds hold molecules in an open lattice, making ice less dense than liquid water. Ice floats on surface water, creating an insulating layer that prevents the liquid habitat beneath from freezing solid.
• Cohesion and Surface Tension: Hydrogen bonds hold water molecules together tightly (cohesion). This allows continuous, unbroken columns of water to be pulled up xylem vessels during transpiration, and provides high surface tension for small invertebrates to walk on water.
Quick Review: Key Takeaway
Water's partial charges create hydrogen bonding, giving rise to thermal stability, solvent abilities, high cohesion, and an insulating ice layer. The four key ions to remember are \(\text{Mg}^{2+}\) (chlorophyll), \(\text{Fe}^{2+}\) (haemoglobin), \(\text{Ca}^{2+}\) (bones/cell walls), and \(\text{PO}_4^{3-}\) (\(\text{DNA}\)/\(\text{ATP}\)/membranes).
---2. Carbohydrates
Carbohydrates consist of the elements Carbon (\(\text{C}\)), Hydrogen (\(\text{H}\)), and Oxygen (\(\text{O}\)), usually in the empirical ratio of \((\text{CH}_2\text{O})_n\).
Monosaccharides (Single Sugars)
Monosaccharides are sweet, soluble single sugar units classified by their number of carbon atoms:
• Triose (\(\text{C}_3\text{H}_6\text{O}_3\)): 3-carbon sugars, such as glyceraldehyde and dihydroxyacetone, which act as metabolic intermediates in respiration and photosynthesis.
• Pentose (\(\text{C}_5\text{H}_{10}\text{O}_5\)): 5-carbon sugars, such as ribose (found in \(\text{RNA}\) and \(\text{ATP}\)) and deoxyribose (\(\text{C}_5\text{H}_{10}\text{O}_4\), found in \(\text{DNA}\)).
• Hexose (\(\text{C}_6\text{H}_{12}\text{O}_6\)): 6-carbon sugars including glucose, fructose, and galactose.
Isomers of Glucose: \(\alpha\)-glucose vs \(\beta\)-glucose
Molecules with the same molecular formula but different structural arrangements are called isomers:
• In \(\alpha\)-glucose, the hydroxyl group (\(-\text{OH}\)) on Carbon-1 points below the plane of the ring.
• In \(\beta\)-glucose, the hydroxyl group (\(-\text{OH}\)) on Carbon-1 points above the plane of the ring.
Memory Trick: Think \(\alpha\) = "Alpha Apart/Below" and \(\beta\) = "Beta Birds/Above"!
Disaccharides (Double Sugars)
Disaccharides (\(\text{C}_{12}\text{H}_{22}\text{O}_{11}\)) are formed when two monosaccharides join together in a condensation reaction, eliminating a single molecule of water (\(\text{H}_2\text{O}\)) and forming a strong covalent glycosidic bond. The reverse reaction (breaking the bond using water) is called hydrolysis.
• Maltose: \(\alpha\text{-glucose} + \alpha\text{-glucose}\) joined by an \(\alpha\text{-1,4-glycosidic bond}\).
• Sucrose: \(\alpha\text{-glucose} + \text{fructose}\) joined by an \(\alpha\text{-1,2-glycosidic bond}\).
• Lactose: \(\beta\text{-galactose} + \alpha\text{-glucose}\) joined by a \(\beta\text{-1,4-glycosidic bond}\).
Polysaccharides (Complex Carbohydrates)
Polysaccharides are large polymers formed from chains of monosaccharide monomers.
1. Starch (Plant Energy Storage)
Starch is an insoluble, compact storage molecule made of two \(\alpha\)-glucose polymers:
• Amylose: An unbranched chain linked exclusively by \(\alpha\text{-1,4-glycosidic bonds}\) that coils into a tight helix.
• Amylopectin: A branched chain containing \(\alpha\text{-1,4-glycosidic bonds}\) with \(\alpha\text{-1,6-glycosidic bonds}\) occurring every 24 to 30 glucose units.
• Why is starch ideal for storage? It is compact and insoluble, meaning it does not alter the osmotic potential of the plant cell.
2. Glycogen (Animal Energy Storage)
The main carbohydrate storage molecule in animals and fungi, stored in liver and muscle cells. It is structurally similar to amylopectin but has much more frequent \(\alpha\text{-1,6-glycosidic branches}\). This highly branched structure creates many terminal ends, allowing rapid hydrolysis into glucose when energy is urgently needed.
3. Cellulose (Plant Structural Support)
Cellulose forms the rigid cell walls of plants:
• Composed of straight, unbranched chains of \(\beta\)-glucose monomers joined by \(\beta\text{-1,4-glycosidic bonds}\).
• Because the \(-\text{OH}\) group on Carbon-1 is above the ring, every alternate \(\beta\)-glucose monomer must be inverted by \(180^\circ\) to form the bond.
• Thousands of parallel cellulose chains form extensive hydrogen bonds cross-linking with one another, bundling together into strong microfibrils, which bundle into macrofibrils to provide enormous tensile strength.
Quick Review: Key Takeaway
Alpha-glucose polymers (starch, glycogen) serve as energy storage molecules with helical or branched chains (\(\alpha\text{-1,4}\) and \(\alpha\text{-1,6}\) bonds). Beta-glucose polymers (cellulose) form straight, rigid, unbranched structural microfibrils (\(\beta\text{-1,4}\) bonds with alternating inverted units).
---3. Lipids
Lipids are organic molecules containing carbon, hydrogen, and oxygen (with a much lower proportion of oxygen than carbohydrates). They are non-polar and insoluble in water.
Triglycerides
Triglycerides are composed of 1 molecule of glycerol bonded to 3 fatty acid chains:
• Formed via three condensation reactions, each producing an ester bond and releasing a total of 3 molecules of \(\text{H}_2\text{O}\).
• Saturated Fatty Acids: Contain only single carbon-carbon bonds (\(\text{C}-\text{C}\)) in their hydrocarbon chain. The chains are straight and pack tightly together, making saturated lipids solid at room temperature (fats).
• Unsaturated Fatty Acids: Contain one (monounsaturated) or more (polyunsaturated) double carbon-carbon bonds (\(\text{C}=\text{C}\)). These double bonds introduce kinks in the hydrocarbon chains, preventing tight packing and making unsaturated lipids liquid at room temperature (oils).
Biological Functions of Triglycerides:
• High-Density Energy Storage: Yields more energy per gram than carbohydrates upon oxidation.
• Thermal Insulation: Subcutaneous adipose layers reduce heat loss.
• Electrical Insulation: Forms myelin sheaths around neurons to speed up nerve impulses.
• Metabolic Water: High ratio of hydrogen atoms releases significant metabolic water when oxidised during cellular respiration.
• Protection: Surrounds and cushions delicate internal organs.
Phospholipids
A phospholipid consists of 1 glycerol molecule, 2 fatty acid tails, and 1 negatively charged phosphate group (\(\text{PO}_4^{3-}\)).
• Amphipathic Nature: The charged phosphate head is hydrophilic (water-loving) and soluble, whereas the non-polar fatty acid tails are hydrophobic (water-repelling) and insoluble.
• In aqueous environments, phospholipids spontaneously orient themselves to form a phospholipid bilayer, creating the fundamental basis of all biological cell membranes.
⚠️ Examiner Warning: Lipids are large macromolecules, but they are NOT polymers because they are not composed of identical repeating monomer units linked in a chain!
Quick Review: Key Takeaway
Triglycerides = 1 glycerol + 3 fatty acids linked by ester bonds (energy, insulation, protection). Phospholipids = 1 glycerol + 2 fatty acids + 1 phosphate group (amphipathic membrane bilayers).
---4. Proteins
Proteins are polymers of amino acids. There are 20 naturally occurring amino acids, each with a shared core structure.
Amino Acid Structure
Every amino acid has a central alpha-carbon (\(\text{C}_\alpha\)) attached to four groups:
1. An amino group (\(-\text{NH}_2\))
2. A carboxyl group (\(-\text{COOH}\))
3. A hydrogen atom (\(-\text{H}\))
4. A variable side chain or R-group (\(-\text{R}\))
The Peptide Bond
When two amino acids combine, a condensation reaction occurs between the amino group (\(-\text{NH}_2\)) of one amino acid and the carboxyl group (\(-\text{COOH}\)) of another. This forms a covalent peptide bond (\(-\text{CO}-\text{NH}-\)) and releases a molecule of \(\text{H}_2\text{O}\).
Levels of Protein Structure
Proteins achieve their specific 3D shape across four structural levels:
• Primary Structure (\(1^\circ\)): The specific sequence and number of amino acids in a polypeptide chain, determined by genetic code and held entirely by covalent peptide bonds.
• Secondary Structure (\(2^\circ\)): Localised folding of the polypeptide chain into an \(\alpha\)-helix or a \(\beta\)-pleated sheet. This level is held and stabilised exclusively by hydrogen bonds between the \(-\text{C}=\text{O}\) and \(-\text{N}-\text{H}\) groups of the peptide backbone.
• Tertiary Structure (\(3^\circ\)): The overall three-dimensional folding of the entire polypeptide chain into a complex, functional conformation. This shape is maintained by four distinct bond types between the R-groups:
1. Disulfide bridges: Strong covalent bonds between sulfur-containing cysteine R-groups.
2. Ionic bonds: Electrostatic attractions between oppositely charged R-groups (easily broken by changes in pH).
3. Hydrogen bonds: Numerous weak bonds between polar R-groups (broken by high temperatures).
4. Hydrophobic interactions: Non-polar R-groups clumping together in the interior of the protein away from water.
• Quaternary Structure (\(4^\circ\)): The association of two or more polypeptide chains (and sometimes non-protein prosthetic groups) into a complete, functional protein complex. Examples include haemoglobin (4 polypeptide chains: \(2\alpha + 2\beta\) plus 4 haem groups) and collagen (3 polypeptide chains wound into a triple helix).
Globular vs Fibrous Proteins
• Globular Proteins: Compact, spherical, and soluble in water (hydrophilic R-groups point outward). They perform metabolic roles—e.g., enzymes, haemoglobin, antibodies, and protein hormones.
• Fibrous Proteins: Long, repetitive, ribbon-like, and insoluble in water. They perform structural roles—e.g., collagen (in tendons and bones), keratin, and elastin.
⚠️ Common Misconception: Protein denaturation disrupts secondary, tertiary, and quaternary bonds (hydrogen, ionic, disulfide), destroying the functional 3D shape. Denaturation does NOT break the primary peptide bonds!
Quick Review: Key Takeaway
Primary = sequence of amino acids (peptide bonds). Secondary = \(\alpha\)-helices and \(\beta\)-sheets (backbone hydrogen bonds). Tertiary = 3D shape (R-group bonds). Quaternary = multiple polypeptide chains. Globular = soluble/metabolic; Fibrous = insoluble/structural.
---5. Nucleic Acids (\(\text{DNA}\) and \(\text{RNA}\))
Nucleic acids are information-carrying polymers made of nucleotide monomers.
Nucleotide Structure
Each nucleotide contains three components:
1. A pentose sugar (deoxyribose in \(\text{DNA}\), ribose in \(\text{RNA}\))
2. A nitrogenous base attached to Carbon-1' of the sugar
3. A phosphate group attached to Carbon-5' of the sugar
Nitrogenous Bases
• Purines (double-ring structure): Adenine (\(\text{A}\)) and Guanine (\(\text{G}\)).
• Pyrimidines (single-ring structure): Cytosine (\(\text{C}\)), Thymine (\(\text{T}\) - \(\text{DNA}\) only), and Uracil (\(\text{U}\) - \(\text{RNA}\) only).
Structure of \(\text{DNA}\)
• Consists of two polynucleotide strands arranged in an antiparallel orientation (one strand runs \(5' \rightarrow 3'\), the other runs \(3' \rightarrow 5'\)), wound into a double helix.
• The sugar-phosphate backbone is formed by strong phosphodiester bonds (condensation between the 3'-OH of one nucleotide and the 5'-phosphate of the next).
• Bases from opposite strands pair via complementary base pairing held by hydrogen bonds:
• \(\text{Adenine} = \text{Thymine}\) (\(\text{A}=\text{T}\)) linked by 2 hydrogen bonds.
• \(\text{Cytosine} \equiv \text{Guanine}\) (\(\text{C}\equiv\text{G}\)) linked by 3 hydrogen bonds.
Structure of \(\text{RNA}\)
• \(\text{RNA}\) is a single-stranded polynucleotide chain.
• Contains the pentose sugar ribose instead of deoxyribose.
• Contains the base Uracil (\(\text{U}\)) instead of Thymine (\(\text{T}\)), which pairs complementarily with Adenine (\(\text{A}\)).
Quick Review: Key Takeaway
\(\text{DNA}\) is a double-stranded antiparallel double helix with deoxyribose, phosphodiester backbones, and base pairs \(\text{A}=\text{T}\) (2 H-bonds) and \(\text{C}\equiv\text{G}\) (3 H-bonds). \(\text{RNA}\) is single-stranded, contains ribose, and uses \(\text{U}\) instead of \(\text{T}\).
---6. Practical Biochemical Food Tests
You must know the exact procedures, conditions, and colour changes for qualitative biochemical tests examined in AS 1 and AS 3.
1. Test for Starch
• Reagent: Iodine solution (iodine dissolved in potassium iodide, \(\text{I}_2\text{ in KI}\)).
• Procedure: Add a few drops of iodine solution to the sample at room temperature.
• Positive Result: Colour changes from yellow-brown (orange-brown) to blue-black.
2. Test for Reducing Sugars (e.g., Glucose, Maltose, Lactose)
• Reagent: Benedict's reagent (alkaline copper(II) sulfate).
• Procedure: Add an equal volume of Benedict's reagent to the sample and heat in a water bath above \(80\,^\circ\text{C}\) (or boiling) for 5 minutes.
• Positive Result: Colour progression from clear blue \(\rightarrow\) green \(\rightarrow\) yellow \(\rightarrow\) orange \(\rightarrow\) brick-red precipitate.
3. Test for Non-Reducing Sugars (e.g., Sucrose)
Sucrose gives a negative (blue) result with an initial Benedict's test. To test for non-reducing sugars:
• Step 1: Confirm a negative result by heating fresh sample with Benedict's reagent.
• Step 2: Add dilute hydrochloric acid (\(\text{HCl}\)) to a new sample and boil in a water bath (this hydrolyses the glycosidic bonds, splitting sucrose into reducing monosaccharides).
• Step 3: Slowly add sodium hydrogen carbonate (\(\text{NaHCO}_3\)) until the solution is neutralised (Benedict's reagent cannot work in acidic conditions!).
• Step 4: Add Benedict's reagent and heat above \(80\,^\circ\text{C}\).
• Positive Result: A brick-red precipitate confirms the original presence of a non-reducing sugar.
4. Emulsion Test for Lipids
• Reagent: Absolute ethanol and cold water.
• Procedure: Add ethanol to the sample, shake thoroughly to dissolve the lipid, then decant the mixture into a test tube containing cold water.
• Positive Result: Formation of a cloudy, milky-white emulsion.
5. Biuret Test for Proteins
• Reagent: Biuret reagent (dilute \(\text{NaOH}\) and dilute \(\text{CuSO}_4\)).
• Procedure: Add Biuret reagent to the liquid sample at room temperature and gently agitate.
• Positive Result: Colour changes from light blue to purple / lilac.
Summary of Common Exam Pitfalls
• Forgetting Heat: The Benedict's test requires heating (\(>80\,^\circ\text{C}\)); all other food tests occur at room temperature.
• Skipping Neutralisation: When testing for non-reducing sugars, you must neutralise the acid with \(\text{NaHCO}_3\) before adding Benedict's reagent.
• Calling Lipids Polymers: Lipids are macromolecules, not polymers.
• Imprecise Bond Names: Always specify the exact glycosidic bond type (\(\alpha\text{-1,4}\), \(\alpha\text{-1,6}\), or \(\beta\text{-1,4}\)).
• Hydrogen Bond Counts: Remember that \(\text{A}=\text{T}\) has two hydrogen bonds, while \(\text{C}\equiv\text{G}\) has three.