Molecules

Q: What is the difference between a condensation reaction and a hydrolysis reaction in biology?

A condensation reaction joins two molecules by forming a chemical bond and eliminating a molecule of water ($\text{H}_2\text{O}$), whereas a hydrolysis reaction breaks a chemical bond through the addition of a molecule of water.

Q: What are the structural differences and functional benefits of amylose and amylopectin in starch?

Amylose is an unbranched, coiled helix with $\alpha$-$1,4$-glycosidic bonds (compact for storage), whereas amylopectin is branched with both $\alpha$-$1,4$ and $\alpha$-$1,6$-glycosidic bonds (allowing rapid enzymatic hydrolysis to release glucose).

Q: What is the difference in chemical structure between saturated and unsaturated fatty acids?

Saturated fatty acids have no $\text{C}=\text{C}$ double bonds in their hydrocarbon chains, whereas unsaturated fatty acids contain one or more $\text{C}=\text{C}$ double bonds that cause kinks in the chain.

Q: How does the amphipathic structure of a phospholipid allow it to form biological cell membranes?

A phospholipid contains a polar, hydrophilic phosphate head and two non-polar, hydrophobic fatty acid tails , which spontaneously arrange into a double layer (bilayer) in aqueous environments.

Q: What are the four levels of protein structure and the primary bonds that stabilise each level?

Primary ($1^\circ$): amino acid sequence (peptide bonds); Secondary ($2^\circ$): $\alpha$-helices/$\beta$-sheets (backbone hydrogen bonds); Tertiary ($3^\circ$): 3D folding (R-group hydrogen, ionic, disulfide bonds, hydrophobic interactions); Quaternary ($4^\circ$): multiple polypeptide chains joined together.

Q: What structural and functional differences separate globular proteins from fibrous proteins?

Globular proteins are compact, spherical, and water-soluble with metabolic roles (e.g., enzymes, haemoglobin), whereas fibrous proteins form long, insoluble, parallel chains with structural roles (e.g., collagen, keratin).

Biology 1010 · CCEA AS Level · 13 min read · Updated: 31 Aug 2026

Welcome to Biological Molecules!

Welcome to the building blocks of life! Everything in biology—from the tiniest bacterium to the largest blue whale—is constructed from chemical elements bonded together into biological molecules. In this chapter of AS 1: Molecules and Cells, we will explore the major groups of molecules: carbohydrates, lipids, proteins, nucleic acids, water, and key inorganic ions, along with the essential practical biochemical tests used to identify them.

Don't worry if biochemistry feels a bit daunting at first! We will break every concept down into clear, manageable steps with simple memory tricks and everyday analogies.

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1. Building Blocks & Chemical Reactions: The Basics

Before diving into individual molecules, it helps to understand two core concepts:

• Monomers: Small, individual repeating units (like single Lego bricks). Examples include monosaccharides, amino acids, and nucleotides.
• Polymers: Large, complex molecules made by joining many monomers together in a repeating chain (like a long Lego tower). Examples include polysaccharides, polypeptides (proteins), and nucleic acids.

How Bonds Form and Break

Biological polymers are assembled and broken down by two fundamental types of reactions:

1. Condensation Reaction: A chemical reaction in which two molecules join together with the elimination (release) of a molecule of water ($\text{H}_2\text{O}$). A new chemical bond is formed.
2. Hydrolysis Reaction: A chemical reaction where a chemical bond is broken by the addition of a molecule of water ($\text{H}_2\text{O}$). (Memory trick: "Hydro" = water, "lysis" = splitting).

Quick Review: Condensation vs. Hydrolysis

• Condensation: Monomer + Monomer $\to$ Polymer + $\text{H}_2\text{O}$ (Bond formed)
• Hydrolysis: Polymer + $\text{H}_2\text{O}$ $\to$ Monomer + Monomer (Bond broken)

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2. Carbohydrates

Carbohydrates are organic compounds containing three elements: Carbon ($\text{C}$), Hydrogen ($\text{H}$), and Oxygen ($\text{O}$). They usually have the general formula $\text{C}_n(\text{H}_2\text{O})_n$.

A. Monosaccharides (Simple Sugars)

Monosaccharides are the simplest carbohydrate units. They are sweet, soluble in water, and classified by how many carbon atoms they contain:

• Triose ($3\text{C}$): e.g., Glyceraldehyde (crucial in respiration and photosynthesis pathways).
• Pentose ($5\text{C}$): e.g., Ribose ($\text{C}_5\text{H}_{10}\text{O}_5$) in RNA and ATP; Deoxyribose ($\text{C}_5\text{H}_{10}\text{O}_4$) in DNA.
• Hexose ($6\text{C}$): e.g., Glucose, Fructose, Galactose ($\text{C}_6\text{H}_{12}\text{O}_6$).

$\alpha$-Glucose vs. $\beta$-Glucose: Spot the Difference!

Glucose is an isomer—it exists in different structural forms with the same molecular formula ($\text{C}_6\text{H}_{12}\text{O}_6$). The two main ring isomers you need to know are $\alpha$-glucose and $\beta$-glucose:

• On Carbon 1 ($\text{C}_1$) of $\alpha$-glucose, the hydroxyl group ($\text{-OH}$) points down (below the ring).
• On Carbon 1 ($\text{C}_1$) of $\beta$-glucose, the hydroxyl group ($\text{-OH}$) points up (above the ring).

Memory Trick: ABBA $\to$ Alpha = Below, Beta = Above!

B. Disaccharides (Double Sugars)

Disaccharides consist of two monosaccharides joined by a condensation reaction, forming a glycosidic bond and releasing a molecule of water.

• $\alpha$-glucose + $\alpha$-glucose $\to$ Maltose + $\text{H}_2\text{O}$ (joined by an $\alpha$-$1,4$-glycosidic bond)
• $\alpha$-glucose + Fructose $\to$ Sucrose + $\text{H}_2\text{O}$ (the main transport sugar in plants)
• $\alpha$-glucose + Galactose $\to$ Lactose + $\text{H}_2\text{O}$ (sugar found in milk)

C. Polysaccharides

Polysaccharides are polymers formed by many monosaccharide units joined by glycosidic bonds. Because they are large and insoluble, they do not affect the water potential of cells, making them ideal storage and structural molecules.

1. Starch (Plant Energy Storage)

Starch is a mixture of two polysaccharides made exclusively of $\alpha$-glucose:

• Amylose: Unbranched, coiled helix of $\alpha$-glucose joined only by $\alpha$-$1,4$-glycosidic bonds. Its compact spiral structure makes it great for storing large amounts of glucose in a small space.
• Amylopectin: Branched chain of $\alpha$-glucose joined by both $\alpha$-$1,4$-glycosidic bonds and $\alpha$-$1,6$-glycosidic bonds (branch points occur roughly every $20\text{–}30$ glucose units). Its branching provides multiple ends that enzymes can hydrolyse quickly to release glucose when energy is needed.

2. Glycogen ("Animal Starch")

Glycogen is the primary carbohydrate storage molecule in animals and fungi, stored in liver and muscle cells. Like amylopectin, it consists of $\alpha$-glucose with $\alpha$-$1,4$ and $\alpha$-$1,6$-glycosidic bonds. However, glycogen is much more highly branched and has shorter chains than amylopectin. This gives it a huge number of free ends for rapid hydrolysis into glucose when animals need quick energy for muscle contraction.

3. Cellulose (Plant Cell Wall Structure)

Cellulose is a structural polymer that provides high tensile strength to plant cell walls:

• Made of long, unbranched chains of $\beta$-glucose joined by $\beta$-$1,4$-glycosidic bonds.
• To form these bonds, every alternating $\beta$-glucose monomer must be rotated $180^\circ$ relative to its neighbour.
• This rotation keeps the chain straight and uncoiled.
• Multiple straight chains run parallel to each other and are linked together by thousands of cross-linking hydrogen bonds, forming strong bundles called microfibrils.
• Microfibrils bundle together into fibres, giving cell walls immense strength to withstand internal osmotic pressure (turgor).

Key Takeaway: Carbohydrates

• Storage: Starch (plants) and Glycogen (animals) $\to$ made of $\alpha$-glucose, compact, branched, insoluble.
• Structure: Cellulose (plant cell walls) $\to$ made of $\beta$-glucose, straight chains, hydrogen cross-links form rigid microfibrils.

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3. Lipids

Lipids are non-polar, hydrophobic organic molecules composed of Carbon ($\text{C}$), Hydrogen ($\text{H}$), and Oxygen ($\text{O}$) (with significantly less oxygen than carbohydrates). They are insoluble in water but soluble in organic solvents (like ethanol).

A. Triglycerides

A triglyceride consists of one molecule of glycerol linked to three fatty acid chains by three condensation reactions, forming three ester bonds.

$$\text{Glycerol} + 3\text{ Fatty Acids} \xrightarrow{\text{Condensation}} \text{Triglyceride} + 3\text{H}_2\text{O}$$

Saturated vs. Unsaturated Fatty Acids

• Saturated Fatty Acids: Contain no $\text{C}=\text{C}$ double bonds in the hydrocarbon chain. Every carbon is saturated with hydrogen atoms. The chains are straight, pack tightly together, and usually form solid fats at room temperature (e.g., animal fats, butter).
• Unsaturated Fatty Acids: Contain one or more $\text{C}=\text{C}$ double bonds.
— Monounsaturated: Exactly one double bond.
— Polyunsaturated: Two or more double bonds.
The double bond causes a kink (bend) in the chain, preventing molecules from packing closely together. As a result, they have lower melting points and remain liquid at room temperature (e.g., plant oils).

Functions of Triglycerides

• Energy Storage: Yield more than twice as much energy per gram as carbohydrates due to high proportions of $\text{C-H}$ bonds.
• Thermal Insulation: Subcutaneous fat reduces heat loss in mammals.
• Protection: Acts as a shock absorber around delicate organs (e.g., kidneys).
• Buoyancy: Low density helps aquatic organisms stay afloat.
• Metabolic Water Source: Oxidation of lipids during respiration produces significant amounts of metabolic water (vital for desert animals like camels).

B. Phospholipids

Phospholipids are modified triglycerides where one fatty acid is replaced by a phosphate group ($\text{PO}_4^{3-}$).

• Hydrophilic Head: The phosphate group is electrically charged and polar, meaning it interacts favourably with water (water-loving).
• Hydrophobic Tails: The two fatty acid hydrocarbon chains are non-polar and repel water (water-hating).

Because they are amphipathic (having both hydrophilic and hydrophobic parts), phospholipids spontaneously arrange themselves into a phospholipid bilayer in aqueous environments—forming the fundamental basis of all biological cell membranes.

Common Mistake to Avoid

Do not confuse triglycerides with phospholipids! Triglycerides have three fatty acids and cannot form membranes. Phospholipids have two fatty acids and one phosphate group.

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4. Proteins

Proteins are polymers made up of amino acid monomers. They contain Carbon ($\text{C}$), Hydrogen ($\text{H}$), Oxygen ($\text{O}$), Nitrogen ($\text{N}$), and often Sulfur ($\text{S}$).

Structure of an Amino Acid

All $20$ naturally occurring amino acids share a common central structure bonded to four distinct groups:

• A central Carbon atom ($\alpha$-carbon)
• An Amine group ($\text{-NH}_2$)
• A Carboxyl group ($\text{-COOH}$)
• A Hydrogen atom ($\text{-H}$)
• A variable $\text{R}$ group (side chain) that differs in each amino acid, giving it unique chemical properties (e.g., acidic, basic, polar, or non-polar).

The Peptide Bond

Two amino acids join together via a condensation reaction between the amine group of one amino acid and the carboxyl group of another. This forms a peptide bond ($\text{-CO-NH-}$) and releases a molecule of water ($\text{H}_2\text{O}$).

• Dipeptide: Two amino acids joined together.
• Polypeptide: Many amino acids joined in a chain.

Levels of Protein Structure

A functional protein is not just a loose string of amino acids; it folds into a precise three-dimensional shape through four organizational levels:

1. Primary ($1^\circ$) Structure: The unique sequence of amino acids in a polypeptide chain, held together purely by peptide bonds. Determined by the genetic code in DNA. A change in even one amino acid can alter the entire $3\text{D}$ shape of the protein!

2. Secondary ($2^\circ$) Structure: The local folding of the polypeptide chain caused by regular hydrogen bonds forming between the $\text{-C=O}$ and $\text{-N-H}$ groups of the polypeptide backbone. Two main patterns form:
• $\alpha$-helix: A tight, coiled spiral.
• $\beta$-pleated sheet: A flat, sheet-like folded structure.

3. Tertiary ($3^\circ$) Structure: The overall, precise $3\text{D}$ folding of the entire polypeptide chain into a complex shape. This structure is stabilized by interactions between the variable $\text{R}$ groups:
• Hydrogen bonds: Weak bonds between polar $\text{R}$ groups; easily broken by high temperatures or $\text{pH}$ shifts.
• Ionic bonds: Electrostatic attractions between oppositely charged $\text{R}$ groups; broken by changes in $\text{pH}$.
• Disulfide bridges (disulfide bonds): Strong, covalent bonds formed between sulfur-containing $\text{R}$ groups (e.g., cysteine). Resistant to moderate heat.
• Hydrophobic interactions: Non-polar $\text{R}$ groups cluster tightly together inside the core of the protein away from surrounding water.

4. Quaternary ($4^\circ$) Structure: Occurs when a functional protein consists of two or more polypeptide chains bonded together (e.g., haemoglobin, which has four polypeptide chains). May also incorporate non-protein parts called prosthetic groups (e.g., the iron-containing haem group in haemoglobin).

Fibrous vs. Globular Proteins

• Globular Proteins: Compact, spherical, water-soluble molecules with metabolic roles. Hydrophilic $\text{R}$ groups face outward, while hydrophobic groups point inward. Examples: Enzymes (e.g., catalase, amylase), Haemoglobin, Antibodies, Insulin.
• Fibrous Proteins: Long, parallel polypeptide chains forming rope-like fibres with structural roles. Insoluble in water and tough. Examples: Collagen (provides high tensile strength in tendons, skin, and bones; formed from a triple helix), Keratin (hair, nails).

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5. Water: The Medium of Life

Water ($\text{H}_2\text{O}$) makes up $65\text{–}95\%$ of the mass of most organisms. Its unique biological importance stems from its polarity.

The Dipolar Nature of Water

Oxygen has a greater electronegativity (pull on electrons) than hydrogen. This gives oxygen a partial negative charge ($\delta^-$) and hydrogen atoms a partial positive charge ($\delta^+$). As a result, water is a dipolar molecule.

Oppositely charged regions attract one another, forming many weak hydrogen bonds between neighbouring water molecules. Together, these hydrogen bonds give water remarkable properties:

• Universal Solvent: Water dissolves polar and ionic substances (e.g., glucose, salts, amino acids) by surrounding them with hydration shells, allowing biochemical reactions to occur in solution.
• High Specific Heat Capacity: It takes a large amount of energy to raise the temperature of water because many hydrogen bonds must first be broken. This buffers aquatic habitats and internal body fluids against sudden temperature fluctuations.
• High Latent Heat of Vaporisation: Evaporation of water requires large amounts of heat energy to break hydrogen bonds. This provides an effective cooling mechanism via sweating or transpiration without excessive water loss.
• Cohesion and Surface Tension: Water molecules stick together via hydrogen bonding (cohesion), allowing continuous columns of water to be pulled up xylem vessels in plants. Cohesion at the surface creates surface tension, supporting small organisms (like pond skaters).
• Maximum Density at $4^\circ\text{C}$: Ice is less dense than liquid water because water molecules form a rigid, open lattice structure. Ice floats, insulating the liquid water beneath and allowing aquatic organisms to survive winter freezes.
• Metabolite: Directly participates in chemical reactions as a reactant in hydrolysis and a product in condensation reactions.

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6. Inorganic Ions

Inorganic ions occur in solution in the cytoplasm and bodily fluids of organisms, each serving specialized biological roles:

• Magnesium ($\text{Mg}^{2+}$): Essential constituent of chlorophyll for light absorption in photosynthesis. In mammals, it is needed for healthy bone development and serves as an enzyme cofactor.
• Iron ($\text{Fe}^{2+}$): Central component of the haem group in haemoglobin, binding reversibly with oxygen for transport in red blood cells.
• Phosphate ($\text{PO}_4^{3-}$): Key component of nucleotides (DNA and RNA), ATP (energy currency), and phospholipids in cell membranes.
• Calcium ($\text{Ca}^{2+}$): Structural component of bones and teeth in animals, calcium pectate in plant middle lamellae (cementing cell walls together), and essential for muscle contraction and blood clotting.

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7. Practical Skills: Biochemical Food Tests

Mastering the standard biochemical tests is an essential requirement for AS Biology practical examinations.

A. Test for Reducing Sugars (Benedict's Test)

Reducing sugars include all monosaccharides (e.g., glucose, fructose) and some disaccharides (e.g., maltose, lactose).

Procedure:
1. Add an equal volume of blue Benedict's reagent to the liquid sample in a test tube.
2. Heat the mixture in a boiling water bath ($\ge 80^\circ\text{C}$) for $5$ minutes.
Result: If reducing sugar is present, the solution changes colour from blue $\to$ green $\to$ yellow $\to$ orange $\to$ brick-red precipitate. (The colour indicates the sugar concentration: green = low, brick-red = high).

B. Test for Non-Reducing Sugars (e.g., Sucrose)

Sucrose gives a negative result (stays blue) with the standard Benedict's test.

Procedure:
1. Perform the standard Benedict's test. If it stays blue, take a fresh sample.
2. Add dilute hydrochloric acid ($\text{HCl}$) and heat in a boiling water bath for a few minutes (this hydrolyses glycosidic bonds, splitting sucrose into glucose and fructose).
3. Allow to cool and neutralise with sodium hydrogen carbonate ($\text{NaHCO}_3$) (Benedict's reagent cannot work in acidic conditions).
4. Re-test by adding Benedict's reagent and heating for $5$ minutes.
Result: A brick-red precipitate confirms that a non-reducing sugar was originally present.

C. Test for Starch (Iodine Test)

Procedure: Add a few drops of yellow/brown iodine in potassium iodide solution to the sample.
Result: A positive result produces an immediate colour change from orange/brown to blue-black.

D. Test for Lipids (Emulsion Test)

Procedure:
1. Add absolute ethanol to the dry test sample and shake thoroughly to dissolve any lipid present.
2. Pour the ethanol solution into a test tube containing cold distilled water.
Result: A positive result is the formation of a milky-white emulsion (tiny lipid droplets suspended in water).

E. Test for Proteins (Biuret Test)

Procedure: Add Biuret reagent (or sodium hydroxide followed by dilute copper(II) sulfate) to the liquid sample at room temperature and shake gently.
Result: A colour change from blue to lilac / purple / violet confirms the presence of peptide bonds.

Semi-Quantitative Testing & Colorimetry

• Semi-Quantitative: Comparing the visual colour intensity (e.g., shades of Benedict's orange/red) against known standards.
• Quantitative (Colorimeter): To determine exact concentrations:
1. Carry out Benedict's tests on known concentrations of glucose.
2. Centrifuge or filter out the red precipitate.
3. Place samples into a colorimeter to measure light absorbance or transmission.
4. Plot a calibration curve (concentration on $x$-axis, absorbance/transmission on $y$-axis).
5. Test an unknown sample, measure its absorbance, and read its exact concentration directly off the calibration curve!

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Chapter Summary Checklist

Before moving on to Cell Structure, make sure you can confidently:
• Distinguish between $\alpha$-glucose and $\beta$-glucose using the ABBA rule.
• Describe how condensation and hydrolysis reactions form and break glycosidic, ester, and peptide bonds.
• Explain the structural differences and functional roles of starch (amylose and amylopectin), glycogen, and cellulose.
• Contrast the structure of triglycerides and phospholipids.
• Outline the four levels of protein structure ($1^\circ, 2^\circ, 3^\circ, 4^\circ$) and the bonds maintaining them.
• Differentiate between globular (e.g., haemoglobin) and fibrous (e.g., collagen) proteins.
• Recall the unique properties of water and specific roles of $\text{Mg}^{2+}$, $\text{Fe}^{2+}$, $\text{PO}_4^{3-}$, and $\text{Ca}^{2+}$.
• Describe the steps and positive results for Benedict's, Iodine, Biuret, and Emulsion tests.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the difference between a condensation reaction and a hydrolysis reaction in biology?

A condensation reaction joins two molecules by forming a chemical bond and eliminating a molecule of water ($\text{H}_2\text{O}$), whereas a hydrolysis reaction breaks a chemical bond through the addition of a molecule of water.

How can you distinguish between the structures of $\alpha$-glucose and $\beta$-glucose?

On Carbon 1 ($\text{C}_1$), the hydroxyl group ($\text{-OH}$) points down in $\alpha$-glucose and up in $\beta$-glucose.

What are the structural differences and functional benefits of amylose and amylopectin in starch?

Amylose is an unbranched, coiled helix with $\alpha$-$1,4$-glycosidic bonds (compact for storage), whereas amylopectin is branched with both $\alpha$-$1,4$ and $\alpha$-$1,6$-glycosidic bonds (allowing rapid enzymatic hydrolysis to release glucose).

How does the structure of cellulose provide high tensile strength to plant cell walls?

Cellulose consists of straight, unbranched chains of $\beta$-glucose joined by $\beta$-$1,4$-glycosidic bonds (with alternate units inverted by $180^\circ$), which cross-link via hydrogen bonds to form strong microfibrils.

What is the difference in chemical structure between saturated and unsaturated fatty acids?

Saturated fatty acids have no $\text{C}=\text{C}$ double bonds in their hydrocarbon chains, whereas unsaturated fatty acids contain one or more $\text{C}=\text{C}$ double bonds that cause kinks in the chain.

How does the amphipathic structure of a phospholipid allow it to form biological cell membranes?

A phospholipid contains a polar, hydrophilic phosphate head and two non-polar, hydrophobic fatty acid tails, which spontaneously arrange into a double layer (bilayer) in aqueous environments.

What are the four levels of protein structure and the primary bonds that stabilise each level?

Primary ($1^\circ$): amino acid sequence (peptide bonds); Secondary ($2^\circ$): $\alpha$-helices/$\beta$-sheets (backbone hydrogen bonds); Tertiary ($3^\circ$): 3D folding (R-group hydrogen, ionic, disulfide bonds, hydrophobic interactions); Quaternary ($4^\circ$): multiple polypeptide chains joined together.

What structural and functional differences separate globular proteins from fibrous proteins?

Globular proteins are compact, spherical, and water-soluble with metabolic roles (e.g., enzymes, haemoglobin), whereas fibrous proteins form long, insoluble, parallel chains with structural roles (e.g., collagen, keratin).

Why is water a dipolar molecule and how does this enable hydrogen bonding?

Oxygen is more electronegative than hydrogen, creating a partial negative charge ($\delta^-$) on oxygen and a partial positive charge ($\delta^+$) on hydrogen, which allows electrostatic attraction (hydrogen bonds) between adjacent water molecules.

What are the key biological roles of magnesium ($\text{Mg}^{2+}$), iron ($\text{Fe}^{2+}$), phosphate ($\text{PO}_4^{3-}$), and calcium ($\text{Ca}^{2+}$) ions?

$\text{Mg}^{2+}$ is part of chlorophyll; $\text{Fe}^{2+}$ is in the haem group of haemoglobin; $\text{PO}_4^{3-}$ forms nucleotides, ATP, and phospholipids; and $\text{Ca}^{2+}$ strengthens bones/middle lamellae and aids muscle contraction.

How is a test for non-reducing sugars (such as sucrose) carried out using Benedict's reagent?

Heat the sample with dilute $\text{HCl}$ to hydrolyse glycosidic bonds, neutralise with $\text{NaHCO}_3$, add Benedict's reagent, and boil; a brick-red precipitate indicates a positive result.

How are the emulsion test for lipids and the Biuret test for proteins performed?

For lipids, dissolve sample in ethanol and pour into cold water (positive: milky-white emulsion). For proteins, add Biuret reagent (positive: blue to lilac/purple).

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