Chemistry for Biologists – Biological Molecules (Carbohydrates, Lipids & Proteins)

A comprehensive set of question-and-answer flashcards covering key definitions, structures, properties and comparisons of carbohydrates, lipids and proteins as presented in the lecture notes.

What is a macromolecule?

A very large molecule (≥1 000 atoms) with a high molecular mass, e.g. proteins, polysaccharides, lipids, nucleic acids.

Define a polymer and give two biological examples.

A molecule made of many repeating sub-units (monomers) joined together; examples: proteins (amino-acid monomers) and polysaccharides (monosaccharide monomers).

Name the four main biological macromolecules.

Carbohydrates, lipids (fats/oils), proteins, nucleic acids.

Which biological macromolecule is NOT a true polymer and why?

Lipids, because they are built from glycerol and fatty acids rather than an endlessly repeating monomer sub-unit.

State the general formula for a monosaccharide.

CₙH₂ₙOₙ or (CH₂O)ₙ.

Give three common hexose monosaccharides.

Glucose, fructose, galactose.

What is the empirical formula of glucose?

C₆H₁₂O₆.

Compare the number of sugar units in mono-, di- and polysaccharides.

Mono: 1 unit; Di: 2 units; Poly: ≥11 units (usually thousands).

What is the formula pattern for a disaccharide?

CₙH₂ₙ₋₂Oₙ₋₁ (loss of one H₂O during condensation).

Which bond joins two monosaccharides?

A glycosidic bond.

Which two monosaccharides form maltose and what bond links them?

Two α-glucose molecules linked by a 1→4 α-glycosidic bond.

Identify the monomers of sucrose and the type of glycosidic bond.

α-glucose + β-fructose joined by a 1→2 glycosidic bond.

Identify the monomers of lactose and the glycosidic linkage.

β-galactose + β-glucose joined by a 1→4 β-glycosidic bond.

Why is sucrose a non-reducing sugar?

Both potential reducing ends are involved in the 1→2 glycosidic bond, leaving no free aldehyde/ketone group.

Give the three main polysaccharides discussed.

Starch, glycogen, cellulose.

What two polysaccharides make up starch?

Amylose and amylopectin.

Describe the structure of amylose.

Unbranched spiral chain of α-glucose linked by 1→4 glycosidic bonds.

Describe the structure of amylopectin.

Branched α-glucose polymer with 1→4 glycosidic bonds in chains and 1→6 bonds at branch points.

Why is starch a good storage molecule? (Give two reasons.)

It is compact and insoluble (no osmotic effect); branched amylopectin allows rapid hydrolysis to glucose.

How does glycogen differ from amylopectin?

Glycogen is more highly branched, enabling even faster hydrolysis; stored mainly in liver and muscles.

Define ‘reducing sugar’.

A sugar that possesses a free aldehyde or ketone group able to reduce Benedict’s reagent, giving a positive test.

Name three common reducing sugars.

Glucose, fructose, galactose (also maltose and lactose).

Which common disaccharide is non-reducing?

Sucrose.

State four properties of lipids.

Hydrophobic (water-insoluble), lower O:H ratio than carbs, less dense than water, high energy content.

What reaction forms an ester bond in lipids?

Condensation between the –COOH of a fatty acid and –OH of glycerol, producing water.

Describe the structure of a triglyceride.

One glycerol molecule esterified with three fatty acids, forming three ester bonds.

Differentiate saturated and unsaturated fatty acids (structure).

Saturated have no C=C double bonds (straight chains); unsaturated have one or more C=C bonds, causing kinks.

How do C=C bonds affect melting point of fatty acids?

More C=C bonds → weaker packing, weaker hydrophobic interactions → lower melting point (more liquid).

Why are fatty acids poorly soluble in blood?

Their long non-polar hydrocarbon tails cannot form hydrogen bonds with water, so they aggregate.

How are triglycerides transported in blood?

They attach to proteins forming water-soluble lipoproteins.

Explain why phospholipids are amphipathic.

They contain a polar hydrophilic phosphate head and two non-polar hydrophobic fatty-acid tails.

List two key differences between triglycerides and phospholipids.

Triglyceride: 3 fatty acids, no phosphate, 3 ester bonds; Phospholipid: 2 fatty acids, 1 phosphate group, 2 ester bonds.

Give the general formula of an amino acid.

H₂N–CH(R)–COOH (amine group, central carbon with R side chain, carboxyl group).

What determines an amino acid’s chemical properties?

Its R (side-chain) group.

Name the covalent bond linking amino acids.

Peptide bond (formed by condensation of –COOH and –NH₂ groups).

Define the primary structure of a protein.

The specific linear sequence of amino acids in a polypeptide chain.

What type of bonds stabilise secondary structure?

Hydrogen bonds between C=O and N–H groups of the backbone.

Name the two main secondary structures.

α-helix and β-pleated sheet.

What interactions stabilise tertiary structure? (four types)

Hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions between R groups.

Define quaternary structure.

The 3-D arrangement of two or more polypeptide subunits held by R-group interactions.

Rank protein bonds from strongest to weakest.

Peptide > disulfide > ionic > hydrogen > hydrophobic interactions.

Explain how primary structure dictates a protein’s 3-D shape.

The amino-acid order fixes the positions of specific R groups, determining which R-group bonds form, which in turn fold the chain into its unique tertiary/quaternary shape.

Describe a globular protein’s general features.

Spherical, soluble, irregular amino-acid sequence, hydrophilic R groups outside, metabolic functions (e.g. enzymes, haemoglobin).

Why are globular proteins water-soluble?

Hydrophilic R groups project outward and form hydrogen bonds with water.

Describe fibrous proteins.

Long, parallel polypeptide strands with repetitive sequences, water-insoluble, structural function, few tertiary interactions.

Give two differences between globular and fibrous proteins.

Globular: compact, soluble, metabolic; Fibrous: elongated, insoluble, structural.

Name two examples of fibrous proteins.

Collagen (tendons) and keratin (hair).

Why are fibrous proteins less sensitive to pH/temperature changes?

They lack extensive tertiary R-group bonds; their strength comes from many hydrogen (and covalent) cross-links along extended chains.

What type of bond links cysteine residues?

Disulfide bond (–S–S–).

Which R-group interaction involves non-polar side chains clustering to avoid water?

Hydrophobic interaction.

State the role of hydrogen bonds in proteins.

They stabilise secondary, tertiary, and quaternary structures via attractions between polar groups.

What is a conjugated protein?

A protein containing a non-protein (prosthetic) group, e.g. haemoglobin (with haem), lipoproteins.

Why is haemoglobin classified as a globular conjugated protein?

It has four polypeptide subunits (globular) each bound to a haem prosthetic group (conjugated).

Give two locations where glycogen is stored in humans.

Liver and muscle tissue.

Explain ‘osmotic effect’ in relation to storage polysaccharides.

Because starch/glycogen are insoluble, they do not lower water potential inside cells, preventing water influx.

What is Benedict’s test used for?

Detecting reducing sugars via colour change upon reduction of Cu²⁺ ions.

Why does sucrose give a negative Benedict’s test unless hydrolysed?

Its reducing ends are involved in the 1→2 bond; hydrolysis frees glucose/fructose which are reducing.

State two functional groups present in all amino acids.

Amine group (–NH₂) and carboxyl group (–COOH).

Which R-group property promotes ionic bonding?

Side chains that can become ionised (e.g. –NH₃⁺, –COO⁻).

Describe hydrophobic amino-acid side chains.

Non-polar, often hydrocarbons, tend to orient towards the protein interior away from water.

How many ester bonds are in a triglyceride?

Three.

What covalent bond joins glycerol to a fatty acid?

Ester bond.

Name the reaction that breaks a glycosidic bond.

Hydrolysis (addition of water).

Explain why amylopectin can be hydrolysed faster than amylose.

Its branched structure provides many enzyme attack points (1→6 branches) for simultaneous hydrolysis.

Which lipid class is a major component of cell membranes?

Phospholipids.

Describe a micelle.

Spherical aggregate of fatty acids in water with hydrophobic tails inward and hydrophilic heads outward.

What is the basic building block (monomer) of nucleic acids?

Nucleotide.

Define ‘esterification’.

Formation of an ester bond via condensation between an acid (–COOH) and an alcohol (–OH).

Why are unsaturated fats typically liquid at room temperature?

C=C bonds introduce kinks, preventing tight packing and lowering melting point.

Give one health significance of lipoproteins.

They transport hydrophobic lipids such as cholesterol and triglycerides through the aqueous bloodstream.

What element allows disulfide bridges to form?

Sulfur (in cysteine residues).

What is the role of peptide bonds in protein primary structure?

They link amino acids covalently, creating the polypeptide backbone.

Which type of protein typically shows catalytic activity?

Globular proteins (e.g. enzymes).

Which bond type is most easily disrupted by mild temperature changes?

Hydrophobic interactions.

How does water’s polarity influence lipid behaviour?

Polar water molecules cannot form hydrogen bonds with non-polar lipid tails, causing lipids to be insoluble and aggregate.

Why do phospholipids form bilayers in water?

Hydrophilic heads interact with water, while hydrophobic tails avoid it, arranging tails inward and heads outward.

What causes the spiral shape of amylose?

Hydrogen bonding within the unbranched α-1,4 linked chain forces it to coil into a helix.

Which sugar unit ratio defines carbohydrates?

Atoms in a 1 : 2 : 1 ratio of C:H:O.

Give the chemical change during condensation of two α-glucose molecules.

Removal of H from one –OH and OH from the other to form water and a glycosidic bond.

What functional groups must be free for a sugar to act as a reducing sugar?

A free aldehyde (–CHO) or ketone (C=O) group at the reducing end.

Which bond type contributes most to protein tertiary stability after peptide and disulfide bonds?

Ionic bonds between charged side chains.

How does branching in glycogen benefit animals?

Provides rapid release of glucose during high metabolic demand because many enzymes can act simultaneously.

Why is cellulose not digested by human enzymes?

It consists of β-1,4 linked glucose forming straight chains; humans lack cellulase to hydrolyse β-glycosidic bonds.

Which factor primarily determines a fatty acid’s hydrophobicity?

Length and non-polarity of its hydrocarbon tail.

What determines whether an amino acid is hydrophilic or hydrophobic?

The chemical nature of its R group (polar/charged vs non-polar).

Identify four elements found in proteins but not always in carbohydrates or lipids.

Carbon, hydrogen, oxygen (common), plus nitrogen (always in proteins) and sometimes sulfur.