Biological Molecules – Carbohydrates, Lipids, Proteins & Water

These flashcards review key concepts on carbohydrates, lipids, proteins, and water properties for the upcoming exam, covering definitions, structures, bonding, and biological significance.

What is a macromolecule?

A very large molecule (≥1000 atoms) with high molecular mass, e.g., proteins, carbohydrates, lipids, nucleic acids.

Define a polymer.

A macromolecule made of many repeating sub-units (monomers) joined by covalent bonds.

Give one example of a macromolecule that is NOT a polymer and explain why.

Lipids; they are large but are built from glycerol and fatty acids, not a long chain of identical repeating monomers.

Name the monomer of proteins.

Amino acid.

Name the monomer of polysaccharides.

Monosaccharide.

Name the monomer of nucleic acids.

Nucleotide.

State the general formula of a monosaccharide.

CnH2nOn (or (CH2O)n).

How many sugar units does an oligosaccharide contain?

3–10 sugar units.

Formula of a disaccharide derived from two hexoses.

C12H22O11 (general form CnH2n−2On−1).

General formula of a polysaccharide produced from glucose.

(C6H10O5)n (because each condensation removes H2O).

List three common monosaccharides.

Glucose, fructose, galactose.

Which monosaccharide exists as α- and β-anomers important in biology?

Glucose.

Compare solubility of mono-, di- and polysaccharides.

Mono: highly soluble; Di: soluble but less than mono; Poly: insoluble (no osmotic effect).

Why do monosaccharides taste sweet but starch does not?

Monosaccharides interact with sweet taste receptors; large polysaccharides cannot, so they are not sweet.

What two monosaccharides form maltose?

Two α-glucose molecules.

Type of glycosidic bond in maltose.

α(1→4) glycosidic bond.

Monomers that form lactose.

β-galactose and β-glucose.

Type of glycosidic bond in lactose.

β(1→4) glycosidic bond.

Monomers that form sucrose.

α-glucose and β-fructose.

Type of glycosidic bond in sucrose.

α(1→2) glycosidic bond (often written just 1→2).

Define a reducing sugar.

A sugar with a free aldehyde or ketone group able to reduce Benedict’s reagent.

Give one disaccharide that is non-reducing.

Sucrose.

Why is sucrose non-reducing?

Its aldehyde/ketone groups are involved in the 1→2 glycosidic bond, so no free reducing end.

Name the three main polysaccharides in plants and animals covered.

Starch, glycogen, cellulose.

Which two polysaccharides make up starch?

Amylose and amylopectin.

Bonding pattern in amylose.

α(1→4) glycosidic bonds only; unbranched helix.

Bonding pattern in amylopectin.

α(1→4) main chain with α(1→6) branch points; branched.

Why is amylose more compact than amylopectin?

It coils into a spiral helix with no branches, packing tightly.

Give two advantages of branching in amylopectin.

(1) Many terminal glucose units for rapid hydrolysis, (2) more compact storage.

State two reasons starch is an ideal storage molecule.

Insoluble (no osmotic effect) and highly compact store of chemical energy.

Why is glycogen broken down faster than starch?

It is more highly branched, providing more ends for enzyme action.

Where is glycogen stored in the human body?

Liver and muscle cells.

State the three elements present in all lipids.

Carbon, hydrogen and oxygen (with relatively little oxygen).

Why are lipids insoluble in water?

They are non-polar/hydrophobic and cannot form hydrogen bonds with water.

Define a triglyceride.

A lipid formed by condensation of one glycerol molecule with three fatty acids, producing three ester bonds.

What type of bond links fatty acids to glycerol?

Ester bond.

Difference between saturated and unsaturated fatty acids regarding C=C bonds.

Saturated: no C=C double bonds; Unsaturated: one or more C=C double bonds.

Why do unsaturated fatty acids have lower melting points?

Cis double bonds create kinks, preventing close packing, weakening hydrophobic interactions and lowering melting point.

State two features shared by saturated and unsaturated fatty acids.

Both contain C, H, O and possess a carboxyl (COOH) group.

Explain why fatty acids are poorly soluble in blood.

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

How are triglycerides transported in blood?

Attached to proteins as lipoproteins (LDL or HDL).

What replaces one fatty acid in a phospholipid?

A phosphate group, creating a hydrophilic head.

Number of ester bonds in a phospholipid.

Two ester bonds (because only two fatty acids).

Compare triglycerides and phospholipids by presence of phosphate.

Triglycerides lack phosphate; phospholipids contain a phosphate group.

Define lipoprotein.

A conjugated protein–lipid complex that transports triglycerides and cholesterol in blood.

Role difference between LDL and HDL.

LDL delivers cholesterol to tissues/arteries; HDL removes excess cholesterol back to liver.

General formula parts of an amino acid.

Central carbon attached to an amino group (NH2), carboxyl group (COOH), hydrogen, and variable R group.

Primary protein structure definition.

Linear sequence of amino acids in a polypeptide, linked by peptide bonds.

Type of bond characteristic of the primary structure.

Peptide bond (covalent, strong).

Secondary structure definition.

Regular folding (α-helix or β-pleated sheet) stabilized by hydrogen bonds between C=O and N–H groups of backbone.

Tertiary structure definition.

Overall 3-D folding of a single polypeptide due to interactions between R groups (H-bonds, ionic, disulfide, hydrophobic).

Quaternary structure definition.

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

List four types of R-group interactions in tertiary/quaternary structure.

Hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions.

Which bond is strongest in proteins?

Peptide bond.

Which interaction is weakest in proteins?

Hydrophobic interaction.

Define a globular protein.

Compact, spherical protein with hydrophilic R groups facing outwards; soluble and metabolically active.

Define a fibrous protein.

Long, parallel polypeptide chains forming insoluble structural proteins with limited tertiary structure.

Give one example of a globular protein.

Enzyme (e.g., amylase) or haemoglobin.

Give one example of a fibrous protein.

Collagen.

Why are globular proteins water-soluble?

Hydrophilic R groups on surface form hydrogen bonds with water.

Describe the basic structure of collagen.

Three helical polypeptides rich in glycine form a triple helix, molecules cross-linked into fibrils and fibres for tensile strength.

Why is glycine at every third residue in collagen?

Its small size allows tight packing of the three helices.

Describe the quaternary structure of haemoglobin.

Four polypeptides (2α, 2β) each with a haem prosthetic group containing Fe²⁺ that binds O2.

Maximum number of O₂ molecules one haemoglobin can carry.

Four.

What makes haemoglobin a conjugated protein?

Presence of non-protein haem groups (prosthetic groups).

Define a glycoprotein and give one role.

Protein covalently linked to carbohydrate; functions as receptor, antigen, or transport protein.

State two properties of water arising from hydrogen bonding important to life.

High specific heat capacity and high latent heat of vaporisation.

Explain why water is an excellent solvent for ions.

Dipolar water molecules surround ions; Oδ− attracts cations, Hδ+ attracts anions, separating and dissolving them.

How does high specific heat capacity benefit organisms?

Buffers temperature changes, maintaining stable internal and aquatic environments.

Why does ice float on water?

Water becomes less dense below 4 °C as hydrogen bonds arrange molecules into an open lattice.

Give one biological advantage of ice floating.

Insulates water beneath, preventing aquatic life from freezing.

Define cohesion in water and state its effect.

Attraction between water molecules via hydrogen bonds creates high surface tension.

State the heat required to raise 1 kg of water by 1 °C.

4,200 J (high specific heat capacity).

Why is evaporation of sweat effective at cooling?

High latent heat of vaporisation means large heat energy is removed when water molecules escape.

Explain how primary structure determines tertiary structure.

Sequence dictates R-group positions, which decide which bonds form, leading to specific folding.

Which level(s) of protein structure involve disulfide bonds?

Tertiary and quaternary structures.

What type of bond joins two cysteine residues?

Disulfide (covalent) bond.

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

They lack extensive tertiary interactions that are easily disrupted; stability relies on strong cross-linking.

List two reasons triglycerides provide more energy per gram than carbohydrates.

Higher proportion of C–H bonds and lower mass of water associated (they are hydrophobic).

Name the bond formed in a condensation reaction between two amino acids.

Peptide bond.

What is the melting point trend with increasing C=C bonds in fatty acids?

More double bonds ⇒ lower melting point.

Explain the term ‘hydrophobic interaction’ in proteins.

Non-polar R groups cluster away from water, stabilising the protein’s interior.

State two reasons starch does not affect cell water potential.

It is insoluble and has no significant osmotic effect.

Why can amylopectin be hydrolysed faster than amylose?

Branch points offer multiple sites for enzyme attachment, speeding hydrolysis.

Give two structural similarities between triglycerides and phospholipids.

Both contain glycerol backbone and ester bonds linking fatty acids.