2.2: Biological molecule

What type of bonding occurs between water molecules?

Hydrogen bonding.

Why is water a good solvent?

Its polarity allows it to surround and dissolve ions and other polar molecules.

How does water act as a transport medium?

Many ions and polar molecules dissolve in water, allowing substances to be transported in organisms.

How does water act as a coolant?

It has a high specific heat capacity, requiring large amounts of energy to raise its temperature; evaporation also removes heat.

Why is water important as a habitat?

Its properties provide a stable environment for aquatic organisms, including temperature stability and a medium for dissolving substances.

What is a monomer?

A small molecule that can join with other similar molecules to form a polymer.

What is a polymer?

A large molecule formed from many repeating monomers joined by covalent bonds.

What is a condensation reaction?

A reaction that forms a covalent bond between molecules with the removal of a molecule of water.

What is hydrolysis?

A reaction in which a covalent bond is broken by the addition of water.

Which elements are found in carbohydrates, lipids, proteins and nucleic acids?

Carbohydrates: C,H,O; Lipids: C,H,O; Proteins: C,H,O,N,S; Nucleic acids: C,H,O,N,P.

What is a monosaccharide?

A single sugar molecule that cannot be hydrolysed into a simpler carbohydrate.

What is the difference between a hexose and a pentose?

A hexose has six carbon atoms; a pentose has five carbon atoms.

What is the difference between α-glucose and β-glucose?

They differ in the orientation of the OH group attached to carbon 1.

What are the structures of glucose and ribose classified as?

Glucose is a hexose monosaccharide; ribose is a pentose monosaccharide.

How are disaccharides formed and broken down?

Two monosaccharides join by condensation, forming a glycosidic bond; hydrolysis breaks the bond using water.

What are sucrose, maltose and lactose made from?

Sucrose = glucose + fructose; maltose = glucose + glucose; lactose = glucose + galactose.

How are polysaccharides formed?

Many monosaccharides join by condensation reactions, forming glycosidic bonds.

What is a glycosidic bond?

A covalent bond formed between monosaccharides during a condensation reaction.

What is the structure and function of amylose?

Amylose is an unbranched α-glucose polymer with α-1,4 glycosidic bonds; its coiled structure makes it compact for energy storage.

What is the structure and function of amylopectin?

Amylopectin is a branched α-glucose polymer with α-1,4 and α-1,6 glycosidic bonds; branching allows glucose to be released rapidly.

What is the structure and function of glycogen?

Glycogen is a highly branched α-glucose polymer with α-1,4 and α-1,6 glycosidic bonds; extensive branching allows rapid glucose release for respiration.

What is the structure and function of cellulose?

Cellulose consists of β-glucose chains joined by β-1,4 glycosidic bonds; chains form strong microfibrils through hydrogen bonding, providing plant cell-wall strength.

Why is starch suitable for energy storage?

It is compact, insoluble and does not significantly affect water potential; its branching allows glucose to be released when needed.

Why is glycogen more suitable than starch for energy storage in animals?

Glycogen is more highly branched, allowing glucose to be released more rapidly to meet animals' high energy demands.

What is a triglyceride made from?

One glycerol molecule and three fatty acids joined by three ester bonds.

What is a phospholipid made from?

A glycerol molecule, two fatty acids and a phosphate-containing group.

What is the difference between saturated and unsaturated fatty acids?

Saturated fatty acids have no C=C double bonds; unsaturated fatty acids have one or more C=C double bonds.

How are triglycerides formed and broken down?

Three fatty acids react with glycerol by condensation/esterification to form three ester bonds; hydrolysis breaks the ester bonds using water.

Why are triglycerides good energy stores?

They contain many C-H bonds, releasing much energy when oxidised, and are insoluble so they do not affect cell water potential.

Why are phospholipids important in cells?

They form the phospholipid bilayer of biological membranes because they have hydrophilic and hydrophobic regions.

What are hydrophilic and hydrophobic regions?

Hydrophilic regions are attracted to water; hydrophobic regions repel water and are attracted to non-polar substances.

What is cholesterol and what are its roles?

Cholesterol is a lipid that helps regulate membrane fluidity and reduces membrane permeability; it is also a precursor for steroid hormones.

What is the general structure of an amino acid?

An amino acid has a central carbon bonded to an amino group, carboxyl group, hydrogen atom and variable R group.

How are dipeptides and polypeptides formed?

Amino acids join by condensation reactions forming peptide bonds; many amino acids form polypeptides.

What is a peptide bond?

A covalent bond formed between the amino group of one amino acid and the carboxyl group of another during condensation.

How are peptide bonds broken?

By hydrolysis, with water added across the peptide bond.

What is the primary structure of a protein?

The specific sequence of amino acids in a polypeptide chain, determined by the genetic code.

What is the secondary structure of a protein?

Local folding of a polypeptide into α-helices or β-pleated sheets, stabilised mainly by hydrogen bonds.

What is the tertiary structure of a protein?

The overall three-dimensional shape of one polypeptide caused by interactions between R groups, including hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions.

What is the quaternary structure of a protein?

The arrangement and interaction of two or more polypeptide chains, or subunits, in a functional protein.

What is a globular protein?

A compact, roughly spherical protein with a hydrophobic interior and hydrophilic exterior, generally soluble and suited to metabolic functions.

What is a conjugated protein?

A protein containing a non-protein component called a prosthetic group; haemoglobin is an example.

What are haemoglobin, insulin and an enzyme examples of?

Haemoglobin is a conjugated globular protein for oxygen transport; insulin is a globular protein involved in blood-glucose regulation; enzymes are globular proteins that act as biological catalysts.

What are fibrous proteins and what are their functions?

They are long, insoluble proteins adapted for strength and support; examples include collagen, keratin and elastin.

Which key inorganic ions must you know for OCR A?

Cations: Ca²⁺, Na⁺, K⁺, H⁺, NH₄⁺; Anions: NO₃⁻, HCO₃⁻, Cl⁻, PO₄³⁻ and OH⁻.

What are important biological roles of Ca²⁺, Na⁺, K⁺, H⁺ and NH₄⁺?

Ca²⁺: bones, muscle contraction and cell signalling; Na⁺/K⁺: nerve impulses and membrane potentials; H⁺: pH and proton gradients; NH₄⁺: nitrogen source for amino-acid synthesis in plants.

What are important biological roles of NO₃⁻, HCO₃⁻, Cl⁻, PO₄³⁻ and OH⁻?

NO₃⁻: nitrogen source for plants; HCO₃⁻: blood pH buffering; Cl⁻: nerve impulses and osmotic balance; PO₄³⁻: ATP, nucleic acids and phospholipids; OH⁻: affects pH.

How is the Biuret test used to test for protein?

Add Biuret reagent to the sample; a lilac/purple colour indicates peptide bonds and therefore protein. A blue result is negative.

How is Benedict's test used to test for reducing sugars?

Add Benedict's reagent and heat in a water bath; blue changes through green/yellow/orange to brick-red precipitate as reducing sugar concentration increases.

How can a non-reducing sugar be tested for?

First hydrolyse the sample with dilute hydrochloric acid and heat, then neutralise it and perform Benedict's test; a positive result indicates a non-reducing sugar was present.

How are reagent test strips used to detect reducing sugars?

Apply the sample to the strip and compare the resulting colour with the manufacturer's calibration scale; the colour indicates the approximate concentration.

How is the iodine test used to test for starch?

Add iodine solution; a blue-black colour indicates starch, while iodine remains yellow-brown if starch is absent.

How is the emulsion test used to test for lipids?

Add ethanol to the sample and shake, then add water; a white/milky emulsion indicates lipid is present.

What is colorimetry used for in biology?

It determines the concentration of a coloured substance by measuring absorbance; a calibration curve made from known concentrations can be used to determine an unknown concentration.

What is a biosensor and how is it used?

A device containing a biological component that detects a specific substance and produces a measurable signal related to its concentration.

What are the principles of paper/TLC chromatography and how is Rf calculated?

A mixture separates because substances have different solubilities and affinities for the stationary and mobile phases. Rf = distance travelled by solute ÷ distance travelled by solvent front.

How can chromatography be used to analyse a biological solution?

Apply the sample to the stationary phase, allow the solvent to move through it, identify separated spots and compare their positions/Rf values with known substances; paper or TLC can separate proteins, carbohydrates, vitamins or nucleic acids.