biological molecules alevel

Why is water a polar molecule?

Oxygen is slightly negative (δ-), and hydrogens are slightly positive (δ+), creating a dipole.

Importance of hydrogen bonds

There are so many that collectively they give water unique properties like high heat capacity and cohesion.

High specific heat capacity

A large amount of energy is needed to change the temperature of water — because energy goes into breaking H bonds.

Why is ice less dense than water?

Hydrogen bonds hold water molecules further apart in ice → forms an open lattice structure.

Benefits of ice floating

It insulates water below, allowing aquatic organisms to survive in winter.

High latent heat of vaporisation

It takes a lot of energy to evaporate water.

Importance of high latent heat of vaporisation for organisms

Allows cooling by evaporation (e.g. sweating) without losing much water.

Why is water a good solvent?

It's polar — positive and negative parts attract ions and other polar molecules, allowing them to dissolve.

Why is water a good transport medium in plants?

It's cohesive (water molecules stick together) → forms continuous columns in xylem.

What causes cohesion in water?

Hydrogen bonding between molecules.

what does cohesion in water molecules lead to?

Surface tension — water forms a "skin" at the surface which allows insects like pondskaters to live on it

Metabolic reactions involving water

Hydrolysis reactions (used up), Condensation reactions (produced), Photosynthesis (used), Respiration (produced).

What are monosaccharides?

Single sugar molecules — simplest carbohydrates (e.g. glucose, fructose, galactose).

Elements in all carbohydrates

Carbon, hydrogen, and oxygen (C, H, O).

Why are monosaccharides soluble in water?

Contain many hydroxyl (-OH) groups that form hydrogen bonds with water — they're hydrophilic.

Difference between α-glucose and β-glucose

The position of the hydroxyl (-OH) group on carbon 1.

alpha=below the ring

beta=above the ring

What forms a disaccharide?

Two monosaccharides joined by a glycosidic bond during a condensation reaction (releasing H₂O).

Maltose

α-glucose + α-glucose

Sucrose

α-glucose + fructose

Lactose

α-glucose + galactose

1,4 Glycosidic Bond

Forms between carbon 1 of one glucose and carbon 4 of another.

Starch

Made up of amylose and amylopectin; insoluble, preventing water entry by osmosis.

Amylose

α-glucose molecules linked by 1,4 glycosidic bonds, forming a compact helical structure held by hydrogen bonds.

Amylopectin

α-glucose with 1,4 and 1,6 glycosidic bonds, resulting in a branched structure.

Glycogen

α-glucose with 1,4 and 1,6 glycosidic bonds; similar to amylopectin but more branched and compact (because metabolic demand is higher in animals).

Glycogen Storage

Stored in liver and muscle cells.

Cellulose

Made of β-glucose monomers joined by 1,4 glycosidic bonds.

Cellulose Arrangement

Every other β-glucose molecule flips 180°, forming straight, unbranched chains.

Hydrogen Bonds in Cellulose

Form between cellulose chains, creating strong microfibrils, macrofibrils, and cellulose fibres.

Amino Acids Elements

Carbon, hydrogen, oxygen, nitrogen (and sometimes sulfur).

Peptide Bond Formation

Occurs between the carboxyl group of one amino acid and the amino group of another via a condensation reaction, releasing water.

Primary Structure of Protein

The sequence of amino acids in a polypeptide chain, determined by the gene (DNA sequence).

Shapes of secondary structure

α-helix and β-pleated sheet.

Tertiary structure

The overall 3D shape of a single polypeptide formed by further folding due to interactions between R groups.

Quaternary structure

The structure formed when two or more polypeptide chains (subunits) combine, sometimes with a prosthetic group.

Example of quaternary structure

Haemoglobin (4 subunits + haem group).

Bonds in tertiary and quaternary protein structure

Hydrogen bonds, ionic bonds, disulfide bonds, hydrophilic/hydrophobic interactions.

Ionic bonds

Bonds between oppositely charged R groups.

Disulfide bonds

Strong covalent bonds between two sulfur atoms from cysteine.

Hydrophilic/hydrophobic interactions

Interactions between polar and nonpolar R groups.

Bonds broken by heat or pH change

Hydrogen and ionic bonds.

Conjugated protein

A protein with a prosthetic group (non-protein) attached to it.

Globular proteins

Compact, roughly spherical proteins with hydrophilic R groups on the outside → soluble in water (enzymes are globular proteins)

how do globular proteins act?

Hydrophilic R groups face outward and interact with water; hydrophobic R groups cluster inside.

Haemoglobin structure

4 polypeptide chains (2 α, 2 β) + 4 haem prosthetic groups (each with Fe²⁺).

Haemoglobin as a conjugated protein

It contains a non-protein prosthetic group (haem).

what does haemoglobin do?

Binds reversibly to oxygen in lungs → releases O₂ in tissues.

number of O₂ molecules carried by haemoglobin

4 (one per haem group).

Insulin structure

Two polypeptide chains joined by disulfide bonds.

what does insulin do?

Hormone that regulates blood glucose levels; binds to specific protein receptors on cell membranes.

what does lysozyme do?

Catalyse the breakdown of bacterial cell walls- found in tears and saliva.

Fibrous proteins

Long, rope-like, insoluble proteins with high tensile strength.

Function of fibrous proteins

Structural — provide strength, elasticity, or support.

Collagen location

Found in tendons, ligaments, skin, and connective tissue.

Collagen structure

Three polypeptide chains wound into a triple helix with hydrogen bonds and crosslinks between chains.

Glycine in collagen

Every third amino acid in collagen tends to be glycine due to its small R group (H) allowing tight winding of the triple helix.

Keratin

Found in hair, skin, nails.

Strength of Keratin

Contains many disulfide bonds between cysteine amino acids.

Is keratin soluble?

No- it has a lot of hydrophobic amino acids.

Elastin

Found in skin, lungs, and walls of blood vessels (e.g. arteries).

Property of Elastin

Elasticity - can stretch and recoil.

Cause of Elasticity in Elastin

Crosslinks between long, hydrophobic elastin molecules that can move apart and reassociate.

Lipids

A group of nonpolar, hydrophobic biological molecules including fats, oils, phospholipids, and steroids.

Main Elements in Lipids

Carbon, hydrogen, and oxygen (less oxygen than in carbohydrates).

Key Biological Roles of Lipids

•Energy storage

•Insulation and protection (around organs, under skin)

•Membrane structure (phospholipids in bilayers).

Triglycerides

Made of 1 glycerol molecule + 3 fatty acids.

Reaction Forming Triglycerides

Condensation reaction between hydroxyl (-OH) groups on glycerol and carboxyl (-COOH) groups on fatty acids, forming ester bonds and releasing 3 H₂O molecules.

Unsaturated Fatty Acid

Contains a C=C double bond, causing kinks in the chain

Polyunsaturated Fatty Acid

Has multiple double bonds in the hydrocarbon chain.

Energy Storage of Triglycerides

•Contain many C-H bonds → release lots of energy when oxidised;

•Insoluble → do not affect osmotic balance

•Compact → store large energy

Waterproofing of Triglycerides

Nonpolar and hydrophobic — e.g. oils on feathers or leaves.

Phospholipids

Made of 1 glycerol + 2 fatty acids + 1 phosphate group.

Hydrophilic Part of Phospholipid

The phosphate head (polar, negatively charged).

Hydrophobic Part of Phospholipid

The fatty acid tails (nonpolar).

Behavior of Phospholipids in Water

They form a bilayer — heads face water, tails face inward away from water.

Cholesterol

A sterol lipid — small molecule with a hydrophilic OH group and a hydrophobic ring structure.

how does Cholesterol interact in Membranes

The hydrophilic OH group interacts with the phospholipid heads, and the hydrophobic rings interact with the tails.

Role of Cholesterol in Cell Membranes

Regulates fluidity and stability — prevents membranes from becoming too fluid or too rigid.

Substances Made from Cholesterol

hormones-oestrogen, testosterone, vitamin D, and bile (making them lipid-soluble)

Function of Bile

Emulsifies lipids → increases surface area for lipase action during digestion.

how do you prepare a food sample for tests?

Grind up the food with a mortar and pestle with some distilled water then filter the mixture with filter paper to remove the food particles

For which test do you not filter the solution?

Lipids test because lipids can stick to filter paper

What is the test for starch?

Place 1cm cubed of a solution containing iodine and potassium iodide

What is the test for proteins?

Biurets test: dilute sodium hydroxide solution and dilute copper sulphate solution.

What is the positive result of starch?

Blue-black

What is the positive result of proteins?

Lilac

What exactly does biurets solution detect?

Peptide bonds- if it were only amino acids the result would be negative (blue)

What is the test for lipids?

Shaking the solution with 3cm cubed of ethanol and 3cm cubed of water.

What is the positive result for lipids?

A cloudy white emulsion.

What is a reducing sugar?

a sugar that can donate electrons to another chemical

what type of sugars are reducing sugars?

all monosaccharides, some disaccharides

what type of sugar is sucrose?

a disaccharide, made up of alpha glucose and fructose joined by a glycosidic bond- to test for nonreducing sugars, we break up this bond and test fpr the monosaccharides individually

how to test for reducing sugars

add 3cm cubed of benedicts solution and place in a beaker of boiling water

what is the positive result for reducing sugars

(depending on the concentration) green- yellow- orange- brick red

what type of monosaccharides are glucose and ribose?

hexose and pentose

test for nonreducing sugars

•first carry out Benedict's

•separate a fresh sample into monosaccharides with dilute HCL acid(in water bath)

•neutralise with an alkaline

•carry out benedicts again