3.1.1 Biological molecules

what are monomers

theyre the smaller units which may be joined together to form polymers

what is polymerisation

the reaction used to convert monomers into polymers

what is the basic monomer unit

sugar/ saccharide

what is a single monomer unit called

monosaccharide

what is a pair of monosaccharides called

disaccharides

what is multiple monosaccharides called

polysaccharides

how does the condensation reaction occur

• when 2 or more molecules form bonds to make a larger polymer

• there is a release of a water molecule between the bonds

how does the hydrolysis reaction occur

• when larger polymers break bonds to form a smaller sub-unit monomer

• a water molecule is used to seal the bond

what is an isomer

same molecular formula but different structure

whats the difference between alpha and beta glucose

in alpha glucose the OH is below hydrogen whereas in beta its above the hydrogen

what is fructose

1. common and soluble monosaccharide

2. its the main sugar in fruits and nectar

how is maltose formed

2 alpha glucose

how is sucrose formed

alpha glucose + fructose

how are disaccharides formed

1. 2 monosaccharide molecules join together with a glycosidic bond

2. a glycosidic bond forms between a hydroxyl group on one monosaccharide with another

3. in maltose, sucrose, and lactose the bond is formed between carbon 1 and carbon 4 forming 1-4 glycosidic bonds

what does the formation of glycosidic bonds produce

water (condensation reaction)

how can glycosidic bonds be broken down

through the addition of water in a hydrolysis reaction

what is amylopectin

A branched polysaccharide of α-glucose that makes up part of starch.

which bonds are found in amylopectin

α-1,4 glycosidic bonds (straight chains) and α-1,6 glycosidic bonds (branches).

Why does amylopectin’s structure suit its function?

The branches provide many ends for enzyme action → glucose is released quickly.

what is starch

The main storage polysaccharide/ carbohydrate storage in plants, made of α-glucose.

What two polysaccharides make up starch?

Amylose and Amylopectin.

What bonds are in amylose?

α-1,4 glycosidic bonds → forms a coiled, compact helix.

What bonds are in amylopectin?

α-1,4 glycosidic bonds (chains) and α-1,6 glycosidic bonds (branches).

Why is starch a good storage molecule?

• Insoluble → doesn’t affect water potential.

• Compact (amylose) → lots stored in small space.

• Branched (amylopectin) → glucose released quickly by enzyme action.

Where is starch stored in plant cells?

As starch grains in plastids (e.g. chloroplasts, amyloplasts).

what is cellulose

A structural polysaccharide in plants made of β-glucose.

What type of glycosidic bonds are in cellulose?

β-1,4 glycosidic bonds.

Why are alternate β-glucose molecules flipped 180° in cellulose?

To allow β-1,4 glycosidic bonds to form a straight chain.

What kind of chains does cellulose form?

Long, straight, unbranched chains of β-glucose.

How are cellulose chains held together?

By many hydrogen bonds forming microfibrils, which bundle into fibres.

What properties do microfibrils give cellulose?

High tensile strength and rigidity.

What is the function of cellulose in plants?

forms strong cell walls to provide support, maintains shape, and prevents cells from bursting when they take in excess water

Is the cellulose cell wall permeable?

Yes, it allows water and solutes to pass through.

What is glycogen?

The main storage polysaccharide in animals and fungi, made of α-glucose.

What type of glycosidic bonds are in glycogen?

α-1,4 glycosidic bonds (chains) and α-1,6 glycosidic bonds (branches).

How does glycogen’s structure compare to amylopectin?

Glycogen is more highly branched than amylopectin.

Why is glycogen highly branched?

Provides many ends for enzyme action → glucose can be released very rapidly when needed (e.g. during exercise).

Why is glycogen a good storage molecule?

• Insoluble → doesn’t affect water potential.

• Compact → lots of glucose stored in a small space.

• Highly branched → rapid glucose release.

Where is glycogen stored in animal cells?

As small granules in the cytoplasm, especially in liver and muscle cells.

describe the benedict’s test for reducing sugars

1. Add Benedict’s reagent (blue solution of copper(II) sulfate) to the test sample.

2. Heat the mixture in a water bath at ~95 °C.

3. A positive result = colour change from blue → green → yellow → orange → brick-red (depending on amount of reducing sugar present).

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

describe the benedict’s test for non reducing sugars

• If the Benedict’s test is negative, hydrolyse the sample by adding dilute hydrochloric acid and boiling.

• Neutralise the acid with sodium hydrogencarbonate (NaHCO₃).

• Add Benedict’s reagent again and heat.

• A positive result = same colour change (blue → brick-red).

  • Example: sucrose is a non-reducing sugar. It only gives a positive result after being hydrolysed into glucose + fructose.

what are lipids

they’re a diverse group of compounds that are insoluble in water but soluble in organic solvents

what are lipids made out of

mainly of carbon, hydrogen, and a small amount of oxygen.

What is the structure of a triglyceride?

1 glycerol molecule + 3 fatty acids joined by ester bonds (via condensation reactions).

What are the functions of triglycerides?

Long-term energy storage, insulation,

what is glycerol

• Glycerol is a small alcohol molecule.

• It has three hydroxyl groups (-OH).

what is the role of glycerol in triglycerides

Glycerol bonds with three fatty acids via ester bonds (formed in condensation reactions)

what is the role of glycerol in phospholipids

Glycerol bonds with two fatty acids and one phosphate group.

why is glycerol important

• Provides the backbone for triglycerides and phospholipids.

• Its hydroxyl groups react with fatty acids to form ester bonds.

what are saturated fatty acids

• they contain single bonds between carbon atoms, this makes the molecules straight

• Pack closely together → usually solid at room temperature (e.g., butter)

what are unsaturated fatty acids

• One or more double bonds between carbon atoms

• Double bonds create kinks in the chain → molecules cannot pack tightly.

• Usually liquid at room temperature (e.g., vegetable oils).

what are the 2 types of fatty acids

monounsaturated and polyunsaturated

what are monounsaturated fatty acids

they contain one double carbon bond in the hydrocarbon chain

what are polyunsaturated fatty acids

they contain more than one double carbon bond in the hydrocarbon chain

What is the cis configuration in fatty acids?

A type of unsaturated fatty acid where the hydrogen atoms on a C=C double bond are on the same side, causing a bend in the hydrocarbon chain.

How does the cis configuration affect the properties of lipids?

• Prevents fatty acids from packing tightly.

• Makes lipids more fluid, important for cell membrane flexibility.

How does cis compare to trans configuration?

Trans = hydrogens on opposite sides → straight chain → packs tightly → more solid.

What is the basic structure of a phospholipid?

Glycerol backbone + 2 fatty acids (hydrophobic tails) + 1 phosphate group (hydrophilic head).

What does “amphipathic” mean in phospholipids?

It has both hydrophilic (head) and hydrophobic (tails) parts.

How do phospholipids behave in water?

Heads face water (hydrophilic), tails point away (hydrophobic)

What is the main function of phospholipids in cells?

Form cell membranes, acting as a partially permeable barrier.

describe the emulsion test

• Add ethanol (organic solvent) to the sample to dissolve any lipid.

• Shake the mixture.

• Add water and shake again.

• Positive result: a white, cloudy emulsion forms.

• Negative result: solution remains clear.

what is a protein?

A polymer of amino acids linked by peptide bonds, containing C, H, O, N

What is the primary structure of a protein?

The specific sequence of amino acids in a polypeptide chain, held by peptide bonds.
Function: Determines all higher-level structures and protein function.

What is the secondary structure of a protein?

• Local folding of the polypeptide chain into α-helices or β-pleated sheets.

• Stabilised by hydrogen bonds between the C=O of one amino acid and the N-H of another.

• provides stability and contributes to the proteins shape

What is the tertiary structure of a protein?

• The 3D folding of the polypeptide into a specific shape.

• Interactions: Hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions

• Function: Determines specific shape and therefore function (e.g., active site of enzymes).

What is the quaternary structure of a protein?

• The arrangement of more than one polypeptide chain into a functional protein.(may or may not have prosthetic groups)

• May include prosthetic groups (non-protein components that are permanently attached).

• Proteins with prosthetic groups are called conjugated proteins.

  (eg heamoglobin)

what are non polar molecules

• Molecules where electrons are shared evenly, so there is no positive or negative end.

• No overall charge difference across the molecule.

• Because they have no charges, they cannot form hydrogen bonds with water.

• Hydrophobic = repelled by water → non-polar molecules do not dissolve in water.

• Soluble in organic solvents (e.g., ethanol, chloroform).

• Example: fatty acid tails in lipids, oils, and fats.

How are fibrous proteins structured?

Made of parallel polypeptide chains held together by cross-links (hydrogen bonds or disulfide bridges), giving high tensile strength.

Properties of fibrous proteins?

insoluble in water, strong, stable, repetitive amino acid sequences.

function of fibrous proteins?

Provide structural support in tissues.

Examples of fibrous proteins?

Collagen (connective tissue), keratin (hair, nails, skin).

How are globular proteins structured?

Polypeptide chains fold into compact, spherical shapes (tertiary or quaternary structure), with hydrophilic groups on the outside.

Examples of globular proteins?

• Enzymes → catalyse reactions.

• Haemoglobin → carries oxygen.

• Insulin → regulates blood glucose.

• Antibodies → bind pathogens.

describe the biuret test

• Make the sample alkaline by adding NaOH.

• Add a few drops of copper(II) sulfate solution and sodium hydroxide

• Positive result: solution changes from blue → lilac/purple.

• Negative result: solution stays blue.

why does the biuret test work

Copper ions form a complex with peptide bonds in the protein, causing the colour change.