CHAP 2: BIOLOGICAL MOLECULES

benedict’s test for reducing sugars

1. add benedict’s reagent (contains copper (II) sulfate ions)

2. heat the sample using water bath at 80ºC

Result: blue → green → yellow → orange → red (→ increasing conc. of reducing sugar)

iodine test for starch

present = blue-black

not present = orange/brown/no change

emulsion test for lipids

add ethanol → shake

present = milky emulsion, solution cloudy

not present = no change/ clear

biuret test for lipids

add biuret reagent (CuSO4 + hydroxide)

present = lilac/purple

not present = no colour change/blue

acid hydrolysis & Benedict’s test for non-reducing sugars

1. hydrolyse with HCl and heat

2. neutralise with alkali (NaOH)

Result: blue → green → yellow → orange → red (→ increasing conc. of non-reducing sugar)

monomer

one of many small molecules that combine together to form a polymer

polymer

a giant molecule made from many similar repeating subunits joined together in a chain

macromolecule

• a large molecule formed by condensation reactions between smaller molecules

→ Polymers are a type of macromolecule , but not all macromolecules are formed from repeating units to be polymer

monosaccharide

a molecule consisting of a singular sugar unit with the general formula (CH2O)n

Main types: trioses(3C), pentoses(5C-ribose, deoxyribose), hexoses (6C-glucose, fructose, galactose)

disaccharide

a sugar molecule consisting of two monosaccharides joined together by a glycosidic bond

polysaccharide

a polymer whose sub-units are monosaccharides joined together by glycosidic bonds

isomer

organic molecules that have the same molecular formula but different structures which result in different properties

glycosidic bond

a H-O-H link between two sugar molecules ; formed by condensation reaction ; it is a covalent bond

maltose

1,4 linked alpha-glucose + alpha-glucose

sucrose

• reducing sugar

  • 1,2 linked alpha-glucose + beta-fructose → sucrose + H2O

amylopectin

• made of 1,4 glycosidic bonds linked α-glucose

• branches are formed by 1-6 linkages

• the branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to storage

amylose

• Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules

• The helix shape enables it to be more compact and thus it is more resistant to digestion

• Hydrogen bonds within molecules stabilise the unbranched helix shape

starch

made up of amylose and amylopectin

glycogen

• storage polysaccharide of animals & fungi

• made of chains of 1,4 linked α-glucose with 1,6 linkages forming branches

→ similar structure to amylopectin but more branched

How molecular structure of glycogen makes it suitable for storage

• highly branched and not coiled

• more branching than amylopectin → more compact → animals store more

• more branched = more free ends where glucose molecules can be added or removed, allowing condensation and hydrolysis reactions to occur more rapidly

• storage or release of glucose can suit the demands of the cell

functions of amylose, amylopectin and glycogen

• Compact :many molecules fit into small space, so large volumes can be stored

• Insoluble : don't dissolve in the cell cytoplasm → no osmotic effect

cellulose

a polysaccharide made from B-glucose subunits ; used as a strengthening

material in plant cell walls

structure of cellulose

• unbranched

• long chains of beta-glucose joined by 1,4 glycosidic bonds

• to form 1,4 glycosidic bonds, the beta-glucose will be inverted

• due to the inversion, many hydrogen bonds are formed → more strength

functions of cellulose

• Cellulose gives strength to plant cell walls through many hydrogen bonds in parallel microfibrils, allowing walls to withstand turgor pressure.

• Cell walls are supportive and permeable, as the cellulose–lignin matrix provides structural support while still allowing water and solutes to pass through.

• Cellulose is indigestible to most organisms, due to the lack of cellulase, making it an important source of dietary fibre.

cellulose fibres

• hydrogen bonds result in a strong molecule

• cellulose molecules become tightly cross linked to form bundles called microfibrils

• microfibrils are held together in bundles called fibres by hydrogen bonding

• cellulose fibres have very high tensile strength - makes it possible for a cell to withstand high turgopressures

polar molecules

1) have groups with dipoles

2) they're attracted to H2O molecules as they also have dipoles therefore are hydrophilic

3) soluble in water

Eg- amino acids, glucose, NaCl

non-polar molecules

1) do not have dipoles

2) not attracted to water and hydrophobic

3) insoluble in water

Eg- oils, cholesterol

triglycerides

• non-polar, hydrophobic

monomers:

• glycerol: an alcohol

• fatty acid: contain a methyl group at one end of the a hydrocarbon chain

saturated fatty acids

• no C=C double bonds in the hydrocarbon chain

• mainly in animal fat

unsaturated fatty acids

monounsaturated → 1 C=C

polyunsaturated → >1 C=C 

trans-fatty acid and cis-fatty acid

trans:

• H on opposite sides of C=C

• can’t be metabolised because it cannot forms enzyme-substrate complex

cis:

• H on the same side of C=C

• can be metabolised by enzymes

ester bond

• An ester bond (-COO-) forms when the hydroxyl group (-OH) of the glycerol bonds with the carboxyl group (-COOH) of the fatty acid

  • For each ester bond formed a water molecule is released

  • Therefore, for one triglyceride to form three water molecules are released

formation of triglycerides

1 glycerol + 3 fatty acid → triglyceride + 3H2O

functions of triglycerides

• energy storage

• insulation

• ability to float

• protection

structure of phospholipids

2 fatty acids + 1 glycerol + 1 phosphate ion (PO₄^3-) → 1 phospholipids

• phosphate → polar → hydrophilic

• fatty acid → non-polar → hydrophobic

=> amphipathic (they have both hydrophobic and hydrophilic parts) so they form monolayers or bilayers in water

role of phospholipids

1. Main component of cell membrane

2. Structure & Function: Hydrophobic tails form a core that blocks water-soluble substances; hydrophilic heads interact with water to create compartments for organelles.

3. Fluidity: More saturated tails → less fluid; more unsaturated tails → more fluid.

4. Protein Positioning: Hydrophobic interactions with phospholipids keep membrane proteins in place while allowing lateral movement.

general structure of amino acid

peptide bond

The chemical bond that forms between the carboxyl group of one amino acid and the amino group of another amino acid

primary structure

Sequence of amino acids in a polypeptide or protein

secondary structure

the structure of a protein molecule resulting from the regular coiling or folding of the chain of amino acids (an α-helix or β-pleated sheet)

α-helix

occurs when the hydrogen bonds form between every fourth peptide bond

β-pleated sheet

forms when the protein folds so that two parts of the polypeptide chain are parallel to each other, enabling hydrogen bonds to form between parallel peptide bonds

tertiary structure

=> the compact structure of a protein molecule resulting from the 3D coiling of the chain of amino acids

1) hydrogen bonds (form between strongly polar groups (NH2, CO, OH)

2) disulphide bonds (form between cysteine molecules, strongest)

3) ionic bonds (between ionised amine (NH3+) and carboxylic acid groups (COOH-). Broken by pH changes.

4) hydrophobic interactions (between non polar R groups)

quaternary structure

the 3D arrangement of two or more polypeptides, or of a polypeptide & non-protein component

hydrophobic interactions

form between the non-polar (hydrophobic) R groups within the interior of proteins

→ weak

ionic bonds

• forms between cations and anions

• stronger than hydrogen bonds

• can be broken by pH

hydrogen bonds

• weakest bond

• dp-dp forces due to the polarity of R groups

disulfide bonds

• strong covalent bonds that form between two cysteine R groups

• can be broken by reduction

globular proteins

• Roughly spherical

• Irregular w wide range of R groups

• Functional

• E.g: Haemoglobin, insulin

• Generally soluble in (water hydrophobic R groups inside, hydrophilic R groups outside)

fibrous proteins

• Long strands

• Little to no tertiary structure

• Repetitive w a limited range of R groups

• Structural

• E.g: Collagen, keratin

• Generally insoluble in water

haemoglobin

1) made of 4 polypeptide chains
2) 2 chains are alpha goblin, other 2 are β goblin
3) each chain has a haem group (contains Fe2+) attached
4) held tgt by disulphide bonds
5) globular protein

haemoglobin functions

• Haemoglobin binds and transports oxygen because O₂ is poorly soluble in water.

• The haem group with Fe²⁺ allows reversible binding of oxygen.

• Binding of each O₂ molecule changes haemoglobin’s shape, increasing its affinity for the next O₂

• Amino acids alone cannot bind oxygen effectively → needs haem

collagen

• fibrous protein

• consists of 3 polypeptide chains each in a helical shape

• 3 polypeptides are wound together creating a triple helix

• strands are held together by H and console by bonds

• every 3rd amino acid is glycine

• each 3 stranded molecule interacts with other collagen molecules running parallel to it

• covalent bonds form between R groups of amino acids forming fibrils

• many fibrils lie alongside each other forming strong bonds called fibre

• flexible but has high tensile strength

collagen functions

• Flexible structural protein found in connective tissues.

• Triple helix with many hydrogen bonds → very high tensile strength.

• Staggered molecule arrangement in fibrils adds extra strength.

• High proline and hydroxyproline content increases stability.

• Long molecules dissolve slowly → collagen is insoluble in water.

properties of water to its roles in

living organisms

1. solvent action

→ polar so both ionic and covalent compounds will dissolve

2. high specific heat capacity

→ due to the many hydrogen bonds , it takes a lot of thermal energy to raise 1kg of water by 1ºC

3. high latent heat of vaporisation

→ Only a little water is required to evaporate from the surface of the organism in order to lose a great amount of thermal energy

glycerolycerol