2.1.2: Biological molecules

Question 1

How do hydrogen bonds form between water

molecules?

Answer 1

Water is polar: O more electronegative than H, so attracts electron density in covalent bond more strongly.
Forms O 𝛿- (slightly negative) & H 𝛿+ (slightly positive).
There are intermolecular forces of attraction between a lone pair on O 𝛿- of one molecule & H 𝛿+ on an adjacent molecule.

Question 2

State 7 biologically important properties of water.

Answer 2

● reaches maximum density at 4℃
● high surface tension
● incompressible
● metabolite/ solvent for chemical reactions in the body
● high specific heat capacity
● high latent heat of vaporisation
● cohesion between molecules

Question 3

Why is the incompressible nature of water important for organisms?

Answer 3

Provides turgidity to plant cells.

Provides hydrostatic skeleton for some small animals e.g. earthworms.

Question 4

Explain why ice floats on water. Why is this important

for organisms?

Answer 4

Ice is less dense than water because H-bonds hold molecules in fixed positions further away from each other.
Insulates water in arctic climates so aquatic organisms can survive. Water acts as a habitat.

Question 5

Why is the high surface tension of water important

for organisms?

Answer 5

Slows water loss due to transpiration in plants.
Water rises unusually high in narrow tubes, lowering demand on root pressure.
Some insects can ‘skim’ across the surface of water.

Question 6

Why is water an important solvent for organisms?

Answer 6

Polar universal solvent dissolves & transports charged particles involved in intra & extracellular reactions e.g. PO4 3- for DNA synthesis.

Question 7

Why are the high specific heat capacity and latent of

vapourisation of water important for organisms?

Answer 7

Acts as a temperature buffer which enables endotherms to resist fluctuations in core temperature to maintain optimum enzyme activity.
Cooling effect when water evaporates from skin surface as sweat/ from mouth when panting.

Question 8

Define monomer and polymer. Give some examples.

Answer 8

monomer: smaller units that join together to form larger molecules
● monosaccharides (glucose, fructose, galactose, ribose)
● amino acids
● nucleotides

polymer: molecules formed when many monomers join together
● polysaccharides
● proteins
● DNA/ RNA

Question 9

What happens in condensation and hydrolysis

reactions?

Answer 9

Condensation: chemical bond forms between 2 molecules & a molecule of water is produced.
Hydrolysis: a water molecule is used to break a chemical bond between 2 molecules e.g. peptide bonds in proteins, ester bonds between fatty acids & glycerol in lipids.

Question 10

Name the elements found in carbohydrates, lipids,

proteins and nucleic acids.

Answer 10

carbohydrates & lipids: C, H, O
proteins: C, H, O, N, S
nucleic acids: C, H, O, N, P

Question 11

What’s the difference between ⍺-glucose and 𝛽-glucose?

Answer 11

If the -OH group attached to it is below the ring, the molecule is alpha glucose.
If the -OH group is above the ring, the molecule is beta glucose.

Question 12

Describe the properties of 𝛼 glucose.

Answer 12

● Small & water soluble = easily transported in bloodstream.
● Complementary shape to antiport for co-transport for absorption in gut.
● Complementary shape to enzymes for glycolysis = respiratory substrate.

Question 13

Difference between ribose and deoxyribose?

Answer 13

ribose is a pentose sugar, whereas deoxyribose is a hexose sugar.
deoxyribose is found in RNA, whereas ribose is found in DNA.
ribose binds a phosphate at the 3′ position, whereas deoxyribose binds a phosphate at the 2′ position. deoxyribose has one less oxygen molecule than ribose

Question 14

What type of bond forms when monosaccharides react?

Answer 14

(1,4 or 1,6) glycosidic bond
● 2 monomers = 1 chemical bond = disaccharide.
● Multiple monomers = many chemical bonds = polysaccharide.

Question 15

Name 3 disaccharides. Describe how they form.

Answer 15

Condensation reaction forms glycosidic bond between 2 monosaccharides.
● maltose: glucose + glucose
● sucrose: glucose + fructose
● lactose: glucose + galactose
all have molecular formula C12H22O11

Question 16

Describe the structure and functions of starch.

Answer 16

Storage polymer of 𝛼-glucose in plant cells:
● insoluble = no osmotic effect on cells
● large = does not diffuse out of cells

made from amylose:
● 1,4 glycosidic bonds
● helix with intermolecular
H-bonds = compact

and amylopectin:
● 1,4 & 1,6 glycosidic bonds
● branched = many terminal ends
for hydrolysis into glucose

Question 17

Describe the structure and functions of glycogen.

Answer 17

Main storage polymer of 𝛼-glucose in animal cells
(but also found in plant cells):
● 1,4 & 1,6 glycosidic bonds.
● Branched = many terminal ends for hydrolysis.
● Insoluble = no osmotic effect & does not diffuse out of cells.
● Compact.

Question 18

Describe the structure and functions of cellulose.

Answer 18

Polymer of 𝛽-glucose gives rigidity to plant cell walls
(prevents bursting under turgor pressure, holds stem up).
● 1,4 glycosidic bonds.
● Straight-chain, unbranched molecule.
● Alternate glucose molecules are rotated 180°.
● H-bond crosslinks between parallel strands form microfibrils = high tensile strength.

Question 19

How do triglycerides form?

Answer 19

Condensation reaction between 1 molecule of glycerol & 3 fatty acids which forms ester bonds.

Question 20

Contrast saturated and unsaturated fatty acids.

Answer 20

Saturated:
● contain only single bonds
● straight-chain molecules have many contact points
● higher melting point = solid at room temperature
● found in animal fats

Unsaturated:
● contain C=C double bonds
● ‘kinked’ molecules have fewer contact points
● lower melting point = liquid at room temperature
● found in plant oils

Question 21

Relate the structure of triglycerides to their functions.

Answer 21

● High energy:mass ratio = high calorific value from oxidation (energy storage).
● Insoluble hydrocarbon chain = no effect on water potential of cells & used for waterproofing.
● Slow conductor of heat = thermal insulation e.g. adipose tissue.
● Less dense than water = buoyancy of aquatic animals.

Question 22

Describe the structure and function of phospholipids.

Answer 22

Amphipathic: glycerol backbone attached to 2 hydrophobic fatty acid tails & 1 hydrophilic polar phosphate head.
● Forms phospholipid bilayer in water = component of membranes.
● Tails can splay outwards = waterproofing e.g. for skin.

Question 23

Are phospholipids and triglycerides polymers?

Answer 23

No; they are not made from a small repeating unit. They are macromolecules.

Question 24

Describe the structure and function of cholesterol.

Answer 24

Steroid structure of 4 hydrocarbon rings.
Hydrocarbon tail on one side, hydroxyl group (-OH) on the other side.
Adds stability to cell surface phospholipid bilayer by connecting molecules & reducing fluidity.

Question 25

What is the general structure of an amino acid?

Answer 25

• COOH: carboxyl / carboxylic acid group.
• R: variable side group consists of carbon chain & may include other functional groups e.g. benzene ring or -OH (alcohol).
• NH2: amine/ amino group.

Question 26

How do polypeptides form?

Answer 26

Condensation reactions between amino acids form peptide bonds (-CONH-).
There are 4 levels of protein structure.

Question 27

Define ‘primary structure’ of a protein.

Answer 27

Primary: sequence, number & type of amino acids in the polypeptide, determined by sequence of codons on mRNA.
Secondary: hydrogen bonds form between O 𝛿- attached to ‒C=O & H 𝛿+ attached to ‒NH.

Question 28

Describe the 2 types of secondary protein structure.

Answer 28

α-helix:
● All N-H bonds on same side of protein chain.
● Spiral shape.
● H-bonds parallel to helical axis.
β-pleated sheet:
● N-H & C=O groups alternate from one side to the other.

Question 29

Define ‘tertiary structure’ of a protein. Describe the bonds present.

Answer 29

3D structure formed by further folding
● Disulfide bridges: strong covalent S-S bonds between molecules of the amino acid cysteine.
● Ionic bonds: relatively strong bonds between charged R groups (pH changes cause these bonds to break).
● Hydrogen bonds: numerous & easily broken.

Question 30

Define ‘quaternary structure’ of a protein.

Answer 30

● Functional proteins may consist of more than one polypeptide.
● Precise 3D structure held together by the same types of bond as tertiary structure.
● May involve addition of prosthetic groups e.g metal ions or phosphate groups.

Question 31

Describe the structure and function of globular proteins.

Answer 31

● Spherical & compact.
● Hydrophilic R groups face outwards & hydrophobic R groups face inwards = usually water-soluble.
● Involved in metabolic processes e.g. enzymes such as amylase, insulin (2 polypeptide chains linked by 2 disulfide bonds), haemoglobin.

Question 32

Describe the structure of haemoglobin.

Answer 32

● Globular conjugated protein with prosthetic group.
● 2 𝛼-chains, 2 𝛽-chains, 4 prosthetic haem groups.
● Water-soluble so dissolves in plasma.
● Fe2+ haem group forms coordinate bond with O2.
● Tertiary structure changes so it is easier for subsequent O2 molecules to bind (cooperative binding).

Question 33

Describe the structure and function of fibrous proteins.

Answer 33

● Can form long chains or fibres.
● Insoluble in water.
● Useful for structure and support e.g. collagen in skin.

Question 34

List the functions of collagen, elastin and keratin.

Answer 34

Collagen: component of bones, cartilage, connective tissue, tendons.
Elastin: provides elasticity to connective tissue, arteries, skin, lungs, cartilage, ligaments.
Keratin: structural component of hair, nails, hooves/claws, horns, epithelial cells of outer layer of skin.

Question 35

Describe how to test for proteins in a sample.

Answer 35

Biuret test confirms presence of peptide bond

1. Add equal volume of sodium hydroxide to sample at room temperature.
2. Add drops of dilute copper (II) sulfate solution. Swirl to mix. (steps 1 & 2 make Biuret reagent).
3. Positive result: colour changes from blue to purple Negative result: solution remains blue

Question 36

Describe how to test for lipids in a sample.

Answer 36

1. Dissolve solid samples in ethanol.
2. Add an equal volume of water and shake.
3. Positive result: milky white emulsion forms

Question 37

Describe how to test for reducing sugars.

Answer 37

1. Add an equal volume of Benedict’s reagent to a sample.
2. Heat the mixture in an electric water bath at 100℃ for 5 mins.
3. Positive result: colour changes from blue to orange & brick-red precipitate forms.
   Or use test strip coated in a reagent that changes colour if reducing sugar is present

Question 38

Describe the Benedict’s test for non-reducing sugars.

Answer 38

1. Negative result: Benedict’s reagent remains blue.
2. Hydrolyse non-reducing sugars e.g. sucrose into their monomers by adding 1cm3 of HCl. Heat in a boiling water bath for 5 mins.
3. Neutralise the mixture using sodium carbonate solution.
4. Proceed with the Benedict’s test as usual.

Question 39

Describe the test for starch.

Answer 39

1. Add iodine solution.

2. Positive result: colour changes from orange to blue-black.

Question 40

State the role and chemical symbol of nitrates and

ammonium.

Answer 40

NO3- is used to make DNA, amino acids, NADP
for photosynthesis & NAD for respiration.
NH4+ can be converted to NO3- by saprobionts during nitrogen cycle. Produced by deamination of amino acids during ornithine cycle in liver

Question 41

State the role and chemical symbol of hydroxide and

phosphate ions.

Answer 41

OH- ions affect pH & can interact with bonds in 3° protein structure to cause denaturation.
PO43- is a component of ATP/ ADP for energy release & NADP.

Question 42

State the role and chemical symbol of sodium, potassium and chloride ions.

Answer 42

Na+ & K+ are involved in maintenance of resting potential of neurons/ generation of action potentials.
Na+ is also involved in co-transport mechanisms.
Cl- is involved in inhibitory synapses to cause hyperpolarisation

Question 43

State the role and chemical symbol of hydrogen and

hydrogencarbonate ions.

Answer 43

H+ & HCO3- form in organisms when CO2 dissolves in water.
H+ regulates pH & can interact with bonds in 3° protein structure to cause denaturation. H+ pump is involved in chemiosmosis & active loading in translocation.

Question 44

State the role and chemical symbol of calcium ions.

Answer 44

Ca2+ is used to make calcium pectate to add stability to middle lamella of plant cell walls. Regulates exocytosis of neurotransmitter. Binds to troponin to stimulate muscle contraction.

Question 45

How can the concentration of a solution be measured quantitatively?

Answer 45

● Use colorimetry to measure absorbance/ %transmission. Interpolate a calibration curve from solutions of known concentration.
● Use biosensors. A bioreceptor detects the presence of a chemical. A transducer converts the response into a detectable electrical signal

Question 46

Outline the principles and process of paper/thin-layer chromatography

Answer 46

1. Use capillary tube to spot solution onto pencil ‘start line’ (origin) 1 cm above bottom of paper.
2. Place chromatography paper in solvent. (origin should be above solvent level).
3. Allow solvent to run until it almost touches other end of the paper. Molecules in mixture move different distances based on relative solubility in solvent/attraction to paper.

Question 47

What are Rf values? How can they be calculated?

Answer 47

Ratios that allow comparison of how far molecules have moved in chromatograms.
Rf value = distance between origin and centre of pigment spot / distance between origin and solvent front