2.2 Biological molecules

Question 1

Elements in carbohydrates, lipids, proteins and nucleic acids

Answer 1

CHO and OH – carbohydrates, lipids, + NS for proteins, + NP for nucleic acids

Question 2

Role of lipids

Answer 2

thermal insulation, energy store, protection, component of cell membranes/phospholipid bilayer, steroid hormones, buoyancy, waterproofing, source of water from respiration, electrical insulation as forms myelin sheath around neurones, aid production of vitamins

Question 3

Role of cholestrol and why RBCs have more

Answer 3

regulates fluidity of phospholipid bilayer/supports structure of membranes, converted in steroid, waterproofing the skin, making vitamin D, making bile
* RBCs have more cholesterol because they are free in the blood / not supported by other cells, so cholesterol helps to maintain shape

Question 4

Role of mRNA

Answer 4

carries the copy of the gene

Question 5

Role of glucose

Answer 5

respiratory substrate / used in respiration, source of energy, formation of ATP

Question 6

Why glycogen is a good storage molecule

Answer 6

• Insoluble so it doesn’t change the water potential of a cell
• Can be broken down / hydrolysed quickly: lots of branches for enzymes to attach
• Compact = high energy content for mass / energy dense

Question 7

Ways glucose molecule is well suited to its function

Answer 7

• Soluble so can be easily transported around organism
• Small so can be transported / diffused across cell membranes
• Easily / quickly respired / broken down to release energy / produce ATP
• Molecules can join to produce di / polysaccharides

Question 8

Ways starch suited for its function

Answer 8

• Helical so compact
• Insoluble so doesn’t affect water potential
• Large molecule so doesn’t leave cell

Question 9

Cellulose and chitin structure

Answer 9

• Cellulose: long straight chains is linked together by many H- bonds to form fibrils provide strength
• Chitin: glycosidic bonds between glucose monomers formed by condensation, produces water. Long straight chains with H bonding. Alternative monomers are flipped - like cellulose

Question 10

Saccharides and the polysaccharides they form

Answer 10

• Alpha glucose (x2) = maltose
• Alpha glucose + beta fructose = sucrose
• Glucose + galactose = lactose
• Amylose: alpha glucose 1,4 and amylopectin: alpha glucose 1,4 and 1,6 every 25 glucose subunits. Forms starch (plants), glycogen (in animals and fungi)
  Condensation to form glycosidic bonds

Question 11

Protein structure

Answer 11

• Peptide bond: between H from the amine group and OH from the carboxyl group, condensation reaction so water is lost / produced –> covalent bond, opposite = hydrolysis (catalysed by peptidyl transferase)
• Primary: sequence of amino acids joined by peptide bonds
• Secondary: H bonds between NH group and C=O group of 2 amino acids –> α helix / β-pleated sheets
• Tertiary: secondary structure undergoes further coiling / folding, 3D shapes
• R groups attract/repel, disulfide bonds between cysteine atoms, H bonds, ionic bonds between oppositely charged R groups, hydrophilic / hydrophobic interactions
• Quaternary: arrangement of 2(+) polypeptide chains in a protein

Question 12

Simple vs conjugated protein

Answer 12

• Contains non-protein groups, has prosthetic group which is attached by covalent bonds / ionic interactions / H-bonds
• Prosthetic group is required for the protein to be functional

Question 13

Structure of a collagen (+keratin/elastin) molecule

Answer 13

• Peptide bonds between amino acids to form 3 polypeptide chains that are coiled into a left-handed helix. Every 3rd amino acid is glycine (small R group) allowing the closeness of polypeptide chains. H bonds between polypeptide chains to form fibrils
• Adjacent molecules are joined by crosslinks, crosslinks / end of molecules is staggered = high mechanical strength
• Few hydrophilic R groups on the outside
• Properties: strong, has structural role, forms parts of cartilage / connective tissue
• Found in ligaments: strong, not elastic, insoluble, unreactive, and flexible

Question 14

Haemoglobin structure

Answer 14

• Spherical - globular protein
• Hydrophilic R groups on outside, forms H-bonds with water, soluble
• Contains prosthetic group: haem / Fe2+ which allows oxygen to be carried. Polypeptide chains within haemoglobin have 3o structure. 2 alpha and 2 beta subunits – 1 haem group per polypeptide / 4 per molecule

Question 15

Haemoglobin and collagen structure (similarities and differences)

Answer 15

• Similarities: amino acid chain, peptide bonds, helical, 4o structure: 1+ polypeptide, H-bonds…
• Differences: haemoglobin is globular, hydrophilic R groups on outside, 2 alpha and 2 beta chains, alpha helix (?)

Question 16

Fibrous proteins - keratin: structure and properties

Answer 16

• In hair, skin, and nails – contains lots of cysteine so forms strong disulfide bridges (more in nails so less flexible)
• Properties: strong, inflexible, insoluble, mechanical protection, impermeable barrier to infection, waterproof

Question 17

Fibrous proteins - actin/myosin: function

Answer 17

in muscle for contraction / mechanical movement + found in microtubules in cilia / flagella / spindle / cytoskeleton

Question 18

Fibrous proteins - fibrin: role

Answer 18

involved in formation of blood clots

Question 19

Fibrous proteins - elastin: structure and properties

Answer 19

• Found in walls of blood vessels and alveoli in lungs
• Quaternary structure: made of many soluble, stretchy tropoelastin molecules –> v. large, insoluble, and stable cross-linked structure
• Oroperties: strong and elastic / flexible (can stretch and recoil w/o breaking)

Question 20

Globular proteins - properties

Answer 20

specific / complementary to another molecule, pH / temp sensitive

Question 21

Globular proteins - enzyme example

Answer 21

pepsin: 1 polypeptide chain folded into tertiary structure with many acidic R groups, soluble (hydrophilic R groups on outside), specific 3D shape to form AS

Question 22

Globular proteins - hormone example

Answer 22

insulin: hormone = globular protein with chain a and b, soluble, specific shape to bind to specific receptors on muscle, fat and liver cells to increase glucose uptake, hormone to lower blood glucose conc

Question 23

Alpha and beta glucose structure

Answer 23

image

Question 24

Chemical test for reducing sugars

Answer 24

• Add Benedict’s reagent and heat. Forms precipitate – colour changes from blue to red
• Estimate the concentration: use different colours and compare with standard solutions OR filter, centrifuge and weigh precipitate and greater mass = more sugar present

Question 25

Describe how the concentration of a reducing sugar can be measured using a colorimeter

Answer 25

• Use same volume of known concentrations of reducing sugar each time and heat with excess Benedict’s solution
• Forms a precipitate that changes to blue to red
• Remove precipitate / obtain filtrate by filtering/centrifuging and decanting
• Calibrate colorimeter using water, use red filter
• Read transmission of red light, more transmission of filtrate = more sugar present
• Obtain calibration curve, plotting transmission against reducing sugar concentration
• Use transmission reading of unknown sugar conc. and read off graph to find concentration

Question 26

Chemical test for non-reducing sugars

Answer 26

add (HCl) acid and boil, then add sodium carbonate / alkali. Then carry out reducing sugar test

Question 27

Chemical test for proteins

Answer 27

add biuret reagent or alkali and CuSO4 solution, result: purple solution

Question 28

Emulsion test for lipids

Answer 28

shake sample with ethanol, add to water – result: milky white

Question 29

Chromatography (spots / amino acids / samples) – improved method & reason

Answer 29

• Put pencil line / origin higher than solvent / 1cm + to stop spots dissolve/mix/touching solvent
• Put spots further apart / on separate plates + to stop them merge/touching
• Touch plate edges/not middle / wear gloves / use forceps + to prevent contamination / transfer of substances from hands
• Place lid / cover over beaker + to prevent evaporation of solvent
• Support the plate / attach to beaker + to keep plate vertical / still
• Use ninhydrin + to see amino acids
• Repeat and find mean / average (Rf value) + to improve accuracy / check for repeatability / exclude anomalies
• Label spots in pencil / on beaker + to know which is which / avoid confusion

Question 30

H-bonding: in water (explained) and other examples

Answer 30

• between delta- O and delta+ H of adjacent water molecules – possible if the molecule is polar
• Examples: protein 2o structure (alpha helix / beta-pleated sheet) and 3o structure, between polypeptide chains in 4o structure, between chains of cellulose, between strands of / bases in DNA

Question 31

Glycogen vs cellulose

Answer 31

• No H-bonding vs H-bonds between long straight chains –> fibrils