2 — molecular biology

Question 1

describe the structure of proteins, including features that are common to all proteins and features that vary (7)

Answer 1

• composed of one or more polypeptides
• a polypeptide is a chain of amino acids
• amino acids linked by peptide bonds
• 20 different amino acids, differentiated by their R groups
• primary structure is the sequence of amino acids in the polypeptide
• secondary structure is the formation of alpha helices and beta pleated sheets
• secondary structure stabilised by hydrogen bonding
• tertiary structure is the further folding of the polypeptide
• tertiary structure stabilised by interactions between r groups
• fibrous or globular are the two main classes of 3D structure
• 3D structure determined by amino acid sequence
• quaternary structure if 2 or more poly peptides are linked to form a single protein

Question 2

explain the roles of proteins in the movement of substances across membranes (5)

Answer 2

• pump proteins for active transport
• movement of particles against the concentration gradient
• requires ATP
• channel proteins for facilitated diffusion
• particles diffuse along the concentration gradient
• channel proteins allow ions to diffuse across
• aquaporins for movement of water
• greater membrane permeability to water with more aquaporins
• proteins that move substances across membranes are embedded in the phospholipid bilayer

Question 3

draw a diagram to show the ring structure of living organisms (3)

Answer 3

• ring with 4 carbons and 1 oxygen atom
• CH2OH attached to C4
• OH and H attached by single bonds to C1, C2 and C3 with OH facing downwards on C2 and C3

Question 4

outline, using graphs, the effect of different factors that influence enzyme activity (5)

Answer 4

• enzymes have active sites that bind specific substances
• act as catalysts to speed up reactions
• rate increases with temperature
• up to an optimum temperature
• sharp decline in activity above optimum temperature
• rate declines at a pH above and below the optimum pH
• rate increases sharply as substrate concentration goes up
• above a certain concentration, the rate reaches a maximum
• competitive inhibitor present, as substrate concentration increases, enzyme requires higher concentration to achieve maximum rate

Question 5

outline how the properties of water make it an effective coolant for the body (3)

Answer 5

• hydrogen bonds hold water molecules together
• evaporation requires breaking of hydrogen bonds
  water has a high heat of vaporisation
• evaporation of water removes heat from the skin

Question 6

describe how changes in weather conditions affect the transport and loss of water in plants (5)

Answer 6

• water vapour lost by transpiration
• transpiration causes transport of water in xylem
• more water loss in hotter weather
• because there is more heat for evaporation
• less water loss in more humid weather
• faster diffusion of water out of the leaf in low humidity
• more water loss in windier weather
• wind carries away water vapour from around the leaf
• high winds can cause stomata closure
• drought causes stomata to close so reduces water loss

Question 7

outline the process of protein denaturation (4)

Answer 7

• change to conformation
• bonds within the protein changed
• pH and temperature can cause denaturation
• heat at high temperatures breaks bonds
• extreme pH alters ionisation of amino acids and breaks ionic bonds
• protein cannot carry out its function
• usually permanent/ irreversible change

Question 8

distinguish between competitive and non competitive enzyme inhibition (4)

Answer 8

competitive:
- binds to active site
- inhibitor and substrate are chemically similar
- binding of substrate prevented
- inhibition reduced by increasing substrate concentration
- useful as pharmaceuticals

non competitive:
- binding away from active site
- inhibitor different from substrate
- active site changed
- inhibition not affected by increased substrate concentration
- useful as end product inhibitors

Question 9

draw labelled diagrams to show the structure of RNA nucleotides and how they are linked together to form a molecule of DNA (4)

Answer 9

• ribose drawn as pentagon and labelled sugar
• base drawn with correct link to ribose and labelled base
• phosphate drawn with correct link to ribose and labelled P
• two or more ribonucleotides drawn with correct link

Question 10

identify four examples of proteins that illustrate the wide range of functions of this group of biochemicals in living organisms (4)

Answer 10

• insulin receptors start the cellular signalling pathway
• leptin is a hormone
• antibodies defend body from disease
• photosystems convert light energy to electrical impulses
• keratin/ collagen is a structural protein
• microtubules involved in cell division
• ATP synthase is an enzyme that catalyses the formation of ATP
• fibrin/ fibrinogen is a protein involved in clotting
• haemoglobin/ sodium potassium pump is a transport protein
• actin is involved in muscle contraction
• spider silk is used to form spider webs

Question 11

describe the structure of starch (5)

Answer 11

• starch is a carbohydrate
• starch is formed by carbon, hydrogen and oxygen
• it is a polymer
• formed from monosaccharides
• linked together by condensation
• consists of amylose and amylopectin
• amylose is unbranched
• amylopectin is branched

Question 12

explain how cells produce a large variety of proteins with different sequences of amino acids (7)

Answer 12

• protein is produced when a gene is expressed
• genetic code consists of 4 nucleotides
• genetic code in DNA is transcribed to mRNA
• mRNA exits the nucleus
• mRNA is translated into a polypeptide
• amino acid sequence formation occurs at a ribosome
• one codon translates to one amino acid
• tRNA carries code for specific amino acids
• tRNA anticodon matches with specific codon in mRNA
• amino acids joined to form polypeptide
• sequence of amino acids determined by order of bases
• proteins very based on which amino acids are used and their order

Question 13

outline how hydrogen bonds form in water (3)

Answer 13

• water molecules are polar and dipolar
• attractive between positive and negative poles
• hydrogen bond formed between hydrogen and oxygen
• bond between different water molecules

Question 14

outline the range of functions of proteins in cells (5)

Answer 14

• enzymes that catalyse metabolic reactions
• proteins can be hormones which are chemical messengers to cells
• proteins that transport through the membrane e.g. channel
• haemoglobin in red blood cells that transports oxygen
• membrane proteins for tissue recognition
• structural elements of muscle fibre for movement
• histones condense DNA into chromosomes

Question 15

outline how hydrogen bonds form in water (3)

Answer 15

• water molecules are polar and dipolar
• attractive between positive and negative poles
• hydrogen bond formed between hydrogen and oxygen
• bond between different water molecules

Question 16

draw a molecular diagram to show the formation of a peptide bond (3)

Answer 16

• 2 amino acids correctly drawn
• removal of H2O molecule shown
• peptide bond labelled between C of C=O and N of N-H

Question 17

ADH is a peptide hormone that is produced in the hypothalamus. explain its action in the human body (8)

Answer 17

• ADH plays a role in osmoregulation
• acts on the collecting ducts of the kidney
• acts in distal convoluted tubule
• hypothalamus detects solute concentration
• if plasma is too concentrated, pituitary releases ADH
• ADH stimulates insertion of aquaporins in the collecting duct
• water moves through aquaporins by osmosis into the blood
• urine becomes more concentrated
  if blood is hypotonic, no ADH is released
  water is not reabsorbed from the collecting ducts
  urine becomes more dilute

Question 18

explain the action of enzymes in digestion and the different roles of at least two named enzymes that are produced in the pancreas (8)

Answer 18

• enzymes catalyse/ speed up chemical reactions
• have specific active sites to which specific substrates bind
• enzyme catalysis involves molecular motion and the collision of substrates with the active site
• enzymes break macromolecules into monomers in digestion
• smaller molecules become more readily absorbed
  pancreas secretes enzymes into the lumen of small intestine
• the small intestine has an alkaline pH
• enzymes have maximum action at specific pHs
• amylase breaks down starch into sugars
• lipase breaks lipids into monoglycerides
• protease breaks peptide bonds in proteins

Question 19

explain the importance of hydrogen bonding for living organisms (7)

Answer 19

• cohesion in water molecules stick together
• cohesion helps transport under tension of water
• adhesion between water and polar molecules
• adhesion helps water to rise in plants
• solvent properties due to hydrogen bonds with polar molecules
• solvent properties exemplified by glucose
• high latent heat of evaporation so water is useful as a coolant
• high specific heat capacity so water temperature changes less
• base pairing between nucleotides by hydrogen bonding
• base pairing between bases in RNA and DNA for transcription
• proteins have hydrogen bonding in secondary structure
• proteins have hydrogen bonding between R groups

Question 20

explain the process of photosynthesis (8)

Answer 20

• autotrophs perform photosynthesis
• carbon dioxide and water are the reactants
• light splits water molecules
• photolysis releases oxygen as a waste product
• light energy is converted into chemical energy
• photosynthesis produces organic compounds
• photosynthesis occurs in chloroplasts
• chlorophyll absorbs light
• different pigments absorb different wavelengths of light
• chlorophyll absorbs red and blue light
• carbon dioxide concentration, temperature, light intensity are limiting factors

Question 21

outline the production of carbohydrates in photosynthesis (4)

Answer 21

• light is absorbed by chlorophyll
• light energy is converted to chemical energy
• some of the energy is used for production of ATP
• water molecules are split
• produces oxygen as waste product
• plants fix CO2
• ATP is needed to produce carbohydrates

Question 22

distinguish between anabolism, catabolism and metabolism (3)

Answer 22

• metabolism is all enzyme catalysed reactions in a cell
• anabolism is the synthesis of polymers
• catabolism is breaking down complex molecules into simpler ones

Question 23

distinguish between the structures of the different types of fatty acids in food (4)

Answer 23

• fatty acids can be saturated or unsaturated
• unsaturated can be monounsaturated or polyunsaturated
• saturated fats have no double bonds
• cis-form has hydrogen atoms on the same side of carbon double bond
• trans-form has hydrogens on opposite sides of
• carbon double bond
• length of hydrocarbon chain can vary

Question 24

explain how animal cells produce the ATP that they need (5)

Answer 24

• respiration
• energy released from glucose
• anaerobic respiration does not require oxygen
• anaerobic respiration does not require oxygen
• lactate is produced in anaerobic respiration
• oxygen used in aerobic respiration
• carbon dioxide and water produce in aerobic respiration
• mitochondria used for aerobic respiration
• larger yield of ATP from aerobic than anaerobic respiration

Question 25

explain how temperature affects enzymes (4)

Answer 25

• speed of reaction increases as temperature rises
• faster molecular motion so more collisions between substrate and active site
• denaturation at higher temperatures
  active site damaged
• an enzyme works fastest at its optimum temperature
• inactivation at lower temperatures
• sketch graph to model the effect of temperature on enzyme activity