Topic 2: Molecular Biology

Question 1

Outline the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis. [6]

Answer 1

light:
 rate of photosynthesis increases as light intensity increases
 photosynthetic rate reaches plateau at high light levels
CO2:
 photosynthetic rate reaches plateau at high carbon dioxide levels
 up to a maximum when rate levels off
temperature:
 rate of photosynthesis increases with increase in temperature
 up to optimal level / maximum
 high temperatures reduce the rate of photosynthesis

Question 2

Compare how pyruvate is used in human cells when oxygen is available and when oxygen is not available. [5]

Answer 2

 aerobic cell respiration if oxygen available and anaerobic if unavailable;
 pyruvate enters mitochondrion for aerobic respiration;
 whereas pyruvate stays in the cytoplasm for processing under anaerobic conditions;
 pyruvate converted aerobically into carbon dioxide and water;
 whereas pyruvate converted anaerobically to lactate;
 large ATP yield when oxygen available/from aerobic cell respiration;
 no (further) ATP yield without oxygen;

Question 3

Outline the role of condensation and hydrolysis in the relationship between amino acids and dipeptides. [4]

Answer 3

 diagram of peptide bond drawn
 condensation / dehydration synthesis: water produced (when two amino acids joined)
 hydrolysis: water needed to break bond
 dipeptide –> amino acids - hydrolysis occurs
 amino acids –> dipeptide - condensation occurs

Question 4

Outline the thermal, cohesive and solvent properties of water. [5]

Answer 4

 water has a high specific heat capacity;
 a large amount of heat causes a small increase in temperature;
 water has a high latent heat of vaporization;
 a large amount of heat energy is needed to vaporize/evaporate water;
 hydrogen bonds between water molecules make them cohesive/stick together;
 this gives water a high surface tension / explains how water rises up xylem;
 water molecules are polar;
 this makes water a good solvent;

Question 5

Describe the significance of water to living organisms. [5]

Answer 5

 surface tension - allows some organisms (e.g. insects) to move on water’s surface
 polarity / capillarity / adhesion - helps plants transport water
 (excellent) solvent - capable of dissolving substances for transport in organisms
 (excellent) thermal properties (high heat of vaporization) - excellent coolant
 ice floats - lakes / oceans do not freeze, allowing life under the ice
 buoyancy - supports organisms
 structure - turgor in plant cells / hydrostatic pressure
 habitat - place for aquatic organisms to live

Question 6

Describe the use of carbohydrates and lipids for energy storage in animals. [5]

Answer 6

carbohydrates: 3 max
 stored as glycogen (in liver)
 short-term energy storage
 more easily digested than lipids so energy can be released more quickly
 more soluble in water for easier transport
lipids: 3 max
 stored as fat in animals
 long-term energy storage
 more energy per gram than carbohydrates
 lipids are insoluble in water so less osmotic effect

Question 7

List three functions of lipids. [3]

Answer 7

	energy storage / source of energy / respiration substrate
	(heat) insulation
	protection (of internal organs)
	water proofing / cuticle
	buoyancy
	(structural) component of cell membranes
	electrical insulation by myelin sheath
	(steroid) hormones
	glycolipids acting as receptors

Question 8

List four functions of proteins, giving an example of each. [4]

Answer 8

 storage - zeatin (in corn seeds)/casein (in milk)
 transport - hemoglobin/lipoproteins (in blood)
 hormones - insulin/growth hormone/TSH/FSH/LH
 receptors - hormone receptor/neurotransmitter receptor/receptor in chemoreceptor cell
 movement - actin/myosin
 defense - antibodies/immunoglobin
 enzymes - catalase/RuBP carboxylase
 structure - collagen/keratin/tubulin/fibroin
 electron carriers - cytochromes
 pigments - rhodopsin
 active transport - sodium potassium pumps/calcium pumps
 facilitated diffusion - sodium channels/aquaporins

Question 9

Describe the structure of proteins. [9]

Answer 9

 (primary structure is a) chain of amino acids/sequence of amino acids
 (each position is occupied by one of) 20 different amino acids
 linked by peptide bonds
 secondary structure formed by interaction between amino and carboxyl/-NH and -C=O groups
 (weak) hydrogen bonds are formed
 (α-) helix formed / polypeptide coils up
 or (ß-) pleated sheet formed
 tertiary structure is the folding up of the polypeptide
 stabilized by disulfide bridges / hydrogen / ionic / hydrophobic bond
 quaternary structure is where several polypeptide subunits join
 conjugated proteins are proteins which combine with other non-protein molecules
 for example metals / nucleic acids / carbohydrates / lipids

Question 10

Lactase is widely used in food processing. Explain three reasons for converting lactose to glucose and galactose during food processing. [3]

Answer 10

 it allows people who are lactose intolerant/have difficulty digesting lactose to consume milk (products);
 galactose and glucose taste sweeter than lactose reducing need for additional sweetener (in flavored milk products);
 galactose and glucose are more soluble than lactose / gives smoother texture / reduces crystallization in ice cream;
 (bacteria) ferment glucose and galactose more rapidly (than lactose) shortening production time (of yoghurt/cottage cheese);

Question 11

Simple laboratory experiments show that when the enzyme lactase is mixed with lactose, the initial rate of reaction is highest at 48 °C. In food processing, lactase is used at a much lower temperature, often at 5 °C. Suggest reasons for using lactase at relatively low temperatures. [2]

Answer 11

 less denaturation / enzymes last longer at lower temperatures;
 lower energy costs / less energy to achieve 5 °C compared to 48 °C;
 reduces bacterial growth / reduces (milk) spoilage;
 to form products more slowly / to control the rate of reaction;

Question 12

Outline how enzymes catalyze reactions. [4]

Answer 12

 they increase rate of (chemical) reaction;
 remains unused/unchanged at the end of the reaction;
 substrate joins with enzyme at active site;
 to form enzyme-substrate complex;
 active site/enzyme (usually) specific for a particular substrate;
 enzyme binding with substrate brings reactants closer together to facilitate chemical reactions (such as electron transfer);
 making the substrate more reactive;

Question 13

Explain the effect of pH on enzyme activity. [3]

Answer 13

 enzymes have an optimal pH
 lower activity above and below optimum pH / graph showing this
 too acidic / base pH can denature enzyme
 change shape of active site / tertiary structure altered
 substrate cannot bind to active site / enzyme-substrate complex cannot form
 hydrogen / ionic bonds in the enzyme / active site are broken / altered

Question 14

Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together. [8]

Answer 14

 subunits are nucleotides
 one base, one deoxyribose and one phosphate in each nucleotide
 description/ diagram showing base linked to deoxyribose C1 and phosphate to C5
 four different bases - adenine, cytosine, guanine and thymine
 nucleotides linked up with sugar-phosphate bonds
 covalent/ phosphodiester bonds
 two strands (of nucleotides) linked together
 base to base
 A to T and G to C
 hydrogen bonds between bases
 antiparallel strands
 double helix drawn or described

Question 15

Describe the genetic code. [6]

Answer 15

 composed of mRNA base triplets
 called codons
 64 different codons
 each codes for the addition of an amino acid to a growing polypeptide chain
 the genetic code is degenerate
 meaning more than one codon can code for a particular amino acid
 the genetic code is universal
 meaning it is the same in almost all organisms
 (AUG is the) start codon
 some (nonsense) codons code for the end of translation

Question 16

Explain briefly the advantages and disadvantages of the universality of the genetic code to humans. [4]

Answer 16

 genetic material can be transferred between species/ between humans
 one species could use a useful gene from another species
 transgenic crop plants/ livestock can be produced
 bacteria/ yeasts can be genetically engineered to make a useful product
 viruses can invade cells and take over their genetic apparatus
 viruses cause disease

Question 17

Distinguish between RNA and DNA. [3]

Answer 17

 DNA is double-stranded while RNA is single-stranded;
 DNA contains deoxyribose while RNA contains ribose;
 the base thymine found in DNA is replaced by uracil in RNA;
 one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);

Question 18

Describe the roles of mRNA, tRNA and ribosomes in translation. [6]

Answer 18

 mRNA with genetic code/ codons
 tRNA with anticodon
 tRNA with amino acid attached
 ribosome with two sub-units
 mRNA held by ribosome
 start codon
 two tRNA molecules attached with mRNA on ribosome
 peptide bond between amino acids on tRNA
 polypeptide forms
 continues until a stop codon is reached
 polypeptide is released

Question 19

Explain the similarities and differences in anaerobic and aerobic cellular respiration. [8]

Answer 19

 aerobic requires oxygen and anaerobic does not utilize oxygen
similarities: 3 max
 both can start with glucose
 both use glycolysis
 both produce ATP/energy(heat)
 both produce pyruvate
 carbon dioxide is produced
 (both start with glycolosis) aerobic leads to Krebs’ cycle and anaerobic leads to fermentation
differences: 5 max
anaerobic:
 (fermentation) produces lactic acid in humans
 (fermentation) produces ethanol and CO2 in yeast
 occurs in cytoplasm of the cell
 recycles NADH (NAD+)
aerobic cellular respiration
 pyruvate transported to mitochondria
 further oxidized to CO2 and water (in Krebs cycle)
 produces a larger amount of ATP (36-38 ATP)/anaerobic produces less ATP (2)
 can use other compounds / lipids / amino acids for energy

Question 20

Explain briefly the advantages and disadvantages of the universality of the genetic code to humans. [4]

Answer 20

 genetic material can be transferred between species/ between humans
 one species could use a useful gene from another species
 transgenic crop plants/ livestock can be produced
 bacteria/ yeasts can be genetically engineered to make a useful product
 viruses can invade cells and take over their genetic apparatus
 viruses cause disease

Question 21

Distinguish between RNA and DNA. [3]

Answer 21

 DNA is double-stranded while RNA is single-stranded;
 DNA contains deoxyribose while RNA contains ribose;
 the base thymine found in DNA is replaced by uracil in RNA;
 one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);

Question 22

Describe the roles of mRNA, tRNA and ribosomes in translation. [6]

Answer 22

 mRNA with genetic code/ codons
 tRNA with anticodon
 tRNA with amino acid attached
 ribosome with two sub-units
 mRNA held by ribosome
 start codon
 two tRNA molecules attached with mRNA on ribosome
 peptide bond between amino acids on tRNA
 polypeptide forms
 continues until a stop codon is reached
 polypeptide is released

Question 23

Explain the similarities and differences in anaerobic and aerobic cellular respiration. [8]

Answer 23

 aerobic requires oxygen and anaerobic does not utilize oxygen
similarities: 3 max
 both can start with glucose
 both use glycolysis
 both produce ATP/energy(heat)
 both produce pyruvate
 carbon dioxide is produced
 (both start with glycolosis) aerobic leads to Krebs’ cycle and anaerobic leads to fermentation
differences: 5 max
anaerobic:
 (fermentation) produces lactic acid in humans
 (fermentation) produces ethanol and CO2 in yeast
 occurs in cytoplasm of the cell
 recycles NADH (NAD+)
aerobic cellular respiration
 pyruvate transported to mitochondria
 further oxidized to CO2 and water (in Krebs cycle)
 produces a larger amount of ATP (36-38 ATP)/anaerobic produces less ATP (2)
 can use other compounds / lipids / amino acids for energy

Question 24

Compare how pyruvate is used in human cells when oxygen is available and when oxygen is not available. [5]

Answer 24

 aerobic cell respiration if oxygen available and anaerobic if unavailable;
 pyruvate enters mitochondrion for aerobic respiration;
 whereas pyruvate stays in the cytoplasm for processing under anaerobic conditions;
 pyruvate converted aerobically into carbon dioxide and water;
 whereas pyruvate converted anaerobically to lactate;
 large ATP yield when oxygen available/from aerobic cell respiration;
 no (further) ATP yield without oxygen;

Question 25

Explain how the rate of photosynthesis can be measured. [7]

Answer 25

 CO2 + H2O –> (CH2O)n + O2/ suitable photosynthesis equation
 amount of CO2 absorbed (per unit time) can be measured
 increase in biomass (per unit time) can be measured
 O2 excretion (per unit time) can be measured
methods for measuring the above:
 volume of O2 (bubbles) produced per unit time can be measured
 dry mass can be measured
 increase in starch concentration in leaves (as measured by iodine)
 use of pH indicator can monitor CO2 uptake in water
 the rate of photosynthesis measured is relative because some of the CO2 is produced by the plant internally through respiration
 the rate of photosynthesis measured is relative because some of the carbohydrates are used internally by the plan for respiration

Question 26

Explain the role of water in photosynthesis [4]

Answer 26

 water is a substrate / reactant / raw material / for photosynthesis / equation for photosynthesis
 water is a source of electrons
 to replace those lost by chlorophyll / photosystem II
 water is a source of H+ needed to produce NADPH + H
 photolysis / splitting / breaking of water
 water for non-cyclic photophosphorylation / ATP production
 water is transparent so photosynthesis can take place underwater / light can penetrate to chloroplasts