Question
.
| Amino acid Yellow mealworms / g kg–1 dry matter | Cattle / g kg–1 dry matter |
Isoleucine | 25 | 16 |
Leucine | 52 | 42 |
Lysine | 27 | 45 |
Methionine | 6 | 16 |
Phenylalanine | 17 | 24 |
Threonine | 20 | 25 |
Valine | 29 | 20 |
Birds and mammals maintain constant body temperature despite considerable losses of body heat to the environment. In insects such as mealworms, body temperature is variable and is often the same as the temperature of the environment or only slightly above it. Analyse the data in the bar charts, using this information. [2]
Using all the relevant data in this question, discuss the use of insects as a major food source for humans. [3]
▶️Answer/Explanation
Ans: 1. a i cricket a ii 25 − 4 = 21kg «more required for cattle» Must state unit kg.
1. a iii cricket 1. b i a. mealworms have more isoleucine/leucine/valine than cattle
b. cattle have more lysine/methionine/phenylalanine/threonine
c. the total proportion of these amino acids is «slightly» greater in cattle (188 to 176)
1. b ii a. cattle as they are more closely related to humans b. cattle as they are more likely to have proteins with a similar amino acid composition to humans OWTTE.
c. cattle as they contain a «slightly» higher proportion of «essential» amino acids required in human diet (188 to 176)
1. c a. mealworms contribute much less to global warming than other traditional farm livestock for protein production Accept converse or OWTTE.
b. mealworms require less land use than other traditional farm livestock for protein production
1. d a. cell respiration required to generate heat (lost to environment) to maintain body temperature b. birds/chickens and mammals/cattle carry out cell respiration at higher rate than insects c. «chickens/cattle therefore» generate more\( CO_{2}\) per kg protein produced
d. «chickens/cattle» need more food/land area to produce body mass
e. feed conversion ratios are lower in mealworms/insects/cold blooded animals as they do not need to maintain a constant body temperature (accept converse)
1. e a. insects/crickets have the highest edible percentage
b. insects have the lowest feed conversion ratio/produce the most edible mass per kg of food they eat
c. insects supply amino acids required in the human diet d. insects cause less global warming/use less land area per kg of protein produced
e. in western countries, there is a disgust factor/cultural factors about eating insects which would need to be overcome before they could be used as a significant food source
f. insects may supply less proportions/content of amino acids required in human diets
Question
(a) Label the parts of the DNA diagram indicated by I, II, III and IV. [2]
b. (i) Explain how model making helped Watson and Crick to establish the structure of DNA. [2]
(ii) Distinguish between the chromosomes of eukaryotic cells and prokaryotic cells. [1]
(This question continues on the following page)
(Question 2 continued)
c. Outline the role of the following enzymes in replication.
Helicase [1]
DNA polymerase [2]
▶️Answer/Explanation
Ans: a. I. cytosine Award [1] for any two correct responses.
b. II. sugar-phosphate/covalent/phosphodiester bond
c. III. phosphate
d. IV. deoxyribose 2. b i a. decided to combine what was known about chemical content of DNA with information from X-ray diffraction studies OWTTE.
b. built scale models of components of DNA c. then attempted to fit them together in a way that agreed with the data «from separate
sources» d. made several arrangements of scale model until found best one that fitted all the data
2. b ii a. associated with «histone» proteins in eukaryotes but not prokaryotes
b. is linear in eukaryotes but circular in prokaryotes c. in cytoplasm in prokaryotes, but within nucleus in eukaryotes.
2. c i unwinds/separates strands/double helix (by breaking hydrogen bonds) 2. c ii a. links nucleotides together to form a new strand of DNA
b. uses pre-existing strand of DNA as template
c. makes covalent bonds between nucleotides
Question
(a) The electron micrograph shows a palisade mesophyll cell.
State the name of the structures labelled I and II. [1]
Outline the function of the structure labelled III. [2]
The plant from which this cell was taken is in the group angiospermophyta.
State one characteristic that is unique to this group of plants. [1]
(This question continues on the following page)
(Question 3 continued)
b. Distinguish between autotrophic nutrition and heterotrophic nutrition. [2
c. Explain how energy and nutrients are transferred in ecosystems. [3]
▶️Answer/Explanation
Ans: a i I. cell wall II. nucleus/chromatin Both needed.
a ii a. necessary for photosynthesis/converts light energy into chemical energy
b. contains chlorophyll to absorb light
c. (contains enzymes) for production of carbohydrate/glucose/starch
a iii a. produce flowers b. enclosed seeds/have fruit
b autotroph a. inorganic source of carbon organic source of carbon compounds
b. synthesizes organic molecules from inorganic sources obtains c. autotrophs photosynthesise/require light (or chemicals) for building its own nutrients chemical energy from ingested nutrients heterotroph organic molecules from other organisms/cannot make organic molecules from inorganic heterotrophs require chemical energy from ingested nutrients
c a. energy enters ecosystems from the Sun / continuous supply from the Sun
b. light energy is converted into chemical energy and lost with movement along foodchains OR energy needs to be «constantly» added «to ecosystem» as lost with movement along food chains / energy lost as heat with movement along food chains
c. nutrients are recycled within ecosystems / nutrients in an ecosystem are finite and limited
d. nutrients not lost but transformed into different compounds e. nutrients «carbon compounds»/energy flow through food chains by means of feeding
Question
(a) Sketch a graph to show the effect of temperature on the activity of enzymes. [2]
(b) Explain enzyme–substrate specificity. [3]
Answer one question. Up to one additional mark is available for the construction of your
answer. Answers must be written within the answer boxes provided
▶️Answer/Explanation
Ans:
a a. axes labelled correctly: x-axis as temperature AND y-axis as rate of reaction/enzyme activity eg: rate of reaction temperature Fall should be at least twice as steep as rise.
b. correct shape of graph: increases gradually to max and then decreases more rapidly
b a. enzymes are proteins with specific 3-D geometry/shape
b. enzymes with active site that binds with the substrate/reactants Accept marks from clear annotated diagrams.
c. active site shape only allows it to bind with specific substrates «with complementary shapes»
d. when enzyme-substrate complex formed allows reaction to occur
e. products are released and enzyme returns to original shape and can be reused OR denaturing changes shape «of active site» so changes ability to bind with substrate
Question
5. Every cell is surrounded by a cell surface membrane which regulates the movement
of materials into and out of the cell.
a. Draw an annotated diagram of the fluid mosaic model of membrane structure. [4]
b. Describe the processes involved in absorbing different nutrients across the cell membrane of villus epithelium cells lining the small intestine. [4]
c. Explain the events that occur during a nerve impulse and how the impulse is propagated along an axon. [7]
▶️Answer/Explanation
Ans: a Award [1] for each structure clearly drawn and correctly labelled.
a. phospholipid bilayer – with head and tails
b. hydrophilic/phosphate/polar heads AND hydrophobic/hydrocarbon/fatty acid/non-polar tails labelled
c. integral/intrinsic protein – embedded in the phospholipid bilayer
d. protein channel – integral protein showing clear channel/pore e. peripheral/extrinsic protein – on the surface
f. glycoprotein with carbohydrate attached
g. cholesterol – shown embedded in bilayer
b a. «simple» diffusion of nutrients along/down a concentration gradient
b. example of simple diffusion, eg: fatty acids c. facilitated diffusion of nutrients involves movement through channel proteins d. example of nutrient for facilitated diffusion eg: fructose e. active transport of nutrients against a concentration gradient / involving protein pumps
f. example of active transport, eg: (iron) ions/glucose/amino acids
g. endocytosis / by means of vesicles h. example of nutrient for endocytosis, eg: cholesterol in lipoprotein particles
c a. nerve impulses are action potentials propagated along axons of neurons Accept any of the points clearly explained in an annotated diagram. b. resting potential is –70 mV OR relatively negative inside in comparison to the outside
c. \(\mathrm Na{+}/ \mathrmK{+}\)pumps maintain/re-establish «the resting potential» d. more sodium ions outside than inside «when at the resting potential»
OR more potassium ions inside than outside «when at the resting potential»
e. action potential stimulates «wave of» depolarization along the membrane/axon
f. «when neuron is stimulated» if threshold potential is reached Na+ channels open
g. sodium ions diffuse/move in
h. «\(Na{+}\) move in» causing depolarization / inside of the neuron becomes more positively charged than the outside of the neuron 
i. potassium ion channels open OR potassium ions diffuse/move out
j. «K+ move out» causing repolarization
k. local currents OR description of Na+ ion diffusion between depolarized region and next region of axon to depolarize
l. myelination increases propagation speed/allows saltatory conduction
Question
6. Although simple in structure, bacteria as a group show a wide range of diversity.
Outline the roles bacteria play in the carbon cycle. [3]
Describe the evolution of antibiotic resistance in bacteria. [4]
Explain the process of genetically modifying bacteria. [8]
▶️Answer/Explanation
Ans: a. decomposition of dead organic material «by saprotrophic bacteria»
b. «decomposition» leads to CO2 formation/regeneration due to respiration c. «saprotrophic bacteria only» partially decompose dead organic matter in acidic/anaerobic conditions in waterlogged soil d. results in peat formation in bogs/swamps e. photosynthetic bacteria/cyanobacteria fix CO2 in photosynthesis
b a. problem results from excessive use of antibiotics by doctors/veterinarians/in livestock OR low antibiotic doses taken by patients (not finishing treatment) b. natural variation exists in any population of bacteria making some resistant to a specific antibiotic c. variation arises from mutation OR antibiotic resistance can be transferred between bacteria by plasmids d. antibiotic kills all bacteria except those that are resistant e. resistant bacteria survive, reproduce and pass on resistance to offspring f. soon population is made of mainly antibiotic resistant bacteria g. this is an example of natural selection «increasing frequency of characteristics that make individuals better adapted to environment»
6. c a. genetic modification carried out by gene transfer between species Accept any of the points clearly explained in an annotated diagram.
b. genes transferred from one organism to another produce the same protein/amino acid sequence
c. due to universality of genetic code OR organisms use same codons of mRNA to code for specific amino acids
d. mRNA for required gene extracted/identified e. DNA copies of mRNA made using reverse transcriptase
f. PCR used (to amplify DNA to be transferred) g. genes/DNA transferred from one species to another using a vector h. plasmid acts as vector to transfer genes to bacteria/E. coli
i. plasmid cut open at specific base sequences using restriction endonuclease OR plasmid cut to produce blunt ends then extra cytosine/C nucleotides added OR sticky ends made by adding extra guanine/G nucleotides OR mention of sticky ends if not gained already
j. cut plasmids mixed with DNA copies stick together (due to complementary base pairing) k. DNA ligase makes sugar-phosphate bonds to link nucleotides of gene with those of plasmid
l. bacteria that take up plasmid are identified m. (genetically modified) bacteria will reproduce carrying the transferred gene n. example – eg: as production of human insulin using E. coli bacteria