1 02107020/CAPE/KMS 2021 C A R I B B E A N E X A M I N A T I O N S C O U N C I L CARIBBEAN ADVANCED PROFICIENCY EXAMINATION® BIOLOGY UNIT 1 - Paper 02 KEYS AND MARK SCHEME MAY/JUNE 2021 - 2 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME Question 1 (a) Diagram N.B.- Candidates are not expected to draw in the several less dense areas within the cytoplasm in their diagram 1. Faithfulness of representation - Clear accurate line representation of specimen- single line for cell membrane and cell wall separately 2. Clean continuous lines of even thickness 3. Use of label lines that do not cross and must touch structure. 4. Accurate labelling (at least three correct labels) 5. Annotations are written beside the labels 6. Features correctly proportioned 7. Suitable title below the drawing- name of organism 5-7 points 4 marks 3-4 points 3 marks 2 points 2 marks 1 point 1 mark 4 marks XS Magnification Reasonable value (140,000-500,000). If magnification is placed beside title, award mark for magnification, provided it is correct 1 mark XS - 3 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME Annotations N.B.-Annotations should be written beside the labels. They are placed below for clarity purposes. Annotations can be structural/functional. • • • • • • Cell/plasma membrane - controls the movement of substances in and out of cells/partially permeable/acts as a barrier/allows only certain substances to pass through/composed of a phospholipid bilayer. Cell wall – composed of peptidoglycan/murein/gives structural strength /provide protection/ stops cells from bursting if it expands Cytoplasm – contains all the enzymes needed for metabolic reactions/composed of water, salts and proteins/ no membrane bound organelles / site of metabolic reactions/cellular reactions Cytosol- aqueous component of cell/gel like Nucleoid - region containing naked/circular DNA Ribosomes - synthesizes proteins/translates mRNA/70S Any 3 correct annotations given – 1 mark each 3 marks KC [8 marks] Question 1 cont’d (b) (i) Similarities • • • • • • • • (Have a) nucleus/ nuclear membrane (Have a) cytoplasm (Have a) cell/plasma membrane (Contain) mitochondria (Contain) ribosomes (Contains) vacuoles (Contains) endoplasmic reticulum/ER / Smooth ER / Rough ER (Contains) Golgi body/apparatus Any 3 similarities – 1 mark each [3 marks KC] (ii) • Plant cells have chloroplasts/ thylakoid (1) which contain a pigment called chlorophyll which captures the energy from the sun through a series of chemical reactions/photosynthesis, the - 4 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME • • • • sun’s energy is converted into ATP to provide energy / food (starch, sugars) / photosynthate for the plant/ Site of photosynthesis/site of starch production/ starch storage. (1) Plant cells, have a rigid cell wall (1) that gives the cell strength and structure/ The cell wall prevents the cells from bursting (in the presence of excess water)/resists turgor pressure/provides protection. (1) Central/large/permanent vacuole (which is surrounded by tonoplast) (1) which stores cell sap (sugars/ pigments/enzymes/ waste products) / regulates the osmotic properties of cell (1) Plant cells can have plasmodesmata (1) which allows movement of substances between adjacent plant cells (1). Plant cells can have an amyloplast (1), which is used for the storage of starch granules (1). For cell sap, accept either cell sap OR any two components Any two Identification of 2 structure =2mks, function of each structure= 2x 1mk =2mks [4 marks KC] Question 1 cont’d (c) Marking Instructions For any THREE differences fully discussed, award 5 marks for EACH as follows: Award mark Award - 1 mark for identifying the key difference (seen or implied) – 1 2 marks for expanding on the difference in starch Describing the structure of starch – 1 mark each Explaining the importance of the structural difference – 1 mark Award 2 marks for expanding on the difference in cellulose - Describing the structure of cellulose – 1 mark each - Explaining the importance of the structural difference – 1 mark Annotated drawing may be used to describe structure of starch/cellulose for 1 mark each Sample Responses There is a difference in composition / monosaccharides from which it is formed /type of polysaccharides that makes it (1). Starch is made up of alpha glucose (monomers)(1). Cellulose is made up of beta glucose (monomers) (1) / Starch is a mixture of two types polysaccharides (Amylose and Amylopectin) (1). Cellulose is made up of only one polysaccharide (1). Presence of amylose and amylopectin makes starch a good energy storage molecule (1). Monomers that make up the structure of cellulose allows it to give strength to the cell/cell wall (1). - 5 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME There is a difference in type of chain formed / type of bond between monomers (glucose units)/type of (glycosidic) bonding (1). Starch has a mixture of branched and linear chains (1) whereas cellulose has only linear/unbranched chains (1). Starch is composed of alpha-1,4glycosidic bonds (which makes it linear) and alpha-1,6-glycosidic bonds (which allows for branching)(1). Cellulose is composed of beta1,4 glycosidic bonds (linkages), which allows the molecules to form long and straight chains (1). There is a difference in the orientation of monomers (1). In cellulose alternate beta glucose molecules are rotated 180 degrees (1). In starch the glucose molecules have the same orientation/are not rotated 180 degrees (1). In cellulose, rotation of alternate beta-glucose molecules allows glycosidic bonds (beta-1,4) to be formed between alternate glucose molecules and gives the molecule its strength/rigidity (1). Absence of rotation contributes to the function of starch as a storage molecule as it can be packed into a smaller area /uses less space in the cell (1). There is a difference in overall shape of molecule (1). Amylose molecules coil up into a spiral/helix and is compact (while amylopectin molecule spirals to a lesser extent) (1). Cellulose molecules do not coil but lie straight (linear)/parallel chains (1). Hydrogen bonding occurs within the molecules of amylose and amylopectin, which allows it to act as a storage molecule (1). Hydrogen bonding occurs between molecules of cellulose/between -OH of neighbouring cellulose molecules allowing it form fibers and fibrils /Cellulose forms parallel chains, which are held together by hydrogen bonds which gives the tensile strength (rigidity) (1). Any 3, 5 marks each, [15 marks UK] Total 30 marks - 2 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME Question 2 (a) (i) Labels to identify the parts of the DNA molecule S T U V - phosphate deoxyribose cytosine thymine Accuracy (with label lines) 3-4 correct labels – 3 marks 2 correct labels – 2 marks 1 correct label - 1 mark 1 mark for label lines not crossing (at least two drawn) 1 mark for no arrowheads on all line IF NO LABEL LINES: 3-4 correct labels – 2 marks 1-2 correct labels - 1 mark [5 marks XS] (ii) Identify a nucleotide. - 3 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME Any 1 shown on diagram – 1 mark – KC Question 2 cont’d (iii) How two nucleotides in a DNA molecule are held together 1. Held together by hydrogen bonds between bases 2. A to T and C to G / purine to pyrimidine / base pairing 3. 2 hydrogen bonds between A and T/ 3 hydrogen bonds between C to G 4. Phosphodiester bonds between nucleotides on a chain/backbone DO NOT ACCEPT HYDROGEN BONDS OR PHOSPHODIESTER BONDS ONLY (iv) Any 1, 1 mark KC Roles of messenger RNA (mRNA) and transfer RNA (tRNA) in the production of polypeptides mRNA 1. Provides instructions/information copied from the DNA (base sequence)/transcription /gene/ genetic code for the formation of a chain of amino acids in a polypeptide/protein (protein synthesis) 2. Used to transfer genetic code (from nucleus (DNA)) to ribosomes (in cytoplasm to make proteins / polypeptide) - 4 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME 1 mark each = 2 marks tRNA 3. Deciphers/translates the codes / codons on mRNA sequence into a sequence of amino acids in a polypeptide/protein /attaches anticodon which are complementary to codons on mRNA and specific amino acids 4. tRNA binds/attaches to a specific amino acid (on its amino acid binding site according to anticodon and complementary codon) 5. tRNA transfers the specific/appropriate amino acid (to the end of growing amino acid chain/polypeptide/protein) on the ribosome Any 2 points 1 mark each = 2 marks 4 marks KC Question 2 cont’d (b) (i) (ii) Change in the gene which results in the production of abnormal haemoglobin • one base is changed/substituted in the sequence of DNA(coding for normal Hb) / Adenine /A replaces thymine/T/ CTT is changed to CAT /single base substitution 1 mark How low oxygen saturation results in a change of the shape of red blood cells • • • • Valine forms a bond between themselves/hydrophobic amino acidcannot interact with water-but can interact with each other This sticks the haemoglobin molecule together Long fibres of stuck together haemoglobin molecules are produced As fibres are formed inside the red blood cells(erythrocytes), they pull the cells out of the biconcave shape/ deform the biconcave disc into sickle shaped cells Any 3 points, 1 mark each [3 marks] (c) (i) Success of gene therapy of sickle cell anaemia (From data) - 5 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME • • • Gene therapy was effective in treating sickle cell anaemia After 12 months, patient sickled red cell was much lower than untreated people with the diseases (controls 3,4,5) /there was 15-19% lower sickled red cells than untreated people with the disease. After 12 months, sickled red cell were not significantly different from his asymptomatic mother /there was only 1-4% difference in sickled red cells compared to the asymptomatic mother. 1 mark each 3 marks Question 2 cont’d (c) (ii) Marking Instructions For any THREE challenges fully discussed, award 4 marks for EACH as follows: Award 1 mark for stating a relevant challenge Award 3 marks for expanding on the challenge - Award 3 marks for a complete discussion with at least THREE relevant and accurate supporting points. - Award 2 marks for a partial discussion with at least TWO relevant and accurate supporting points. - Award 1 mark for a limited discussion with at least ONE relevant and accurate supporting point. Sample Responses: Delivering the gene to the right place (1). Gene therapies only work if a normal gene is delivered to a large number of cells in the correct tissue/crucial new gene reaches correct cells for success of gene therapy (1). There may be challenges with the vectors which results in the gene being delivered to the wrong cells (1). If delivered to the - 6 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME wrong cells, it would be inefficient and cause health problems for patient (1). Activating the gene/Switching on the gene (1). Gene therapies require the inserted gene results in the production of new proteins (1). Once the gene is delivered to the right cell, it must be activated or turned on to make required proteins (1). Gene once activated must remain on to shut down genes that are too active or show unusual activity (1). Avoiding the immune response/delivering gene without triggering immune system (1). The role of the immune system is to fight off pathogens such as bacteria and viruses (1). Gene delivery agents are usually viruses which may trigger an immune response in patients (1) this could cause serious illness or even death (case in USA of death cause by inflammatory response to adenovirus used to transfer gene) (1). Making sure the new gene does not disrupt the function of other genes (1). Good gene therapy involves the new gene introduced will integrate into patient’s genome and work for rest of their lives (1). There is a risk that the gene will insert itself into path of another gene disrupting its activity (1) this could interfere with important genes involved in regulating cell division which leads to cancer (tumour formation)/ leukaemia formation in SCID patients when gene stitched itself to a gene regulating cell division / insertion of gene into protooncogene/cancer causing gene (1). The effects of gene therapy are usually short lived (1). Cells containing therapeutic DNA should be long lived and stable in order to have a permanent cure (1). Only treatment of somatic cells is allowed and somatic cells die and are replaced frequently (e.g lung cells) (1). Patients therefore need to undergo multiple treatments to achieve consistent relief (1). Type of genetic diseases that can be treated (1). Diseases that are caused by a (single) recessive gene are usually treated because it does not have to be removed/inactivated/switched off (1) The dominant allele can mask defective recessive allele when added to the genome (1) however a disease caused by a dominant allele would have to be inactivated/turned off which is difficult to do (since many processes have to be turned off)(1). Cost of gene therapy/commercial viability/ Ethical issue of equal access to treatment (1). Many genetic disorders that can be targeted by gene therapy are extremely rare and requires an individual case by case approach (1). Treatment becomes costly since there is a need for repeated treatments/ many patients not able to afford treatment (1). Developing new therapy is very expensive (1) pharmaceutical firms may not want to develop treatment because they cannot make a profit (1). Any 3, 4 marks each [12 marks UK] - 2 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME Question 3 (a) (i) 1. 2. 3. 4. Plot points correctly for each hormone - 2 marks Correct labelling for both X- and Y-axes with units - 1 mark Appropriate key (legend) labelled/curves identified - 1 mark Appropriate scale used (25% to 100% of area used) - 1 mark [5 marks XS] (ii) 1. Increased FSH (around Day 5) stimulate maturation of follicle (secondary follicle to graafian follicle/ dominant follicle) 2. LH/FSH stimulate dominant follicle to secrete oestrogen, 3. oestrogen has negative feedback on LH and FSH/surge in LH and FSH as oestrogen continues to increase. 4. LH builds up pressure in the antrum/weakens follicular walls leading to bursting of the follicle. 5. surge of the hormones stimulates release of secondary oocyte from follicle into oviduct) Any 4 points = 1 mark each = [4 marks KC] (b) 1. progesterone stimulates thickening of the uterine lining (endometrium), 2. cells on surface of blastocyst (trophoblasts) secrete enzymes to partially digest cells in endometrium /cause tissues in the endometrium to break down, 3. blastocyst burrows into the endometrium, 4. trophoblast cells produce tiny/finger-like projections (villi)/trophoblast cells differentiate (into an outer layer, the chorion), forming the chorionic villi, 5. villi project/grow into endometrium, 6. trophoblast secretes hCG, which stimulates corpus luteum to continue producing progesterone, 7. endometrium also forms blood spaces (sinuses) around villi, (whole structure called placenta) - 3 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME Any 4 points [4 marks KC] Question 3 cont’d (c) (i) • • A/Ovary wall develops into pericarp / fruit wall of fruit B/Ovule/embryo sac develops into seed 1 mark for each correct description [2 marks KC] (ii) Marking Instructions For any THREE advantages fully discussed, award 5 marks for EACH as follows: Award 1 mark for stating a relevant advantage Award 3 marks for expanding on the challenge - Award 3 marks for a complete discussion with at least THREE relevant and accurate supporting points. - Award 2 marks for a partial discussion with at least TWO relevant and accurate supporting points. - Award 1 mark for a limited discussion with at least ONE relevant and accurate supporting point. Award 1 mark for providing a limitation linked to the advantage discussed (Any one of the limitations underlined) Sample Responses: Maintenance of cultivar characteristics/preservation of useful parental characteristics in next generation/Desirable characteristics are maintained in the offspring (an example: flowers of same colour, tea leaves, apples, mangoes) (1). New plants contain genetic material of only one parent /clones of parent plant (which is not the case in sexual reproduction) (1). This is especially important for commercial growers who want to reproduce the highest quality plants and ensure consistency of plant or crop for sale (1). It helps maintain consistent quality and taste in produces made from plants or crops/ batch after batch of standard plants can be produced ensuring reliability and quality control (1). There is an increased risks due to pest and diseases due to limited genetic diversity OR if a particular plant clone is susceptible to certain diseases there is a potential to lose the entire crop OR The method cannot be applied to all plant species OR Decreased genetic diversity that may have advantageous characteristics OR Decreased ability to adapt in changing environment (1) - 4 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME Rapid propagation/large number of plants in a short period of time (1). Plantlet formation is induced from cultured tissue cells, resulting in rapid multiplication by tissue culture (1). This allows rapid production of a large numbers of plants from just one or a few stocks plants (1). Plants can be produced anytime of the year/ allyear round availability of planting material OR It allows the balancing of crop losses as asexual reproduction makes it possible to rapidly regenerate a current generation of crops and minimize losses(1). There is a high cost of tissue culture facilities and training / must be carried out in sterile environment / labour intensive OR There is the Possibility of off-types due to mutation during cell culture / possibility of plants being sterile OR There is competition for resources between parent plant and offspring (1). Propagation of species that do not produce seeds/non-viable seeds/ too few seeds/ where seed productions is uncommon / process makes possible production of seedless varieties(1). The seedless plants (examples: banana, orange, rose, jasmine) can be propagated by several natural/artificial methods (tissue culture, fragmentation, runners, stolons, suckers, grafting and budding) without germination of seeds/does not need to depend on fertilization to reproduce (1). These methods reduce the complexity of obtaining offspring for market, as the difficult processes of seed harvesting and germination are avoided (1). Plants can be produced artificially by cuttings (e.g. sugarcane, African Violets) / Exotic plants such as orchids (that are hard to produce by seeds) can be cloned in large numbers by tissue culture (1). The methods are Labour intensive to plant large numbers of plants needed compared to mechanical drilling of a large field of seeds OR Training required for methods like grafting/tissue culture (1). Easier management of plants (1). All plants have identical genetic makeup therefore need the same environmental conditions/all plants of the same cultivar can be taken care of under the same environmental conditions (1) Therefore, field operations such as pest and disease management and fertilizers can be synchronized / Disease free plants/virus free plants can be produced from parental plants (1). Plants grown using tissue culture are light weight and small in size mean the plants can be air-freighted easily/takes up very little space compared to fields (1). The root systems may not be as robust as plants propagated from seeds OR The method cannot be applied to all plants OR Difficulty in adapting to change in environmental conditions (1). - 5 - 02107020/CAPE/KMS 2021 BIOLOGY UNIT 1 – PAPER 02 KEY AND MARK SCHEME Question 3 cont’d Faster maturity of crops (1). Plants reach the mature phase/flowering sooner than using seeds (1). This shortened growing time makes it possible for multiple yields in some environments (1). Faster maturity of crops save a lot of time and money for commercial plant production (1). The root systems may not be as robust as plants propagated from seeds OR Faster maturation leads to nutrient deficiency similar to the parent plant (1). **any other plausible advantages discussed. Any 3, 5 marks each 15 marks UK Total 30 marks
