RI N21 H2 P2 Answers

2021 H2 Biology Paper 2 9744/02 1 (a) Name the structures labelled A, B and C in Fig. 1.1. [3] A middle lamella of cell wall B cytoplasm C chloroplasts (b) The magnification of the photomicrograph is x560. Calculate the length of cell X in m, along the line Y–Z. Show your working. magnification = Picture length Y-Z in mm x1000/ actual length = x 560 actual length = picture length in um/560 length of cell X =………… m [2] (c) The leaves of mosses typically consist only of a single sheet of photosynthetic tissue that is one cell thick across its entire surface. There are no air spaces. (i) Describe how carbon dioxide in the air travels to the site of photosynthesis in the leaves of the mosses. [3] 1. Carbon dioxide dissolves at the film of water surrounding/ at the cell wall and diffuses into the cytoplasm; 2. down a carbon dioxide concentration gradient; 3. Dissolved carbon dioxide further diffuses into stroma across the double membrane of chloroplast; 4. where carbon fixation takes place via Calvin cycle of photosynthesis; R: across the stomata as the leaf is single cell thick (ii) Suggest why moss plants require damp conditions in which to grow. [2] 1. Absence of vascular bundle/xylem and thus moss plants depend on water moving into cells from the surrounding via osmosis; 2. Damp conditions prevent water from being lost to surrounding via evaporation thus preventing the plant from drying up; 3. Water is needed for photolysis* during non-cyclic photophosphorylation* 4. It is also needed as a medium for diffusion of molecules and other biochemical reactions within the cell; [Total: 10] 2 (a) escribe the relationship between the speed of the ants and the temperature, as shown in Fig. 2.1. [3] 1. As temperatures increase from 9oC to 26oC, speed of ant movement increase gradually from 0.4 cms-1 to 2.8 cms-1; 2. As temperatures increase further from 26oC to 32 oC, speed of ant movement increase more sharply from 2.8 cms-1 to 4 cms-1; 3. As temperatures increase from 32oC to 38oC, speed of ant movement experience the steepest increase from 3.8 cms-1 to 6.6 cms-1;

(b) Explain the effect of temperature on the speed of the ants, as shown in Fig. 2.1. [4] 1. As temperature increase from 90C to 380C, there is increase in kinetic energy; 2. which increases frequency of effective collisions* between substrate and enzyme active sites thus rate of formation of enzyme-substrate complex* increases; 3. increasing temperature also increases number of molecules having sufficient energy to overcome activation energy barrier to form products of metabolic reaction; 4. Rate of reaction could possibly be at maximum 380C with most of active sites* of enzymes being saturated with substrate; (c) Suggest and explain what will happen to the speed of the ants as the temperature continues to increase above 40oC. [4] 1. As the temperature increases above 40°C, there will be further increase in kinetic energy thus resulting in greater (intramolecular) vibrations/ thermal agitation of the enzymes; 2. which breaks hydrogen, ionic bonds and other weak interactions that stabilizes the 3D conformation resulting in denaturation*; 3. Active site* of enzymes may no longer complementary in conformation* and charge to substrates and; 4. speed of ant movement may decrease steeply from 6.6 cms-1; [Total: 11] 3 (a) Describe the features and functions of myeloid stem cells. [3] 1. Myeloid stem cells are undifferentiated* and unspecialized* cells which are multipotent*; 2. They have ability to do self-renew* and proliferate and to differentiate* into specialised cells; 3. They are able to do self renewal to ensure a constant pool of stem cells; 4. for growth and development; 5. Their ability to differentiate into specialised cells helps to regenerate/replace cells that are lost due cell death and injury; (b) Outline the series of events that occurs during the formation of plasma cells from a B lymphocyte. [4] 1. Each B lymphocyte has one specific type of B cell receptor (BCR) on its cell surface that can recognise an epitope which is complementary* in conformation and bind to the specific antigen; 2. BCR together with bound antigen are taken into the B lymphocyte by endocytosis; 3. The antigen is processed into short peptides and attached to MHC proteins; 4. The peptide:MHC complexes are transported to cell surface membrane of B lymphocyte where the peptide is recognised by antigen-specific helper T cell; 5. Interaction between helper T cell and B lymphocyte stimulate cytokine secretion from helper T cell; 6. In turn, these cytokines activates the antigen-specific B lymphocyte to undergo clonal expansion and differentiation* into antibody-secreting plasma cells;

(c) State the main differences between myeloid stem cells and lymphoid stem cells. [3] 1. Myeloid stem cells can be differentiated into many different types of blood cells including megakaryocytes, erythrocyte, myeloblast and mast cell whereas lymphoid stems cells can be differentiated into T lymphocytes and B lymphocytes; 2. Myeloid stem cells form blood cells which have multi-lobed nucleus, e.g. neutrophil and eosinophil whereas lymphoid stem cells form blood cells which do not have a lobed nucleus; 3. Myeloid stem cells form blood cells which have granular cytoplasm e.g. basophil and macrophage whereas lymphoid stem cells form blood cells which do not have a granular cytoplasm; 4. Myeloid stem cells form blood cells which are circulated in the blood but lymphoid stem cells form lymphocytes are circulated in the lymph; [Total: 10] 4 (a) Name the structures labelled D, E and F in Fig. 4.1. [3] D: (Icosahedral) capsid* head E: Double-stranded DNA F: Tail fibre (b) Fig. 4.2 represents the two types of reproductive cycle that lambda phage viruses can carry out. (i) Name the two types of lambda phage reproductive cycle labelled P and Q in Fig. 4.2. [2] P: Lytic* cycle Q: Lysogenic* cycle (ii) Describe what happens in stage R of the lambda phage reproductive cycle shown in Fig. 4.2. [3] 1. When tail fibres of lambda phage attaches to specific receptors with complementary* shape on the bacterial host cell wall; 2. It will release lysozyme*, an enzyme which digests the bacterial cell wall resulting in release of molecules that changes base plate conformation; 3. This in turn causes the tail sheath to contract and thrust hollow tube through the bacterial cell wall; (R bacterial cell membrane); 4. Viral DNA genome will then be injected into the bacterial host cell via the hollow tube. The original linear phage DNA forms a circle; (iii) Describe the results of the two types of reproductive cycle shown in Fig. 4.2, for the host cells and for the virus. [4] Reproductive cycle P 1. Enzyme lysozyme* which is released by phage to digest bacterial cell wall to trigger lysis* of host cell; 2. Newly formed/ assembled viruses are released to infect other host bacterial cells; Reproductive cycle Q 1. Lambda phage DNA is integrated into bacterial DNA forming a prophage*; 2. every time the host cell’s machinery replicates bacterial chromosome, it also replicates prophage DNA along with it; 3. Host bacteria is not killed, and the prophage will be found in all progeny bacterial cells, where it remains latent; [Total: 12]

5 (a) Name the stages of meiosis labelled J and K in Fig. 5.1 and describe the events that occur in each of these stages. [4] name of stage J Anaphase I/ anaphase of meiosis I description of events in J separation of a pair of homologous chromosomes to opposite poles of a cell name of stage K Anaphase II/ anaphase of meiosis II description of events in K separation of sister chromatids to opposite poles to opposite poles of a cell (b) Describe the main differences between meiosis and mitosis. [4] 1.