RI 2020 A level H2 P2

2020 H2 A level Paper 2 2020 H2 Biology A Level Paper 2 [Turn over Answer all questions. 1 Fig. 1.1 is an electronmicrograph showing parts of two liver cells. (a) Name the structures labelled A, B and C in Fig. 1.1. A Mitochondrion B Nucleus C Cell surface membrane or plasma membrane [3] (b) Membranes are used within cells to create separate compartments. Explain the advantages to cells of having separate compartments. [3] 1. Within a cell, compartmentalization allows the formation of unique environments optimal for specialized activities; 2. such as enzyme reactions where enzyme and substrate can accumulate at high concentration to increase the rate of enzyme-catalyzed reaction; 3. Enzymes and substrate involved in related metabolic reactions can be localized in the same compartment to increase the rate of enzyme-catalyzed reaction; 4. e.g Enzymes and substrates involved in Krebs cycle in mitochondria matrix or Calvins cycle in stroma of chloroplast; 5. Maintenance of optimal pH for enzyme action; 6. e.g. acidic pH for lysosomal enzymes in lysosomes; 7. For the establishment of proton gradients within specialized organelles such as mitochondria and chloroplasts for chemiosmosis and formation of ATP; 8. Allows storage of molecules within a separate environment to prevent degradation by other enzymes; 9. e.g. storage of starch in amyloplast; Any 3 of the above

2 2020 H2 Biology A Level Paper 2 Table 1.1 shows the percentage of the total area of all membranes of a liver cell contributed by each type of membrane. Table 1.1 Table 1.2 shows the percentage of the total volume of a liver cell contributed by each type of membrane-bound compartment. Table 1.2 (c) Explain the difference between the percentage of total cell membrane area contributed by the inner mitochondrial membrane and the percentage of total cell membrane area contributed by the outer mitochondrial membrane, as shown in Table 1.1. [3] 1. Inner mitochondria membrane take up 25% more total cell membrane area than the outer mitochondria membrane; 2. Inner mitochondria membrane/Cristae is highly folded to increase surface area; 3. Allowing more electron carriers* of electron transport chain and ATP synthase* to be embedded in it; 4. To increase the rate of oxidative phosphorylation/ATP* production; 5. Outer mitochondria membrane is not folded, hence smaller total cell membrane area;

3 2020 H2 Biology A level Paper 2 [Turn over (d) The ratio of the percentage of total cell membrane area to percentage of total cell volume for the nucleus is approximately 0.07 : 1. (Note to student: 0.4:6 = 4:60 = 0.07:1) (see on tables 1.1 and 1.2) (i) Use Table 1.1 and Table 1.2 to calculate the ratio of the percentage of total cell membrane area to percentage of total cell volume for the smooth endoplasmic reticulum and Golgi body combined. ratio = …………………………………………… [1] 24 : 6, then reduce to lowest term = 4:1 (see on tables 1.1 and 1.2) (ii) State how the structure of the smooth endoplasmic reticulum and Golgi body accounts for the ratio calculated in (d)(i). [2] 1. Smooth ER consist of tubular network of membranes, cisternae and the Golgi body consist of a stacks of flattened sac of membranes which contributes to their high surface area; 2. Both have small cisternal space that contain small volume resulting in a higher surface area to volume ratio; [Total: 11] 2 Fig. 2.1 shows the main structural features of the human immunodeficiency virus (HIV). (a) Name the structures labelled D, E and F in Fig. 2.1.[3] D viral envelope E capsid protein F viral RNA genome (b) Describe how an enveloped virus such as HIV enters a host cell.[4] Attachment 1. The viral glycoprotein *, gp120* recognizes and binds to the CD4 receptor* on the membrane of the host cell e.g. helper T cells or macrophages ; 2. that it is complementary * in shape/conformation and charge to; Penetration & Uncoating 3. gp41* binds to co receptor ; 4. and facilitiates fusion* of the viral envelope with plasma membrane of the host cell to release the capsid into the cell leaving the envelope behind ;

4 2020 H2 Biology A Level Paper 2 (c) Anti-retroviral therapy (ART) uses multiple anti-retroviral drugs to control HIV in individuals who are infected with HIV (HIV positive individuals). Fig. 2.2 shows the number of copies of HIV RNA per cm3 plasma and the number of copies of HIV DNA per 106 T lymphocytes in an HIV positive individual over a period of 20 years. The start of anti-retroviral therapy (ART) is shown with an arrow. (i) Describe the effect of anti-retroviral therapy (ART) on the number of copies of HIV RNA per cm3 plasma in an HIV positive individual, as shown in Fig. 2.2.[2] 1. When ART started, there was a steep decrease in number of copies of HIV RNA per cm3 plasma from 100,000 copies per cm3 to 0.6 copies per cm3 within 1 year from year 2 to year 3; (compulsory point) 2. Thereafter remained low between 0.6 copies per cm3 to and 300 copies per cm3 from year 2 to year 20; A: Thereafter there was a gradual increase with some fluctuation up to 300 copies per cm3 until year 14, then a gradual decrease to about 12 copies per cm3 at year 20; (ii) Suggest why HIV RNA is measured in the plasma and HIV DNA is measured in the T lymphocytes.[2] 1. HIV has RNA genome and the amount of RNA is a measure of HIV viruses in the plasma; 2. The amount of DNA in T lymphocytes is a measure of HIV-infected cells as T lymphocytes are the host cell of the virus and upon successful infection, HIV RNA genome is reverse transcribed to form DNA which is integrated into host cell genome as a provirus* during the latent phase; [Total: 11] 3 Proteins are one constituent of cell membranes. Fig. 3.1 shows the molecular structure of three other constituent biomolecules of cell membranes, J, K and L. (a) Name biomolecules J, K and L. J Phospholipid; K Glycolipid; L Cholesterol;

5 2020 H2 Biology A level Paper 2 [Turn over (b) Describe the formation of the bonds labelled M in Fig. 3.1.[2] 1. An ester * linkage is formed between a hydroxyl* group (-OH) of a glycerol molecule and a carboxyl* group (-COOH) of a fatty acid chain; 2. via a condensation reaction* whereby a molecule of water* is removed; (c) Explain the roles of biomolecules K and L in the cell surface membrane.[3] 1. K - glycolipid acts as markers for cell-cell recognition* to distinguish cells as ‘self’/’non-self’ as the basis of the immune system; Or Cell-cell recognition* also result in cell adhesion allowing cells to be attached to one another to form tissues and organs; 2. Acts as receptor* for certain pathogens and toxins to bind to host cells and gain entry into cell; 3. L - Cholesterol regulates membrane fluidity * by preventing the membrane from being overly fluid at warmer temperatures as cholesterol’s rigidity restricts phospholipids’ lateral movement; 4. The membrane is prevented from being overly firm at lower temperatures as cholesterol prevents the close packing of phospholipids and hence prevents its solidification/ crystallization; [Total: 8] 4 Sickle cell anaemia is a genetic disease caused by a recessive mutant allele of the β-globin gene. Scientists investigated the inheritance of the disease in one family. Fig. 4.1 shows the inheritance of sickle cell anaemia in this family. (a) Describe how PCR is carried out to amplify only the region of DNA that includes the β- globin gene and not other regions of the DNA. [2] 1. Design and use primers* with nucleot