A Level H1 Biology Practice Paper 3

Answer Key - Biology H1 Practice Paper (Version 3)

Section A: Cell Structure and Biomolecules

Question 1 (a) RER is the site of protein synthesis [1]; ribosomes on the RER synthesize proteins that are destined for secretion or membrane insertion [1]. (b) Cardiac muscle cells have a higher metabolic demand for ATP [1] to power continuous contractions [1]. Mitochondria are the site of aerobic respiration/ATP production [1]. (c) Phospholipids are arranged in a bilayer [1] with hydrophilic heads facing the aqueous environments (extracellular/cytosol) [1] and hydrophobic tails facing inward [1]. This prevents the free passage of polar/charged molecules [1], allowing the cell to control the internal environment via transport proteins.

Question 2 (a) X: Protein (Collagen), Y: Protein (Hemoglobin), Z: Lipid (Triglyceride). [3] (b) A change in primary sequence (amino acid sequence) [1] alters the folding/tertiary structure of the protein [1]. This changes the shape of the active site/binding site [1], preventing the molecule (e.g., oxygen) from binding effectively [1]. (c) Triglycerides have one glycerol molecule bonded to three fatty acids [1]. Phospholipids have one glycerol, two fatty acids, and one phosphate group [1]. Phospholipids are amphipathic, whereas triglycerides are entirely hydrophobic [1].

Question 3 (a) Increase in temperature increases the kinetic energy of molecules [1]. This leads to more frequent successful collisions between enzyme and substrate [1]. (b) High temperatures break the hydrogen/ionic bonds maintaining the tertiary structure [1]. The active site changes shape (denaturation) [1], so the substrate can no longer fit/bind [1]. (c) Non-competitive inhibitors bind to an allosteric site [1]. This changes the shape of the active site, reducing the maximum rate of reaction ($V_{max}$ decreases) [1]. Since the inhibitor does not compete for the active site, the affinity for the substrate ($K_m$) remains unchanged [1].

Section B: Genetics and Cellular Processes

Question 4 (a) Affected: aa [1]; Carrier: Aa [1]. (b) Parents: Aa x Aa. Gametes: A, a and A, a. Punnett square showing AA, Aa, Aa, aa [2]. Probability of affected (aa) = 1/4 or 25% [2]. (c) Males are hemizygous (XY) [1]. They only have one X chromosome, so a single recessive allele on the X chromosome will be expressed [1]. Females (XX) need two copies of the recessive allele to express the trait [1].

Question 5 (a) DNA helicase unwinds the double helix [1]. Each strand serves as a template [1]. DNA polymerase adds complementary nucleotides (A-T, C-G) [1] in the 5' to 3' direction [1]. Result is two identical DNA molecules, each with one original and one new strand [1]. (b) The reading frame is the specific grouping of three nucleotides into codons [1]. A deletion shifts this frame for all subsequent codons [1]. This leads to a different sequence of amino acids [1] and often a premature stop codon [1], resulting in a non-functional protein [1].

Question 6 (a) DNA is replicated exactly once during S phase [1]. Sister chromatids are formed [1]. During anaphase, spindle fibers pull identical sister chromatids to opposite poles [1]. Each daughter cell receives an identical set of chromosomes [1]. This ensures the genotype remains constant across cell generations [1]. (Max 6: focus on replication, separation, and identity). (b) Mitosis: 2 genetically identical daughter cells [1], diploid (2n) [1]. Meiosis: 4 genetically distinct daughter cells [1], haploid (n) [1].

Question 7 (a) CO2 enters the leaf via diffusion across the stomata and cell membranes [2] down a concentration gradient. Water enters root cells via osmosis [2] across semi-permeable membranes. Ions (e.g., Mg2+ for chlorophyll) are taken up via active transport [2] against a gradient using ATP. Glucose is exported from chloroplasts/cells via facilitated diffusion or active transport [2]. (b) Thylakoids are stacked into grana [1], increasing the surface area for photosystems/chlorophyll [1]. The thylakoid membrane maintains a proton gradient [1] for ATP synthesis via chemiosmosis [1]. The stroma provides a space for the Calvin cycle [1]. (Max 6).

Question 8 (a) B-cell binds specific antigen via surface receptors [1]. Antigen is processed and presented [1]. Helper T-cells recognize the antigen and release cytokines [1]. B-cell is activated and undergoes clonal expansion [1]. B-cells differentiate into plasma cells [1] and memory cells [1]. Plasma cells secrete antibodies [1]. (b) Active: Body produces its own antibodies after exposure to antigen/vaccine [1]. Example: Recovery from flu [1]. Passive: Antibodies are transferred from another source [1]. Example: Colostrum/Breast milk or anti-venom [1]. (Max 5).

Question 9 (a) Variation exists in bacteria due to mutation [1]. Antibiotics kill susceptible bacteria [1]. Bacteria with resistance alleles survive [1]. These survivors reproduce (binary fission) [1], passing the resistance gene to offspring [1]. Over time, the resistant strain becomes dominant in the population [1]. (b) Allopatric: Speciation due to geographic isolation [1]. Mechanism: A physical barrier (e.g., mountain) divides a population, leading to different selective pressures [1]. Sympatric: Speciation within the same geographic area [1]. Mechanism: Behavioral or temporal isolation (e.g., different mating seasons) [1]. (Max 6).

Question 10 (a) Energy is lost as heat during respiration [1]. Not all biomass is consumed by the next level [1]. Some energy is lost as undigested waste/faeces [1]. Only about 10% of energy is transferred to the next trophic level [1]. (Max 5). (b) Pyruvate is converted to Acetyl-CoA [1]. Acetyl-CoA enters the Krebs cycle [1]. Through a series of redox reactions, NAD+ and FAD are reduced to NADH and FADH2 [1]. These carriers transport high-energy electrons to the ETC [1] to drive ATP synthesis [1].