A Level H1 Biology Practice Paper 3

TuitionGoWhere Exam Practice (AI) - Biology H1 A-Level

Answer Key and Marking Scheme Topic: Cells and Biomolecules
Version: 3 of 5

Section A: Structured Questions

1. (a) A: Phosphate head / Hydrophilic head [1] B: Fatty acid tails / Hydrophobic tails [1]

(b) Phospholipids form a bilayer [1]. The hydrophilic heads face outward towards the aqueous environment (cytoplasm/tissue fluid) and the hydrophobic tails face inward, away from water [1].

(c) The hydrophobic interior prevents the passage of water-soluble (polar/charged) substances [1]. This allows the cell to control what enters and leaves (selective permeability) / maintains distinct internal environment [1].

2. (a) 40°C [1]

(b) High temperature causes the enzyme to denature [1]. The hydrogen bonds (and other bonds) maintaining the tertiary structure break [1]. The shape of the active site changes, so the substrate no longer fits / enzyme-substrate complexes cannot form [1].

(c) At low temperatures, molecules have low kinetic energy [1]. There are fewer successful collisions between enzyme and substrate per unit time [1].

3. (a) Method X: Active Transport [1] Method Y: Facilitated Diffusion [1] (Note: Assuming Fig 3.1 shows X moving against gradient with ATP, and Y moving down gradient via channel/carrier)

(b) Method X requires ATP / energy; Method Y does not [1].

(c) Glucose is a large / polar molecule [1]. It cannot pass through the hydrophobic fatty acid tails of the phospholipid bilayer [1].

4. (a) Pyruvate can enter the mitochondria and undergo the Link Reaction and Krebs cycle, which produce reduced NAD/FAD for the Electron Transport Chain (ETC) where oxygen is used [1]. Glucose cannot enter the mitochondria directly [1]. Glucose must first be broken down into pyruvate via glycolysis, which occurs in the cytoplasm, not in isolated mitochondria [1].

(b) Krebs Cycle / Citric Acid Cycle [1]

5. (a) Water molecules are polar (dipole) [1]. They form hydrogen bonds with ions/polar molecules, surrounding them and keeping them in solution [1].

(b) Any two of:

1. High specific heat capacity [1]
2. High latent heat of vaporisation [1]
3. High thermal conductivity [1]

Section B: Data Response and Extended Structured Questions

6. (a) Both contain glycerol and fatty acids [1]. Triglycerides have 3 fatty acids; Phospholipids have 2 fatty acids and 1 phosphate group [1]. Triglycerides are non-polar/hydrophobic; Phospholipids are amphipathic (have hydrophilic head and hydrophobic tails) [1].

(b) The fatty acid tails contain many C-H bonds which store high energy [1]. They are insoluble in water, so they do not affect the water potential of cells/osmosis [1]. They can be packed densely without water, providing more energy per gram than carbohydrates [1].

(c) The kinks prevent the fatty acid tails from packing closely together [1]. This increases the fluidity of the membrane [1]. At low temperatures, this prevents the membrane from becoming too rigid/solidifying [1].

7. (a) Primary: Sequence of amino acids [1]. Secondary: Alpha-helix or beta-pleated sheet formed by hydrogen bonds [1]. Tertiary: 3D folding of polypeptide chains held by ionic, hydrogen, disulphide bonds [1]. Quaternary: Association of multiple polypeptide subunits (4 in haemoglobin) [1].

(b) Change in primary structure changes the sequence of amino acids [1]. This alters the R-group interactions, changing the tertiary structure/shape of the protein [1]. This changes the shape of the heme group or oxygen binding site, reducing oxygen carrying capacity / causing polymerization (sickling) [1].

(c) Globular: Compact, spherical, soluble, metabolic function (e.g., Haemoglobin/Insulin) [1.5 for description + example]. Fibrous: Long, insoluble, structural function (e.g., Collagen/Keratin) [1.5 for description + example].

8. (a) Sugar: Deoxyribose (DNA) vs Ribose (RNA) [1]. Bases: Thymine (DNA) vs Uracil (RNA) [1]. Structure: Double helix / Double stranded (DNA) vs Single stranded (RNA) [1].

(b) Ensures accurate replication / fidelity [1]. Each strand acts as a template for the new strand [1]. Complementary base pairing ensures the new DNA molecule is identical to the original [1].

(c) 1200 nucleotides / 3 (bases per codon) = 400 codons [1]. One codon codes for one amino acid (stop codon does not code for amino acid, so max is 399 or 400 depending on interpretation, but usually 1200/3 = 400 is accepted for 'maximum' unless stop is specified. Accept 399-400). Answer: 400 (or 399) [1].

9. (a) As oxygen concentration increases, the rate of potassium uptake increases [1]. The rate levels off / reaches a maximum at higher oxygen concentrations [1].

(b) Uptake is by active transport [1]. Active transport requires ATP / energy [1]. ATP is produced by aerobic respiration, which requires oxygen [1].

(c) All carrier proteins are saturated / working at maximum rate [1]. Limited number of carrier proteins available [1].

10. (a) The region on the enzyme where the substrate binds [1].

(b) The active site is not rigid/complementary to the substrate initially [1]. The substrate induces a change in the shape of the active site [1]. This creates a precise fit for the substrate (enzyme-substrate complex) [1].

(c) Competitive inhibitor has a similar shape to the substrate [1]. It binds to the active site [1]. It blocks the substrate from binding / reduces the frequency of successful collisions between enzyme and substrate [1].

11. (a) To reduce enzyme activity / prevent digestion of organelles by lysosomal enzymes [1].

(b) To maintain pH / prevent denaturation of enzymes/proteins [1].

(c) To remove unbroken cells / large tissue debris [1].

(d) Spin at low speed to pellet heaviest organelles (nuclei) [1]. Remove supernatant and spin at higher speed to pellet next heaviest (mitochondria/chloroplasts) [1]. Repeat at increasing speeds to separate lighter organelles (ribosomes) [1].

12. (a) Any two: Nucleus, Membrane-bound organelles (mitochondria, Golgi, ER), Linear DNA, Histones, 80S ribosomes [1 each].

(b) 70S [1]

(c) Human cells have 80S ribosomes which have a different structure [1]. The antibiotic specifically targets the 70S ribosome structure / binding site [1].

13. (a) Alpha-glucose / Glucose [1]

(b) It is insoluble, so it does not affect water potential [1]. It is compact/coiled, allowing storage of many units in small space [1]. It is highly branched, allowing rapid hydrolysis/release of glucose when needed [1].

(c) Cellulose contains beta-glucose [1]. Humans lack the enzyme (cellulase) to break the beta-1,4-glycosidic bonds [1].

14. (a) Fluid: Phospholipids and proteins can move laterally [1]. Mosaic: Proteins are embedded in the bilayer in a scattered pattern [1].

(b) Any two:

1. Regulates membrane fluidity (prevents packing at low temp / stabilizes at high temp) [1].
2. Reduces permeability to small water-soluble molecules [1].
3. Mechanical stability [1].

(c) Cell recognition / Cell signalling / Antigen receptor [1].

15. (a) The tendency of water molecules to move from one region to another / The pressure exerted by water molecules [1].

(b) 0 [1]

(c) Water leaves the cell by osmosis [1]. The vacuole shrinks and the cytoplasm pulls away from the cell wall [1]. The cell becomes plasmolysed [1].

16. (a) Adenine (nitrogenous base) [1]. Ribose (pentose sugar) [1]. Three phosphate groups [1].

(b) Releases a small, manageable amount of energy suitable for cellular reactions [1]. Hydrolysis is a single-step reaction / releases energy quickly [1].

(c) Any one: Active transport, Muscle contraction, Protein synthesis, DNA replication, Nerve impulse transmission [1].

17. (a) Rate increases as substrate concentration increases [1]. More enzyme-substrate complexes form per unit time / More frequent collisions [1].

(b) All active sites are saturated / occupied [1]. Enzyme concentration is the limiting factor [1].

(c) Curve rises more steeply initially and plateaus at a higher rate (Vmax doubles) [1].

18. (a) Three polypeptide chains wound together to form a triple helix [1]. Held together by hydrogen bonds [1]. Many triple helices bundle together to form fibrils/fibres [1].

(b) High tensile strength [1]. Resists pulling forces / stretches without breaking [1].

19. (a) DNA replication / DNA synthesis [1].

(b) Cell grows in size [1]. Synthesis of organelles and proteins required for division [1].

(c) Chromatin: Uncondensed, long thin threads, present during interphase [1]. Chromosomes: Condensed, thick, visible structures, present during mitosis [1].

20. (a) Genetic material (DNA or RNA) [1]. Protein coat / Capsid [1].

(b) They cannot reproduce independently / require a host cell [1]. They do not carry out metabolism / do not have cellular structure [1].

(c) Derived from the host cell membrane (during budding) [1].