A Level H1 Biology Practice Paper 2

TuitionGoWhere Practice Paper - Biology H1 A-Level

Answer Key and Marking Scheme Topic: Cells and Biomolecules Version: 2 of 5

Section A: Structured Questions

1. (a) A: Phosphate head (hydrophilic) [1]; B: Fatty acid tails (hydrophobic) [1] (b) Phospholipids form a bilayer [1]; Hydrophilic heads face outward towards the aqueous environment, while hydrophobic tails face inward, away from water [1]. (c) It is permeable to non-polar molecules / The interior is hydrophobic [1].

2. (a) As temperature increases, kinetic energy of enzyme and substrate molecules increases [1]; This leads to more frequent collisions [1]; More enzyme-substrate complexes are formed per unit time [1]. (b) High temperature breaks hydrogen/ionic bonds holding the tertiary structure [1]; The active site changes shape (denaturation), so the substrate no longer fits [1].

3. (a) X: Cristae (inner membrane fold) [1]; Y: Matrix [1] (b) Muscle cells require large amounts of ATP for contraction [1]; Mitochondria are the site of aerobic respiration/ATP production [1]. (c) Krebs cycle (Citric Acid Cycle) / Link Reaction [1].

4. (a) Hemoglobin consists of four polypeptide chains (subunits) [1]; These chains are held together by bonds (e.g., hydrogen bonds) to form a specific 3D shape [1]. (b) The specific 3D shape creates a binding site for heme groups/oxygen [1]; This allows reversible binding of oxygen for transport [1].

5. (a) Metaphase [1] (b) Chromosomes align at the equator/center of the cell [1]; Spindle fibers attach to the centromeres of chromosomes [1]. (c) Produces genetically identical cells for growth [1]; Replaces damaged/dead cells for repair [1].

6. (a) Glycogen is branched/helical; Cellulose is straight/unbranched [1] OR Glycogen has alpha-glucose; Cellulose has beta-glucose [1]. (b) Branched structure provides many ends for enzymes to act on, allowing rapid release of glucose [1]; Compact structure allows storage of many glucose units in a small space [1]. (c) Humans lack the enzyme (cellulase) to hydrolyze beta-1,4-glycosidic bonds [1].

7. (a) Transmembrane protein / Channel protein / Carrier protein [1] (b) "Fluid": Phospholipids and proteins can move laterally within the layer [1]; "Mosaic": Proteins are embedded in the bilayer in a scattered pattern [1].

8. (a) The tendency of water molecules to move from one region to another / Measure of the free energy of water molecules [1]. (b) Water is a polar molecule [1]; It forms hydrogen bonds with ions and other polar molecules, surrounding them and keeping them in solution [1].

9. (a) The net movement of water molecules [1] from a region of higher water potential to a region of lower water potential [1] through a partially permeable membrane. (b) Water potential of potato cells is lower (more negative) than the sucrose solution [1].

10. (a) DNA contains deoxyribose sugar; RNA contains ribose sugar [1]; DNA is double-stranded; RNA is single-stranded [1] OR DNA contains Thymine; RNA contains Uracil [1]. (b) mRNA carries the genetic code from DNA in the nucleus [1] to the ribosomes in the cytoplasm for translation [1].

Section B: Data Interpretation and Application

11. (a) The rate of reaction increases linearly/proportionally with substrate concentration [1]; Because there are more substrate molecules available to collide with active sites [1]. (b) All active sites are saturated/occupied [1]; Adding more substrate does not increase the rate because enzymes are working at maximum capacity ($V_{max}$) [1]. (c) Curve should start at the same origin, rise more slowly, and plateau at the same $V_{max}$ as the original curve [1].

12. (a) Histones allow DNA to be packed tightly into chromosomes [1]; This protects DNA and regulates gene expression [1]. (b) Prokaryotes have cell walls made of peptidoglycan (murein) [1] OR Prokaryotes lack membrane-bound organelles [1].

13. (a) Peptide bond [1] (b) Condensation [1] (c) The specific sequence of amino acids in the polypeptide chain [1]; Determined by the sequence of bases in DNA/gene [1].

14. (a) Regulates membrane fluidity / Stabilizes the membrane at high temperatures [1]. (b) Cholesterol is insoluble in water (hydrophobic) [1]; Like other lipids, it is soluble in organic solvents [1].

15. (a) pH 7 (or value indicated by peak on graph) [1] (b) Change in pH alters the charge on amino acid R-groups [1]; This disrupts ionic/hydrogen bonds maintaining the tertiary structure [1]; The active site changes shape, reducing enzyme-substrate complex formation [1].

Section C: Extended Response

16. Marking Points (Max 4):

• High specific heat capacity: Water absorbs/releases large amounts of heat with little temperature change [1]; Helps organisms maintain stable internal temperature/homeostasis [1].
• High latent heat of vaporization: Evaporation of water (sweating/transpiration) removes large amounts of heat [1]; Provides an effective cooling mechanism for organisms [1].
• Note: Accept other valid thermal properties such as high boiling point allowing liquid state over a wide range.

17. Marking Points (Max 6):

• Facilitated Diffusion: Movement of molecules down a concentration gradient [1]; Requires channel or carrier proteins [1]; Does not require energy (ATP) [1].
• Active Transport: Movement of molecules against a concentration gradient [1]; Requires carrier proteins (pumps) [1]; Requires energy (ATP) from respiration [1].
• Comparison: Both involve specific membrane proteins; Both are selective/specific to certain molecules.
• Contrast: Direction of gradient (down vs. against); Energy requirement (none vs. ATP).

18. Marking Points (Max 5):

• Named Protein: e.g., Collagen [1].
• Structure: Three polypeptide chains wound into a triple helix [1]; Held together by hydrogen bonds [1]; Cross-links between chains provide strength [1].
• Function: Provides tensile strength to tissues (e.g., tendons, skin, bone) [1]; Resists pulling forces [1].
• Alternative: Keratin (alpha-helix, disulfide bonds, waterproofing/protection).

19. Marking Points (Max 4):

• Adenine pairs with Thymine (2 H-bonds); Guanine pairs with Cytosine (3 H-bonds) [1].
• Ensures the two strands are held together securely but can be separated for replication/transcription [1].
• Ensures accurate replication: Each strand serves as a template for the new strand [1].
• Maintains the constant width of the DNA double helix [1].

20. Marking Points (Max 6):

• Membranes create separate compartments allowing incompatible reactions to occur simultaneously [1].
• Example 1: Lysosomes: Contain hydrolytic enzymes [1]; Membrane prevents enzymes from digesting the cell itself [1]; Maintains acidic pH for enzyme activity [1].
• Example 2: Mitochondria: Inner membrane creates compartments for electron transport chain [1]; Establishes proton gradient for ATP synthesis [1].
• Example 3: Nucleus: Nuclear envelope separates transcription (DNA to RNA) from translation (RNA to Protein) [1]; Allows for regulation of gene expression [1].
• General: Concentrates substrates/enzymes to increase reaction efficiency [1].