A Level H2 Biology Practice Paper 3

TuitionGoWhere Practice Paper - Biology H2 A-Level

Answer Key and Marking Scheme (Version 3)

Topic: Cells and Biomolecules

Section A: Structured Questions

1. (a) Amphipathic nature / Has hydrophilic head and hydrophobic tail. [1] (b)

• The interior of the bilayer is hydrophobic (non-polar). [1]
• Non-polar molecules are soluble in the lipid layer and can diffuse through. [1]
• Ions are charged/polar and are repelled by the hydrophobic core / cannot dissolve in the lipid layer. [1] (c)
• Restricts the movement of phospholipids. [1]
• Reduces membrane fluidity / maintains stability. [1]

2. (a)

• As temperature increases, kinetic energy of enzyme and substrate molecules increases. [1]
• More frequent collisions between enzyme and substrate. [1]
• More enzyme-substrate complexes are formed per unit time. [1] (b)
• High temperature breaks hydrogen bonds / ionic bonds holding the tertiary structure. [1]
• The active site changes shape / denatures. [1]
• Substrate can no longer bind to the active site / no enzyme-substrate complexes formed. [1] (c) Some enzyme molecules may not have denatured yet / reaction is slowing but not stopped instantly. [1]

3. (a) A: Adenine (base) [1] B: Ribose (sugar) [1] C: Phosphate group [1] (b)

• Releases small, manageable amounts of energy suitable for cellular processes. [1]
• Can be rapidly regenerated / recycled. [1] (c)
• Electrons from reduced NAD/FAD pass along the electron transport chain (ETC). [1]
• Energy released is used to pump protons ($H^+$) from the matrix into the intermembrane space. [1]
• This creates an electrochemical gradient / proton motive force. [1]
• Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi. [1]

4. (a)

• Primary: Sequence of amino acids. [1]
• Secondary: Hydrogen bonding forms alpha-helices or beta-pleated sheets. [1]
• Tertiary: 3D folding due to interactions between R-groups (hydrogen, ionic, disulfide bonds). [1]
• Quaternary: Four polypeptide chains (subunits) associated together. [1] (b)
• Contains haem groups with iron ($Fe^{2+}$) that bind oxygen. [1]
• Cooperative binding / conformational change allows efficient loading and unloading of oxygen. [1]

5. (a) (i) Positive / Orange-red precipitate. [1] (ii) Negative / Remains brown/yellow. [1] (b)

• Glucose is a small molecule / monosaccharide. [1]
• It can diffuse through the pores of the dialysis tubing. [1] (c)
• Add respiratory inhibitor (e.g., cyanide) to stop ATP production. [1]
• Or use a living tissue model (e.g., root hair cells) instead of dialysis tubing. [1]

Section B: Data and Diagram Interpretation

6. (a) X: Mitochondrion [1] Y: Rough Endoplasmic Reticulum (RER) [1] (b) Site of aerobic respiration / ATP production. [1] (c)

• Liver cells synthesize many proteins (e.g., enzymes, plasma proteins). [1]
• RER has ribosomes for protein synthesis. [1]

7. (a) P: Phospholipid bilayer [1] Q: Channel protein / Carrier protein [1] (b)

• Fluid: Phospholipids and proteins can move laterally. [1]
• Mosaic: Proteins are embedded in the bilayer like tiles in a mosaic. [1] (c)
• Cell recognition / antigen. [1]
• Cell signalling / receptor for hormones. [1]

8. (a)

• Histones allow DNA to be packed tightly into chromosomes. [1]
• Regulates gene expression / controls access to DNA for transcription. [1] (b)
• Prokaryotic ribosomes (70S) have a different structure/size from human ribosomes (80S). [1]
• Antibiotics specifically bind to 70S ribosomes, inhibiting bacterial protein synthesis without affecting human cells. [1]

9. (a) The maximum rate of reaction when the enzyme is saturated with substrate. [1] (b)

• At high substrate concentrations, substrate molecules outcompete the inhibitor for the active site. [1]
• All active sites can still be occupied by substrate, so the same maximum rate is achieved. [1] (c)
• Inhibitor molecules occupy some active sites. [1]
• Fewer active sites are available for substrate binding, reducing the rate of ES complex formation. [1]

10. (a) Metaphase. [1] (b)

• Spindle fibres shorten. [1]
• Sister chromatids are pulled apart to opposite poles. [1] (c)
• Growth / repair of tissues. [1]
• Produces genetically identical cells / maintains chromosome number. [1]

Section C: Extended Response

11. (a)

• Water is a polar molecule. [1]
• Forms hydrogen bonds with other polar molecules and ions. [1]
• Dissolves substances allowing them to react in solution. [1] (b)
• Water absorbs a large amount of heat energy for a small rise in temperature. [1]
• Buffers temperature changes in organisms/environment. [1]

12.

• Similarities: Both are polysaccharides / polymers of glucose. [1]
• Similarities: Both contain glycosidic bonds. [1]
• Difference (Structure): Starch has $\alpha$-glucose (helical); Cellulose has $\beta$-glucose (straight chains). [1]
• Difference (Bonding): Starch has 1,4 and 1,6 glycosidic bonds (branched/unbranched); Cellulose has 1,4 glycosidic bonds with alternating orientation. [1]
• Difference (Function): Starch is for energy storage (compact, insoluble); Cellulose is for structural support (cell wall, high tensile strength due to H-bonds between chains). [1]

13.

• Transcription: DNA unwinds; RNA polymerase binds to promoter; mRNA synthesized complementary to template strand (U replaces T); mRNA processed (splicing) and leaves nucleus. [3 marks for detail]
• Translation: mRNA binds to ribosome; tRNA brings specific amino acids; anticodon pairs with codon; peptide bonds form between amino acids; polypeptide chain grows; stops at stop codon. [3 marks for detail]

14.

• Membrane contains receptor proteins specific to signalling molecules (ligands). [1]
• Ligand binds to receptor, causing conformational change. [1]
• Triggers signal transduction pathway / second messenger system (e.g., cAMP). [1]
• Leads to cellular response (e.g., gene expression, enzyme activation). [1]

15.

• Enzymes lower activation energy, allowing reactions to occur at body temperature. [1]
• Specificity ensures correct metabolic pathways occur. [1]
• Regulation (e.g., end-product inhibition) controls rate of metabolism. [1]
• Allows compartmentalization of reactions. [1]

16.

• Pentose sugar (ribose or deoxyribose). [1]
• Phosphate group. [1]
• Nitrogenous base (adenine, guanine, cytosine, thymine/uracil). [1]

17.

• Passive: No energy (ATP) required; down concentration gradient. [1]
• Active: Requires energy (ATP); against concentration gradient. [1]
• Active: Requires carrier proteins/pumps; Passive can be simple diffusion or facilitated. [1]

18.

• Receives proteins from RER in transport vesicles. [1]
• Modifies proteins (e.g., glycosylation). [1]
• Packages proteins into secretory vesicles for transport to cell surface membrane. [1]

19.

• Inner membrane folded into cristae: increases surface area for ETC/ATP synthase. [1]
• Matrix contains enzymes for Krebs cycle. [1]
• Intermembrane space allows formation of proton gradient. [1]
• Double membrane maintains distinct environments. [1]

20.

• Pro: Potential to cure diseases / regenerate tissues. [1]
• Pro: Reduces need for organ donors. [1]
• Con: Ethical issues regarding destruction of embryos (if embryonic stem cells used). [1]
• Con: Risk of tumour formation / immune rejection. [1] (Accept any balanced discussion with valid points)