O Level Biology Practice Paper 1

TuitionGoWhere Practice Paper - Biology O-Level

Answer Key and Marking Scheme

Subject: Biology (6093) | Level: O-Level | Paper: Practice Paper 1 (Cells & Biomolecules) | Version: 1 of 5 | Total Marks: 60

Section A: Multiple Choice (10 marks)

Section B: Structured Questions (30 marks)

Question 11 (7 marks)

(a) Identify structures P, Q, R, and S. [4]

Accept: "Golgi complex" or "Golgi" for S.

(b) State one function of structure Q (mitochondrion). [1]

• Site of aerobic respiration [1]
• Releases energy (ATP) for cellular activities [1]

Any one correct function accepted.

(c) Explain why structure R appears rough in the electron micrograph. [2]

• Ribosomes are attached to the surface of the endoplasmic reticulum [1]
• Ribosomes appear as small, dark granules/dots on the membrane surface, giving it a rough appearance [1]

Question 12 (6 marks)

(a) Explain why the potato cylinder in 0.0 mol/dm³ sucrose solution increased in length. [3]

• The 0.0 mol/dm³ solution is distilled water, which has a higher water potential than the cell sap of the potato cells [1]
• Water molecules move from the solution into the potato cells by osmosis, from a region of higher water potential to lower water potential, across the partially permeable cell membrane [1]
• The cells become turgid as water enters the vacuoles, causing the cytoplasm to push against the cell wall, increasing the length of the cylinder [1]

(b) What does the result for the 0.4 mol/dm³ sucrose solution indicate? [1]

• The water potential of the sucrose solution is equal to the water potential of the potato cell sap / the solution is isotonic to the potato cells [1]
• There is no net movement of water into or out of the cells [1]

Any one correct answer accepted.

(c) Predict what would happen if a potato cylinder were placed in a 1.0 mol/dm³ sucrose solution. Explain your answer. [2]

• The potato cylinder would decrease in length / become shorter and softer [1]
• The 1.0 mol/dm³ solution has a lower water potential than the potato cells, so water moves out of the cells by osmosis, causing the cells to become flaccid/plasmolysed [1]

Question 13 (6 marks)

(a) Define the term enzyme. [1]

• An enzyme is a biological catalyst that speeds up the rate of a chemical reaction without being chemically changed at the end of the reaction [1]
• Accept: "A protein that acts as a biological catalyst."

(b) Explain how enzymes lower the activation energy of a chemical reaction. [3]

• The substrate molecule has a specific three-dimensional shape complementary to the active site of the enzyme [1]
• The substrate binds to the active site, forming an enzyme-substrate complex [1]
• This binding brings the substrate molecule(s) into the correct orientation / puts stress on chemical bonds, reducing the amount of energy needed to start the reaction (activation energy) [1]

(c) Amylase is an enzyme that digests starch. Explain why amylase cannot digest proteins. [2]

• Enzymes are specific in their action [1]
• The active site of amylase has a specific three-dimensional shape that is complementary only to the starch molecule (substrate). The protein molecule has a different shape and cannot fit into the active site of amylase, so no enzyme-substrate complex can form [1]

Question 14 (6 marks)

(a) Which food sample contains starch? Give a reason for your answer. [1]

• Food sample A [1]
• Iodine solution turned blue-black, which is the positive result for starch [1]

Mark for correct sample and correct reason.

(b) Which food sample contains both reducing sugar and protein? Explain your answer. [2]

• Food sample B [1]
• Benedict's test gave a brick-red precipitate (positive for reducing sugar) AND Biuret test gave a purple colour (positive for protein) [1]

Note: Sample B shows brown for iodine (negative for starch) and colourless for ethanol (negative for fats).

(c) Describe how you would carry out the ethanol emulsion test for fats. [3]

• Add about 2 cm³ of ethanol to the food sample and shake thoroughly to dissolve any fat present [1]
• Pour/decant the ethanol solution into a test tube containing an equal volume of water [1]
• If fat is present, a white/milky emulsion forms. If absent, the solution remains clear/colourless [1]

Question 15 (5 marks)

(a) State the optimum temperature for this enzyme. [1]

• 40°C [1]

(b) Explain why the rate of reaction increases between 0°C and the optimum temperature. [2]

• As temperature increases, the kinetic energy of the enzyme and substrate molecules increases [1]
• This results in more frequent collisions between enzyme and substrate molecules, and a greater proportion of collisions have sufficient energy to overcome the activation energy, increasing the rate of enzyme-substrate complex formation [1]

(c) Explain why the rate of reaction decreases rapidly after the optimum temperature. [2]

• High temperatures disrupt the hydrogen and ionic bonds that maintain the specific three-dimensional (tertiary) structure of the enzyme [1]
• The active site loses its complementary shape (the enzyme is denatured), so the substrate can no longer bind, and the rate of reaction decreases [1]

Section C: Free Response Questions (20 marks)

Question 16 (6 marks)

Compare the structure of a typical plant cell with that of a typical animal cell.

Marking scheme: Award up to 2 marks for each correctly identified structure with its function. Maximum 6 marks.

Accept any three of the above. Other acceptable structures: plasmodesmata (allow communication/transport between adjacent cells).

Note: Students must clearly state that these structures are present in plant cells but absent from animal cells. Award marks for correct identification and explanation of function.

Question 17 (8 marks)

Describe the processes of diffusion, osmosis, and active transport.

Marking scheme: Award marks for correct description and example with importance for each process.

Diffusion [3 marks]:

• Description: Net movement of particles/molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient; does not require energy; passive process [1]
• Example: Oxygen diffuses from the alveoli (high oxygen concentration) into the blood capillaries (low oxygen concentration) in the lungs [1]
• Importance: Allows efficient gas exchange for respiration; oxygen is delivered to cells for aerobic respiration to produce energy [1]

Osmosis [3 marks]:

• Description: Net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane; passive process; does not require energy [1]
• Example: Water is absorbed by root hair cells from the soil (higher water potential) into the root cells (lower water potential) [1]
• Importance: Enables plants to absorb water needed for photosynthesis, transpiration, and maintaining turgidity for support [1]

Active Transport [2 marks]:

• Description: Movement of molecules/ions from a region of lower concentration to a region of higher concentration, against the concentration gradient; requires energy (ATP) and carrier proteins [1]
• Example: Absorption of mineral ions (e.g., nitrates) by root hair cells from the soil, where ion concentration in the soil is lower than inside the root cells / Absorption of glucose by villi in the small intestine [1]
• Importance: Allows organisms to absorb essential nutrients even when external concentrations are low [1]

Note: Maximum 8 marks. Accept any valid biological examples.

Question 18 (6 marks)

Discuss the accuracy of the statement: "all enzymes are proteins, and all proteins are enzymes."

Marking scheme: Award marks for addressing both parts of the statement with correct biological reasoning.

Part 1: "All enzymes are proteins" — Largely accurate [3 marks]:

• Enzymes are biological catalysts that are globular proteins [1]
• They are made up of amino acids linked by peptide bonds, folded into a specific three-dimensional (tertiary) structure [1]
• The specific shape of the active site, determined by the sequence of amino acids, is essential for enzyme function [1]
• Note: Some RNA molecules (ribozymes) can also act as biological catalysts, but at O-Level, enzymes are considered proteins. Accept this qualification for full marks.

Part 2: "All proteins are enzymes" — Inaccurate [3 marks]:

• Proteins have many diverse functions in living organisms beyond catalysis [1]
• Examples of non-enzymatic proteins include:
  • Structural proteins: collagen in skin and bones, keratin in hair and nails [1]
  • Transport proteins: haemoglobin in red blood cells transports oxygen [1]
  • Hormonal proteins: insulin regulates blood glucose concentration [1]
  • Contractile proteins: actin and myosin in muscles enable movement [1]
  • Antibodies: proteins of the immune system that defend against pathogens [1]

Accept any two valid examples of non-enzymatic proteins with their functions for full marks in Part 2.

Conclusion: The statement is partially accurate. While all enzymes (at O-Level) are proteins, proteins serve many other essential functions in living organisms and are not limited to enzymatic activity. [1]

Maximum 6 marks. Award marks for clear reasoning and appropriate examples.

END OF ANSWER KEY

This answer key was generated by TuitionGoWhere AI. Mark allocations reflect O-Level Biology (6093) assessment standards.