O Level Biology Practice Paper 2

TuitionGoWhere Practice Paper – Biology O-Level

Answer Key and Marking Scheme

Paper: PRACTICE – Version 2 of 5
Total Marks: 80

Section A: Structured Questions (30 marks)

1. (a) P: Rough endoplasmic reticulum (RER) [1]
Q: Mitochondrion [1]

(b) The RER has ribosomes attached to its surface [1]; ribosomes are the site of protein synthesis, and the RER transports the synthesised proteins within the cell [1].

(c) Mitochondrion is the site of aerobic respiration / releases energy (ATP) for cellular activities [1].

2. (a) A: Starch [1]
B: Reducing sugar (e.g., glucose) [1]
C: Protein [1]
D: Fat / Lipid [1]

(b) Heating provides the energy needed for the reaction between Benedict's solution and reducing sugars to occur [1]; without heating, the colour change (brick-red precipitate) would not be observed [1].

(c) The brick-red precipitate indicates the presence of a reducing sugar [1].

3. (a) As temperature increases from 0°C to 40°C, enzyme activity increases [1]; the rate of increase is initially slow, then becomes steeper as the optimum temperature is approached [1].

(b) Above 50°C, the high temperature causes the enzyme to denature [1]; the bonds maintaining the three-dimensional shape of the active site are broken [1]; the substrate can no longer fit into the active site, so enzyme activity decreases rapidly [1].

(c) Lock and key hypothesis [1].

4. (a) pH 7 [1].

(b) At pH 4 and pH 9, the pH is far from the optimum pH for amylase [1]; the shape of the active site is altered / the enzyme is denatured [1]; the substrate (starch) can no longer fit into the active site, so the rate of digestion decreases and more time is needed [1].

(c) Temperature / concentration of amylase / concentration of starch / volume of starch solution [1] (any one).

5. Difference 1: Bacterial cells have a cell wall made of peptidoglycan, while plant cells have a cell wall made of cellulose [2].
Difference 2: Bacterial cells lack a true nucleus (have a nucleoid region), while plant cells have a membrane-bound nucleus [2].

(Accept other valid differences: bacterial cells have plasmids / 70S ribosomes / no membrane-bound organelles; plant cells have chloroplasts / large central vacuole / 80S ribosomes.)

Section B: Data-Based and Diagram Questions (30 marks)

6. (a) Protein / Polypeptide [1].

(b) Amino acids [1].

(c) Growth and repair of tissues / formation of enzymes / formation of hormones / formation of antibodies [1] (any one).

7. (a) The nucleus appears as a darkly stained, roughly spherical structure within the cell [1]; it is surrounded by lighter-staining cytoplasm [1].

(b) The concentrated salt solution has a lower water potential than the cell contents [1]; water moves out of the cell by osmosis [1]; the cell loses water and the cytoplasm shrinks [1]; the cell wall is rigid and does not shrink, so the cell membrane pulls away from the cell wall (plasmolysis) [1].

(c) Osmosis [1].

8. (a) Active transport [1]; nitrate ions are moving from a region of lower concentration (soil) to a region of higher concentration (root hair cell) [1]; this movement against the concentration gradient requires active transport [1].

(b) Active transport requires energy (ATP) because ions are moved against the concentration gradient [1]; energy is needed to change the shape of carrier proteins in the cell membrane [1].

(c) Calcium ions may be required in smaller amounts by the plant [1]; or calcium ions may enter the cell by facilitated diffusion as the concentration gradient is smaller [1].

9. (a) Mitosis [1]; cell X has the same number of chromosomes as the parent cell / cell X is genetically identical to the parent cell [1].

(b) (Answer depends on the cell type shown in the diagram. Example for a red blood cell:) Cell Y lacks a nucleus [1]; this provides more space for haemoglobin [1], allowing the cell to carry more oxygen [1].

(Accept other valid adaptations with correct function link.)

(c) Cell differentiation / Specialisation [1].

10. (a) The concentration where the mass of the potato strip does not change (0% change in mass) [1]; this is the point where the line crosses the x-axis [1]; at this concentration, there is no net movement of water into or out of the cells, so the water potential of the sucrose solution equals the water potential of the potato cells [1].

(b) The mass of the potato strip would increase [1]; distilled water has a higher water potential than the potato cells [1]; water enters the cells by osmosis, causing the cells to become turgid and increasing the mass of the strip [1].

(c) Use potato strips of the same initial mass / same surface area / same length / same variety of potato [1] (any one).

Section C: Free Response Questions (20 marks)

11. Describe the roles of enzymes in human digestion. Give two named examples in your answer. [6]

Marking scheme:

• Enzymes are biological catalysts that speed up the rate of digestion without being used up in the reaction [1].
• They break down large, insoluble food molecules into smaller, soluble molecules that can be absorbed into the bloodstream [1].
• Example 1: Amylase (in saliva/pancreatic juice) breaks down starch into maltose [1]; this occurs in the mouth and small intestine [1].
• Example 2: Protease (e.g., pepsin in the stomach) breaks down proteins into polypeptides/amino acids [1]; this occurs in the stomach and small intestine [1].
• (Accept other valid examples: lipase breaks down fats into fatty acids and glycerol; maltase breaks down maltose into glucose.)

12. Explain how the structure of a villus in the small intestine is adapted for the absorption of digested food substances. [6]

Marking scheme:

• The villus has a large surface area due to its finger-like shape [1]; this increases the rate of absorption of digested food substances [1].
• The epithelium of the villus is only one cell thick [1]; this provides a short diffusion distance for absorbed substances [1].
• The villus contains a dense network of blood capillaries [1]; this allows rapid transport of absorbed glucose and amino acids away from the small intestine, maintaining a concentration gradient [1].
• The villus contains a lacteal (lymphatic vessel) [1]; this absorbs fatty acids and glycerol (as lipoproteins) and transports them away [1].
• The epithelial cells have microvilli on their surface [1]; this further increases the surface area for absorption [1].

(Award up to 6 marks for any valid structural adaptation linked to its function.)

13. A person consumes a meal high in protein. Describe how the body processes the excess amino acids and explain the role of the liver in this process. [8]

Marking scheme:

• Excess amino acids cannot be stored in the body [1].
• In the liver, excess amino acids undergo deamination [1].
• Deamination is the removal of the amino group (–NH₂) from the amino acid molecule [1].
• The amino group is converted to ammonia (NH₃) [1].
• Ammonia is toxic and is quickly converted to urea (a less toxic compound) in the liver [1].
• Urea is transported in the blood to the kidneys, where it is excreted in urine [1].
• The remaining part of the amino acid (carbon skeleton) can be converted to glucose (for energy) or stored as glycogen/fat [1].
• The liver thus plays a central role in removing toxic nitrogenous waste and regulating amino acid levels in the blood [1].

(Award marks for: deamination, ammonia production, urea conversion, excretion pathway, fate of carbon skeleton, role of liver.)

END OF ANSWER KEY

TuitionGoWhere Secondary School (AI) – Practice Paper Version 2 of 5