O Level Biology Practice Paper 4

TuitionGoWhere Practice Paper - Biology O-Level

Answer Key and Marking Scheme

Version 4 of 5

Section A: Structured Questions (40 marks)

1. Animal cell structures

(a) P: Nucleus [1]
Q: Cell membrane [1]
R: Cytoplasm [1]

(b) Contains genetic material / DNA / controls cell activities / controls cell division [1] (Accept any one correct function)

(c) The cell membrane (Q) is very thin / transparent [1]; it is difficult to see under a light microscope because it does not absorb much stain / is not easily resolved at low magnification [1].

[Total: 6 marks]

2. Enzyme and pH investigation

(a) Graph:

• Axes correctly labelled: x-axis = pH, y-axis = Rate of reaction (arbitrary units) [1]
• Appropriate scales used [1]
• All points plotted correctly and connected with a smooth curve [1]

(b) Optimum pH: 2 [1]

(c) At pH values above 4, the shape of the active site of the enzyme is altered / the enzyme is denatured [1]; the substrate can no longer fit into the active site [1]; therefore, fewer / no enzyme-substrate complexes are formed, and the rate of reaction decreases [1].

(d) Enzyme: Pepsin [1]
Reason: Pepsin works in the stomach where the pH is acidic (around pH 2) / pepsin has an optimum pH of around 2, which matches the results [1].

[Total: 9 marks]

3. Cell membrane

(a) X: Phospholipid / Phospholipid bilayer [1]
Y: Protein / Channel protein / Carrier protein [1]

(b) Structure Y (protein) acts as a channel or carrier [1] to allow specific substances (e.g., ions, glucose) to pass through the membrane by facilitated diffusion or active transport [1].

(c) Oxygen / Carbon dioxide / Water (Accept any one correct example) [1]

[Total: 5 marks]

4. Osmosis investigation

(a) Completed table:

Award [1] for all change in mass values correct; [1] for all percentage change calculations correct; [1] for correct positive/negative signs.

(b) The 0.0 mol/dm³ solution is distilled water / has a higher water potential than the potato cells [1]; water enters the potato cells by osmosis [1]; from a region of higher water potential (solution) to a region of lower water potential (potato cells) through a partially permeable membrane [1].

(c) The water potential of the potato tissue is approximately equal to that of a 0.4 mol/dm³ sucrose solution [1]; because at this concentration, there is no net movement of water / no change in mass, indicating the water potentials are equal [1].

[Total: 8 marks]

5. Digestive system

(a) A: Stomach [1]
B: Pancreas [1]
C: Liver [1]

(b) Bile emulsifies fats [1]; it breaks large fat droplets into smaller droplets [1]; this increases the surface area for lipase to act on, speeding up the digestion of fats [1].

(c) The ileum is long, providing a large surface area for absorption [1]; the inner surface is folded and has villi, further increasing surface area [1]; the epithelial cells of the villi have microvilli, increasing surface area even more [1]; the villi contain a dense network of blood capillaries to absorb glucose and amino acids, and a lacteal to absorb fatty acids and glycerol [1].

(d) Deamination of excess amino acids / breakdown of hormones / breakdown of alcohol / regulation of blood glucose concentration / storage of glycogen / formation of urea (Accept any one correct function) [1]

[Total: 11 marks]

6. Diffusion experiment

(a) Temperature of the water [1]

(b) Volume of water / size/mass of potassium permanganate crystal / size of beaker (Accept any one correct variable) [1]

(c) Increasing temperature increases the rate of diffusion [1]; because particles gain more kinetic energy [1]; they move faster and spread out more quickly through the water [1].

[Total: 5 marks]

Section B: Data-Based Questions (20 marks)

7. Enzyme concentration investigation

(a) Completed table:

Award [1] for correct formula (1/time); [1] for all values correctly calculated (accept values to 3 or 4 decimal places).

(b) As the volume of amylase solution increases, the rate of reaction increases [1]; the relationship is directly proportional / linear [1].

(c) Increasing the volume of amylase solution increases the number of enzyme molecules present [1]; more active sites are available for the substrate (starch) to bind to [1]; more enzyme-substrate complexes are formed per unit time, so the rate of reaction increases [1].

(d) Repeat the experiment and calculate a mean / average [1].

(e) At 60°C, the rate of reaction would be very low or zero / the starch would not be broken down [1]; because the high temperature would denature the enzyme [1]; the active site of the enzyme would change shape, and the substrate could no longer bind / no enzyme-substrate complexes would form [1].

[Total: 11 marks]

8. Mitochondrion

(a) D: Crista / Cristae [1]
E: Matrix [1]

(b) The inner membrane is folded into cristae [1]; this increases the surface area for the attachment of enzymes involved in aerobic respiration [1]; the matrix contains enzymes for the reactions of aerobic respiration / the Krebs cycle [1].

(c) Cell type: Muscle cell / Sperm cell / Liver cell (Accept any one correct example) [1]
Explanation: The cell requires a lot of energy / ATP for contraction / movement / metabolic activities [1].

(d) Glucose + Oxygen → Carbon dioxide + Water (+ Energy) [2]
(Award [1] for correct reactants; [1] for correct products. Accept 'energy' or 'ATP'.)

[Total: 9 marks]

Section C: Free Response Question (20 marks)

9. Cell structure and specialisation

(a) Structure of a typical animal cell under an electron microscope [6 marks]

Award marks for the following points:

• Cell membrane: composed of a phospholipid bilayer with embedded proteins; controls movement of substances in and out of the cell [1]
• Nucleus: contains nucleolus and chromatin; surrounded by a nuclear envelope with pores; controls cell activities and contains genetic material [1]
• Cytoplasm: contains organelles suspended in cytosol; site of many metabolic reactions [1]
• Mitochondria: rod-shaped with double membrane; inner membrane folded into cristae; site of aerobic respiration / ATP production [1]
• Ribosomes: small organelles (80S); site of protein synthesis; may be free in cytoplasm or attached to rough endoplasmic reticulum [1]
• Rough endoplasmic reticulum: network of membranes studded with ribosomes; transports proteins synthesised by ribosomes [1]
• Golgi body / apparatus: stack of flattened membrane-bound sacs; modifies, packages, and transports proteins and lipids for secretion [1]
• Vesicles: small membrane-bound sacs; transport substances within the cell [1]

(Award up to [6] for any six correct structures with functions. Accept smooth endoplasmic reticulum, lysosomes, centrioles.)

(b) Structure-function relationships [8 marks]

(i) Red blood cell [4]

• Contains haemoglobin [1]; which binds to oxygen to transport it from the lungs to respiring tissues [1]
• Biconcave disc shape [1]; increases surface area to volume ratio for faster diffusion of oxygen [1]
• No nucleus when mature [1]; provides more space for haemoglobin / more oxygen can be carried [1]
• Flexible / elastic cell membrane [1]; allows the cell to squeeze through narrow capillaries [1]

(Award up to [4] for any four correct points.)

(ii) Root hair cell [4]

• Long, narrow extension / root hair [1]; increases surface area for absorption of water and mineral ions from the soil [1]
• Thin cell wall [1]; reduces the diffusion distance for water and mineral ions [1]
• Many mitochondria [1]; provide energy / ATP for active transport of mineral ions against the concentration gradient [1]
• Large vacuole [1]; maintains turgor pressure and stores absorbed water and ions [1]

(Award up to [4] for any four correct points.)

(c) Importance of cell specialisation [6 marks]

Award marks for the following points:

• Cell specialisation allows cells to perform specific functions efficiently [1]
• In multicellular organisms, different tasks are divided among different cell types (division of labour) [1]
• Specialised cells have structures adapted to their specific functions, increasing efficiency [1]
• Examples: red blood cells specialised for oxygen transport; nerve cells specialised for transmitting impulses; muscle cells specialised for contraction [1]
• Groups of similar specialised cells form tissues; tissues form organs; organs form systems [1]
• This organisation allows complex multicellular organisms to carry out all life processes effectively [1]
• Without specialisation, no single cell could perform all necessary functions efficiently [1]

(Award up to [6] for a well-developed discussion with appropriate examples and clear reasoning.)

[Total: 20 marks]

10. Enzymes

(a) Lock-and-key hypothesis [6 marks]

Award marks for the following points:

• The enzyme has a specific three-dimensional shape with an active site [1]
• The active site has a shape that is complementary to the shape of the specific substrate [1]
• The substrate fits into the active site like a key fits into a lock [1]
• An enzyme-substrate complex is formed [1]
• The reaction takes place at the active site, and products are formed [1]
• The products are released, and the enzyme remains unchanged and can be reused [1]
• Enzymes are specific; each enzyme catalyses only one type of reaction [1]

(Award up to [6] for a clear, logical description with correct terminology.)

(b) Effect of temperature and pH on enzyme activity [8 marks]

Temperature:

• At low temperatures, enzyme and substrate molecules have low kinetic energy [1]; fewer collisions occur, so the rate of reaction is low [1]
• As temperature increases, kinetic energy increases; more frequent collisions between enzyme and substrate [1]; more enzyme-substrate complexes formed per unit time; rate of reaction increases [1]
• At the optimum temperature, the rate of reaction is at its maximum [1]
• Above the optimum temperature, the enzyme is denatured [1]; the bonds maintaining the three-dimensional shape of the enzyme break; the active site changes shape [1]; the substrate can no longer fit; the rate of reaction decreases rapidly [1]

pH:

• Each enzyme has an optimum pH at which it works best [1]
• At pH values above or below the optimum, the enzyme's shape is altered [1]; the active site changes shape, and the substrate cannot bind [1]; the enzyme is denatured [1]
• Different enzymes have different optimum pH values depending on their location (e.g., pepsin in stomach has optimum pH ~2; amylase in mouth has optimum pH ~7) [1]

(Award up to [8] for a thorough explanation covering both temperature and pH effects with correct terminology. Marks should be allocated based on the depth and accuracy of the explanation.)

(c) Importance of enzymes in living organisms [6 marks]

Award marks for the following points:

• Enzymes are biological catalysts that speed up metabolic reactions without being used up [1]
• Without enzymes, most metabolic reactions would occur too slowly to sustain life [1]
• Enzymes lower the activation energy required for reactions to occur [1]
• Digestion: Amylase breaks down starch to maltose in the mouth and small intestine; protease breaks down proteins to amino acids in the stomach and small intestine; lipase breaks down fats to fatty acids and glycerol in the small intestine [1]
• Respiration: Enzymes in mitochondria catalyse the reactions of aerobic respiration to release energy / ATP [1]
• Photosynthesis: Enzymes in chloroplasts catalyse the reactions of photosynthesis to produce glucose [1]
• DNA replication: DNA polymerase catalyses the synthesis of new DNA strands during cell division [1]
• Enzymes allow reactions to occur at body temperature, preventing damage to cells that would occur at higher temperatures [1]

(Award up to [6] for a well-developed discussion with at least two named examples and clear explanation of their roles.)

[Total: 20 marks]

END OF ANSWER KEY

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