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O Level Biology Practice Paper 1

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TuitionGoWhere Practice Paper - Biology O-Level (Answer Key)

Subject: Biology (6093)
Paper: Practice Paper 1 (Version 1 of 5)
Topic: Cells & Biomolecules

Section A: Multiple Choice & Short Structured Questions

1. C
Reasoning: Animal cells are eukaryotic (nucleus present). Bacterial cells are prokaryotic (nucleus absent/nucleoid region). [1]

2. B
Reasoning: Palisade mesophyll cells contain chloroplasts (green), a large vacuole, and a cell wall. Root hair cells have no chloroplasts. Liver and RBCs are animal cells (no wall/chloroplasts). [1]

3. B
Reasoning: The lock and key hypothesis describes the specific fit between an enzyme's active site and its substrate. [1]

4. C
Reasoning: Haemoglobin is a protein. Glycogen and cellulose are carbohydrates (polysaccharides). Triglycerides are lipids. [1]

5. B
Reasoning: Iodine negative = no starch. Benedict's negative = no reducing sugar. Biuret positive = protein present. Ethanol positive = fat present. [1]

6.
(a) Osmosis [1]
(b) Plant cells have a rigid cell wall [1]. The cell wall prevents the cell from bursting by exerting back pressure (turgor pressure) when water enters [1]. [2]

7.
(a) A substance that increases the rate of a chemical reaction [1] without being used up/changed in the process [1]. (Accept: "speeds up reaction" for 1 mark if context implies biological). [1]
(b) Enzymes have a specific active site shape [1]. Only substrates with a complementary shape can bind to the active site [1]. [2]

8.
(a) 40°C [1]
(b) At 60°C, the high temperature causes the enzyme to denature [1]. The bonds holding the enzyme structure break, changing the shape of the active site [1]. The substrate can no longer fit/bind to the active site [1]. [3]

9.
(a) Any two from:

  1. Active transport moves substances against the concentration gradient (low to high), while diffusion moves down the gradient (high to low).
  2. Active transport requires energy (ATP/respiration), while diffusion does not.
  3. Active transport requires carrier proteins, while simple diffusion does not.
    [2]
    (b) Uptake of mineral ions by root hair cells [1] OR Uptake of glucose/amino acids by villi epithelial cells [1]. [1]

10.
(a) Site of aerobic respiration / Production of energy (ATP) [1].
(b) Muscle cells require a large amount of energy for contraction [1]. Mitochondria produce ATP via aerobic respiration to supply this energy [1]. [2]

Section B: Structured Response Questions

11.
(a) Carbon, Hydrogen, Oxygen, Nitrogen [1]. (Sulfur is also acceptable but not required for "all"). [1]
(b) Add Biuret solution [1] to the food sample. Shake/mix. A positive result is indicated by a colour change from blue to purple/violet [1]. [3] (1 mark for reagent, 1 mark for method detail, 1 mark for colour change).
(c) The enzyme is pepsin [1]. It breaks down proteins into peptides/polypeptides [1]. It works in the acidic conditions of the stomach [1]. [3]

12.
(a) A: Cell Wall [1]
B: Chloroplast [1]
C: Nucleus [1]
D: Mitochondrion [1] [4]
(b) Animal cells do not have a cell wall / Animal cells do not have a large central vacuole / Animal cells have centrioles (plant cells usually don't) [1]. Explanation: Animal cells are irregular in shape / Plant cells are fixed shape due to wall [1]. [2]
(c) Chloroplasts contain chlorophyll [1] which traps light energy for photosynthesis [1]. OR Chloroplasts have many thylakoids/grana to increase surface area for light absorption [1]. [2]

13.
(a) Glucose [1].
(b) Starch: Used for energy storage [1]. It is insoluble so does not affect water potential/osmosis [1].
Cellulose: Used for structural support in cell walls [1]. It is strong/fibrous to maintain cell shape [1]. [4]
(c) Glycogen is insoluble [1], so it does not draw water into cells by osmosis (preventing swelling) [1]. It can be rapidly broken down into glucose when energy is needed [1]. (Any 2 points). [2]

14.
(a) The net movement of water molecules [1] from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) [1] through a partially permeable membrane [1]. (Max 2 marks). [2]
(b) The 5.0% salt solution has a lower water potential than the potato cell cytoplasm [1]. Water moves out of the potato cells by osmosis [1]. The vacuole shrinks and the cytoplasm pulls away from the cell wall (plasmolysis), causing the strip to become flaccid [1]. [3]

15.
(a)

  1. Feature: Biconcave shape. Adaptation: Increases surface area for faster diffusion of oxygen [1].
  2. Feature: No nucleus. Adaptation: More space for haemoglobin to carry oxygen [1].
    (Other valid answers: Thin membrane for short diffusion distance). [4]
    (b) Root hair cells have a long hair-like projection [1]. This greatly increases the surface area for absorption of water and minerals [1]. [2]

Section C: Free Response / Data Analysis

16.
(a) Graph Requirements:

  • X-axis labeled "pH" with linear scale [1].
  • Y-axis labeled "Time taken (s)" with linear scale [1].
  • Points plotted correctly at (5, 120), (7, 30), (9, 180) [1].
  • Smooth curve drawn (bell-shaped inverted for rate, but here it is time, so U-shaped curve) [1].
    Note: Since Y is Time, the curve should be high at pH 3/11 and low at pH 7. [4]
    (b) Approximately 60 seconds (Accept 50-70s) [1].
    (c) At pH 3 and 11, the enzyme is denatured [1]. The extreme pH changes the shape of the active site [1]. The substrate cannot bind, so no reaction occurs (starch remains) [1]. [3]
    (d) Temperature affects enzyme activity [1]. Keeping it constant ensures that only pH is the variable affecting the rate, making it a fair test [1]. [2]

17.
(a) Glycerol [1] and Fatty acids [1]. [2]
(b)

  1. Energy storage: Lipids store more energy per gram than carbohydrates [1].
  2. Insulation: Subcutaneous fat reduces heat loss [1].
    (Other valid: Protection of organs, component of cell membranes). [4]
    (c) High levels of saturated fats/cholesterol lead to deposition in artery walls [1]. This narrows the lumen of the coronary arteries [1]. Blood flow to the heart muscle is reduced [1]. This can lead to a heart attack if blockage is complete [1]. [4]

18.
(a) The membrane consists of a phospholipid bilayer [1]. The phosphate heads are hydrophilic (face outward) and fatty acid tails are hydrophobic (face inward) [1]. Proteins are embedded in the bilayer (mosaic) [1]. The membrane is fluid because phospholipids/proteins can move laterally [1]. [4]
(b) The interior of the membrane is hydrophobic (non-polar) due to fatty acid tails [1]. Non-polar molecules like oxygen can dissolve in and pass through this layer easily [1]. Large polar molecules like glucose are repelled by the hydrophobic core and are too large to pass between phospholipids, requiring transport proteins [1]. [3]

19.
(a) Any three:

  1. DNA is double-stranded; RNA is single-stranded.
  2. DNA contains deoxyribose sugar; RNA contains ribose sugar.
  3. DNA contains Thymine; RNA contains Uracil.
  4. DNA is a long helix; RNA is shorter.
    [3]
    (b) DNA contains the genetic code/base sequence [1]. This sequence determines the order of amino acids in a protein [1]. DNA is transcribed into mRNA, which carries the code to ribosomes for translation [1]. [3]

20.
(a) Perform a Benedict’s test on the juice [1]. Compare the colour of the precipitate (green/yellow/orange/brick-red) to a set of standard solutions with known sugar concentrations tested in the same way [1]. The closer the colour to brick-red, the higher the concentration [1]. Alternatively, filter and weigh the precipitate or use a colorimeter [1]. [4]
(b) Volume of Benedict’s solution used [1] OR Temperature of water bath [1] OR Heating time [1]. [1]