O Level Biology Practice Paper 5

TuitionGoWhere Exam Practice (AI) - Biology O-Level

Practice Paper - Version 5 of 5 - Answer Key & Marking Scheme

Subject: Biology (6093)
Level: O-Level
Total Marks: 40

Section A: Structured Questions

1. Cell Structure (a)

• B: Chloroplast [1]
• D: Mitochondrion [1] (b)
• Controls cell activities / Contains genetic material (DNA) [1] (c)
• Animal cells do not need structural support from a rigid wall as they have a cytoskeleton / are supported by the skeleton (in vertebrates) [1].
• Plant cells need a cell wall to maintain turgidity and shape against osmotic pressure [1]. (Note: Accept "Animal cells need to be flexible for movement" vs "Plant cells need rigidity".)

2. Enzymes and Temperature (a)

• As temperature increases from 10°C to 40°C, the time taken for starch breakdown decreases [1].
• This indicates that the rate of enzyme activity increases [1]. (b)
• At 60°C, the high temperature causes the enzyme (amylase) to denature [1].
• The shape of the active site changes / is destroyed [1].
• The substrate (starch) can no longer fit into the active site / no enzyme-substrate complexes can form [1]. (c)
• At 50°C, the enzyme is beginning to denature / activity is decreasing from the optimum [1].

3. Absorption in the Small Intestine (a)

• Diffusion [1] (Note: If facilitated diffusion is specified, it is also acceptable, but simple diffusion is the general term expected if no carrier is specified in the prompt context, though glucose usually uses facilitated diffusion or active transport. Given the gradient description "higher in lumen", diffusion/facilitated diffusion is correct.) (b)
• Active transport [1] (c)
• Microvilli greatly increase the surface area for absorption [1].
• This allows for a faster rate of diffusion/active transport of nutrients into the blood [1]. (d)
• Fatty acids and glycerol / Fats / Lipids [1]

4. Proteins (a)

• Amino acids [1] (b)
• Carbon, Hydrogen, Oxygen, Nitrogen [1]
• (Sulfur is sometimes present, but C, H, O, N are essential) [1] (1 mark for listing C, H, O; 1 mark for including N) (c) (i)
• Biuret solution / Biuret reagent [1] (ii)
• Colour change from blue to purple / violet [1]

5. Osmosis (a)

• Osmosis is 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) through a partially permeable membrane [1]. (b)
• Bag A has a higher concentration of sucrose (10%) than Bag B (5%) [1].
• Therefore, the water potential gradient between the distilled water and Bag A is steeper than for Bag B [1].
• Water enters Bag A at a faster rate / more water enters Bag A by osmosis, causing the liquid level to rise higher [1].

Section B: Data Interpretation and Application

6. Enzymes and pH (a)

• pH 2 [1] (b)
• Location: Stomach [1]
• Reason: The stomach contains hydrochloric acid, creating an acidic environment (low pH) suitable for Enzyme P [1]. (c)
• The small intestine has an alkaline pH (due to bile/pancreatic juice) [1].
• Enzyme P is adapted to acidic pH and would denature / have reduced activity in alkaline conditions [1].

7. Food Tests (a)

• Sample J: Starch [1]
• Sample K: Reducing Sugar (and Protein) [1] (Note: Biuret is purple, so Protein is also present. Question asks for "main nutrient" or implies identifying positive tests. J is Starch only. K is Reducing Sugar + Protein. L is Fat only. Accept "Reducing Sugar" for K if only one is required, but "Protein" is also correct. Usually, these questions look for the positive result. Let's assume identifying the positive test result.) Correction for marking:
  • J: Starch [1]
  • K: Reducing Sugar AND Protein [1] (Award mark if either is identified, but ideally both). For O-Level, usually, they want the specific positive test identification. Let's award 1 mark for identifying Reducing Sugar and 1 mark for Protein if split, but here it is 1 mark per sample. Accept "Reducing Sugar" or "Protein" or both.
  • L: Fat / Lipid [1] (b)
• Glucose / Maltose / Fructose [1] (Not Sucrose) (c)
• Mix the food sample with ethanol [1].
• Pour the mixture into water [1].
• A white emulsion indicates the presence of fat.

8. Red Blood Cells (a)

• Transport oxygen [1] (b)
• Absence of nucleus creates more space [1].
• This allows for more haemoglobin to be packed into the cell, increasing oxygen carrying capacity [1]. (c)
• Description: The cells shrink / become crenated [1].
• Explanation: The salt solution has a lower water potential than the cell cytoplasm [1].
• Water leaves the red blood cells by osmosis [1].

9. Mitochondria (a)

• Site of aerobic respiration / Production of energy (ATP) [1] (b)
• Muscle cells require a lot of energy for contraction [1].
• Mitochondria provide the ATP needed for this energy demand [1]. (c)
• Uptake of mineral ions often occurs by active transport against the concentration gradient [1].
• Active transport requires energy (ATP) produced by mitochondria [1].

10. Membrane Transport (a)

• Facilitated diffusion [1] (Note: Diagram shows protein channel, high to low, no energy. If diagram showed simple lipid bilayer passage, it would be simple diffusion. Given "protein channel", facilitated is precise.) (b)
• Glucose / Amino acids / Ions [1] (c)
• Active transport moves substances against the concentration gradient (low to high), whereas this method moves with the gradient [1].
• Active transport requires energy (ATP), whereas this method does not [1].

Section C: Extended Response

11. Enzymes (a) Lock and Key Hypothesis [4 marks]

• Enzymes have a specific shape called the active site [1].
• The substrate has a complementary shape to the active site [1].
• The substrate fits into the active site like a key into a lock, forming an enzyme-substrate complex [1].
• The reaction occurs, and products are released, leaving the enzyme unchanged and ready to catalyze another reaction [1].

(b) Digestion of Fats [6 marks]

• Fat digestion begins in the small intestine (duodenum) [1].
• Bile is produced by the liver and stored in the gall bladder, then released into the duodenum [1].
• Bile emulsifies fats, breaking large droplets into smaller droplets to increase surface area for enzyme action [1].
• The pancreas secretes the enzyme lipase into the small intestine [1].
• Lipase breaks down fats (triglycerides) into fatty acids and glycerol [1].
• These products are then absorbed by the villi in the small intestine [1].

12. Cells (a) Comparison [3 marks]

• Plant cells have a cell wall; animal cells do not [1].
• Plant cells have chloroplasts (in green parts); animal cells do not [1].
• Both have a cell membrane / nucleus / cytoplasm / mitochondria [1]. (Note: Large permanent vacuole in plants vs small/temporary in animals is also a valid difference.)

(b) Cell Specialization [4 marks]

• Cell specialization allows cells to perform specific functions efficiently in multicellular organisms [1].
• Example: Root Hair Cell
  • It has a long hair-like projection to increase surface area [1].
  • This maximizes the absorption of water and mineral ions from the soil [1].
  • It has many mitochondria to provide energy for active transport of ions [1].
• OR Example: Red Blood Cell
  • Biconcave shape increases surface area for gas exchange [1].
  • No nucleus allows more space for haemoglobin [1].
  • Contains haemoglobin to bind and transport oxygen [1]. (Award marks for clear link between structure and function for the chosen example.)

End of Marking Scheme