Secondary 3 Biology Semestral Assessment 2 (End of Year) Paper 1

TuitionGoWhere Practice Paper - Biology Secondary 3

SA2 Practice Paper (Version 1 of 5) - ANSWER KEY

Total Marks: 60

Section A: Multiple Choice (10 marks)

Marking notes:

• Q1: Radioactive amino acids are incorporated into proteins at ribosomes on the rough endoplasmic reticulum (RER). The RER is the first organelle to show radioactivity. Golgi body receives proteins later for modification.
• Q2: Cell P with long, thin extensions is a nerve cell (neuron), adapted for rapid transmission of electrical signals along its axon.
• Q3: A group of similar cells performing the same function is a tissue. Insulin-producing cells in the pancreas form the islets of Langerhans, which is a tissue.
• Q4: Golgi body modifies and packages proteins. Mitochondrion releases energy; ribosomes synthesise proteins; chloroplasts carry out photosynthesis.
• Q5: Cell wall, chloroplasts, and large central vacuole are characteristic of plant cells. A leaf cell would possess all three features.
• Q6: Enzymes lower activation energy. They are not used up (A), are specific (B), and have different optimum pH values (D).
• Q7: Transmembrane proteins function as channels or carriers for transporting specific substances across the membrane.
• Q8: Concentrated sugar solution has lower water potential than potato cells. Water moves out of cells by osmosis, causing them to become flaccid.
• Q9: Iodine test negative (no starch), Benedict's test positive (reducing sugar present), Biuret test negative (no protein), Ethanol test negative (no fat). Only reducing sugar is present.
• Q10: Heterozygous parents (Hb^A Hb^S × Hb^A Hb^S). Punnett square gives 25% chance of homozygous recessive (Hb^S Hb^S), which causes sickle cell anaemia.

Section B: Structured Questions (30 marks)

Question 11: Cell Organelles and Protein Synthesis [8 marks]

(a) Identify structures B and C. [2]

• B: Rough endoplasmic reticulum (RER) [1]
• C: Golgi body / Golgi apparatus [1]

(b) Explain how the structure of B is related to its function in protein synthesis. [2]

• The RER has ribosomes attached to its surface [1];
• Ribosomes are the site of protein synthesis, so the RER provides a surface for protein synthesis to occur / the RER transports synthesised proteins through its channels [1].

(c) Describe the role of structure C in the processing and transport of proteins. [2]

• The Golgi body chemically modifies proteins (e.g., by adding carbohydrate groups to form glycoproteins) [1];
• It packages the modified proteins into secretory vesicles for transport to the cell membrane / for secretion out of the cell [1].

(d) Explain why a cell that actively secretes enzymes would contain large numbers of structures B and C. [2]

• Enzymes are proteins, so the cell needs many RER (B) to synthesise large quantities of protein [1];
• The cell needs many Golgi bodies (C) to process and package the enzymes into vesicles for secretion [1].

Question 12: Enzyme Activity and Temperature [8 marks]

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

• 37°C [1]

(b) Explain why the rate of reaction increases as the temperature rises from 10°C to 37°C. [2]

• As temperature increases, the kinetic energy of enzyme and substrate molecules increases [1];
• This increases the frequency of effective collisions between enzyme and substrate molecules / more enzyme-substrate complexes form per unit time [1].

(c) Explain why the rate of reaction decreases rapidly above 45°C. [3]

• High temperatures cause the enzyme to denature [1];
• The hydrogen bonds / ionic bonds that maintain the specific three-dimensional shape of the enzyme are broken [1];
• The active site loses its specific shape and can no longer bind to the substrate / the enzyme-substrate complex cannot form [1].

(d) Suggest what would happen to the enzyme if the temperature were lowered back to 37°C after it had been heated to 60°C. Explain your answer. [2]

• The enzyme would remain inactive / the rate of reaction would not recover [1];
• Denaturation is irreversible; the enzyme's three-dimensional shape has been permanently altered and the active site cannot be restored [1].

Question 13: Specialised Cells [6 marks]

(a) State the main function of each cell. [2]

• Red blood cell: Transport of oxygen (from lungs to body tissues) [1]
• Root hair cell: Absorption of water and mineral ions (from the soil) [1]

(b) Describe one structural adaptation of the red blood cell and explain how this adaptation helps it carry out its function. [2]

Award [1] for adaptation and [1] for explanation. Any one of:

• Biconcave shape: Increases surface area to volume ratio for faster diffusion of oxygen in and out of the cell [2].
• No nucleus: Provides more space to contain haemoglobin, maximising oxygen-carrying capacity [2].
• Contains haemoglobin: Haemoglobin binds to oxygen, enabling the cell to transport oxygen efficiently [2].
• Small and flexible: Can squeeze through narrow capillaries to deliver oxygen to all body tissues [2].

(c) Describe one structural adaptation of the root hair cell and explain how this adaptation helps it carry out its function. [2]

Award [1] for adaptation and [1] for explanation. Any one of:

• Long, narrow extension (root hair): Increases surface area to volume ratio for faster absorption of water and mineral ions [2].
• Thin cell wall: Reduces the diffusion distance for water and mineral ions to enter the cell [2].
• Many mitochondria: Provide energy (ATP) for active transport of mineral ions against the concentration gradient [2].

Question 14: Enzyme Investigation [8 marks]

(a) State the independent variable and the dependent variable. [2]

• Independent variable: pH (of the buffer solution) [1]
• Dependent variable: Time taken for starch to be digested / time taken for iodine test to remain brown [1]

(b) Explain why a buffer solution was added to each test tube. [1]

• To maintain / keep the pH constant throughout the experiment / to ensure that the pH does not change during the reaction [1].

(c) Using the lock-and-key hypothesis, explain why enzyme activity is low at pH 3 and pH 11. [3]

• At extreme pH values (pH 3 and pH 11), the enzyme denatures [1];
• The hydrogen bonds / ionic bonds that maintain the specific three-dimensional shape of the active site are disrupted [1];
• The active site loses its specific shape and can no longer bind to the substrate / the substrate no longer fits the active site, so fewer enzyme-substrate complexes form [1].

(d) State two variables, other than pH, that must be kept constant. [2]

Award [1] each for any two of:

• Temperature (of the reaction mixture) [1]
• Concentration / volume of amylase solution [1]
• Concentration / volume of starch solution [1]
• Volume of buffer solution [1]

Section C: Free Response Questions (20 marks)

Question 15: Comparison of Animal and Plant Cells [8 marks]

Marking scheme:

Award marks for correct structures and functions as follows:

Structures found in BOTH animal and plant cells (award up to 3 marks):

Award [1] for each correctly named structure with its function. Any three of:

Structures found ONLY in plant cells (award up to 3 marks):

Award [1] for each correctly named structure with its function. Any three of:

Explanation (award up to 2 marks):

• Plant cells are autotrophic / make their own food through photosynthesis, so they need chloroplasts to capture light energy [1];
• Plant cells need structural support because plants do not have skeletons; the cell wall provides rigidity and the large central vacuole maintains turgor pressure to keep the plant upright [1].

Question 16: Adaptations of Red Blood Cell and Muscle Cell [6 marks]

Marking scheme:

Red blood cell (3 marks):

• Function: Transport of oxygen from lungs to body tissues [1]
• Adaptation 1 + explanation: Biconcave shape → increases surface area to volume ratio for faster diffusion of oxygen [1]
• Adaptation 2 + explanation: No nucleus / contains haemoglobin → more space for haemoglobin to maximise oxygen-carrying capacity / haemoglobin binds to oxygen for transport [1]

Muscle cell (3 marks):

• Function: Contraction to produce movement [1]
• Adaptation 1 + explanation: Contains many mitochondria → provides large amounts of energy (ATP) needed for muscle contraction [1]
• Adaptation 2 + explanation: Contains many myofibrils / protein filaments (actin and myosin) → these filaments slide past each other to cause contraction [1]

Alternative acceptable answers for muscle cell:

• Elongated shape → allows the cell to shorten when contracting
• Has many nuclei → enables rapid production of proteins needed for contraction

Question 17: Osmosis Investigation [6 marks]

(a) Explain why the potato strip in 0% sugar solution gained mass. [2]

• The 0% sugar solution (distilled water) has a higher water potential than the potato cells [1];
• Water moved into the potato cells by osmosis, from a region of higher water potential to a region of lower water potential, causing the cells to become turgid and the strip to gain mass [1].

(b) Explain why the potato strip in 20% sugar solution lost mass. [2]

• The 20% sugar solution has a lower water potential than the potato cells [1];
• Water moved out of the potato cells by osmosis, from a region of higher water potential (inside cells) to a region of lower water potential (outside solution), causing the cells to become flaccid / plasmolysed and the strip to lose mass [1].

(c) At which concentration of sugar solution was the water potential of the potato cells equal to the water potential of the surrounding solution? Explain your answer. [2]

• At 10% sugar solution [1];
• There was no net movement of water / no change in mass, indicating that the water potential inside the potato cells was equal to the water potential of the surrounding solution [1].

END OF ANSWER KEY