Secondary 3 Biology Cells Biomolecules Quiz

Secondary 3 Biology Quiz - Cells Biomolecules

Answer Key

Section A: Multiple Choice

1. C — Rough endoplasmic reticulum [1]
Explanation: Radioactive amino acids are first incorporated into proteins by ribosomes on the rough endoplasmic reticulum (RER). The proteins then move to the Golgi body for modification and packaging. A common mistake is selecting the Golgi body first, but the RER is where protein synthesis begins.

2. B — Controls the movement of substances into and out of the cell [1]
Explanation: The cell membrane is partially (selectively) permeable and regulates what enters and leaves the cell. The cell wall (not the membrane) provides rigid support in plant cells. Genetic information is stored in the nucleus. Photosynthesis occurs in chloroplasts.

3. C — Mitochondrion [1]
Explanation: The mitochondrion is identified by its double membrane, inner folds called cristae, and the matrix inside. Chloroplasts have thylakoids and stroma. The Golgi body consists of flattened sacs (cisternae). The endoplasmic reticulum is a network of membranes without cristae.

4. C — Carbohydrates [1]
Explanation: Carbohydrates (especially glucose) are the primary and most readily available source of immediate energy for cellular activities through respiration. Lipids store more energy per gram but are used for long-term energy storage. Proteins are not primarily used for energy. Nucleic acids carry genetic information.

5. B — It increases the surface area to volume ratio, allowing faster diffusion of oxygen [1]
Explanation: The biconcave shape increases the surface area relative to the cell's volume, which increases the rate of oxygen diffusion into and out of the cell. This is a classic cell adaptation question. Students sometimes incorrectly select option A, but the biconcave shape does not increase cell volume.

Section B: Short Answer

6. [2 marks]
(i) Plant cells have a cell wall; animal cells do not. [1]
(ii) Plant cells have chloroplasts; animal cells do not. [1]
Acceptable alternatives: Plant cells have a large central vacuole (animal cells have small or no vacuoles); plant cells are generally rectangular/fixed shape (animal cells are irregular in shape). Award 1 mark per valid difference. Do not accept vague answers like "plant cells are different."

7. [2 marks]
Biomolecule: Reducing sugar (e.g., glucose) [1]
Colour change: From blue to orange-red / brick-red precipitate [1]
Note: Accept "blue to orange," "blue to red," or "blue to brick-red." Benedict's test specifically detects reducing sugars. Students sometimes write "glucose" for the colour change — this is incorrect and should not be awarded the mark.

8. [2 marks]
The rough endoplasmic reticulum (RER) has ribosomes attached to its surface. [1] These ribosomes are the site of protein synthesis, so a well-developed RER allows the cell to synthesise large amounts of enzymes (which are proteins) for secretion. [1]
Marking note: Students must link the RER to protein synthesis. Simply stating "RER makes proteins" without mentioning ribosomes scores only 1 mark.

9. [2 marks]
(a) Mitochondrion: Site of aerobic respiration / produces energy (ATP) for the cell [1]
(b) Cell wall: Provides structural support and protection / maintains cell shape / prevents the cell from bursting in a hypotonic solution [1]
Note: Accept any one valid function for each. For mitochondrion, "makes energy" is acceptable at this level.

10. [1 mark]
The potato tissue contains starch. [1]
Explanation: Iodine solution turns blue-black in the presence of starch. This is a standard food test. Students should state the conclusion (starch is present), not just describe the colour change.

Section C: Structured Response

11. [5 marks]
(a) A: Nucleus [1]
B: Mitochondrion [1]

(b) Structure C (rough endoplasmic reticulum) is involved in the synthesis and transport of proteins. [1]
Accept: "It has ribosomes that make proteins" or "It transports proteins to the Golgi body."

(c) Structure D (Golgi body) receives proteins from the rough endoplasmic reticulum [1] and modifies, packages, and sorts them into vesicles [1] for secretion out of the cell. Since hormones are proteins (or protein-based), the Golgi body is essential for processing and packaging them for release. [1]
Marking note: Award 1 mark for stating the Golgi body receives/processes proteins, 1 mark for packaging into vesicles, and 1 mark for linking this to hormone secretion.

12. [4 marks]
(a) 37 °C [1] because the shortest time (3 minutes) for starch to disappear indicates the fastest rate of enzyme activity, meaning the enzyme works best at this temperature. [1]
Marking note: Both the correct temperature and the explanation (shortest time = fastest rate) are required for full marks.

(b) At 70 °C, the enzyme (amylase) is denatured. [1] The high temperature causes the enzyme's active site to change shape, so the substrate (starch) can no longer fit into it. [1]
Note: Simply stating "the enzyme is denatured" scores 1 mark. The explanation of active site shape change earns the second mark.

(c) Any one of: concentration of starch solution / volume of starch solution / volume of amylase / pH of the solution [1]
Note: Temperature is the independent variable and should not be accepted as a controlled variable.

13. [3 marks]
As a cell increases in size, its volume increases faster than its surface area, so the surface area to volume ratio decreases. [1] This means there is relatively less cell membrane available to transport substances (nutrients, oxygen, waste) into and out of the cell per unit volume. [1] Cells adapt by increasing in number (dividing) rather than increasing in size, or by developing adaptations such as folds/villi/microvilli to increase surface area. [1]
Marking note: Award 1 mark for explaining the SA:V ratio concept, 1 mark for stating the problem (insufficient exchange), and 1 mark for describing an adaptation. Accept "cells divide" or "cells stay small" as valid adaptations.

14. [3 marks]
(a) Sample X [1] because Benedict's solution turned orange-red, indicating the presence of reducing sugar. [1]
Note: The explanation must refer to the colour change observed with Benedict's solution.

(b) Sample Y [1] because the iodine solution turned blue-black, indicating the presence of starch. [1]

(c) Sample Z [1] because the Biuret test turned purple (indicating protein) and the Ethanol Emulsion test produced a cloudy white precipitate (indicating lipid). [1]
Note: Sample W also contains protein and lipid, but the question asks for one sample. Accept either W or Z with correct explanation. However, Z is the clearer answer as W's Benedict's test remained blue (no reducing sugar), making Z the best single answer.

15. [3 marks]
Proteins are synthesised by ribosomes on the rough endoplasmic reticulum (RER). [1] The proteins are then transported in vesicles from the RER to the Golgi body. [1] At the Golgi body, the proteins are modified, packaged into secretory vesicles, and released from the cell via exocytosis at the cell membrane. [1]
Marking note: Award 1 mark for each correct structure in the correct order: RER → Golgi body → secretory vesicle → cell membrane (exocytosis). Students must mention at least three structures in the correct sequence for full marks.

Section D: Extended Response

16. [5 marks]
(a) In distilled water, water moves into the red blood cells by osmosis [1] because the water potential outside the cell is higher than inside. The cells swell and may burst (haemolysis). [1]
Marking note: Award 1 mark for osmosis and direction of water movement, 1 mark for the result (swelling/bursting).

(b) In 10% salt solution, water moves out of the red blood cells by osmosis [1] because the water potential outside the cell is lower than inside. The cells shrink and become crenated. [1]

(c) 0.9% salt solution is isotonic to blood cells, meaning there is no net movement of water into or out of the cells. [1] This prevents the cells from bursting or shrinking, maintaining their normal function.
Note: Students may also say "it has the same water potential as blood cells" — accept this.

17. [4 marks]
(a) Cell P is a plant cell [1] because it has a cell wall, chloroplasts, and a large central vacuole, which are features of plant cells. [1]
Note: Identification (1 mark) + one valid reason (1 mark).

(b) Animal cells (like human cheek cells) do not need a cell wall because they are supported by the skeleton / they have a flexible cell membrane that allows them to change shape. [1]
Accept: "Animal cells do not carry out photosynthesis and do not need the rigid support of a cell wall."

(c) Cell R could be a fungal cell or a bacterial cell [1] because it has a cell wall but lacks chloroplasts and a large central vacuole. [1]
Note: Accept "fungal cell" or "bacterial cell" with a valid reason. Do not accept "plant cell" because plant cells have chloroplasts and a large central vacuole.

(d) All four cell types have mitochondria because mitochondria are the site of aerobic respiration, which produces ATP (energy) needed for all cellular activities. [1]
Note: Students must link mitochondria to energy production. Simply stating "for respiration" is acceptable at this level.

18. [4 marks]
(a) 48 cm³ [1]

(b) Mean rate = Total volume ÷ Time = 48 cm³ ÷ 2 min = 24 cm³/min [1] for working, [1] for correct answer with unit.
Note: If the student reads the graph value at 2 minutes (e.g., 30 cm³) and calculates 30 ÷ 2 = 15 cm³/min, accept this if the graph description supports it. Based on the description (steep curve levelling at 6 min), the rate over the first 2 minutes should be calculated from the graph reading. Award 1 mark for correct method even if the value differs slightly.

(c) The reaction stops because all the substrate has been used up / the enzyme has converted all the substrate into product. [1]
Note: Accept "the limiting reactant is exhausted" or equivalent.

19. [4 marks]
Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration down a concentration gradient. [1] It does not require energy (it is a passive process). An example is the diffusion of oxygen from the alveoli into the blood in the lungs. [1]
Active transport is the movement of molecules from a region of lower concentration to a region of higher concentration against a concentration gradient. [1] It requires energy in the form of ATP. An example is the absorption of mineral ions by root hair cells in plants. [1]
Marking note: Award 1 mark for each of: definition of diffusion, definition of active transport, energy requirement comparison, and a named example. Students who only define both processes without examples or energy comparison score a maximum of 2 marks.

20. [4 marks]
(a) To investigate the effect of pH on the activity of the enzyme pepsin. [1]

(b) (i) Independent variable: pH of the solution [1]
(ii) Dependent variable: Time taken for protein to be completely digested [1]
(iii) Controlled variable (any one): Temperature / volume of protein solution / volume of pepsin solution / concentration of protein solution / concentration of pepsin [1]

(c) At pH 7, pepsin would show very little or no activity [1] because pepsin works best in acidic conditions (optimum pH around 2) and is denatured at neutral pH. [1]
Note: Pepsin is a stomach enzyme that functions optimally at low pH. Students should link the prediction to the enzyme's optimum pH.

Total: 40 marks