Organelles & Their Functions

Question 1 (10 marks)

(a) Secretory pathway (5 marks — 1 per correct structure in order):

1. Ribosomes on rough ER — polypeptide synthesised and threaded into ER lumen.
2. Rough endoplasmic reticulum — folding + initial glycosylation.
3. Transport (transition) vesicles — bud off ER, carry protein.
4. Golgi apparatus (cis → trans) — further modification/glycosylation, sorting, packaging.
5. Secretory vesicles → plasma membrane — exocytosis releases antibody.

Why: This is the endomembrane system protein pathway; a secreted glycoprotein must be membrane-bound throughout, never free in cytosol.

(b) (3 marks):

• Organelle: Golgi apparatus (site of most carbohydrate addition/completion) — (1)
• Consequences (any 2 × 1): antibody would be incompletely/incorrectly glycosylated; misfolded / non-functional / unstable; possibly not properly sorted or secreted / degraded.

(c) (2 marks): Rough ER is the site of synthesis of secreted proteins (1); a high-secretion cell needs vast ribosome/ER capacity to meet demand, so rough ER is extensively developed (1).

Question 2 (10 marks)

(a) (4 marks):

• Sample X = plant — evidence: cell wall + large central vacuole + chloroplasts (any 2 × 1). (2)
• Sample Y = animal — evidence: no cell wall + no chloroplasts + many lysosomes (small hydrolytic vesicles) (any 2 × 1). (2)

(b) (3 marks): Muscle cells do mechanical work (contraction) requiring large amounts of ATP (1). Mitochondria are the site of aerobic respiration / oxidative phosphorylation producing ATP (1). More mitochondria = greater ATP supply to sustain repeated contraction (1).

(c) (3 marks): The vesicles are lysosomes containing hydrolytic enzymes (1). Rupture releases enzymes into the cytoplasm (1), which digest the cell's own components → autolysis / cell death (1).

Question 3 (10 marks)

(a) (6 marks — 3 × [process + reason]): Any three of:

• Mitotic spindle formation — spindle is made of microtubules; chromosome separation fails → cell cycle arrest. (2)
• Intracellular vesicle/organelle transport — microtubules are tracks for motor proteins; transport disrupted. (2)
• Cell shape / structural support — microtubules form part of cytoskeleton; shape collapses. (2)
• Cilia/flagella movement — axoneme is microtubule-based; motility lost. (2)
• Centriole/centrosome function — organise microtubules; MTOC disrupted. (2)

(b) (4 marks): Cilium axoneme has nine outer microtubule doublets surrounding two central single microtubules (9+2) (1). Dynein motor arms on doublets use ATP to make adjacent doublets slide past one another (1); because doublets are anchored, sliding is converted to bending/beating (1). Disrupting microtubule assembly destroys the axoneme so no sliding/bending occurs → cilia cannot beat (1).

Question 4 (10 marks)

(a) (5 marks):

• Process: autophagy (1); organelle: lysosome (1).
• Mechanism (3 × 1): damaged mitochondrion enclosed in a double membrane forming an autophagosome → autophagosome fuses with a lysosome → lysosomal hydrolytic enzymes digest the contents → breakdown products (amino acids, etc.) released to cytoplasm for reuse.

(b) (3 marks):

• Organelle: peroxisome (1).
• Enzyme: catalase; balanced equation (2): $2H_2O_2 \rightarrow 2H_2O + O_2$

(c) (2 marks): $H_2O_2$ is a toxic reactive oxygen species that damages proteins/lipids/DNA, so must be removed fast (1). Compartmentalising the reaction in peroxisomes keeps the toxic intermediate and reaction isolated from the rest of the cell, protecting other components (1).

Question 5 (10 marks)

(a) (2 marks): A = mitochondrion (1); B = chloroplast (1).

(b) (3 marks — any 3 × 1): own circular DNA; own 70S (bacterial-type) ribosomes; double membrane; divide by binary fission independently; size similar to bacteria.

(c) (5 marks):

• In the dark, no photosynthesis, but the cell still respires (1): glucose (stored/imported) is broken down in the mitochondrion via aerobic respiration to make ATP (1).
• In daylight relationship (3): chloroplast produces glucose and $O_2$ by photosynthesis (1); mitochondrion uses glucose and $O_2$ as substrates for respiration (1); mitochondrion releases $CO_2$ and $H_2O$ which the chloroplast reuses for photosynthesis — a cyclic exchange of gases/substrates (1).

[
  {"claim":"Catalase decomposition of hydrogen peroxide is balanced: 2H2O2 -> 2H2O + O2",
   "code":"H,O=symbols('H O'); left={H:2*2, O:2*2}; right={H:2*2, O:2*1+2}; result=(left[H]==right[H] and left[O]==right[O])"},
  {"claim":"Q1 marks sum to 10",
   "code":"result=(5+3+2==10)"},
  {"claim":"Total paper marks equal 50",
   "code":"result=(10+10+10+10+10==50)"},
  {"claim":"Q5 part marks sum to 10",
   "code":"result=(2+3+5==10)"}
]