Organelles & Their Functions

Question 1 (12 marks)

(a) Secretory pathway trace (8 marks) — award 1 mark per correct labelled step (max 8):

1. Free ribosome in cytosol — translation begins; an N-terminal signal peptide emerges. (1)
2. Signal peptide → rough ER (RER) — ribosome docks on RER; protein is threaded into the ER lumen (co-translational translocation). (1)
3. Rough ER lumen — protein folds, glycosylation begins, quality control. (1)
4. Transport (COPII) vesicle buds from ER, carrying protein to Golgi. (1)
5. Golgi apparatus (cis → medial → trans) — protein is modified, further glycosylated, sorted and packaged. (1)
6. Secretory vesicle buds from trans-Golgi. (1)
7. Vesicle fuses with plasma membrane — exocytosis. (1)
8. Protein released outside the cell (secretion). (1)

Why: the signal peptide is the "address label" ensuring entry into the endomembrane system; each organelle performs a distinct modification/sorting role.

(b) Vesicles & membrane flow (2 marks)

• Vesicles bud from one compartment and fuse with the next, transferring both cargo and membrane (1).
• Membrane flows RER → Golgi → plasma membrane; membrane is conserved/recycled, maintaining organelle identity (1).

(c) Cytosolic protein (2 marks)

• Made on free ribosomes in the cytosol (1).
• It lacks a signal peptide, so it is never targeted to the RER and stays in the cytosol (1).

Question 2 (10 marks)

(a) Nucleus structure (5 marks) — 1 mark each, max 5:

• Nuclear envelope — double membrane separating nucleoplasm from cytoplasm.
• Nuclear pores — regulate transport of molecules (mRNA out, proteins in).
• Chromatin (DNA + histones) — stores genetic information / genes.
• Nucleolus — site of rRNA synthesis and ribosome subunit assembly.
• Nucleoplasm — semi-fluid matrix; nuclear lamina — supports envelope (accept either).

(b) Ribosome assembly (5 marks)

1. rRNA is transcribed in the nucleolus from rDNA (1).
2. Ribosomal proteins are made in the cytoplasm on free ribosomes, then imported into the nucleus through nuclear pores (1).
3. In the nucleolus, rRNA + proteins assemble into large (60S) and small (40S) subunits separately (1).
4. Subunits exit through nuclear pores into the cytoplasm (1).
5. Subunits join only during translation to form a functional 80S ribosome (eukaryote) (1).

Question 3 (12 marks)

(a) Mitochondrion (6 marks)

• Diagram labels (award up to 3): outer membrane, inner membrane folded into cristae, matrix, intermembrane space.
• Function link (up to 3): cristae increase surface area for the electron transport chain / ATP synthase (1); intermembrane space accumulates H⁺ giving a proton gradient (1); the matrix holds enzymes of the Krebs cycle and mtDNA; ATP is produced by chemiosmosis / oxidative phosphorylation (1).

(b) Endosymbiotic evidence (6 marks) — 2 marks per point, any three:

• Double membrane — inner membrane derived from the ancestral prokaryote, outer from host engulfment (2).
• Own circular DNA resembling bacterial DNA (2).
• Own ribosomes of the 70S (prokaryotic) type, not 80S (2).
• Divide by binary fission independently of the cell (2).
• (Own size ~ bacterial size) accept any valid.

Question 4 (10 marks)

(a) Lysosomes & autophagy (6 marks)

• Lysosomes contain hydrolytic enzymes active at acidic pH (~5); they digest macromolecules, worn organelles, and material taken in by phagocytosis (2).
• Autophagy steps:
  1. A double membrane / phagophore engulfs the damaged organelle forming an autophagosome (1).
  2. The autophagosome fuses with a lysosome (1).
  3. Lysosomal enzymes degrade the contents (1).
  4. Breakdown products (amino acids, etc.) are released to the cytosol and recycled (1).

(b) Peroxisome vs lysosome (4 marks)

• Lysosome: intracellular digestion by hydrolytic enzymes (1).
• Peroxisome: oxidation reactions — breaks down fatty acids (β-oxidation) and detoxifies, producing H₂O₂ (1).
• Enzyme: catalase (1).
• Reaction: $2H_2O_2 \rightarrow 2H_2O + O_2$ (1).

Question 5 (10 marks)

(a) Cytoskeleton (6 marks) — table, 2 marks per row (subunit + function; ranking):

(b) 9+2 vs centriole (4 marks)

• Motile cilium/flagellum: nine outer doublets of microtubules + two central singlets = "9+2" (2).
• Centriole: nine triplets, no central pair = "9+0" arrangement (2).

Question 6 (6 marks)

• (i) Central vacuole (up to 3): large fluid-filled sac in plant cells; maintains turgor pressure by osmosis, providing structural support; stores water, ions, wastes/pigments. Animal cells have only small/temporary vacuoles.
• (ii) Cell wall (up to 3): rigid layer outside the plasma membrane; provides support, protection, and prevents excessive water uptake/bursting; main polysaccharide is cellulose.

[
  {"claim":"Catalase reaction is balanced: 2 H2O2 -> 2 H2O + O2 (atoms conserve)","code":"H_l=2*2; H_r=2*2; O_l=2*2; O_r=2*1+2; result = (H_l==H_r) and (O_l==O_r)"},
  {"claim":"Cytoskeleton diameters rank microfilaments<intermediate<microtubules","code":"mf=7; inter=10; mt=25; result = mf<inter<mt"},
  {"claim":"Cilium axoneme microtubule count in 9+2 = 20 microtubules","code":"result = (9*2 + 2) == 20"},
  {"claim":"Eukaryotic ribosome subunits 60S + 40S combine to 80S (Svedberg values are non-additive so 60+40 != 80)","code":"result = (60+40) != 80"}
]