B2.2 Organelles and compartmentalization. Practice questions with markscheme.

• C: the membrane-bound compartment concentrates hydrolytic enzymes on the target and protects the cell's own structures from digestion; products are then absorbed across the vacuole membrane;

• A: in a eukaryote transcription (in the nucleus) is separated from translation (in the cytoplasm), so the mRNA is modified/processed and then exported before it meets ribosomes; in the bacterium ribosomes bind the mRNA as soon as it is made (both at 0.5 min); B fails because ribosomes already present in the cytoplasm are used; C fails because ribosomes bind mRNA, not DNA; D fails because the mRNA leaves the nucleus intact rather than being remade;

• (a) [3]
  • use an ice-cold solution, so that enzymes released from broken cells or organelles (e.g. hydrolytic enzymes from lysosomes) are less active and do not break down the organelles / enzyme E, OWTTE;
  • use a buffer solution, so that the pH stays constant and the organelles / enzyme E are not damaged, OWTTE;
  • break the cells open gently / blend or homogenize for a much shorter time, so that the cells are opened but most organelles stay intact, OWTTE;

  Beyond this subtopic. Award if given; the marks above can be earned without it.

  • D2.3 use a solution with the same solute concentration as the cytoplasm (an isotonic solution), so that water does not enter the organelles by osmosis and they do not swell and burst, OWTTE;

  Each mark needs both the improvement and its reason. Mark the first three improvements only.

• (b) [2]
  • at a very high speed nearly all the organelles, large and small (nuclei, mitochondria, lysosomes and ribosomes), sediment together into one pellet, OWTTE;
  • (so) activity in the pellet could come from any of the organelles in it;
  • spinning at a series of increasing speeds, collecting the pellet after each spin, would separate the organelles by size / density, the largest / densest first, so that each fraction holds mainly one type of organelle, OWTTE;
• (c) [2]
  • the fraction may also contain unbroken cells, which hold the enzymes of the cytoplasm and of every organelle, OWTTE;
  • the fraction is unlikely to be pure / may contain other organelles of similar size and density, OWTTE;
  • enzyme E may have leaked from broken organelles and stuck to / been trapped among the organelles in the pellet, OWTTE;
  • the activity must be compared across all the fractions and the final liquid: more of enzyme E might be in another fraction or free in the cytoplasm, OWTTE;
  • the organelles in the fraction have not been identified, e.g. by examining it with an electron microscope, OWTTE;

  Mark the first two only.

• in a eukaryotic cell transcription takes place in the nucleus, whereas ribosomes / translation are in the cytoplasm;
• the nuclear envelope / membrane separates the mRNA from ribosomes (until the mRNA has left through the nuclear pores), OWTTE;
• (so) the mRNA can be modified / processed after transcription and before it meets a ribosome;
• a bacterium has no nucleus, so mRNA may meet ribosomes immediately and cannot be modified first;
• modification allows the mRNA to be altered / different versions of the protein to be made from one gene / faulty transcripts to be removed before translation, OWTTE;

Accept 'post-transcriptional modification', 'splicing' or 'adding a cap and tail' as examples of modification, but examples are not required.

• (a) [2]
  • ribosomes: synthesis of proteins / polypeptides / translation (of mRNA);
  • mitochondria: aerobic (cell) respiration / production of ATP;
• (b) [2 max]
  • the enzymes and substrates of photosynthesis are concentrated together in the small volume of the chloroplast, so the reactions are faster;
  • photosynthesis is kept separate from other processes in the cytoplasm that could interfere with it / incompatible processes can run at the same time in different compartments, OWTTE;
  • conditions (e.g. pH) inside the chloroplast can be kept different from those in the cytoplasm, suited to its enzymes, OWTTE;

  Mark the first two only.

• I, cell wall;
• II, chloroplast;
• III, (large central) vacuole;
• IV, nucleus;

Accept nuclear envelope/nuclear membrane for IV. Do not accept plasma membrane for I: the leader ends on the thick outer layer, not the membrane inside it.

• D: 8.4 + 3.6 = 12.0 kDa, so the polypeptide made on the rough ER is processed by being cut into two, and both pieces are packaged into secretory vesicles for release by exocytosis; A fails because the Golgi apparatus has no ribosomes (and ribosomes do not remove amino acids); B fails because the mass falls rather than rises; D fails because the 3.6 kDa fragment is found in the secretory vesicles;

• B: pumping protons into a small volume raises their concentration quickly, so a steep gradient forms rapidly for ATP production by chemiosmosis; A fails because the Calvin cycle enzymes are in the stroma; B fails because the pigments are in the membranes and the fluid volume does not limit the light reaching them; D fails because ATP is released into the stroma, where the Calvin cycle uses it;

• B: clathrin triskelions assemble on the cytoplasmic side of the membrane into a basket/lattice that curves the membrane into coated pits/vesicles (receptor-mediated endocytosis);

• A: in a eukaryote the nuclear envelope separates transcription from translation, so mRNA can be modified before it meets ribosomes; in a prokaryote ribosomes can bind the mRNA immediately, so the unmodified message is translated; A fails because bacteria have their own ribosomes; B fails because translation occurs inside the cell, not in the medium; C fails because the stem states that transcription did occur;

• C: the folded inner membrane (cristae) gives a large surface area for electron transport chains and ATP synthase, so with few folds each mitochondrion makes less ATP by aerobic respiration; A fails because the Krebs cycle enzymes are in the matrix; B fails because most ATP is made at the inner membrane, not by the Krebs cycle; D fails because ATP synthase is in the inner membrane;

• A: the smooth ER has no ribosomes (that defines the rough ER); it makes lipids and handles calcium (and detoxification in liver);

Smooth ER functions go beyond the letter of the 2025 statements (which name only free ribosomes and the rER); kept as an obscure-corner contrast question.

• C: cell fractionation uses the ultracentrifuge to separate organelles of different size and density into fractions, and the function of each organelle can then be tested in isolation; A describes the electron microscope, which reveals structure rather than function; B is wrong because organelles are not cultured as living units; C fails because fractionation separates intact organelles rather than breaking them all down to molecules;

• (a) [2]
  • mRNA (messenger RNA);
  • ribosomal subunits;
  • tRNA;

  Mark the first two only. Do not accept 'RNA' unqualified for both marks.

• (b) [2]
  • spindle microtubules formed in the cytoplasm can then reach and attach to the centromeres/kinetochores;
  • so chromosomes can be moved to opposite poles, allowing an equal separation of chromatids, OWTTE;
• (c) [1]
  • polypeptides made on ribosomes attached to the envelope pass straight into the ER lumen for processing and transport, so no separate transfer step is needed, OWTTE;

• (a) [2 max]
  • if both sets of enzymes were in the same compartment, fatty acids would be broken down as soon as they were made / the two processes would cancel each other out, OWTTE;
  • (so) ATP / energy would be used up with no net product / would be wasted;
  • separate compartments allow each process to go on without interference from the other, so the two incompatible processes can both occur in the same cell, OWTTE;
• (b) [2 max]
  • the mitochondrial membranes do not let (most) fatty acids through, so they can only reach the enzymes that break them down by way of the carrier, OWTTE;
  • when the carrier is inhibited / inactive, fatty acids stay in the cytoplasm, where they are built up / stored rather than broken down;
  • when the carrier is active, fatty acids pass into the matrix and are broken down, releasing energy for ATP production, OWTTE;
  • (e.g.) a substance made while fatty acids are being built up could inhibit the carrier, so newly made fatty acids are not broken down, OWTTE;
• (c) [2 max]
  • fatty acids / fat accumulate in the cytoplasm (as fat droplets), because they cannot enter the matrix;
  • little energy / ATP can be released from fat, because fatty acids cannot reach the enzymes that break them down;
  • the cells run short of ATP / depend entirely on glucose, which is in short supply after many hours without food, OWTTE;

  Mark the first two effects only; each needs its reason.

• rough endoplasmic reticulum drawn as flattened membrane sacs (cisternae) with ribosomes attached to the outer surface, annotated: site of synthesis of proteins for secretion/export;
• vesicles budding from the rough ER and carrying the protein to the Golgi apparatus (arrow showing direction);
• Golgi apparatus drawn as a stack of separate flattened cisternae (cis face receiving vesicles from the ER, trans face releasing vesicles), annotated: modifies/processes/sorts proteins, e.g. adds carbohydrate;
• vesicles budding from the Golgi apparatus, moving to and fusing with the plasma membrane to release the protein by exocytosis;

The nucleus and nuclear envelope may be included but are not required. Do not accept the Golgi drawn as an interconnected network (that is the ER).