A2.2 Cell structure. Practice questions with markscheme.

• D: prokaryotes have naked DNA in a nucleoid, 70S ribosomes and no membrane-bound organelles;

• D: all three are developments in microscopy that revealed new levels of cell ultrastructure and molecular detail;

• B: all cells have a plasma membrane, cytoplasm, ribosomes and DNA; a nucleus is found only in eukaryotic cells;

• the three examples show eukaryotic cells with no nucleus and with many nuclei, so the statement is not correct as a general rule;
• the statement does describe the typical eukaryotic cell, and the great majority of eukaryotic cells do contain a single nucleus;
• the exceptions are specialized cells in which the loss or multiplication of nuclei is related to function, e.g. loss of the nucleus leaves more room for haemoglobin in a red blood cell, and a long muscle fibre formed by cell fusion needs many nuclei to control its large volume of cytoplasm;
• a better statement would be that eukaryotic cells usually contain a nucleus, with named exceptions, OWTTE;

Both an interpretation of the examples and a judgement on the statement are needed for full marks.

• (a) [2]
  • organelle: mitochondrion;
  • projections: cristae / folds of the inner (mitochondrial) membrane;
• (b) [2]
  • rough endoplasmic reticulum, with (attached 80S) ribosomes (as the granules);
  • protein synthesis / translation (of mRNA into polypeptides, e.g. for secretion);

• double membrane / outer and inner membrane drawn and labelled;
• cristae drawn as infoldings of the inner membrane and annotated with function (increase surface area for ATP synthesis / oxidative phosphorylation);
• matrix labelled and annotated with function (site of Krebs cycle / link reaction / contains enzymes);
• 70S ribosomes or naked/circular DNA shown and annotated with function (protein synthesis / codes for mitochondrial proteins);
• overall function annotated (aerobic respiration / ATP production);

Award each mark only where the feature is drawn in a sensible position, labelled, and the annotation includes a function as required by the syllabus.

• (a) [2 max]
  • reads from the diagram that 40 eyepiece divisions align with 0.5 mm of the stage micrometer;
  • converts to 500 µm and divides by 40;
  • 12.5 (µm);

  Award full marks for a correct answer with no working. Accept 12.5 µm per division. ECF from a misread alignment.

• (b) [1]
  • 325 (µm);

  ECF from part (a). Units not required.

• (c) [1]
  • ± 3.1 (µm) / ± 3.13 (µm) / ± 3.125 (µm);

  ECF from part (a): accept ± a quarter of the value found in (a), since a reading to the nearest half division is uncertain by ± 0.25 division. Accept ± 6.25 µm only where the answer explains that both ends of the cell are read, each to ± 0.25 division.

• (d) [2 max]
  • the graticule divisions are arbitrary / have no fixed size of their own;
  • the size a division represents depends on the magnification (of the objective in use), so it changes between objectives;
  • so it must be compared with a scale of known length, the stage micrometer, before any reading means anything, OWTTE;

  Do not accept 'the graticule is not accurate'. Accept 'it is not to scale' only with a reason.

• B: 10 mm represents 2 μm, so 25 mm represents 25 ÷ 10 × 2 = 5 μm;

• D: chitin is the wall polysaccharide of fungi; removing the wall leaves the protoplast unprotected against osmotic swelling; plant walls are cellulose, bacterial walls peptidoglycan, animal cells have no wall (and would not need the enzyme to lyse);

• (a) [3]
  • response to stimuli: the eyespot detects light / the direction of the light;
  • movement: the two flagella beat / lash to pull the cell through the water;
  • the eyespot's signal changes the beating of the flagella, so the cell swims towards moderate light / away from very bright light, OWTTE;
  • this keeps the chloroplast in light suitable for photosynthesis / nutrition, OWTTE;

  For [3], both response to stimuli and movement must be identified.

• (b) [2]
  • difference: in the light it makes its own organic compounds by photosynthesis (in the chloroplast), whereas in darkness it absorbs ready-made organic compounds (acetate) from the water OR in the light it is autotrophic, whereas in darkness it is heterotrophic;
  • similarity: in both it absorbs inorganic substances (e.g. mineral ions such as nitrate) and water from the pond across its cell surface;
  • similarity: in both it obtains organic (carbon) compounds, which it uses for growth and as a source of energy, OWTTE;

  Award [1] max if only differences or only similarities are given.

• (a) [1]
  • ATP synthase;
• (b) [1]
  • its protein is needed for a process that all cells carry out (making ATP), not for one specialized function;
  • every cell needs ATP / a supply of energy for its activities, OWTTE;
• (c) [2]
  • not justified, because (−) shows only that the gene is not expressed / that no mRNA was made from it, not that the gene is absent;
  • all the cells of the mouse arose from one zygote (by cell division) and so contain the same genes / genome;
  • (so) the insulin gene is present in muscle fibres but is not expressed / is switched off, OWTTE;

  No mark for 'not justified' without a reason.

• (d) [2]
  • not expressed / (−);
  • the embryo cells are unspecialized / have not yet differentiated into rod cells;
  • genes for specialized proteins are switched on only as a cell differentiates, often triggered by signals / changes in the cell's surroundings, OWTTE;
  • the rhodopsin gene is nevertheless present in the embryo cells;

  No mark for the prediction without a reason.

• prokaryotic DNA is naked/in a nucleoid, whereas eukaryotic DNA is enclosed in a nucleus (and associated with histones);
• prokaryotes lack membrane-bound organelles, whereas eukaryotes have mitochondria/ER/Golgi etc.;
• prokaryotes have 70S ribosomes, whereas eukaryotes have (cytoplasmic) 80S ribosomes;
• prokaryotic cells are (generally) smaller than eukaryotic cells;

Points must be explicitly comparative. Award converse statements.

• I, nucleus (with nuclear envelope);
• II, mitochondrion;
• III, plasma membrane;
• IV, (free) ribosomes;

Accept nuclear envelope/nucleolus for I only if clearly indicated at the label line.