A2.2 Cell structure. Practice questions with markscheme.

• B: a micrograph is a 2D section through a 3D structure; a plane cutting a sphere away from its centre produces a circle smaller than the true diameter;

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

• C: sieve tube elements lose the nucleus (and most organelles), keeping an open channel for sap; the nucleated companion cell sustains them; red blood cells are also anucleate at maturity but have no supporting neighbour cell, and fungal hyphae and skeletal muscle fibres are multinucleate;

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

• A: plastids include chloroplasts (photosynthesis), chromoplasts (pigment storage) and leucoplasts/amyloplasts (starch storage); mitochondria are a separate endosymbiotic lineage, not plastids;

• peel/cut a single thin layer of epidermis, so that light can pass through and the cells lie in one layer;
• place the tissue flat and unfolded on a clean slide;
• add a drop of liquid (water) so that the specimen is mounted in liquid and does not dry out;
• add a drop of iodine solution to stain the tissue / to make nuclei and cell walls visible;
• lower the coverslip from one edge with a mounted needle, at an angle, to exclude air bubbles;
• blot excess liquid from the edge of the coverslip with filter paper;

Award [3 max]. Accept OWTTE throughout. Do not accept 'cut a section' with no reference to thinness. Design move: procedural understanding with no arithmetic, as M26 SL asked.

• advantage: (far) higher resolution (electrons have much shorter wavelength);
• (so) organelle ultrastructure / detail below ~200 nm can be seen;
• limitation: specimens must be dead / fixed / in a vacuum (no living processes observed);
• (also) no natural colour / elaborate preparation may introduce artefacts, OWTTE;

• metabolism (enzyme-catalysed reactions) / nutrition (obtaining/making food);
• growth and reproduction/division;
• response to stimuli (sensitivity) / homeostasis;
• excretion of wastes / exchange with the environment, OWTTE;

Any 2–3 distinct functions, named sensibly.

• nucleus drawn with nucleolus and a double membrane (nuclear envelope) with pores, annotated: contains the DNA/chromosomes; nucleolus makes ribosomes/rRNA;
• rough endoplasmic reticulum drawn as extensive flattened sacs with ribosomes on the surface, annotated: synthesis of proteins for secretion (e.g. digestive enzymes);
• Golgi apparatus drawn as a stack of curved cisternae with vesicles budding off, annotated: modifies/packages proteins into vesicles;
• secretory vesicles/granules near the plasma membrane and mitochondria (with cristae) drawn, annotated: storage and release of enzymes by exocytosis / ATP production by aerobic respiration respectively;
• magnification = 20 mm ÷ 2 μm = 20 000 μm ÷ 2 μm = ×10 000, stated on the drawing, with structures drawn in proportion using clear continuous lines and no shading, OWTTE;

Award the magnification mark only if the unit conversion is shown or the answer ×10 000 is given. Do not credit organelles that were not described (e.g. chloroplasts, cell wall).

• (cell theory:) organisms are made of cells, the smallest units of life, which come from pre-existing cells;
• all cells share a plasma membrane, cytoplasm, ribosomes and DNA;
• (however) cells are highly diverse in size, shape and internal structure;
• atypical examples challenge the general pattern: red blood cells (no nucleus) / skeletal muscle fibres (very large, multinucleate) / aseptate fungal hyphae (continuous cytoplasm);
• viruses are non-cellular, but they are not considered living / cannot reproduce independently;
• (overall) the theory holds as a generalization, with recognized exceptions, OWTTE;