Plant tissues, fibres and mineral nutrition: exam questions

Answer: A – xylem vessels are dead, lignified tubes; sieve tube elements are living (they lose their nucleus but are supported by companion cells)

• similarity: both are elongated cells / joined end to end forming tubes / found in vascular bundles / involved in transport (any one)
• xylem vessels are dead / sieve tube elements are living
• xylem has thick lignified walls; phloem walls are thin / not lignified
• xylem has no end walls between cells; sieve tubes have sieve plates (perforated end walls)
• xylem transports water and mineral ions; phloem transports organic solutes / sucrose / amino acids
• xylem transports upwards only / phloem transports in both directions; xylem also gives support (any one of these)
• Max 5. Differences must refer to both xylem vessels and sieve tubes; at least one similarity is needed for full marks.

• (a) use a sharp pencil / single clear continuous lines / no shading or colouring
• (a) draw the outline of the section and the boundaries between tissues
• (a) do not draw individual cells
• (a) draw large / in proportion / use most of the space available
• (a) labels drawn with ruled lines that do not cross, touching the structure
• (a) give a title / state the magnification or include a scale
• (a) Max 4
• (b) sclerenchyma fibres (do not accept xylem or phloem)

• magnification = size of image ÷ size of real object (1)
• 84 ÷ 2.4 (1)
• ×35 (1); allow 35 without ×, but do not accept an answer with a unit

• magnesium ions are needed to make chlorophyll / are the central atom of chlorophyll
• less chlorophyll is made, so the leaves are yellow (chlorosis)
• so less light is absorbed, less photosynthesis, less growth

• renewable: crops can be regrown / not a finite resource unlike oil
• biodegradable / broken down by microorganisms, so less waste accumulates
• carbon dioxide absorbed in photosynthesis, so lower net carbon dioxide release / fewer fossil fuels used
• limitation: needs land / water / fertiliser, may compete with food crops or reduce biodiversity (any one)
• Max 4

• cut fibres of the same length from each plant / same age and moisture content
• measure the diameter at several points with a micrometer (or microscope and graticule) and calculate cross-sectional area (πr²)
• clamp one end of the fibre and hang a mass holder from the other end
• add masses in equal increments until the fibre breaks
• record the total mass at which it breaks and convert to force (mass × 9.81)
• calculate tensile strength = breaking force ÷ cross-sectional area
• repeat with several fibres of each plant and calculate a mean
• safety: soft box to catch masses / eye protection
• Max 6

• mean mass = 4.5 kg (1)
• force = 4.5 × 9.81 = 44.1 N (1)
• radius = 0.15 mm, so area = π × 0.15² = 0.0707 mm² (1)
• 44.1 ÷ 0.0707 = 620 N mm⁻² (1); accept 620 to 625; ecf from earlier steps; unit required; do not accept use of the diameter instead of the radius (about 156 N mm⁻²)