(a) Explain what is meant by the term limiting factor and explain how knowledge of limiting factors is used to increase crop yields in glasshouses. (b) Describe the behaviour of chromosomes during meiosis.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

1. [7] any seven from: 1 reaction / photosynthesis, controlled / affected, by more than one / several, factors; 2 factor/variable, nearest its minimum value / in shortest supply; 3 which, prevents increase in rate / restricts rate / makes rate plateau ; to increase crop yield in glass house: 4 light intensity + temperature + carbon dioxide concentration ; 5 artificial lights / lamps / provide lighting at night ; 6 heaters / open windows / control ventilation ; 7 CO2 from, cylinders / dry ice / combustion / (named) fuel heater ; 8 automation / computerised control / digital control ; 9 ref. to optimum, light intensity / temperature / CO2 concentration / conditions ; 10 AVP; e.g. water/irrigation / sprinklers control / optimum, humidity (named) fertiliser / nitrate / manure
2. [8] any eight from: meiosis I 1 chromosomes, condense / thicken / spiralise ; 2 homologous chromosomes pair up / bivalents form; 3 crossing over / chiasma(ta); 4 spindle fibres / microtubules, attach to / pull, centromeres / kinetochores ; 5 bivalents, line up / arranged, on equator ; 6 independent/random, assortment (of homologous pairs); 7 chromosomes, move / pulled, to poles ; meiosis II 8 (single) chromosomes / pairs of chromatids, line up on, equator / mid-line ; 9 at right angles to first equator ; 10 centromeres divide; 11 chromatids separate ; 12 ref. to haploid / chromosome no. halved / one set of chromosomes ;