Draw a labelled diagram of a eukaryotic plant cell as seen in an electron micrograph.

Outline how the energy flow through food chains limits their length.

In hot, dry conditions plants lose water rapidly due to transpiration. Explain how the structures and processes of the plant allow this water to be replaced.

Cell wall shown with two continuous lines to indicate the thickness

Plasma membrane/cell membrane shown as a single continuous line

Accept inner line of wall as membrane if clearly labelled.

Nuclear membrane/nucleus shown with double membrane and nuclear pores

Vacuole «membrane»/tonoplast shown as a single continuous line

Chloroplast/plastid shown with a double line to indicate the envelope and thylakoids/grana

Mitochondrion shown with double membrane/cristae

Only a small proportion/20 %/10 % «of energy» can pass from one trophic level to the next  Accept named trophic levels or named stages in a food chain in place of “trophic levels”.
OR
large proportion/80 %/90 % lost between one trophic level and the next  Accept if clearly shown in a diagram such as a pyramid of energy.

Energy released by respiration AND lost as heat  Not just respiration or heat.

Energy losses due to uneaten parts/undigested parts/feces/egestion

Not enough energy for 4th/5th/later stages of a food chain
OR
more energy available if feeding at an earlier stage in a food chain

Evaporation of water «in leaf/mesophyll» creates tension/low pressure/negative pressure «potential»/pulling force/transpiration pull

Water drawn through cell walls/out of xylem «in leaf» by capillary action/adhesion «to cellulose»

Low pressure/tension/suction/pulling force in xylem

Hydrogen bonds make water cohesive/allow water to be pulled up under tension/allow the transpiration pull «to move water»

Xylem resists tension/low pressure/collapse with thickened/lignified walls

Water travels from the roots to the leaves in xylem

Water absorbed in roots by osmosis

Active transport of ions/solutes into roots «enabling osmosis»

Deep/wide ranging/extensive root systems/taproots/many root hairs

Thick/waxy cuticle reduces transpiration/water loss/evaporation

Small/no leaves/reduced surface area of leaves/thorns instead of leaves

Few stomata/stomata in pits/rolled leaves

Hairs on leaf surface «to reduce air flow near the leaf/reflect sunlight»

Stomata open at night/CAM physiology to reduce water loss

Diagrams of plant cell structure were mostly rather poor and few candidates scored full marks. The question specified ‘as seen in an electron micrograph’. Many diagrams showed the appearance of plant cells in a light micrograph. This allowed marks for cell wall and cell membrane to be awarded, but not for internal structures such as the nucleus as their representation was not detailed or accurate enough. In contrast to the membrane diagrams in 6(a), many of these cell diagrams were carelessly drawn with overlapping, multiple or discontinuous lines used for structures that have a single continuous edge.

This is a familiar question, though there was a slight twist in that candidates were expected to explain specifically why food chains cannot be long. Nearly all candidates wrote about energy losses between trophic levels and many mentioned the '10% rule' though in some cases got it the wrong way round and stated that 10% of energy is lost. Too few candidates mentioned the most important idea – that release of energy by cell respiration and its use is accompanied by loss of energy from a food chain in the form of heat.

This question caused some problems. It was another case where a sentence had been added to set the scene, but it proved a distraction rather than an aid to focus. The wording of the question as a whole was clear, but many candidates seemed not to have read to the end of the second sentence and they did not therefore explain how losses of water by transpiration are replaced. Some answers were concerned exclusively with xerophytic adaptations. An extensive markscheme was devised that allowed these answers to score up to five marks. Those candidates who did actually describe the uptake and transport of water within the plant were able to score full marks. There were few really strong answers and many misunderstandings. One in particular is worth mention: capillary action due to adhesion of water to xylem walls only helps to refill xylem vessels when they are air-filled. If a plant is transpiring the xylem will be filled with water under tension and adhesion cannot cause upward movement.