Q3 Describe the process by which sugar is transported from a plant leaf to a developing fruit.
This question relates to the following part of the syllabus:
2.1 Identify a range of plant tissues and describe their structure and function.
Identify and describe the structure and function of plant tissues, to include:
Complex tissues: xylem (vessels, tracheids, parenchyma, sclerenchyma fibres), phloem (sieve tube elements, companion cells, parenchyma, sclerenchyma fibres).
3.4 Explain the movement of water and solutes through the plant.
Explain how water and solutes enter, move through, and leave the plant. To include: apoplast, symplast, endodermis, Casparian strip, transpirational pull, root pressure, capillary action, guttation.
Describe the significance of the following terms in relation to water movement in the plant: diffusion, osmosis, mass flow, capillarity, adhesion and cohesion.
Explain how environmental factors affect the rate of transpiration: temperature, water availability, relative humidity, wind speed.
Describe the uptake and distribution of mineral nutrients in the plant: nutrients from soil solution/foliar feed, distribution in the xylem and phloem, active uptake against concentration gradient into cells by membrane carriers.
Describe transport of sucrose in the phloem: mass flow hypothesis, phloem loading and unloading.
Describe the concept of sources and sinks in relation to: plant organs; seasonal changes.
Most exam questions are broken down into smaller parts, but there are some with just a straight question for 10 marks. It is important to structure your answer well for this type of question to make sure you are covering relevant points.
Key points in the syllabus here are those on the transport of sucrose, and sources and sinks. This question is looking for an understanding of the leaf being a source of sugar in the form of sucrose, and the fruit being the sink. This should be the starting point of the answer.
The examiners’ comments state that:
This question required candidates to describe the movement of sugar as sucrose from a leaf as a source, to a developing fruit, the sink. Candidates who used these terms and then went on to describe phloem loading, the pressure flow hypothesis of sugar movement in the sieve tubes and phloem unloading at the sink scored highly.
An understanding of the specific plant tissues involved is important for this too. The examiners’ comments mention that the role of specialised phloem parenchyma, as companion cells and transfer cells for loading and unloading sucrose wasn’t always included in answers. It also needed to be included that these are active processes requiring energy.
The examiners’ comments continue to state that:
Water enters the sieve tubes by osmosis at source from the xylem to increase the hydrostatic pressure here. When sucrose leaves the phloem at the sink (to be used or converted to starch or fructose etc. for storage), water will move out to be recirculated in the xylem, thus maintaining a pressure gradient and driving ‘mass flow’ (bulk flow) of sucrose from source to sink.
To summarise in order to break the question down you need to think about the leaf being the source, the fruit being the sink, the specialised tissues involved, mass flow hypothesis, and phloem loading and unloading.
The examiners comments mention that some candidates described photosynthesis or respiration, rather than the points described above. It is easy to go off on the wrong track on a question like this and lose marks. Another point made in the examiners’ comments was that ‘some candidates stated that sugar is transported by osmosis, which is incorrect. Only water moves by osmosis’.