Transport in plants (3.1.3)
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Content (from AS and A-level)
The content from the specification that is covered by this delivery guide is:
|3.1.3 Transport in plants|
|(a)||the need for transport systems in multicellular plants||To include an appreciation of size, metabolic rate and surface area to volume ratio (SA:V).
M0.1, M0.3, M0.4, M1.1, M2.1, M4.1
HSW1, HSW3, HSW5, HSW8
|(b)||(i) the structure and function of the vascular system in the roots, stems and leaves of herbaceous dicotyledonous plants
(ii) the examination and drawing of stained sections of plant tissue to show the distribution of xylem and phloem
(iii) the dissection of stems, both longitudinally and transversely, and their examination to demonstrate the position and structure of xylem vessels
|To include xylem vessels, sieve tube elements and companion cells.
|(c)||(i) the process of transpiration and the environmental factors that affect transpiration rate
(ii) practical investigations to estimate transpiration rates
|To include an appreciation that transpiration is a consequence of gaseous exchange.
To include the use of a potometer.
M0.1, M0.2, M1.1, M1.2, M1.3, M1.6, M1.11, M3.1, M3.2, M3.3, M3.5, M3.6, M4.1
HSW2, HSW3, HSW4, HSW5, HSW6, HSW8
|(d)||the transport of water into the plant, through the plant and to the air surrounding the leaves||To include details of the pathways taken by water
the mechanisms of movement, in terms of water potential, adhesion, cohesion and the transpiration stream.
|(e)||adaptations of plants to the availability of water in their environment||To include xerophytes (cacti and marram grass) and hydrophytes (water lilies).
|(f)||the mechanism of translocation.||To include translocation as an energy-requiring process transporting assimilates, especially sucrose, in the phloem between sources (e.g. leaves) and sinks (e.g. roots, meristem)
details of active loading at the source and removal at the sink.
The Transport in Plants section of the specification covers the following areas:
- basic principles (a)
- histology of transport tissues (b)
- water loss (transpiration) (c)
- water entry (d)
- adaptations to aquatic and dry environments (e)
- sucrose transport (f ).
Theory teaching materials are listed in the first section here, ideas relating to the more challenging mathematical or philosophical aspects in the second (concepts) section, and practical activity ideas and unusual approaches in the third (contexts).
A 1 minute clip with botanist Timothy Walker from 2011. This clip was sourced from the new BBC site which combines video clips and classroom resources with learner materials up to GCSE level (formerly ‘Class Clips’).
The links are to the video clip and the BBC Bitesize home page.
Approaches to teaching the content
One of the biggest problems to overcome is the low level of interest the majority of students have in plant physiology. They also tend to be put off by the language of plant histology (e.g. sclerenchyma, collenchyma) and have trouble seeing differences between different plant cells. On the plus side, plant cells are bigger than animal cells and so easier to see, with distinct cell walls that can be clearly stained to show their thickness (a key point in identification of different types). Students also like practical work. Well-supported microscopy work with labelled images on the white board or on worksheets is essential for students to find their way around plant anatomy. The use of prepared slides should be complemented by some dissection of plants so that students can find their way round the different tissues and organs for themselves. The sheet ‘The weirdness of plants’ gives ideas for raising students’ curiosity about how plants live. Associating water transport in the xylem with desert survival, and sucrose transport in the phloem with products like maple syrup, gives an anthropocentric point of reference for exploration of the mechanisms of transpiration and translocation.
Common misconceptions or difficulties students may have
The main difficulty seen in exam answers is a lack of detail in student answers, and a lack of precision about parts of cells and plant tissues when describing the mechanism of water movement across the root, the transpiration pull and active loading of sucrose at sources. Terms like osmosis, active transport and diffusion need to be used appropriately, and a biochemical level of detail is needed to describe hydrogen ion and sucrose co-transport at companion cells for instance.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
2.1.1 (b) light microscopy
2.1.2 (a) the properties of water
2.1.2 (e) sucrose
2.1.5 (d) and (e) movement of molecules across cell membranes
Later topics which require understanding from 3.1.3
4.1.1 (a), (b) and (c) plant pathogens
4.2.2 (g) adaptation to a habitat (xerophytes, hydrophytes)
5.1.5 (a)-(f) plant hormones (and their modes of transport)
5.2.1 (f) uses of triose phosphate from photosynthesis
5.2.2 (a) why plants need to respire (relate to translocation to sinks, e.g. roots, fruits)
6.2.1(a) production of viable cuttings requires understanding of the significance of the transport tissues and limiting water loss by transpiration
6.3.2(d) syrup tapping in North American forests provides an example of sustainable exploitation of a resource in an ecosystem.
The success of vascular plants based on their transport tissues is animatedly described by gifted communicator Hank Green in this 12 minute clip. It would provide a good introductory context to section 3.1.3 Transport in plants. There is a useful table of contents showing what the video covers under ‘Show More’ on the YouTube platform.
An index to the full selection of 40 Crash Course Biology videos can be seen via the link 'Crash Course Biology index'.
The contexts listed include using bananas as a source of xylem for microscopy, some fun activities with cut flowers, finding out about harvesting phloem translocate from trees, and how to use transpiration as a way of obtaining drinking water from plants.
An easy way to see annular and helical xylem and to practise microscopy skills (supporting Module 1 and PAG1) is the following:
Peel a banana and take the stringy bits that fall away from the banana.
Cut a small section of this string and lay it on a microscope slide.
Squash down well with a coverslip (consider the safest way to do this without the coverslip breaking) to separate out the cells.
Search on low power for longitudinal xylem vessels and then work up to high power to see detail.
Biological drawings can then be made.
The standard material for demonstrating movement of a coloured dye up a plant stem is celery, but it does need to have a leafy top. As supermarkets cut off the leaves of the outer stems, the smaller middle stems are better. The advantage of celery is that if the stem is cut, the dyed vascular bundles are very clear and easy to see by eye and draw. To add interest, students could ‘race’ dyes up different plants, maybe by combining the celery practical with 'Rainbow roses' or Glow In the Dark Flowers ('How to make glowing water').
Another possibility is rhubarb stems (with leaf or part of leaf still attached), perhaps investigating how leaf surface area affects the rate of transpiration up the stem (M4.1). Razor blades can be used to cut thin transverse sections of the dyed celery stem to see the vascular bundles under the microscope and these could then be compared with prepared stem slides with the xylem (lignin) coloured red with phloroglucinol (supporting Module 1 and PAG1).
This biological drawing skills handbook has been developed to support GCE Biology A H020/H420 and GCE Biology B H022/H422.
The ability to draw, label and annotate biological specimens is an important and useful biological skill.
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