How is the structure of plants suited to support and transport, and how do humans use plants?
The structure of plant cells and tissues, plant fibres and their properties, the transport of water in the xylem, and the economic and sustainable use of plants and their products.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on plant structure and economic use, covering plant cell and tissue structure, plant fibres and starch, the transport of water in the xylem, and the sustainable use of plant products.
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What this dot point is asking
Edexcel wants you to describe the structure of plant cells and tissues, explain the properties and uses of plant fibres, describe how water is transported in the xylem, and discuss the economic and sustainable use of plants. The cohesion-tension theory and an evaluation of plant products are recurring exam themes.
Plant cells and tissues
Plant cells share organelles with animal cells but also have a cellulose cell wall (for support), a permanent vacuole (for turgor) and chloroplasts (for photosynthesis). Cells are grouped into tissues such as xylem (water transport and support), phloem (transport of sugars) and sclerenchyma (support).
Plant fibres and starch
Transport of water in the xylem
Water is pulled up the xylem in a continuous column (the cohesion-tension theory). Transpiration at the leaf lowers the water potential, creating tension that pulls water up; water molecules are cohesive (they hydrogen-bond to each other) so they move as an unbroken column, and they adhere to the vessel walls. The xylem vessels are dead, hollow tubes with no end walls, strengthened with lignin so they do not collapse under the tension, suited to this transport role. Factors that increase transpiration (high light, high temperature, low humidity, air movement) increase the rate of water uptake.
To test plant fibre strength practically, Edexcel expects you to hang masses on fibres until they break and compare the breaking force, controlling fibre length and diameter.
Economic and sustainable use
Plants provide food, fibres, fuels, drugs (such as quinine and aspirin precursors) and materials. Using renewable plant products (such as plant fibres for ropes and fabrics, or bioplastics from starch) can reduce reliance on finite oil-based materials and may biodegrade, but growing them needs land, water, fertiliser and energy and may compete with food crops, so their sustainability must be evaluated rather than assumed. Investigating plants also depends on conserving plant biodiversity (for example in seed banks) so that useful species are not lost.
Examples in context
Example 1. Flax and hemp fibres. Sclerenchyma fibres from flax stems are spun into linen and used in composite materials for car panels because the cellulose microfibrils give high tensile strength for low weight. These renewable fibres can replace some glass fibre, reducing reliance on oil-based materials, illustrating the structure-to-use link the specification asks for.
Example 2. Drugs from plants. The drug digoxin, used to treat heart failure, comes from the foxglove, and the cancer drug paclitaxel comes from the yew tree. This shows the economic and medical value of plant biodiversity and why conserving plant species protects a library of potential medicines, linking this dot point to conservation.
Try this
Q1. Explain how the cohesion of water helps water move up the xylem. [2 marks]
- Cue. Water molecules hydrogen-bond to each other, forming a continuous column that can be pulled up as water is lost at the top.
Q2. Suggest one advantage and one disadvantage of using plant fibres instead of oil-based materials. [2 marks]
- Cue. Advantage: renewable and biodegradable. Disadvantage: growing them needs land, water and energy.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksExplain how the cohesion-tension theory accounts for the movement of water up the xylem from the roots to the leaves.Show worked answer →
Markers want the linked chain of cohesion, tension and adhesion.
Water evaporates from the spongy mesophyll cells and is lost from the leaf through the stomata (transpiration). This lowers the water potential at the top of the xylem, creating tension (negative pressure) that pulls water up. Water molecules are polar and hydrogen-bond to each other (cohesion), so they form a continuous unbroken column that is pulled up as one. They also adhere to the lignified walls of the narrow xylem vessels, helping the column resist breaking. So water is pulled, not pushed, up the plant.
Award marks for: transpiration lowers water potential creating tension; cohesion by hydrogen bonds forms a continuous column; adhesion to vessel walls; water pulled up as a continuous column.
Edexcel 20224 marksEvaluate the use of plant-based bioplastics as a sustainable alternative to oil-based plastics.Show worked answer →
An evaluate question: benefits, drawbacks and a judgement.
Benefits: bioplastics are made from renewable plant material (such as starch), so they do not deplete finite crude oil; many are biodegradable, reducing plastic pollution; and growing the plants removes carbon dioxide, potentially lowering net carbon emissions. Drawbacks: growing crops for bioplastics uses land, water and fertiliser that could grow food, and may drive habitat clearance; production can still use fossil-fuel energy; and not all bioplastics are easily biodegradable. Judgement: bioplastics can be more sustainable than oil-based plastics if grown on non-food land with low inputs and genuinely biodegrade, but their sustainability must be assessed case by case rather than assumed.
Markers reward at least two benefits, two drawbacks and a reasoned judgement.
Related dot points
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An Edexcel A-Level Biology B (Salters-Nuffield) answer on biodiversity and classification, covering species, genetic and habitat diversity, the index of diversity, and the classification, naming and grouping of organisms.
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An Edexcel A-Level Biology B (Salters-Nuffield) answer on conservation, covering the economic, ecological and ethical reasons for conserving biodiversity, in-situ and ex-situ methods, seed banks and zoos, and balancing conservation with human needs.
- The fluid mosaic model of the cell membrane, diffusion, osmosis and active transport, the role of water as a solvent and its properties, and the features of efficient exchange surfaces.
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An Edexcel A-Level Biology B (Salters-Nuffield) answer on photosynthesis, covering the light-dependent and light-independent reactions, the structure of the chloroplast, the role of ATP and NADP, and limiting factors.
Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)