International A Level Biology / Plant structure and function

Plant fibres, starch and sustainability revision notes

Edexcel International A Level Biology (YBI11), chapter 10: Plant structure and function, specification point 4.4, 4.7. Free lesson.

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Plant fibre strength comes from cellulose microfibrils arranged in layers and bound by lignin. Comparing plant-based and oil-based products means considering where their materials come from and what happens to them after use.

How a sclerenchyma fibre wall is built

A sclerenchyma fibre has a thick wall that makes it hard to pull apart. The primary cell wall is laid down while the cell grows. After growth stops, the cell adds secondary wall layers inside it. The cell dies after the wall becomes lignified, leaving an empty space called the lumen. Xylem vessels have a similar wall structure. The middle lamella joins the cell to its neighbours. Cellulose microfibrils are bundles of cellulose chains held side by side by hydrogen bonds. These bonds stop the chains sliding, so the microfibrils are difficult to pull apart. Lignin fills the…

How wall structure gives fibres their properties

A physical property describes what a material is like, such as its strength or rigidity. Each part of a fibre wall contributes to these properties. The microfibrils in different layers lie at different angles, like the layers in plywood, so the wall resists forces from more than one direction. Hydrogen-bonded microfibrils give the wall high tensile strength, meaning it is hard to pull apart. Different microfibril angles make the wall strong in more than one direction. Lignin makes the wall rigid, stiff and impermeable to water. This helps stop xylem vessels collapsing. Long, thin…

How flax, hemp and cotton fibres differ

Flax and hemp fibres come from plant stems, while cotton fibres are hairs on seeds. Their cell types and wall contents explain why they are used for different products. Flax and hemp are sclerenchyma fibres. Their walls contain cellulose and some lignin. Flax is used for linen fabric and thread; hemp is used for rope and canvas. Cotton fibres are single, long seed hair cells, not sclerenchyma fibres. Their walls are almost pure cellulose, with little or no lignin, so they are soft and useful for clothing.

How to compare plant-based and oil-based products

Sustainable means that something can keep being used without leaving less for people in the future. To explain why a product is sustainable, give reasons that apply to that product. Plant-based examples include hemp rope, cotton and starch plastics. Oil-based examples include nylon rope, polyester and most plastics. Crops such as maize and potatoes are renewable because they can be grown again each year. Crude oil is finite, so it will eventually run out. Plant-based products are generally biodegradable: microorganisms break them down. Most oil-based plastics are not…

How to explain sustainability for a product

For a sustainability question, connect the material's source to what happens to the product after use. For starch cups made from waste potato peel, you can give several specific reasons. The starch comes from potatoes, a renewable crop, and using waste peel means no extra land is needed for that material. The cup is biodegradable, so microorganisms break it down instead of it persisting as litter. Using starch instead of oil-based plastic saves oil, which is a finite resource.