Plant Fibres and Sustainability

Plant cells have really strong molecules in their cell walls which can be used to produce useful materials, like as ropes and fabrics. Using plants to make these kinds of materials is much more sustainable than producing plastics from fossil fuels. The plant resources aren’t depleted because new plants can be grown and nothing has been used up for the next generation.

Plant Fibres

Plant fibres, such as cotton and hemp, are made up of long tubes of plant cells. Plant fibres are strong which makes them useful for materials such as rope or sacks. Their strength is due to two factors:

1. The arrangement of cellulose microfibrils in the cell wall

2. The secondary thickening of cell walls with lignin

Cellulose Microfibrils

Cellulose is a polysaccharide made up of long chains of beta-glucose monomers joined by 1-4 glycosidic linkages. Cellulose molecules are unbranched but are bonded to each other through hydrogen bonds. This bonding between cellulose molecules forms a net-like arrangement called a microfibril. Microfibrils are in turn hydrogen bonded to other microfibrils to form larger structures called macrofibrils. The net-like arrangement and the strength of the cellulose microfibrils are what makes plant fibres so strong.

Lignin

Lignin is a thick, woody substance which is found in the cell walls of xylem and schlerenchyma cells and provides strength to the plant stem. Lignin forms during a process called secondary thickening - this is when xylem and schlerenchyma cells which have finished growing produce a secondary cell wall between the normal cell wall and the cell membrane. This second cell wall is thicker than the first cell wall and contains a higher proportion of lignin. Secondary thickening is what makes the structural plant fibres (xylem and schlerenchyma vessels) so strong.

Measuring the Tensile Strength of Plant Fibres

Tensile strength is the amount of weight a plant fibre can take before it snaps. It is important to measure the tensile strength to avoid injury when using fibres in things like construction or sports - e.g. a rock climber would need to know if the tensile strength of the rope they were using exceeded their body weight. You can measure the tensile strength of plant fibres using the following method:

1. Set up a clamp stand and attach the plant fibre that you’d like to measure. Tie a weight to the end of the fibre.

2. Add more weights, one at a time, until the plant fibre breaks. Record the mass of weights that the fibre was carrying before it snapped.

3. Repeat with different samples of the same fibre at least three times then calculate a mean.

In this experiment there are some variables that you’d need to keep the same (control variables). These include the length of the plant fibre, the temperature in the room and level of humidity. Safety measures to be considered are to stand away from the area below the weights to avoid them falling on your feet and wearing safety goggles to protect your eyes.

Sustainability

Materials such as rope and fabrics can be made out of plant fibres - but they can also be made out of plastics which are obtained from fossil fuels. Although materials made out of plant fibres are weaker and don’t last as long, they are more sustainable. Sustainability is defined as the use of materials that meets the needs of the current generation and can be replenished so that the same resources are still available for future generations. Sustainable practices use renewable resources - these are resources which can be easily replaced and will never run out - as opposed to non-renewable resources such as fossil fuels which will one day vanish if we carry on using them. Plant fibres are a renewable resource because plants can be easily regrown for future generations.

Making materials from plant fibres as an alternative to plastics has other advantages:

• Converting plants into fibres is a simpler and cheaper process than producing plastics from fossil fuels. This is especially important in developing countries since less technology and expertise are needed to produce the fibres.

• Materials made from plant fibres are biodegradable whereas plastics are non-biodegradable and persist in the environment on landfill sites for hundreds of years.

Along with plant fibres (the xylem and schlerenchyma vessels), starch can also be used as a sustainable resource. Starch can be used to make bioplastics and biofuel. Making plastics and fuel from starch is more sustainable than making it from fossil fuels because less fossil fuel is used up and the crops from which the starch was extracted from can be regrown.

Next Page: Water Transport and Transpiration