Edexcel A Biology A level core practical Write ups. All include an introduction, hypothesis, variables, method, results, evaluations...
CP8 -Determine the tensile strength of plant fibres
Core practical 8 - determining the tensile strength of plant fibres
AIM : To determine and compare the tensile strength of different plant fibres
Introduction
Plants have a high tensile strength due to the arrangement of cellulose microfibrils in the cell
wall and secondary thickening with lignin.
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 forming bundles called microfibrils (the science hive, n.d.)
Hydrogen bonds alone are relatively weak but together, the large number of hydrogen bonds
in the microfibril produces a strong structure
The microfibrils are wound in a helical arrangement around the cell and stuck together with a
polysaccharide glue composed of short branched polysaccharides known as hemicelluloses
and pectins. These short polysaccharides bind both to the surface of the cellulose and to
each other holding the microfibrils together. This arrangement of cellulose microfibrils,
hemicellulose and pectins makes the cell wall very strong and very reinforced. (University Of
York Science Education Group. Nuffield Curriculum Centre, 2015)
The basic chemical structures of cellulose in all plant-based fibres are similar, but they have
different degrees of polymerization whereas the cell geometry of each type of celluloses
varies with the fibres. (Klaus Friedrich and Ulf Breuer, 2015). Plant fibres are more ductile if
the fibrils have a spiral orientation to the fibre axis. Fibres are inflexible, rigid, and have a
high tensile strength if the fibrils are oriented parallel to the fibre axis. They serve as
reinforcement by giving strength and stiffness to the matrix structure (Mizi Fan and Feng Fu,
2017, pp.59–83).Therefore the tensile strength in different plant fibres will vary.
Lignin is found in cell walls of xylem and sclerenchyma and provides strength to the plant. It
forms between the normal cell wall and cell membrane as a thicker secondary cell wall which
strengthens the plant fibres. (the science hive, n.d.) This increases the amount of weight the
fibre can take before snapping thus increasing its tensile strength
Hypothesis : Synthetic fibres will have a higher tensile strength than plant fibres. Plant
fibres will have high tensile strength and different plants will have different values.
Variables
Independent variables : The type of fibre used including cotton, new zealand flax plant,
polyester and celery
Dependent variable : The amount of mass that can be added before the fibre snaps (g)
Control variables:
- Each fibre should be roughly the same length as shorter fibres would be stronger
- the temperature in the room must be kept the same to not affect the bonding of the
polysaccharide
- Level of humidity as a prolonged exposure to higher humidity causes the plant fibres
to weaken
- A set of the same weights can be used to have the exact same mass added each
, Core Practical 8 - 26/01/22
time
- it's best to use a large sample of fibres of roughly the same age from the same
location for fair comparison
- no damage to the fibres we use as this would weaken the fibre
Risk assessment
Hazard Safety precaution
Heavy weights falling which could fall on people feet - clear the area
- stand away when dropping the weights
- Cushion the floor to absorb the force of the falling
mass
- Add the weights slowly
Scalpel is sharp - Take extra precaution when handling sharp objects
- Keep fingers a safe distance from the blade when
separating fibres
- do not leave laying around, put the blade away when
finished using
plant fibres may flick into your eyes when they break Wear goggles to protect
Equipment
- Different plant fibres (cotton, new zealand flax plant, polyester and celery)
- Scalpel → to cut a section of the plant and obtain a fibre
- Forceps → to extract a single fibre from the plant
- 2 Clamp stands → to hold the fibre on each end
- Weight Hanger → to attach the weights onto the fibre
- A set of 10g weights → to create a force on the fibre and measure how
much is needed for the plant to snap
- Cushioning → to absorb the force of the falling mass
- Timer → to place the weights at equal time intervals
Method :
1. Using a scalpel, cut a section of the plant sample and extract a length of plant fibre
2. Clamp the fibres at each end between 2 clamp stands, making sure the fibre is
straight. Place cushioning on the surface under the weights
3. Place a mass hanger in the centre of the fibre and carefully place a 10g mass. Wait
10-20 seconds to seconds before adding another weight.
4. Gradually add masses of 10g onto the mass hanger until the fibre snaps and record
the mass of the weights before the fibre snapped. The mass required to break the
fibre gives a measure of the tensile strength
5. Repeat the experiment again with different samples of the same place and calculate
mean to get more valid results
6. Repeat the experiment with different plant fibres
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