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The key points covered in this video include:
1. Structural Polysaccharides - Cellulose
2. Molecular Structure of Cellulose
3. Hydrogen Bonding in Cellulose
4. Fibres of Cellulose
5. Cellulose as a Fibre
Structural Polysaccharides - Cellulose
Polysaccharides can also have a structural role in organisms such as plants. In plants, the cell wall is made from a polysaccharide called cellulose. Cellulose is very strong and stops plant cells from bursting when too much water enters by osmosis - it makes the cells turgid. This turgidity provides enough strength to each cell to support the whole plant.
Molecular Structure of Cellulose
Cellulose is composed of many thousands of β-glucose molecules joined together by 1,4 glycosidic bonds. To be able to form the 1,4 glycosidic bonds, each β-glucose molecule must be inverted 180˚from the previous molecule. The inversions keep cellulose from coiling and results in a long, straight chain.
Hydrogen Bonding in Cellulose
Cellulose chains are straight - this allows many chains to run parallel to each other. This results in hydroxyl (-OH) groups that are in close proximity. Hydrogen bonds form between the hydroxyl (-OH) groups on adjacent chains. This results in cross-linking between the cellulose chains. While each individual hydrogen bond is weak, the many thousands of hydrogen bonds collectively make cellulose very strong.
Fibres of Cellulose
The hydrogen bonds that cross-link the cellulose chains allow the chains to form into stronger fibres. The cellulose chains first bundle together to form microfibrils. Microfibrils also then bundle together to form larger fibres called macrofibrils. The macrofibrils (fibres) wrap around plant cells in multiple layers at different angles. The wrapping of the strong cellulose fibres around the plant cell provides extra strength to the plant cell wall.
Cellulose as a Fibre
Cellulose is the main component of plant walls, making it the most abundant organic material in the world. However, cellulose is not an easily digestible food source because it is very hard to break down by hydrolysis. This is because most animals lack the cellulase enzyme needed to break the 1,4 glycosidic bonds between β-glucose molecules. Some herbivores have symbiotic bacteria in their guts that produce the enzyme. This allows herbivores to digest a larger proportion of their diet and gain more energy. However, cellulose is still important in the human diet as it provides the fibre needed to keep the digestive system healthy.
Summary
Cellulose is a polysaccharide that is used to strengthen cell walls
Cellulose is composed of long, straight chains of β-glucose with 1,4 glycosidic bonds between each β-glucose molecule
Thousands of hydrogen bonds cross-link between the chains, holding them together
The cellulose chains bundle into stronger microfibrils, which then bundle into even stronger macrofibrils (fibres)
The 1,4 glycosidic bonds are hard to break so cellulose cannot easily be digested
27 фев 2019
