2.2-Carbohydrates 3: Polysaccharides as structural units Flashcards
Where is cellulose found?
Cellulose is found in plants (plastids), forming the cell walls. It is a tough insoluble and fibrous substance. Cellulose is a homopolysaccharides made form long chains of up to 15, 00 B-glucose molecules, bonded together through condensation reactions to form glycosidic bonds.
How is B- glucose structure different from a-glucose?
Rather than spiralling like chains of a- glucose, cellulose chains are straight and lie side by side. This difference in structure is a direct result of bond:
- Hydrogen and hydroxyl groups on carbon 1 are inverted in B-glucose ( as compared with a-glucose). This means that every other B-glucose molecules in the chain is rotated by 180 degrees. This and B1-4 glycosidic bond help to prevent the chain spiralling.
- Hydrogen bonding between the rotated B-glucose molecules in each chain also gives the chain additional strength and stops it spiralling.
- Hydrogen bonding between the rotated B-glucose molecules in different chains gives the while structure additional strength, The hydroxyl group on carbon 2 sticks put, enabling hydrogen bonds to be formed between chains.
What happens when 60 to 70 cellulose chains are bound?
When 60 to 70 cellulose chains are bound together in this way, they form microfibrils, which are 10-30nm in diameter. These then bundle together into macrofibrils containing up to 400 microfibrils which are embedded in pectins (like glue) to form plant cell walls.
What does the arrangement of cellulose chain do?
Arrangement of cellulose chains in the formation of cell wall macrofibrils. The macrofibrils are embedded in pectins to form the wall. Macrofibrils run in all directions criss-crossing the wall for extra strength.
Structure of cellulose chains:
Lots of B-glucose chains————> lots of microfibrils——–> lots of macrofibrils
How is cellulose an excellent material for plant cell walls?
Cellulose is an excellent material for plant cells:
- Microfibrils and macrofibrils have very high tensile strength, both because of the strength of glycosidic bonds but also because of the hydrogen bonds between chains. Macrofibrils are stringer than steel with of same diameter.
- Macrofibrils run in all directions criss-crossing the wall for extra strength.
- It is difficult to digest cellulose because the glycosidic bonds between the glucose molecules are less easily to break, most animals do not even have an enzyme to catalyse the reaction
What are the key features which help plant cell to do its job:
The key features are:
-Because plants do not have a rigid skeleton, each cell needs to have strength to support the whole plant.
There is a space between macrofibrils for water and mineral ions to pass on their way into and out of the cell. This makes the cell wall fully permeable
-The wall has high tensile strength, which prevents plants cell from bursting when they are turgid, helping to support the whole plant. Turgid cells press against each other supporting the structure as a whole. The wall protects the delicate cell membrane.
-The macrofibrils structure can be reinforced with other substances fro extra supporter to make the walls water proof. For example cutin and suberin are waxes the block the spaces in the cell wall, and maker it waterproof. Lignin (polymer of phenylpropane units) performs the same functions for xylem vessels. In woody part of tree trunks, cell walls are extra thick to withstand the weight.
How do humans exploite cellulose?
The structural strength pf cellulose has been exploited by humans. Cotton is 90% cellulose. Cellophane and celluloid (used in photographic film) are also derived from cellulose. One of the main components of paper is cellulose. Rayon (viscose) is a semi- synthetic fibre produced form cellulose. It has similar properties to silk.
What are other structural polysaccharides?
- Bacterial cell wall
- Exoskeletons
What is bacterial cell wall?
Bacteria also have a cell wall, but not made from cellulose. The whole structure surrounding the cell is called a peptidoglycan from long polysaccharide chains that lie in parallel, cross linked by short peptide chains (made form amino acids)
What is exoskeleton?
Insects and crustacean exoskeletons are made of chitin. It differs from cellulose because it ahs an acetylamino group (NH. OCCH3), rather than a hydroxyl group on carbon 2. It forms cross links between long parallel chains of aceyglucosamine similar to cellulose.
