4.9 Cellulose and Starch, 4.10 Uses of Plant Fibres Flashcards
4.9 Understand the structure and function of the polysaccharides starch and cellulose, including the role of hydrogen bonds between β-glucose molecules in the formation of cellulose microfibrils. 4.10 Understand how the arrangement of cellulose microfibrils and secondary thickening in plant cell walls contributes to the physical properties of xylem vessels and sclerenchyma fibres in plant fibres that can be exploited by humans. (22 cards)
What is the major component of cell walls in plants?
cellulose
What is cellulose?
long, unbranched chains of beta glucose
(it is a polysaccharide)
Is cellulose soluble?
no
How is cellulose formed?
beta glucose units join by condensation reaction to form 1-4 glycosidic bonds
What must occur for the formation of 1-4 glycosidic bonds to be possible?
every other beta glucose molecule is rotated 180° (inverted)
Are cellulose chains straight or branched?
straight (1-4 glycosidic bonds are straight)
What happens between cellulose chains?
hydrogen bonds form between the -OH groups in neighbouring cellulose chains, forming bundles of cellulose chains (about 50-80)
What are the bundles of cellulose chains called?
microfibrils
Which are stronger: hydrogen bonds or glycosidic bonds?
glycosidic bonds
How do microfibrils provide structural support?
the large number of hydrogen bonds mean the microfibrils are strong and therefore can strengthen plant cell walls
How are the microfibrils arranged?
- wound in a helical arrangement around the cell and stuck together with a polysaccharide glue
- are laid down at different angles to provide strength and flexibility
What is the polysaccharide glue composed of and how do they work?
- short, branched polysaccharides called hemicelluloses and pectins
- they bind to the surface of the cellulose and to each other, holding the cellulose together
What is starch composed of?
- a mixture of 2 polysaccharides of alpha glucose: amylose and amylopectin
What is amylose?
- a long, unbranched chain of alpha glucose joined by 1-4 glycosidic bonds
- coiled structure
- structure means it is compact and therefore good for storage
What is amylopectin?
- a long, branched chain of alpha glucose joined by 1-4 and 1-6 glycosidic bonds
- side branches allow enzymes to reach the glycosidic bonds easily
- glycosidic bonds are broken down more easily and therefore glucose can be released quickly
Is starch soluble and why?
no
it will not interfere with water potential and osmosis so is good for storage
What is the function of starch in plants?
- the main energy storage in plants
- cells require glucose for energy
- plants store excess glucose as starch
- starch is broken down and glucose is released when the plant requires more energy
What are plant fibres?
long tubes of plant cells
(e.g. the sclerenchyma and xylem are made of tubes of dead cells)
What makes plant fibres strong?
- the arrangement of cellulose microfibrils in the cell wall
- the secondary thickening of cell walls
How does the arrangement of cellulose microfibrils in the cell wall strengthen plant fibres?
- arranged in a net-like arrangement (laid down at different angles like 𐄳)
- this gives plant fibres strength and flexibility
How does the secondary thickening of cell walls strengthen plant fibres?
- some structural plant cells that have finished growing produce a secondary cell wall between the normal cell wall and the cell membrane
- this secondary cell wall is thicker and has more lignin than the normal cell wall
- this makes plant fibres even stronger
How are plant fibres useful?
the strength of plant fibres makes them useful for producing things such as ropes and fabrics (from plant fibres like hemp)
