10.3 Starch, Glycogen and cellulose

• Polysacchride.
• Found in parts of a plant in the form of small grains
• Especially in seeds and storage organs.
• Important component of food.
• Major energy source in most diets.

Starch is made up of, alpha glucose, which form together to form a chain of monosacchrides. Linked together by glycosidic bonds which are formed by condensation reactions. 

The unbranched chain is wounded into a tight coil that amkes the molecule very compact. This arrangement means that:

1. Insouble--> does not effect osmosis.
2. Doesn't easily diffuse out of cells.
3. Compact--> a lot can be stored in a small space.
4. when hydrolysed--> forms an alpha glucose--> easily transported and used in respiration.

Starch is NEVER found in animal cells!

• Similar structure to starch, BUT has shorter chains and is highly branched.
• Sometimes called--> "animal starch"--> major carbonhydate storage product of animals.
• Storaged as small granules mainly in muscles and the liver.
• Structure suits it for storage, (same reasons as starch)--> but made up of smaller chains--> more readily hydolysed to alpha glucose.

It is found in animal cells NEVER plant cells.

Cellulose is different to starch and glycogen for 1 major reason--> it is made by beta glucose.

Beta-glucose:

• The -OH is above rather than below.
• The -H is below rather than above.
• The beta glucose has to flip 180 degrees--> the -CH(2)OH group alternates between each beta glucose.

Cellulose forms straight unbranched chains, these run parrallel to each other, which allows hydrogen bonds to form cross-links between lines. These strengthen cellulose making it the valuable structural matieral.

Cellulose molecules group together to form microfibrils, which arrange into parallel groups called fibres. 

Cellulose is used in the cell wall:

• provides rigidity to the plant cell.
• prevents the cell from bursting while water enters by osmosis--> by exerting an inward pressure-->living plant cells are turgid and push against each other--> making parts of the plant semi-rigid --> this maintains stems and leaves in a turgid state so they can provide maximum surface area for photosynthesis.