IB Biology Topic B1.1.5: Polysaccharides as Energy Storage Compounds Notes

Starch and Glycogen: Compact Glucose Stores for Rapid Energy

1. Plants store energy as starch and animals store energy as glycogen.
2. Both are polysaccharides made entirely of α-glucose monomers, linked by glycosidic bonds.
3. Starch is a mixture of two molecules: amylose and amylopectin.
   1. Amylose
      1. Amylose is an unbranched chain of α-glucose linked by 1,4 glycosidic bonds.
      2. The bond angles cause amylose to coil into a helical structure, making it compact.
      3. Because amylose is unbranched, it has fewer ends where enzymes can act, so glucose release is slower.
   2. Amylopectin
      1. Amylopectin also has 1,4 glycosidic bonds but includes additional 1,6 glycosidic bonds that create branches.
      2. The branches give enzymes access to multiple chain ends at once, so glucose is released faster than from amylose.

Glycogen: More Branched for Faster Glucose Release

1. Glycogen has a structure similar to amylopectin but is even more highly branched.
2. Branches form roughly every 10 glucose units (compared to every 20 in amylopectin), via 1,6 glycosidic bonds.
3. This extensive branching makes glycogen very compact and allows rapid glucose release when energy demands spike.
4. Glycogen is stored mainly in the liver (to regulate blood glucose) and skeletal muscle (to fuel physical activity).

Three Properties That Make Polysaccharides Ideal for Energy Storage

1. Compactness
   1. Coiling and branching during polymerisation make starch and glycogen highly compact.
   2. This lets cells store a large amount of glucose in a small volume.
2. Low Solubility
   1. Starch and glycogen are relatively insoluble because of their large molecular size.
   2. This stops them from dissolving in the cytoplasm and drawing water in by osmosis, which could raise water potential inside the cell and cause it to swell and burst.
3. Easy to Build and Break Down
   1. Glucose can be added by condensation reactions (releasing water) or removed by hydrolysis reactions (adding water).
   2. This makes polysaccharides dynamic reserves that adjust to the cell's energy needs.

Where Organisms Store Their Polysaccharides

1. Starch in Plants
   1. Starch is stored in chloroplasts and amyloplasts (specialised storage organelles).
   2. Potatoes store starch in tubers to provide energy for sprouting.
   3. Seeds like rice and wheat store starch to fuel germination and early growth.
2. Glycogen in Animals
   1. The liver stores glycogen to regulate blood glucose between meals.
   2. Skeletal muscles store glycogen for rapid energy during physical activity.