B1.1.5 Polysaccharides as energy storage compounds Notes

The Structure of Starch and Glycogen: Compact and Efficient

Starch Is The Plant Energy Reserve

1. Plants store energy in the form of starch, a polysaccharide composed entirely of α-glucose molecules.
2. Starch is a mixture of two types of molecules: amylose and amylopectin.
   1. Amylose:
      1. Amylose is an unbranched chain of α-glucose molecules linked by1→4 glycosidic bonds.
      2. Due to the bond angles, amylose forms a helical structure, which makes it compact and efficient for storage.
      3. Its unbranched nature means it has fewer ends where enzymes can act to release glucose, making energy release slower compared to amylopectin.
   2. Amylopectin:
      1. Amylopectin is similar to amylose but includes additional1→6 glycosidic bonds, forming a branched structure.
      2. The branches allow enzymes to access multiple chain ends simultaneously, enabling faster glucose release when energy is needed.

Glycogen Is The Animal Energy Reserve

1. Animals store energy in the form of glycogen, which has a structure similar to amylopectin but is even more highly branched. In glycogen:
   1. 1→4 glycosidic bondslink the α-glucose molecules in the main chain.
   2. 1→6 glycosidic bondsform branches approximately every 10 glucose units, compared to every 20 units in amylopectin.
   3. This extensive branching makes glycogen even more compact and allows rapid mobilization of glucose, which is critical for meeting the high and variable energy demands of animals.

Why Polysaccharides Are Ideal Energy Storage Molecules

Compactness

1. The coiling (in amylose) and branching (in amylopectin and glycogen) during polymerization make these polysaccharides highly compact.
2. This allows a large amount of glucose to be stored in a small space, which is especially important in cells with limited storage capacity.

Low Solubility

1. Unlike glucose, which is highly soluble and would cause osmotic problems if stored in large quantities,
2. starch and glycogen are relatively insoluble due to their large molecular size. This means they:
   1. Do not dissolve in the cytoplasm.
   2. Avoid drawing water into the cell via osmosis, which could cause the cell to swell and burst.

Ease of Adding and Removing Glucose

1. Polysaccharides are dynamic storage molecules. Glucose can be added (via condensation reactions) or removed (via hydrolysis reactions) as needed:
   1. Condensation: When glucose is abundant, enzymes link α-glucose molecules to the growing polysaccharide chain, releasing water as a byproduct.
   2. Hydrolysis: When energy is needed, enzymes break the glycosidic bonds, releasing glucose monomers for cellular respiration.

Applications of Starch and Glycogen as Energy Stores

Starch in Plants

1. Starch is stored in chloroplasts and amyloplasts (specialized storage organelles in plants).
2. It serves as a long-term energy reserve, particularly in seeds, tubers, and roots. For example:
   1. Potatoes store starch in their tubers to provide energy for sprouting new plants.
   2. Seeds like rice and wheat store starch to fuel germination and early growth.

Glycogen in Animals

1. Glycogen is stored in the liver (to regulate blood glucose levels) and muscles (to provide energy during physical activity). For example:
   1. During fasting, glycogen in the liver is broken down to maintain blood glucose levels.
   2. During a sprint, muscle glycogen is rapidly mobilized to meet the immediate energy demands of muscle cells.

Why Branching Matters: Faster Energy Mobilization

1. The branched structure of amylopectin and glycogen provides numerous chain ends where enzymes can act simultaneously.
2. This is especially important for animals, which often need to mobilize energy quickly. For instance:
   1. A bear waking from hibernation relies on glycogen to fuel its initial movements.
   2. A sprinter uses glycogen stores to power their muscles during a race.

Comparing Polysaccharides to Lipids: Energy Storage

1. While polysaccharides like glycogen and starch provide short-term energy storage, lipids serve as long-term energy reserves.
2. The key differences between these two types of molecules in terms of energy storage include:
   1. Energy Density:
      1. Lipids are more energy-dense than polysaccharides, storing approximately 9 kcal/g compared to 4 kcal/g for carbohydrates. This makes lipids a more efficient form of long-term energy storage.
   2. Osmotic Issues:
      1. Glycogen and starch, being hydrophilic, would draw water into the cell if stored in large amounts, leading to potential osmotic problems. In contrast, lipids are hydrophobic, meaning they do not affect cellular water balance.
   3. Mobilization:
      1. Glycogen is more readily mobilized than lipids, which require more complex metabolic processes (e.g., beta-oxidation) for breakdown. This makes glycogen ideal for rapid energy needs (e.g., during exercise or stress), whereas lipids are better suited for slow, sustained energy release.

Key Takeaways

1. Starch (in plants) and glycogen (in animals) are polysaccharides used for energy storage.
2. Both are composed of α-glucose, making them easily broken down for energy.
3. Their compact, branched structures allow efficient storage and rapid mobilization of glucose.
4. Their large size makes them relatively insoluble, avoiding osmotic problems in cells.
5. Glucose can be added or removed through condensation and hydrolysis reactions, making these molecules dynamic energy reserves.