2.2-Carbohydrates 2: Polysaccharides as energy stores Flashcards
What are polysaccharides?
Polysaccharides are polymers of monosaccharides. They are made of hundreds or thousands of monosaccharides monomers bonded together. polysaccharides made solely of one kind monosaccharides called homopolysaccharides. Those made of more than on monomer are called heteropolysaccharides. Starch is an example of homopolysaccharides. Hyaluronic acid (in connective tissue) is an example of a heteropolysaccharides.
What is glucose?
Glucose is a source of energy, as it is a reactant in respiration. The energy released is sued to make ATP, which is the energy currency of the cell:
Glucose + oxygen——>carbon dioxide + water
If you join lots of glucose molecules together into polysaccharides, you can create a store of energy and this is exactly what living things do. Plants store energy as starch in chloroplasts and in membrane bound starch grains, and humans store energy as glycogen in cells of muscles or liver
What does the structure of polysaccharides allow?
The structure of some polysaccharides lends itself to energy storage. Glycogen in animals and starch in plants (amylose and amylopectin) occur within cells in the form of larger granules.
Why do polysaccharides form good stores of monosaccharides?
They form good stores of monosaccharides for the following reasons:
- Glycogen and starch are compact, which means they do not occupy a large amount of space. They both occur in dense granules within the cell.
-Polysaccharides hold glucose molecules in chains, so they can be easily ‘snipped’ off from the end of the chains by hydrolysis when required for respiration. Hydrolysis reactions are always catalysed by enzymes.
-Some chains are unbranched (AMYLOSE) and some are branched (AMYLOPECTIN AND GLYCOGEN).
Branched chains tend to be more compact , but also offer the chance for lots of glucose molecules to be snipped off by hydrolysis at the same time, when lots of energy is required quickly. The enzyme amylase is responsible for hydrolysing 1-4 glycosidic linkages, and glucosidase is responsible for hydrolysing 1-6 glycosidic linkages. A 1-4 glycosidic linkage is one between carbon 1 of one glucose and carbon 4 of the other.
Are polysaccharides soluble? Pol
Polysaccharides are less soluble in water than monosaccharide. If many glucose molecules did dissolve in the cytoplasm, the water potential would reduce, and excess water would diffuse in, disrupting the normal workings of the cell. Polysaccharides are less soluble because of their size, but also because regions which could hydrogen-bond with water are hidden away inside the molecule. Sometimes the amylose molecule may form a double helix, which presents a hydrophobic external surface in contact with surrounding solution
What is amylose?
Amylose (in plants): this molecule is a long chain of a-glucose molecules. Like Maltose, it has glycosidic bonds between carbon 1 and 4
What is the structure of amylose?
Amylose coils into a spiral shape, with hydrogen bonds holding the spiral in place. Hydroxyl groups on carbon 2 are situated on the inside of the coil, making the molecule less soluble and allowing hydrogen bonds to form to maintain coil’s structure.
What is amylopectin?
Amylopectin (in plants) : this molecule is a long chain of a-glucose molecules, with glycosidic bonds between 1 and 4 but in addition it has branches formed by glycosidic bonds between carbon 1 and 6.
What is the structure of amylopectin?
Amylopectin also coils into a spiral shape, held together with hydrogen bonds but with branches emerging from the spiral.
What is glycogen?
Glycogen (in animals): this molecules a long chain of a-glucose molecules, with glycosidic bonds between carbon 1 and 4 and branches formed by glycosidic bonds between carbon 1 and 6.
What is the structure of glycogen?
The 1-4 bonded chains tend to be smaller than in amylopectin, so glycogen has less tendency to coil. However it does have more branches, which makes it more compact. And it is easier to remove monomer units as there are more ends.
