International A Level Biology / Chemistry for biologists
Starch and glycogen: storing energy revision notes
Edexcel International A Level Biology (YBI11), chapter 1: Chemistry for biologists, specification point 1.2, 1.4. Free lesson.
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Starch and glycogen store glucose as large polysaccharides. Their structures let cells store glucose compactly and release it when needed for respiration.
How the glucose chains are joined
Starch and glycogen are polysaccharides, meaning molecules made from many joined monosaccharides. Their monomer is α-glucose. Along a chain, carbon 1 of one glucose joins carbon 4 of the next, forming a 1,4 glycosidic bond. At a branch, carbon 1 of the new chain joins carbon 6 of a glucose in the existing chain, forming a 1,6 glycosidic bond. Amylose has 1,4 bonds only. Amylopectin and glycogen have 1,4 bonds along their chains and 1,6 bonds at branch points.
How amylose, amylopectin and glycogen differ
Starch is a mixture of amylose and amylopectin, both found in plants. Glycogen is the animal glucose store, found in the liver and muscle. Their different branching patterns give them different shapes. Amylose is one unbranched chain that coils into a helix, a spiral shape. Amylopectin is branched, with long stretches between branch points. Glycogen is more branched, with shorter chains between branches. Each branch has a chain end, and glycogen has the most ends.
How their structures help store energy
Each structural feature helps the cell store glucose without disrupting its conditions. In an exam answer, link each feature to what it does. Compact: amylose coils, while branched molecules pack tightly. This allows lots of glucose to fit in a small space. Insoluble: the store does not dissolve in the cell, so it does not lower the cell's water potential or draw water in by osmosis. Large: the molecules are too big to cross the cell membrane, so the store stays inside the cell. Branched: many chain ends are available, so enzymes can remove glucose from many ends at once.…
How hydrolysis releases glucose
Hydrolysis breaks glycosidic bonds using water. It is the reverse of the condensation reaction that builds the store. Enzymes catalyse the bond breaking, and one water molecule is used for each bond broken. The store may be glycogen in an animal cell or starch in a plant cell. Glucose is released from the chains and can be used in respiration. An enzyme that only hydrolyses 1,4 bonds cannot completely break down amylopectin. Its 1,6 bonds remain, leaving small branched pieces rather than only glucose.