Polysaccharides — Starch and Glycogen

Polysaccharides are polymers of monosaccharides joined by glycosidic bonds. They may contain hundreds or thousands of monomer units. This lesson covers the two main α-glucose storage polysaccharides — starch (in plants) and glycogen (in animals and fungi) — and relates their structures to their storage functions. It addresses OCR specification point 2.1.2 (b)(iv).

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1. Why Store Glucose as a Polysaccharide?

Cells store glucose as polysaccharides rather than as free glucose for several key reasons:

1. Osmotic inertness — polysaccharides are large and insoluble (or only slightly soluble), so they do not lower the water potential of the cell. Free glucose would draw water into cells by osmosis, causing them to swell and potentially burst.
2. Compactness — polysaccharides can be coiled or branched, packing enormous amounts of glucose into a small volume.
3. Ease of mobilisation — the terminal glucose residues can be hydrolysed rapidly when energy is needed.
4. Chemically stable — polysaccharides are less reactive than free sugars.

Key Definition — Polysaccharide: A polymer formed by many monosaccharide molecules joined together by glycosidic bonds in a condensation reaction.

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2. Starch — the Storage Polysaccharide of Plants

Starch is the main storage carbohydrate in plants. It is stored as dense starch granules inside chloroplasts (in photosynthetic tissue) and amyloplasts (in storage organs such as potato tubers and cereal grains). Starch is a mixture of two polymers of α-glucose:

• Amylose (unbranched, helical) — typically 20–30% of starch
• Amylopectin (branched) — typically 70–80% of starch

2.1 Amylose

Amylose is an unbranched chain of α-glucose monomers joined exclusively by 1,4-glycosidic bonds. Because of the angle at which α-glucose units join, the chain coils into a left-handed helix, stabilised by hydrogen bonds between residues approximately six glucose units apart. Six glucose units make one complete turn of the helix.

Amylose (coiled helix — 1,4-glycosidic bonds only):

   α-glu — α-glu — α-glu — α-glu — α-glu — α-glu
      \                                      /
       \       coils into a helix          /
        \                                 /
         α-glu — α-glu — α-glu — α-glu

Advantages of the helical structure:

• Very compact — allows large amounts of glucose to be stored in a small volume.
• The interior of the helix is hydrophobic, making amylose insoluble and osmotically inactive.
• The helix binds iodine ions (I₃⁻) inside its coils, producing the characteristic blue-black colour in the iodine test.

2.2 Amylopectin

Amylopectin consists of α-glucose chains joined by 1,4-glycosidic bonds (like amylose) but with additional 1,6-glycosidic bond branches every 20–30 glucose residues.

Amylopectin (branched — mainly 1,4 with 1,6 branches):

       α-glu—α-glu—α-glu—α-glu—α-glu—α-glu—α-glu—α-glu
                           |
                         α-glu
                           |
                         α-glu
                           |
                 α-glu—α-glu—α-glu—α-glu

Advantages of branching:

• Many free terminal ends (non-reducing ends) allow amylases to act on multiple points simultaneously, releasing glucose rapidly when needed.
• Branches create a compact, globular structure suitable for packing into granules.

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3. Glycogen — the Storage Polysaccharide of Animals

Glycogen is the principal storage carbohydrate of animals and fungi. It is stored in large amounts in:

• Liver — around 100 g in an adult human; supplies glucose to blood to maintain blood glucose concentration.
• Skeletal muscle — around 300–400 g in an adult human; provides glucose for muscle contraction.

3.1 Structure

Glycogen is similar to amylopectin but more highly branched — it has 1,6-glycosidic branch points every 8–12 glucose residues (compared with every 20–30 in amylopectin).