Polysaccharides — Starch and Glycogen

Spec mapping — OCR H420 Module 2.1.2 — Biological molecules. This lesson covers the storage polysaccharides of plants (starch — a mixture of amylose and amylopectin) and animals/fungi (glycogen), both built from α-glucose monomers via α-1,4-glycosidic bonds with α-1,6-glycosidic branch points. The structure–function relationship between branching density and mobilisation rate is the central pedagogical thread (refer to the official OCR H420 specification document for exact wording).

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.

1. Why Store Glucose as a Polysaccharide?

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

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.
Compactness — polysaccharides can be coiled or branched, packing enormous amounts of glucose into a small volume.
Ease of mobilisation — the terminal glucose residues can be hydrolysed rapidly when energy is needed.
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.

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.

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.

graph LR
  M1["α-glu"] -- 1,4 --> M2["α-glu"]
  M2 -- 1,4 --> M3["α-glu"]
  M3 -- 1,4 --> M4["α-glu"]
  M4 -- 1,4 --> M5["α-glu"]
  M5 -- 1,4 --> M6["α-glu"]
  M6 -- 1,4 --> M7["α-glu"]
  M3 -- "1,6 branch" --> B1["α-glu"]
  B1 -- 1,4 --> B2["α-glu"]
  B2 -- 1,4 --> B3["α-glu"]
  B3 -- 1,4 --> B4["α-glu"]

Amylopectin is mostly 1,4-linked α-glucose with 1,6-glycosidic branches every 20–30 residues, producing many free non-reducing ends for rapid enzymatic mobilisation.

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.

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).

graph TD
  A[Glycogen core] --> B[1,4 chain]
  B --> C[1,6 branch]
  C --> D[1,4 chain]
  D --> E[1,6 branch]
  B --> F[1,6 branch]
  F --> G[1,4 chain]
  G --> H[1,6 branch]

3.2 Why More Branched Than Amylopectin?

Animals have much higher metabolic rates than plants. They need to mobilise glucose rapidly for contraction of muscles, nerve impulse conduction, and maintenance of blood glucose. The higher branching of glycogen:

Provides even more free ends for glycogen phosphorylase enzymes to act on.
Allows for very rapid hydrolysis during exercise or when blood glucose falls.
Makes glycogen extremely compact and insoluble — essential for cells with a high density of cytoplasmic contents.

4. Comparison of Starch and Glycogen

Feature	Amylose	Amylopectin	Glycogen
Monomer	α-glucose	α-glucose	α-glucose
Main bond	1,4-glycosidic	1,4-glycosidic	1,4-glycosidic
Branching bond	None	1,6-glycosidic	1,6-glycosidic
Branch frequency	N/A	~every 20–30 residues	~every 8–12 residues
Shape	Helical, unbranched	Branched helix	Highly branched globule
Organism	Plants	Plants	Animals, fungi
Hydrolysis rate	Slowest	Faster	Fastest
Iodine test colour	Blue-black	Purple	Red-brown

5. Structure-Function Relationships

Insoluble → osmotically inactive: Both polysaccharides are too large and too non-polar overall to dissolve. Storing glucose this way avoids changing the water potential of the cell.

Compact → efficient storage: Helical amylose and globular glycogen pack enormous amounts of energy into a small volume.

Branched → rapid mobilisation: Branches give many non-reducing ends where enzymes can release glucose. Animals need faster energy release than plants, hence more branches.

α-glucose monomer → easily metabolised: α-glucose can be phosphorylated and enter glycolysis directly once hydrolysed.

Large → cannot cross cell membranes: Polysaccharides remain inside the storage cells or organelles.

6. Hydrolysis of Starch and Glycogen

Starch digestion (humans):

Salivary amylase (mouth) begins hydrolysis of 1,4-glycosidic bonds, producing maltose and short dextrins.
Pancreatic amylase (small intestine) continues hydrolysis to maltose.
Maltase, a membrane-bound disaccharidase on the epithelium of the ileum, hydrolyses maltose to α-glucose, which is absorbed by co-transport with Na⁺.

Glycogen mobilisation (animal cells):

Glycogen phosphorylase cleaves glucose units from the non-reducing ends, releasing glucose-1-phosphate, which is converted to glucose-6-phosphate for entry into glycolysis.
A debranching enzyme handles the 1,6 branch points.
Regulation is hormonal: glucagon and adrenaline stimulate glycogen breakdown; insulin stimulates glycogen synthesis.

Exam Tips

Always specify the type of glycosidic bond (1,4 or 1,6).
When linking structure to function, use the chain: branched → many free ends → rapid hydrolysis → rapid glucose release.
Know specific iodine test colours: starch = blue-black; glycogen = red-brown.
Remember that glycogen is more branched than amylopectin, not the same.

Common Exam Mistakes

Writing "glucose" when you should write "α-glucose".
Saying starch is one molecule — it is a mixture of amylose and amylopectin.
Calling glycogen "animal starch" without explaining branching differences.
Forgetting that starch is insoluble because it is polymeric, not because it is non-polar (glucose residues are hydrophilic).
Writing that branching "increases surface area for bonds to form" — it increases the number of free non-reducing ends available to hydrolysing enzymes.

7. Synoptic Links

This lesson connects across the OCR H420 specification:

ocr-alevel-biology-biological-molecules Lesson 5 — cellulose. Provides the structural contrast: α-1,4 (storage, coiled) vs β-1,4 (structural, extended). One stereochemical inversion produces entirely different biological function.
ocr-alevel-biology-nucleic-acids-enzymes — enzyme specificity. α-amylase, glycogen phosphorylase, debranching enzyme and maltase are tightly specific to α-1,4 vs α-1,6 bonds — illustrating active-site geometric complementarity (induced fit, Koshland 1958).
ocr-alevel-biology-neuronal-hormonal — hormonal control of blood glucose. Insulin (anabolic, promotes glycogen synthesis) and glucagon (catabolic, promotes glycogen breakdown) regulate liver glycogen. Adrenaline acts via cAMP to activate glycogen phosphorylase in muscle.
ocr-alevel-biology-photosynthesis-respiration — Calvin cycle and starch synthesis. Triose phosphates exported from chloroplasts are condensed into starch granules; conversely, starch is hydrolysed at night to maintain sucrose export.

8. Specimen Question (Modelled on the OCR H420 Paper Format)

Q (9 marks): Glycogen is the storage polysaccharide of animals; amylopectin is the major branched component of plant starch. Explain how the differences in their structures relate to differences in metabolic demand between animals and plants.

AO breakdown

AO	Marks	Earned by
AO1	3	Structural features of glycogen and amylopectin
AO2	4	Linking branching density to mobilisation rate and metabolic rate
AO3	2	Evaluating across taxa (motile vs sessile lifestyles)

Mid-band response

Glycogen and amylopectin are both made of α-glucose units joined by 1,4 and 1,6 glycosidic bonds. They are both branched and coiled into compact shapes. Glycogen is more branched than amylopectin: glycogen branches every 8–12 glucose units, while amylopectin branches every 20–30. More branching means more free ends for enzymes to act on, so glucose is released faster. Animals are more active than plants and need glucose quickly, especially in muscle during exercise, so glycogen is more branched. Glycogen is stored in the liver and muscles. Amylopectin is stored in starch granules in chloroplasts and amyloplasts. Both are insoluble so they don't change the water potential of the cell.

Examiner commentary: M1 monomer, M1 1,4+1,6 bonds, M1 branching frequency difference, M1 free ends/rapid hydrolysis, M1 metabolic-rate linkage. Around 5/9. Misses regulatory hormones, the structural compartmentalisation in animal vs plant cells, and the AO3 lifestyle comparison.

Stronger response

Glycogen (animals) and amylopectin (plants, ~70–80% of starch) are both α-glucose polysaccharides with α-1,4-glycosidic backbone bonds and α-1,6-glycosidic branch points. The critical difference is branching density: every 8–12 residues in glycogen, every 20–30 in amylopectin.

This difference is functionally significant because branch points generate non-reducing ends — the substrates of degradation enzymes (glycogen phosphorylase, α-amylase). A more branched molecule has more free ends per unit mass, allowing more simultaneous enzyme attack and therefore faster glucose release per second.

Animals are motile, often endothermic, and have peaks of energy demand (sustained running in mammals, fight-or-flight in vertebrates) that can require ATP turnover increasing tenfold within seconds. Skeletal muscle stores ~300–400 g of glycogen and the liver ~100 g, regulated by insulin (anabolic), glucagon (hepatic catabolic) and adrenaline (rapid catabolic). The high branching density of glycogen permits the explosive glucose release required.

Plants are sessile and metabolically slower. Photosynthetic source tissues store starch during daylight; over a 24-hour cycle the granule is gradually hydrolysed back into triose phosphates and exported as sucrose. Rapid mobilisation is not required, and the lower branching density of amylopectin is sufficient.

Both polysaccharides exploit the same chemistry — α-1,4 + α-1,6 — but the quantitative tuning of branching matches metabolic demand.

Lesson 4: Polysaccharides — Starch and Glycogen