What you need to know
- 1.2(i–ii)Polysaccharides: glycogenThe storage polysaccharide in animals: highly branched chains of α-glucose, so glucose can be released quickly., amyloseThe unbranched part of starch: α-glucose joined by 1,4 glycosidic bonds, coiled into a compact helix. and amylopectinThe branched part of starch: α-glucose joined by 1,4 and 1,6 glycosidic bonds. The branches give many ends for fast hydrolysis., and how their structures suit energy storage.
- 1.4Formation and hydrolysisA reaction that breaks a covalent bond using a molecule of water. of glycosidic bonds in polysaccharides.
β-glucose and cellulose are explicitly outside Topic 1. Core Practical 1 (specification 1.3), including colour standards, is available in the linked practical guide.
By the end, you should be able to…
- Compare amylose, amylopectin and glycogen.
- Connect compactness, insolubility and branching to storage.
- Predict the products of complete hydrolysis.
Build your understanding
Many units, one polymer
A polysaccharideA polymer of many monosaccharides joined by glycosidic bonds, such as starch, glycogen or cellulose. is a large molecule built from many monosaccharideA single sugar unit, such as glucose, fructose or galactose. Monosaccharides are the monomers of carbohydrates. residues joined by glycosidic bonds. starchThe storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble, so it does not affect water potential. and glycogenThe storage polysaccharide in animals: highly branched chains of α-glucose, so glucose can be released quickly. are made from α-glucoseThe form of glucose with the –OH on carbon 1 below the ring. It is the monomer of starch and glycogen.. Repeated condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. reactions join the units; hydrolysisA reaction that breaks a covalent bond using a molecule of water. breaks the bonds.
For one unbranched chain of n glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. units, n − 1 glycosidic bonds are formed and n − 1 water molecules are released. Complete hydrolysis reverses this process and produces glucose.
Two components of starch
Starch is a storage carbohydrate in plants. It contains amyloseThe unbranched part of starch: α-glucose joined by 1,4 glycosidic bonds, coiled into a compact helix. and amylopectinThe branched part of starch: α-glucose joined by 1,4 and 1,6 glycosidic bonds. The branches give many ends for fast hydrolysis.. Amylose has unbranched chains with α-1,4 glycosidic links that coil into a compact shape.
Amylopectin has α-1,4 links along its chains and α-1,6 links at branch points. Its branches provide numerous ends at which enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. involved in mobilisation can act. Compare this with an unbranched chain rather than simply saying that “branches give a large surface areaThe total area of a surface. A larger surface area gives a faster rate of diffusion.”.
Glycogen and the value of storage
Glycogen is an animal storage carbohydrate, notably in liver and muscle cells. It resembles amylopectin but is more highly branched. Numerous accessible chain ends allow rapid mobilisation when glucose is needed.
Storage polysaccharides are compact, so many glucose residues fit into a small volume. They are effectively insoluble and have much less osmotic effect than the equivalent number of free glucose molecules. They can therefore store glucose without producing the large osmotic effect of a concentrated glucose solution. Their glycosidic bonds can be hydrolysed to release glucose for respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP..
Follow the process
Stored starch or glycogen → a hydrolytic enzyme meets an accessible chain end → water breaks one glycosidic bondThe covalent bond that joins two monosaccharides, formed by a condensation reaction. → a glucose unit is released → that glucose can enter respiration. The chain links are α-1,4. A branch point is an α-1,6 link, which creates another end.
More branches mean more ends, so more enzyme molecules can act at once if enzyme is available. The events happen at the polymer. An answer may start with the outcome, rapid release of glucose, and then give the mechanism, many ends. Do not memorise a fixed “every 10 residues” spacing: the specification contrast is that glycogen is more highly branched than amylopectin.
NOT: glycogen is not made of starch. Both are polymers of α-glucose.
- Starch = amylose + amylopectin
- Branch point = α-1,6, which makes another end
- Insoluble store has less osmotic effect than free glucose
- n glucose units in one unbranched chain: n−1 waters released
Two traps this diagram blocks
- Do not say “large surface area” without naming the chain ends and the hydrolysis.
- Do not bring in cellulose or β-glucose. Those belong with plant cell walls, not this storage comparison.
From the diagram alone
Question
Explain one advantage of storing glucose as glycogen rather than as amylopectin in a muscle cell. [3]
Show a model answer
Glycogen is more highly branched, so it has more chain ends. Hydrolytic enzymes can break glycosidic bonds at more ends at the same time. Glucose is therefore released more rapidly for respiration, assuming enough enzyme is present.
| Molecule | Structure | Useful consequence |
|---|---|---|
| Amylose | Unbranched, coiled α-glucose chain | Compact storage |
| Amylopectin | Branched α-glucose chains | More chain ends for mobilisation |
| Glycogen | More highly branched α-glucose chains | Many sites for rapid mobilisation |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. is mobilised quickly from glycogenThe storage polysaccharide in animals: highly branched chains of α-glucose, so glucose can be released quickly.. Y: many branch ends let hydrolytic enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. act at the same time. Z: free glucose is available for respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP.. Incomplete: “Glycogen is branched so it has a large surface areaThe total area of a surface. A larger surface area gives a faster rate of diffusion..” Improved: “Glycogen has many chain ends, so enzymes can hydrolyse glycosidic bonds at several ends at once and release glucose quickly.” The improvement names the ends and the reaction, not a vague surface.
Keep similar names separate
Glycogen is a glucose storage polymer. Glucagon is a hormone studied elsewhere. starchThe storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble, so it does not affect water potential. contains both amyloseThe unbranched part of starch: α-glucose joined by 1,4 glycosidic bonds, coiled into a compact helix. and amylopectinThe branched part of starch: α-glucose joined by 1,4 and 1,6 glycosidic bonds. The branches give many ends for fast hydrolysis..
Avoid this misconception
The claim: Glycogen is made from starch molecules.
The correction: Both are built from α-glucoseThe form of glucose with the –OH on carbon 1 below the ring. It is the monomer of starch and glycogen.. Starch is a mixture of amylose and amylopectin; glycogen is a different, more highly branched polymer.
Why is glycogen so branched?
Starch and glycogen store glucose. Enzymes release glucose from the ends of the chains, so the shape of the molecule changes how fast glucose comes out.
What the model shows
Amylose, amylopectin and glycogen of the same size, with enzymes releasing glucose from the chain ends.
What to look for
How many ends each molecule has, and how fast glucose is released with plenty of enzyme or only one.
What it tells us
More branches give more ends, so glucose is released faster if there is enough enzyme. Polymers store glucose compactly with little osmotic effect.
Be careful what you claim
Illustrative size and branch spacing.
Try it in the model: press Release glucose, then choose Glycogen.
Starch and glycogen as stores
Compare amylose, amylopectin and glycogen of the same size. Release glucose with enzymes at the chain ends and see how branching changes the rate. Sizes are illustrative.
What you see
- A 120-unit molecule of α-glucose (amylose, amylopectin or glycogen) with 1,4 links along chains and 1,6 links at branch points, and enzymes removing glucose from the chain ends.
- A graph of glucose released over time for all three, and counts of bonds, ends and dissolved particles.
How to use it
- Choose a polymer and the number of enzyme molecules, then press Release glucose.
What it shows
- Amylose is unbranched and coiled; amylopectin is branched; glycogen is more highly branched. More ends allow faster release of glucose (1.2).
- A compact, insoluble polymer stores many glucose units with almost no osmotic effect.
Limits of the model
- The branch spacing (every 8 or 4 units) and the 120-unit size are illustrative; real molecules are far larger. The chain is drawn flat, not as a helix.
- The enzyme is simplified: it removes one glucose per step and cannot pass a branch point that still carries units.
Guided tour · step 1 of 7
- In Storage polymer, just below these steps, keep Polymer on Amylose.
- Press Release glucose.
Amylose is an unbranched chain of α-glucose joined by 1,4 glycosidic bonds, coiled into a compact shape. It has only one end where the enzyme can work, so glucose comes off one unit at a time.
Storage polymer
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Explain two advantages of storing glucose as a polysaccharide rather than as free glucose.
Show suggested answer
Its compact structure stores many glucose residues in a small volume. Its low solubility and much smaller osmotic effect reduce the water-potential change that would result from many dissolved glucose molecules.
Practice question
Retrieval and explanation · suggested answer
An unbranched chain contains 80 glucose residues. How many water molecules are required to hydrolyse it completely?
Show suggested answer
79 water molecules: the chain has 79 glycosidic bonds, and each hydrolysis uses one water molecule.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two equal-mass glucose polymers differ only in branching. In a supplied model, an enzyme removes glucose units only from accessible chain ends. Predict which polymer is mobilised faster and state an assumption.
Show a hint
Separate your prediction from the conditions needed for it to hold.
Explore the reasoning
The more branched polymer should be mobilised faster because it has more accessible ends where enzymes can act simultaneously. This assumes enough enzyme is available and that the ends are accessible under otherwise equal conditions. Branching alone does not determine the rate under every condition.
Recall this lesson
Lesson recall questions
Question 1 of 5: How does the branching of glycogen make it a better short-term energy store than an unbranched chain?
Saccharides and energy stores
Practice question
Retrieval and explanation · suggested answer
How does the branching of glycogen make it a better short-term energy store than an unbranched chain?
Lesson: 1A 3: Carbohydrates 2: Polysaccharides
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Could starch become packaging?
1A · Carbohydrates 2: PolysaccharidesResearchers have modified starch to make flexible plastic materials. A University of Leicester study investigated adding a salt to change interactions between starch chains. This is a new…Specification 1.2(i) · 1.2(ii)Read & use the scienceClose article
Why this matters
Researchers have modified starch to make flexible plastic materials. A University of Leicester study investigated adding a salt to change interactions between starch chains. This is a new use for a familiar biological polymer. An energy-storage molecule becomes a material because many polymer chains can interact. Changing those interactions can change flexibility without turning every chain into glucose. This particular research is evidence for a material-making method, not proof that every starch product is environmentally preferable. A materials scientist would also need to measure strength, water sensitivity and what happens after disposal. Knowing the monomers is a starting point; judging a replacement for packaging requires testing the properties of the whole material.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.2(i) know the difference between monosaccharides, disaccharides and polysaccharides, including glycogen and starch (amylose and amylopectin)
- 1.2(ii) be able to relate the structures of monosaccharides, disaccharides and polysaccharides to their roles in providing and storing energy
Use the science
A starch-based film becomes more flexible after treatment, but no extra glucose is detected. Why does this not necessarily show that glycosidic bonds were hydrolysed, and what further evidence would be needed to judge its environmental value?
Explore a possible response
Flexibility can change when interactions between intact polymer chains change; it need not involve hydrolysis into glucose. Testing chain length or hydrolysis products would help distinguish mechanisms. Environmental evaluation also needs evidence about manufacture, useful lifetime and disposal, rather than assuming a starch starting material guarantees a better outcome.
Explore the source
- Abbott and colleagues (2012): salt-modified starch plasticsGreen Chemistry, Royal Society of Chemistry (2012)
What are endurance athletes actually storing?
1A · Carbohydrates 2: PolysaccharidesCarbohydrate loading in endurance sport is about changing the availability of stored fuel, not filling muscles with a pool of free glucose. Research on exercise performance has investigated…Specification 1.2(i) · 1.2(ii) · 1.4Read & use the scienceClose article
Why this matters
Carbohydrate loading in endurance sport is about changing the availability of stored fuel, not filling muscles with a pool of free glucose. Research on exercise performance has investigated muscle glycogen and how its availability relates to prolonged activity. Glycogen is a branched polymer: storing many glucose units in large molecules reduces the number of freely dissolved particles compared with storing the same units separately. Its structure also provides many ends from which units can be mobilised. This explains why molecular architecture matters to a sporting practice, while leaving training, event length and individual responses as additional variables.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.2(i) know the difference between monosaccharides, disaccharides and polysaccharides, including glycogen and starch (amylose and amylopectin)
- 1.2(ii) be able to relate the structures of monosaccharides, disaccharides and polysaccharides to their roles in providing and storing energy
- 1.4 know how monosaccharides (glucose, fructose and galactose) join together to form disaccharides (maltose, sucrose and lactose) and polysaccharides (glycogen, amylose and amylopectin) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions
Use the science
Why would storing an equivalent number of glucose units as free glucose create a different osmotic problem from storing them as glycogen?
Explore a possible response
Many separate glucose molecules exert a larger effect on water potential than far fewer polymer molecules; more water would tend to enter. This is a qualitative comparison, not a claim that glycogen stores contain no water.