IB Biology HLCarbohydrates & LipidsPaper 1 & 2~14 min read
Carbohydrates: Function & Examples
Starch, glycogen and cellulose are all made of glucose. One stores energy in plants, one stores it in animals, and one holds up a tree. The whole difference comes from which isomer of glucose was used, and where the branches are.
📚 What you need to know
The monomers of carbohydrates are monosaccharides. Two join to form a disaccharide; many join to form a polysaccharide.
Monosaccharides join by condensation, forming a glycosidic bond. Their general formula is CnH2nOn.
Glucose exists as two isomers: alpha glucose (OH below the ring at carbon 1) and beta glucose (OH above it).
Starch and glycogen are made from alpha glucose; cellulose is made from beta glucose.
Storage polysaccharides work because they are compact and insoluble; cellulose works because it is strong, durable, insoluble and chemically inert.
Starch is amylose (unbranched helix, 1,4 bonds) plus amylopectin (branched, 1,4 and 1,6 bonds). Glycogen is more branched than amylopectin.
Monosaccharides
Monosaccharides have the general formula CnH2nOn, where n is the number of carbon atoms. That formula applies to monosaccharides only — not to disaccharides or polysaccharides, because water is lost each time a bond forms.
They are colourless crystalline molecules and are soluble in water. Different types are named by how many carbon atoms they contain.
Type
Carbon atoms
Example
Ring shape
Triose
3
Glyceraldehyde
No ring
Pentose
5
Ribose
Five-point ring
Hexose
6
Glucose
Six-point ring
You are expected to recognise ring structures, not to draw every atom. Count the corners: a five-point ring is a pentose such as ribose, and a six-point ring is a hexose such as glucose. That one check answers most structure-recognition questions.
Glucose and its two isomers
The best known carbohydrate monomer is glucose, molecular formula C6H12O6. It is the most common monosaccharide, the main substrate used in respiration to release energy for the production of ATP, and it is produced during photosynthesis.
Glucose exists in two structurally different forms, alpha glucose and beta glucose. These are the isomers of glucose: same atoms, different arrangement.
Always check carbon 1, on the right-hand corner of the ring. Below the ring is alpha; above the ring is beta.
This seemingly minor example of isomerism has far-reaching consequences for the polymers built from it. Different polysaccharides are formed from the two isomers: starch and glycogen from alpha glucose, cellulose from beta glucose.
Properties of glucose
Property
Reason
Stable structure
Covalent bonds are strong and hard to break
Soluble in water
The molecule is polar
Easily transportable
Follows from its water solubility
A source of chemical energy
Energy is released when its covalent bonds are broken
What polysaccharides are for
Carbohydrates function as energy storage molecules and as structural molecules.
Storage: starch and glycogen
Compact. Large quantities can be stored in a small space.
Insoluble. This is essential, because soluble molecules dissolve in the cytoplasm, lowering the water potential and causing water to move into the cell. If too much water enters an animal cell, it will burst.
Structure: cellulose
Strong and durable.
Insoluble and slightly elastic.
Chemically inert: few organisms possess enzymes that can hydrolyse it.
Read the shapes from left to right and the story is one of increasing branching, then a complete change of direction for cellulose.
Starch
Starch is the storage polysaccharide of plants, stored as granules in chloroplasts, and made of alpha glucose monomers. It is constructed from two different polysaccharides.
Amylose (10–30 % of starch). An unbranched, helix-shaped chain with 1,4 glycosidic bonds between alpha glucose molecules. The helix shape makes it more compact, and therefore more resistant to digestion.
Amylopectin (70–90 % of starch). Contains 1,4 glycosidic bonds as well as 1,6 glycosidic bonds, creating a branched molecule. The branches produce many terminal glucose molecules that can be easily hydrolysed for respiration, or added to for storage.
Glycogen
Glycogen is the storage polysaccharide of animals and fungi.
Its monomer is alpha glucose, joined by 1,4 and 1,6 glycosidic bonds.
It is more branched than amylopectin, providing more free ends where glucose can be removed by hydrolysis. This means glycogen can be broken down quickly, supplying the higher metabolic needs of animal cells.
Liver and muscle cells contain glycogen as visible granules, enabling high rates of cellular respiration.
Cellulose
Cellulose is a structural carbohydrate found in plant cell walls. Its molecules are straight and unbranched.
It is a polymer of beta glucose. The hydroxyl group on carbon 1 sits above the ring in beta glucose, whereas it sits below the ring in alpha glucose.
So in order to form a glycosidic bond, every alternate beta glucose molecule must invert itself — flip upside down.
That alternating pattern allows hydrogen bonding between strands of beta glucose, adding strength to the polymer. Hydrogen bonds link many cellulose molecules together to form microfibrils.
Individually a hydrogen bond is weak. The point about cellulose is that there are an enormous number of them, side by side along the whole length of the chains. Thousands of weak bonds acting together is what holds a tree upright.
Summary table
Feature
Amylose
Amylopectin
Glycogen
Cellulose
Monomer
Alpha glucose
Alpha glucose
Alpha glucose
Beta glucose
Branches
No
Yes, about every 20 monomers
Yes, about every 10 monomers
No
Helix shape
Yes
No
No
No
Glycosidic bonds
1,4
1,4 and 1,6
1,4 and 1,6
1,4
Found in
Plant cells
Plant cells
Animal cells
Plant cells
Worked examples
WE 1
Why storage molecules are insoluble
Explain why it is important that starch and glycogen are insoluble. (3 marks)
Point 1: what solubility would do
Soluble molecules would dissolve in the cytoplasm and lower the water potential of the cell.
Point 2: the consequence
Water would then move into the cell by osmosis.
Point 3: the damage
If too much water enters an animal cell, the cell will burst.
Insoluble means no osmotic effect on the cell“compact” is a separate mark — large amounts stored in a small space
WE 2
Glycogen versus amylopectin
Suggest why animals store glycogen rather than a molecule like amylopectin. (3 marks)
Point 1: the structural difference
Glycogen is more branched than amylopectin.
Point 2: what branching gives you
More branches means more terminal glucose molecules, so there are more free ends for enzymes to act on during hydrolysis.
Point 3: why animals need it
Glucose can therefore be released quickly, meeting the higher metabolic demands of animal cells during respiration.
More branches, more free ends, faster releasethe word “terminal” is worth using — it shows you know where hydrolysis happens
WE 3
From isomer to function
Explain how the structure of cellulose makes it suitable for its function in plant cell walls. (4 marks)
Point 1: the monomer
Cellulose is a polymer of beta glucose, so every alternate molecule must be inverted to form a glycosidic bond.
Point 2: the shape
This gives straight, unbranched chains that lie parallel to each other.
Point 3: the bonding
Hydrogen bonds form between the chains, linking them into microfibrils and adding strength.
Point 4: the function
The result is strong, durable, insoluble and chemically inert, so it supports the cell and resists digestion.
Beta glucose → inverted chains → hydrogen bonds → strengthstart from the isomer; answers that begin at “it is strong” miss the first two marks
💡 Exam tips
Learn the general formula CnH2nOn and that it applies to monosaccharides only.
Recognise rings by counting corners: five-point pentose, six-point hexose.
Say which isomer a polysaccharide is built from — it is often the first marking point.
Quote the bond types: 1,4 for straight chains, 1,6 for branch points.
For storage molecules, give both compact and insoluble, and explain the water potential point.
Use the percentages if you can: amylose 10–30 %, amylopectin 70–90 %.
⚠ Common mistakes
Saying cellulose is made from alpha glucose. It is beta.
Applying CnH2nOn to a disaccharide. Water is lost when the bond forms, so the formula no longer holds.
Saying glycogen is found in plants. Animals and fungi.
Writing that starch is one molecule. It is amylose plus amylopectin.
Saying hydrogen bonds are strong. Each one is weak; there are simply a great many of them.
Confusing insoluble with unreactive. Insolubility is about osmosis; inertness is about enzymes.
Up next: The Role of Glycoproteins — what happens when a carbohydrate is attached to a protein and put on the outside of a cell.
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